the development of language in bonobos and chimpanzees
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P e r g a m o n
Language Communication, Vol. 15, No. 2, pp. 1 21-148, 1995
C o p y r i g h t © 1 9 9 5 E l s e v i e r S c i e n c e L td
Pr in ted in Grea t Br i ta in . Al l r ights reserved
0271-5 309/95 9 .50 + 0 .00
0271-5309 95)00001-1
T H E D E V E L O P M E N T O F L A N G U A G E S K I L L S IN B O N O B O A N D
C H I M P A N Z E E - - I . C O M P R E H E N S I O N
K A R E N E . B R A K K E a n d E . S U E S A V A G E - R U M B A U G H
T h e s p e c ie s m o s t c l o s e l y re l a te d t o o u r o w n a r e t h e b o n o b o
P a n p a n i s c u s )
a n d th e c h i m p a n z e e
P a n t r o g l o d y t e s ) , w ho d i ve r g ed f r om t he hom i n i d ev o l u t i ona r y pa t h a s l it tl e as f i ve t o e i gh t
m i l l io n y e a r s ag o ( D i a m o n d , 1 9 8 8 ; M i y a m o t o
e t a L ,
1988) . The t wo s pec i e s o f
P a n ,
in turn ,
s ha r ed a com m on an ces t o r t wo t o t h r ee m i l l i on yea r s ago ( Di am ond , 1988).
A l t h o u g h c h i m p a n z e e s a n d b o n o b o s h a v e d e m o n s t r a te d m a n y c o m p l e x c o g n i t iv e s k il ls ( e .g .
B o y s e n a n d B e r n t s o n , 1 9 90 ; P r e m a c k , 1 9 7 6; R u m b a u g h e t a l . , 1993 ; To t h e t a l . , 1993) , the i r
capac i t y t o acqu i r e and us e l anguag e ha s been a t op i c o f con t r ove r s y ( Te r r ace
e t a l . ,
1979;
Vau c l a i r, 1990). Pa r t o f the d i f f i cu l ty in a s s e s s ing l anguage s k i ll s i n apes a r i s e s f r om t he f ac t
t ha t t hey do no t s peak . Ev en i f t hey pos s e s s ed a l l o f the cogn i t i ve requ i s i t e s f o r l anguage , t he
s t r uc t u r e o f t he i r voc a l appa r a ti wo u l d no t pe r m i t a rt i cu la t e s peech ( D uch i n , 1990 ; L i ebe r m an ,
1991) . Thus , i n o r de r t o exam i ne l anguage p r oduc t i on i n t he g r ea t apes , i nves t i ga t o r s have
t u r ned t o s i gn ( Fou t s , 1973 ; Ga r dne r and Ga r dne r , 1969 ; Mi l e s , 1983 ; Pa t t e r s on , 1978) o r t o
a r t if i c ia l s ym bo l s ys t em s ( P r em ack , 1971 ; Rum baug h , 1977 ; Sava ge - R um baug h , 1986 ;
S a v a g e - R u m b a u g h
e t a l . ,
1986) , e ach o f wh i ch en t a i l m o t o r and / o r vocab u l a r y l im i t a t i ons f o r
t h e n o n - h u m a n s u b je c ts . M a n u a l s ig n s s u c h a s th o s e u s e d in A m e r i c a n S i g n L a n g u a g e ( A S L ) ,
f o r e x a m p l e , o f t e n r e q u ir e f i n e m o t o r c o o r d i n a ti o n t h at p r o v e s a w k w a r d f o r l es s d e x t r o u s a p e
hands . Al s o , ape s ub j ec t s u s i ng non- s i gn s ym bo l s a r e expos ed t o a t m os t s eve r a l hundr ed
s y m b o l s t h a t t h e y c a n p r o d u c e ( P r e m a c k , 1 9 7 6 ; S a v a g e - R u m b a u g h
e t a l . ,
1986) r a t he r t han t he
t h o u s a n d s o f w o r d s h e a r d b y m o s t c h il d r e n as th e y m a t u r e . E v a l u a ti o n o f t h e a p e s p r o d u c t i v e
s k i ll s i s i n l a r ge pa r t, t hen , d e t e r m i ned by m o da l i t y l i m i ta t ions .
C o m p r e h e n s i o n m a y p r o v i d e a b e tt e r i n d e x o f a s p e c i e s c o m p e t e n c e f o r l a n g u a g e t ha n d o e s
p r oduc t i on . As f a r a s we know , no pe r i phe r a l ana t om i ca l l i m i t at i ons app l y to apes ab i l it y t o
c o m p r e h e n d h u m a n s p e e c h . T h e e a r s t ru c t u re a n d t h e s e n s o r y - p e r c e p t u a l a u d i to r y p a t h w a y s o f
t h e t w o g e n e r a a p p e a r t o b e v e r y s i m i l a r ( A n k e l - S i m o n s , 1 9 8 3 ) ; s p e e c h c o m p r e h e n s i o n
d i f f e r ences be t ween s pec i e s , t he r e f o r e , a r e l i ke l y t o r e f l e c t CNS f unc t i on i ng . By eva l ua t i ng
w h i c h o f t h e r e c e p t i v e c a p a c i t i e s u n d e r l y i n g l i n g u i s t i c c o m p e t e n c e a r e w e l l d e v e l o p e d i n
s p e c ie s c l o s e l y r e l a te d t o o u r o w n , a n d w h i c h a r e n o t, w e m a y b e a b le t o b e t t e r u n d e r s t an d h o w
hum ans cam e t o be t he adep t l ingu i s t ic c r ea t u r e s tha t we a r e .
L a n g u a g e c o m p r e h e n s i o n i n h u m a n s
D e v e l o p m e n t a l p s y c h o l o g i s ts a r e fa m i l ia r w i th t h e p r e m i s e th a t c o m p r e h e n s i o n l e a d s
p r o d u c ti o n ( G o l d in - M e a d o w e t a l . , 1 9 76 ; H u t t e n l o c h e r , 1 9 7 4 ) . C o m p r e h e n s i o n o f l a n g u a g e
C o r r e s p o n d e n c e r e l a t i n g to t h i s p a p e r s h o u l d b e a d d r e s s e d t o K a r e n E . B r a k k e , L a n g u a g e R e s e a r c h C e n t e r , G e o r g i a
S t a t e U n i v e r s i t y , U n i v e r s i t y P l a z a , A t l a n t a , G A 3 0 3 0 3 - 3 0 8 3 , U . S . A .
121
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122 KAREN E. BRAKKE and E. SUE SAVAG E RUMB AUGH
b e g i n s t o b e e v i d e n t a r o u n d 8 - 1 0 m o n t h s o f a g e ( B a t e s , 1 9 9 3 ) , a n d a c h i l d s r e c e p t i v e
vocabu l a r y ea r l y i n t he l anguage - l ea r n i ng p r oces s i s l i ke l y t o be l a r ge r t han i t s p r oduc t i ve
v o c a b u l a r y ( B a t es , 1 99 3; G o l d i n - M e a d o w e t a l . 1976 ; Snyde r e t a l . 1981) .
A l t h o u g h l a n g u a g e c o m p r e h e n s i o n b e g i n s t o a p p e a r e a r l y i n l i f e a n d d o e s n o t i n h e r e n t l y
r equ i r e t he f i ne m o t o r ab i l it ie s t ha t s peech and s i gn p r oduc t i ve s k il ls do , i t is by n o m eans a
s i m p l e o r a u t o m a t i c p h e n o m e n o n . T h e d e v e l o p m e n t o f r e c e p t iv e l a n g u a g e s k i ll s i n o u r s p e c i e s
r equ i r e s t ha t s eve r a l m i n i m a l cond i t i ons be m e t . Us i ng s poken l anguage a s an exam pl e , t he
i nd i v i dua l m u s t be ab l e t o d i s ti ngu i s h d i f f e r en t pho nem es , t hen r e l a te t hem t o on e ano t he r a s
w o r d s . M e a n i n g m u s t b e a t ta c h e d t o t h e w o r d s , a n d f i n a l ly t h e w o r d s m u s t b e s y s t e m a t i c a ll y
r e l a t ed i n o r de r f o r the i nd i v i dua l t o de r i ve t he agg r ega t e m ean i ng o f t he u t te r ance . Un de r l y i ng
t hes e l i ngu i s t i c s k i l l s a r e cogn i t i ve capac i t i e s s uch a s t he ab i l i t y t o i n t eg r a t e t em por a l l y o r
s pa t i a l l y d i s t i nc t p i ece s o f i n f o r m a t i on ( Ba t e s , 1993 ; Di am ond , 1991) , t o m en t a l l y r ep r e s en t
abs en t en t i t i e s ( P i age t and I nhe l de r , 1969) , and t o e s t ab l i s h pa r t - who l e r e l a t i ons ( Lange r ,
1 9 8 6 ) . T h e c o n t e x t o f t h e u t t e r a n c e a n d t h e c o m m u n i c a t i v e i n t e n t o f t h e s p e a k e r a r e a l s o
e s s en t i a l com pon en t s o f s ucces s f u l unde r s t and i ng o f o t he r s m e s s ages ( Br une r , 1983 ; Lo ck ,
1980) .
I n add i t i on t o cogn i t i ve r equ i si t e s, app r op r i a te env i r onm en t a l cond i t ions m us t a l s o be ex t an t
f o r l anguag e - l ea r n i ng t o occu r . T he i n f an t c ann o t l e a r n l anguage u n l e s s i t i s expos e d t o it. I t is
b e c o m i n g i n c r e a s in g l y e v i d e n t t h at t h e d e v e l o p m e n t o f la n g u a g e c o m p e t e n c e i s h e a v i ly
s uppor t ed by t he peop l e and cu l t u r e s u r r ound i ng t he i n f an t ( Br une r , 1983 ; Lock , 1980) . Us e
o f ch i l d - d i r ec t ed s peech ( Be r k o - G l eas on , 1989 ; Fe r gus on , 1964 ; Snow , 1977) and j o i n t
a t te n t io n r o u t in e s ( B a k e m a n a n d A d a m s o n , 1 9 8 4; B r u n e r , 1 9 7 5 ; T r e v a r t h e n a n d H u b l e y ,
1978) , f o r exam pl e , h i gh l i gh t t he com m u ni ca t i ve p r oces s and e ngage t he i n f an t s a t ten t ion .
Pa r en t s encour age ch i l d r en t o pa r t i c i pa t e i n and r e s pond app r op r i a t e l y t o u t t e r ances du r i ng
da i l y ac t i v i t i e s l ong be f o r e t hey expec t t o hea r s i m i l a r u t t e r ances f r om t he i n f an t s ( Br une r ,
1 9 8 3 ; G o l d i n - M e a d o w e t a l . 1976) .
C o m p r e h e n s i o n , t h e n , a r is e s i n h u m a n s d u r i n g n o r m a l d a y - t o - d a y in t e r ac t io n s b e t w e e n
i n f an ts and o t he r s . Ch i l d r en a r e no t t augh t l angu age by d r i ll i ng r epea t ed t ri a ls o f wo r ds i n
o r d e r to g e t a f o o d r e w a r d . T h i s m u s t b e c o n s i d e r e d w h e n s t u d y in g l a n g u a g e c o m p e t e n c y i n
o t he r p r i m a t e s . I n o r d e r t o eva l ua t e s k i ll s o f r e l a t ed s pec i e s a t a l eve l t ha t pe r m i t s co m p ar i s on
t o ou r own , we m us t p r ov i de r ea r i ng env i r onm en t s a s s i m i l a r a s pos s i b l e t o t hos e f ound i n
hum an cu l t u r e s . Th i s i nc l udes p r ov i d i ng l i ngu i s t i c and s oc i a l i npu t t ha t i s m ode l ed a f t e r t he
way hum an adu l t s i n t e r ac t wi t h i n f an t s a r ound t he wor l d .
a n g u a g e c o m p r e h e n s i o n in b o n o b o s
A t t h e L a n g u a g e R e s e a r c h C e n t e r ( L R C ) , t w o b o n o b o s ( K a n z i a n d M u l ik a ) , l e a rn e d t o u s e
and und e r s t and s ym bo l s i n m an y d i f f e r en t com m uni ca t i ve capac i ti e s wi t hou t exp l i c i t t r a in i ng
( S a v a g e - R u m b a u g h e t a l . 1990 ; Sev c i k and Savage - R um baug h , 1994) . Th e i r i n s t r uc ti on was
m u ch l e s s s t r uc t u r ed than t ha t o f o t he r apes wh o w er e t augh t t o u s e s ym bo l s i n repea t ed - t r ia l
o r r ewa r d - bas ed pa r ad i gm s ( P r em ac k , 1971 ; Ru m b augh , 1977 ; Sava ge - Ru m b augh , 1986).
L a n g u a g e - l e a r n i n g b y t h e s e b o n o b o s i n v o l v e d o b s e r v a t io n o f h u m a n m o d e l s w h o u s e d t h e
s y m b o l s a s t h e y i n t e ra c t e d w i th t h e a p e s t h r o u g h o u t t h e d a y . T h e y o u n g b o n o b o s w e r e a s k e d
t o a t t end t o and pa r t i c i pa t e i n t he da i l y ac t i v i t i e s o f t he l abo r a t o r y ( i nc l ud i ng p l ay and
s t r uc t u r ed t a s ks ) , bu t we r e no t r equ i r ed t o p r oduce s ym bo l s i n o r de r t o r ece i ve f ood o r o t he r
r ewar ds .
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COMPREHENSION IN P N 23
T h e t w o b o n o b o s s p o n t a n e o u s l y d e v e l o p e d r e c e p t i v e l a n g u a g e s k i l l s a n d r e a d i l y
c o m p r e h e n d e d s e v e r a l E n g l i s h w o r d s u n d e r b l i n d t e s t c o n d i t i o n s ( S a v a g e - R u m b a u g h
e t a l .
1 9 9 3; S e v c i k a n d S a v a g e - R u m b a u g h , 1 9 9 4 ). T h e o l d e r o f th e s e b o n o b o s h a s , i n f a c t,
d e m o n s t r a t e d t h e a b i l i t y t o u n d e r s t a n d s e v e r a l h u n d r e d n o v e l c o m p l e x s p o k e n s e n t e n c e s
( S a v a g e - R u m b a u g h e t a l . 1 9 9 3 ) . F u r t h e r m o r e , t h e c o m p r e h e n s i o n o f s y m b o l s g e n e r a l i z e d t o
s y m b o l p r o d u c ti o n i n th e s e s u b j ec t s ( S a v a g e - R u m b a u g h
e t a l .
1990) . In shor t , the two
b o n o b o s w e r e r e a r e d m u c h l i k e c h i l d re n a n d t h e ir s y m b o l a c q u is i ti o n e v i d e n c e d m a n y o f t h e
s a m e c h a r a c t e ri s t ic s a s t h a t o f y o u n g h u m a n s .
L a n g u a g e - l e a r n i n g a n d r o u t i n e s
C e n t r a l t o th e b o n o b o s l e a r n in g o f n at u r a l la n g u a g e w a s p a r t ic i p a t i o n in i n te r i n d iv i d u a l
r o u ti n e s a c c o m p a n i e d b y a d u l t la n g u a g e u s e . A s o u t li n e d i n S a v a g e - R u m b a u g h
e t a l .
(1993) ,
t h e s e r o u t in e s a r e m o r e o r l es s r e g u l a r ly s e q u e n c e d s e t s o f i n t e ri n d i v id u a l i n t e r a c ti o n s t h a t
o c c u r in a r e l a ti v e l y s i m i l a r m a n n e r o n d i f f e r e n t o c c a s i o n s ( p . 2 5 ). T h e s e i n t e ra c t i o n s a r e
e m b e d d e d w i t h i n a l a r g e r c o n t e x t . F o r e x a m p l e , w i t h i n a n e v e n i n g c o n t e x t w h e n t h e l a b i s
q u i e t , t h e r e w i l l o c c u r r o u t i n e s i n v o l v i n g m i l k p r e p a r a t i o n , q u i e t p l a y , d i a p e r c h a n g i n g , a n d
g r o o m i n g u n t i l t h e i n f a n t fa l ls a s l e e p . E a c h o f th e s e r o u t i n e s , i n tu r n , i s m a d e u p o f a s e q u e n c e
o f a c ts o f c o m p o n e n t s s u c h a s - - f o r m i l k p r e p a r a t i o n - - p u t t i n g t h e li n in g i n t he b o t tl e , o p e n i n g
t h e m i l k f o r m u l a , p o u r i n g t h e f o r m u l a i n to t h e b o t tl e , a d d i n g w a t e r , p u t t in g t h e n i p p l e o n , a n d
f ina l l y d r i nk ing t he mi lk .
W i t h i n t h e s e r o u t i n e s , e a c h p a r t i c i p a n t h a s a r o l e t h a t f r o m d a y t o d a y s t a y s r e l a t i v e l y
c o n s i s t e n t b u t o v e r th e c o u r s e o f t i m e m a y s h i f t, w i t h t h e in f a n t b e c o m i n g m o r e a n d m o r e
ac t i ve i n in i t i a ti ng t he co m pon en t s o f the rou t i ne . I n i t ia l l y , how eve r , t he adu l t pa r t i c i pan t
d i r e c ts t h e f l o w o f t h e i n t e r a c t io n a n d a s s i g n s a r o l e t o t h e i n f a n t t h at c o n s i s ts o f p a y i n g
a t t en t i o n a n d i n d i c a ti n g r e a d i n e s s t o c o n t i n u e w i t h t h e n e x t c o m p o n e n t o f t h e r o u ti n e . S a l i e n t
c o m p o n e n t s o r t r a n si t io n s o f t h e r o u t i n e a r e b e h a v i o r a l l y a n d l i n g u is t i c al l y ( v i a s p e e c h a n d / o r
l e x i g ra m ) e n c o d e d b y th e a du l t. S u c h m a r k i n g o f t h e c o m p o n e n t s a p p e a rs to b e m o s t
e f f e c t i v e w h e n i t o c c u r s p r i o r t o t h e a c t u a l e v e n t ( S a v a g e - R u m b a u g h , 1 9 9 1 ) . T a l k i n g a b o u t
w h a t w e a r e g o i n g t o d o , b e f o r e w e d o i t, a l l o w s a l l p a r t ic i p a n t s t o p r e d i c t w h a t w i l l h a p p e n
n e x t . O n c e a b l e t o p r e d i c t u p c o m i n g e v e n t s , t h e i n f a n t c a n b e g i n t o e x e r t s o m e c o n t r o l o v e r
h o w t h o s e e v e n t s u n f o ld .
Q u i t e o f t en , c o m p o n e n t s o f a s t a n d a r d r o u t i n e w i ll b e a d d e d o r m o d i f i e d . If , w h i l e
p r e p a r i n g m i l k , f o r e x a m p l e , i t is d i s c o v e r e d t h a t t h e b o x o f b o t t le l i n e rs i s e m p t y , a n e w b o x
w i l l h a v e t o b e lo c a t e d a n d o p e n e d . L i n g u i s t ic m a r k i n g b e c o m e s e s p e c i a l l y i m p o r t a n t a t th e s e
t i m e s w h e n t h e r o u t i n e i s a l t e r e d . B y p r o v i d i n g i n f o r m a t i o n t h a t m i g h t o t h e r w i s e b e l a c k i n g ,
t h e v e r b a l i n f o r m a t i o n a l l o w s a l l o f t h e i n d i v i d u a l s i n v o l v e d t o c o o r d i n a t e t h e i r b e h a v i o r s o
t h a t c o n t i n u a t io n o f t h e r o u ti n e m a y p r o c e e d s m o o t h l y ( s e e S a v a g e - R u m b a u g h , 1 9 91 , f o r a f u l l
d i s c u s s io n o f t h e c o n t r i b u t io n s o f r o u t i n e s t o l a n g u a g e - l e a r n i n g i n a p e s a n d c h i ld r e n ).
S p e c i e s o r r e a r i n g d i f f e r e n c e s
P r i o r t o t h e s t u d y w i t h b o n o b o s a t t h e L R C , t w o m a l e c h i m p a n z e e s , S h e r m a n a n d A u s t i n ,
w e r e t a u g h t r e fe r e n ti a l u s e a n d c o m p r e h e n s i o n o f v i s u o g r a p h i c s y m b o l s ( l e x i g r a m s ) v i a
e x p l i c i t t r i a l- b a s e d p a r a d i g m s . E a c h c o m m u n i c a t i v e f u n c t i o n - - r e q u e s t i n g , l a b e l i n g , a n d
c o m p r e h e n s i o n o f o t h e r s u t t e r a n c e s - - w a s t a u g h t s e p ar a t el y a n d r e q ui re d m a n y r e p e a t ed t ri al s
f o r th e c h i m p a n z e e s t o m a s t e r . P r o d u c t i v e s k i l ls d i d n o t g e n e r a l iz e t o c o m p r e h e n s i o n w i t h o u t
e x p l i c i t t r a in i n g ( S a v a g e - R u m b a u g h , 1 9 8 6 ) , a n d c o m p r e h e n s i o n o f s y m b o l s , o n c e le a r n e d , d i d
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1 24 K A R E N E . B R A K K E a n d E . S U E S A V A G E R U M B A U G H
n o t e x t e n d t o s p e e c h ( S a v a g e - R u m b a u g h e t a l . 1 9 8 5 b ) . A l t h o u g h S h e r m a n a n d A u s t i n
a p p e a r e d t o u n d e r s t a n d s o m e v e r b a l r e q u e s t s p r e s e n t e d t o t h e m o n a r e g u l a r b a s i s , w h e n
c o m p a r e d o n a f o r m a l s i n g l e - w o r d r e c e p t i v e l a n g u a g e t a s k ( ' G i v e m e t h e x ' ) , t h e t w o
c h i m p a n z e e s d e m o n s t r a t e d n o c o m p r e h e n s i o n o f s p o k e n E n g l i sh d i v o r c e d o f c o n te x t u al
c u e s .
T h e d a t a f r o m S h e r m a n a n d A u s t i n o n o n e h a n d a n d K a n z i a n d M u l i k a o n t h e o t h e r
s u g g e s t e d a p o s s i b l e s p e c i e s d i f f e r e n c e in t h e a b i li t y to u n d e r s t a n d c o m p l e x s p e e c h ( S a v a g e -
R u m b a u g h e t a l . 1 9 8 5 a) . I f c o n f i r m e d , t h i s d i f f e re n c e w o u l d b e a p o i n t o f d i s t in c t io n b e t w e e n
t h e t w o s p e c i e s o f P a n a n d m i g h t p r o v i d e i m p o r t a n t c lu e s t o t h e e v o l u t i o n o f h u m a n
l a n g u a g e .
T h e c o m p a r i s o n b e t w e e n t h e b o n o b o ( K a n z i a n d M u l i k a ) a n d c h i m p a n z e e ( S h e r m a n a n d
A u s t i n ) s u b j e c t s w a s c o m p r o m i s e d b y t w o d i f f e r e n c e s in t h e ir r ea r i n g h i st o r ie s , h o w e v e r . T h e
c h i m p a n z e e s w e r e 1 t o 2 ' ~ y e a r s o l d w h e n t h e y st a r te d s y m b o l t ra i n in g , w h i l e th e b o n o b o s
w e r e 4 - 6 m o n t h s o ld w h e n t h e y w e r e f i rs t e x p o s e d to s y m b o l u se a n d E n g l i s h - s p e a k in g
c a r e g i v e r s . I t m a y b e t h a t e a r ly e x p o s u r e a n d a t t a c h m e n t t o h u m a n s i s a c ri t ic a l c o m p o n e n t f o r
d e v e l o p i n g s p e e c h c o m p r e h e n s i o n i n c h i m p a n z e e s .
T h e r e i s s o m e e v i d e n c e t h a t t h i s m a y b e t h e c a s e . F o u t s e t a l . ( 1 9 7 6 ) r e p o r t e d a p p r o p r i a t e
r e s p o n s e s t o a t l e a s t 1 0 E n g l i s h w o r d s i n a c h i m p a n z e e t r a i n e d i n A S L . E a r l i e r s t u d i e s w i t h
c h i m p a n z e e s V i k i ( H a y e s , 1 9 5 1 ) a n d G u a ( K e l l o g g a n d K e l l o g g , 1 9 33 ) s u g g e s t e d t h a t s o m e
s p e e c h c o m p r e h e n s i o n m a y h a v e d e v e l o p e d i n ' h o m e - r e a r e d ' s u b j e ct s, b u t t h e se r e p o rt s w e r e
b a s e d o n d a t a c o l l e c t e d w i t h o u t c o n t e x t u a l c o n t r o l s a n d s o a r e in c o n c l u s i v e .
T h e s e c o n d d i f f e r e n c e b e t w e e n t h e L R C s t u d ie s i n v o l v i n g t h e c h i m p a n z e e s o n th e o n e h a n d
a n d t h e b o n o b o s o n th e o t h e r w a s t h e l a n g u a g e - l e a r n i n g e n v i r o n m e n t e x p e r i e n c e d b y t h e a p e s
a f t e r t h e i r in c l u s io n i n t h e p r o j e c t. T h e b o n o b o s l e a r n e d s y m b o l u s e o b s e r v a t i o n a l l y , w i t h a n
e m p h a s i s o n s y m b o l c o m p r e h e n s i o n . T h e c h i m p a n z e e s , o n t h e o t h e r h a n d , w e r e f i r s t t a u g h t
s y m b o l i c re q u e s t s k i ll s i n a s t ru c t u re d , t r i a l- b a s e d p a r a d i g m . A s S a v a g e - R u m b a u g h e t a l .
( 1 9 90 ) n o t ed , it ' m a y b e t h at a n e a r ly e m p h a s i s o n p r o d u c t iv e s y m b o l t r a i n i n g . . , m a y , in
i t s e l f , i n t e r f e r e w i th acqu i s i t i on p roces se s t ha t r e l y on obse rva t i on ' ( p . 250 ) .
I n o r d e r t o e v a l u a t e b o t h p r o d u c t i v e a n d r e c e p t i v e l a n g u a g e d i f f e r e n c e s i n t h e t w o s p e c i e s
o f P a n t h e n , t h e n e x t s t ep w a s t o r ai s e a m e m b e r o f e a c h s p e c i e s t o g e t h e r f r o m a n e a r l y a g e ,
p r o v i d i n g t h e m w i t h th e o p p o r t u n i t y t o l e a r n l a n g u a g e s k i l ls o b s e r v a t i o n a l l y a n d e m p h a s i z i n g
t h e c o m p r e h e n s i o n o f v e r b a l an d v i s u a l s y m b o l u s e in s t e a d o f r e q u i ri n g s y m b o l p r o d u c t i o n
f r o m t h e o u t s e t . ' T h i s i s t h e s a m e p a r a d i g m u s e d w i t h t h e o l d e r b o n o b o s , K a n z i a n d M u l i k a .
I f u n d e r s u c h c o n d i t i o n s t h e c h i m p a n z e e f a il s to d e v e l o p s p e e c h c o m p r e h e n s i o n s k il ls s i m i l a r
t o t h o s e t h a t b o n o b o s h a v e e x h i b i t e d , t h e n t h is s p e c i e s d i f f e r e n c e m a y r e p r e s e n t a k e y , a b r u p t
d e v e l o p m e n t i n t he p h y l o g e n y o f li n g u is t ic c o m p e t e n c e . I f a t th e o t h e r e x t r e m e , h o w e v e r , t h e
c h i m p a n z e e a n d b o n o b o p r o f i t e q u a l l y f r o m o b s e r v a t i o n a l l e a r n i n g , t h i s w o u l d t e s t i f y t o t h e
i m p o r t a n c e o f e a r l y e x p o s u r e t o a li n g u i st i c c u l tu r e i n f o s t e r i n g c o m p r e h e n s i o n s k il ls e v e n i n
s p e c i e s o t h e r t h a n o u r o w n .
T h i s c o m p a r i s o n h a s n o w b e e n e f f e c t e d w i th a s i n gl e b o n o b o a n d c h i m p a n z e e p a i r a n d t h e
r e s u lt s o f th e s t u d y r e l e v a n t t o c o m p r e h e n s i o n a r e r e p o r t e d h e r e in . T w o g e n e r a l a s p e c t s o f t h e
s u b j e c t s ' r e c e p t i v e l a n g u a g e d e v e l o p m e n t p r o v i d e d t h e f o c i o f t h e p r e s e n t s t u d y : th e i r in i ti a l
e n t r y i n t o t h e s y s t e m ( i. e. h o w t h e y f i rs t c a m e t o r e s p o n d a p p r o p r i a t e l y to u t t e r an c e s ) a n d t h e
e m e r g e n c e o f c o m p e t e n c e ( i. e. t h e g r o w t h o f th e i r s k il ls r e s p o n d i n g t o s i n g l e w o r d s a n d
c o m p l e x u t t er a n c e s ).
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COMPREHENSION IN P N 25
e t h o d
Subjec ts
P a n b a n i sh a , t h e b o n o b o Pan pan i s cus ) , w a s b o r n a t t h e L a n g u a g e R e s e a r c h C e n t e r (L R C )
o n 1 7 N o v e m b e r 1 9 8 5 . S h e w a s r e a r e d b y h e r m o t h e r , M a t a t a , u n t i l s h e w a s 7 w e e k s o f a g e
w h e n h e r c a r e w a s t r a n s f e r r e d t o h u m a n c a r e g i v e r s . P a n p a n z e e , t h e c h i m p a n z e e P a n
troglody tes ) , w a s b o r n a t t h e m a i n s t a t io n o f th e Y e r k e s R e g i o n a l P r i m a t e R e s e a r c h C e n t e r o n
3 1 D e c e m b e r 1 98 5 ( th e f i rs t c h i m p a n z e e b o r n t h e r e a f t e r P a n b a n i s h a s b i r th ) a n d t r a n s f e r r e d
t o t h e L R C o n 8 J a n u a r y 1 98 6. F r o m t h a t d a y t h r o u g h t h e e n d o f th e s t u d y p e r i o d ( O c t o b e r
1 9 8 9) , P a n b a n i s h a a n d P a n p a n z e e w e r e n e a r l y a l w a y s t o g e t h e r a n d in t h e c o m p a n y o f a t l e a s t
o n e h u m a n c a r e g i v e r , e x c e p t f o r t h e l a s t m o n t h s o f th e s t u d y w h e n t h e s u b j e c ts s l e p t w i t h o l d e r
b o n o b o s a t n i g h t. T h e s u b j e c t s f o r m e d c l o s e a t t a c h m e n t b o n d s w i t h t h e c a r e g i v e r s w h o w o r k e d
w i t h t h e m c o n s i s te n t l y .
Rear ing and communica t i ve env i ronmen t
P a n b a n i s h a a n d P a n p a n z e e w e r e r e a r e d t o g e t h e r i n a n e n v i r o n m e n t s i m i l a r t o t h a t i n w h i c h
m a n y h u m a n c h i l d r e n a r e r a i s e d , b u t o n e t h a t t o o k i n t o a c c o u n t t h e a c t i v i t i e s a n d i n t e r e s t s
t o w h i c h y o u n g a p e s a r e p r o n e . E a t i n g , s l e e p i n g , b a t h i n g , c h a n g i n g d i a p e r s , t r a v e l i n g i n
t h e w o o d s a n d c l i m b i n g t r e e s , s o c i a l i n t e r a c t i o n w i t h o t h e r a p e s a n d h u m a n s , a n d q u i e t
w o r k s e s s i o n s ( d u r in g w h i c h v a r i o u s a s s e s s m e n t s m i g h t b e a d m i n i s te r e d ) f il le d th e y o u n g
a p e s d a y .
Th e d i f f e r en t r ou t i ne s o f the day , i n t e r spe r sed w i th occas ion a l tw i s t s o r su rp r i s e s , len t
s t ru c t u r e to th e y o u n g a p e s l i v e s a n d a l s o c r e a t e d a n o p t i m a l e n v i r o n m e n t f o r c o m m u n i c a t i o n
a b o u t o n g i n g a n d u p c o m i n g e v e n t s . S u c h a n e n v i r o n m e n t w a s c o n s i d e r e d c r u c i a l t o t h e
e m e r g i n g t h e o r y o f n o n - h u m a n l a n g u a g e a c q u i s i t i o n t h a t h a d b e e n e s t a b l i s h e d a f e w y e a r s
e a rl ie r w i t h th e b o n o b o s K a n z i a n d M u l i k a ( S a v a g e - R u m b a u g h , 1 99 1 ; S a v a g e - R u m b a u g h e t
al . , 1 9 8 6) . P a n p a n z e e a n d P a n b a n i s h a w e r e i n c l u d e d i n a ll a s p e c t s o f l a b o r a t o r y l if e , a n d e v e r y
e f f o r t w a s m a d e t o c o n v e y i n f o r m a t i o n t o th e m w i t h l e x i g r a m s a s w e l l a s w i th s p o k e n E n g l i sh .
I t w a s i n th i s w a y , t h r o u g h o b s e r v a t i o n o f ad u l t m o d e l s a n d s u b s e q u e n t c o m p r e h e n s i o n o f
u t t e r an c e s , t h a t th e a p e s d e v e l o p e d l a n g u a g e s k il ls .
T h r o u g h o u t t h e s t u d y , c a r e g i v e r s s p o k e t o t h e s u b j e c t s i n E n g l i s h , m u c h a s t h e y w o u l d
s p e a k t o h u m a n i n fa n t s. N o s p e c i f ic p r o c e d u r e s w e r e f o l l o w e d i n s p e a k i n g w i t h t h e in f a n ts , b u t
s p e e c h c o u l d o ft e n b e c h a r a c t e r i z e d a s c h i l d - d i r e c t e d ( a l s o k n o w n a s m o t h e r e s e ) ( B e r k o -
G l e a s o n , 1 9 8 9 ), e s p e c i a l l y w h e n t h e s u b j e c ts w e r e v e r y s m a l l . I n f le c t io n , t o n e o f v o i c e , e y e
c o n t a c t , a n d g e s t u r e s w e r e a l l u s e d a s n e c e s s a r y t o c o m m u n i c a t e t h e d e s i r e d m e s s a g e d u r i n g
da i l y i n t e r ac t i on .
T h e p r in c i p al d i f f er e n c e b e t w e e n c o m m u n i c a t i o n w i t h t h e se a p e s a n d c o m m u n i c a t i o n w i t h
h u m a n i n fa n t s w a s t h at , w h e n e v e r p o s s ib l e , v is u o g r a p h i c s y m b o l s ( ca l le d l e x i g r a m s ) w e r e
u s e d t o s u p p l e m e n t o r c o m p l e m e n t t he E n g l i s h u t t er a n ce s . T h e s e s y m b o l s h a d b e e n c r e a t e d
a n d a s s i g n e d m e a n i n g s d u r i n g e a r l i e r s t u d y w i t h o t h e r a p e s ( R u m b a u g h , 1 9 7 7 ; S a v a g e -
R u m b a u g h e t aL, 1 9 8 6) . E a c h l e x i g r a m w a s g l o s s e d a s t h e e q u i v a l e n t o f a n E n g l i s h w o r d , s u c h
a s m i l k , d o g o r g o o d . A p p r o x i m a t e l y 2 5 6 l e x i g r a m s w e r e a v ai l a b le f o r u s e t h r o u g h o u t t h e
s t u d y p e r i o d . T h e s y m b o l s w e r e a r r a n g e d o n f a c e p l a t e s t h r o u g h o u t t h e l a b b u t t h e y
s o m e t i m e s a l so a p p e a r e d o n i n d i v i d u a l p h o t o g r a p h s o f t h e re f e r e n t s o r w e r e m o u n t e d
i n d i v i d u a l ly in a p p r o p r i a t e p l a c e s ( f o r e x a m p l e , t h e b e d r o o m l e x i g r a m w a s t a p e d t o t h e
b e d r o o m d o o r ) .
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1 26 K A R E N E . B R A K K E a nd E . S U E S A V A G E R U M B A U G H
Data collection procedures
Daily data T h r o u g h o u t t h e 4 y e a r s o f th e c o - r e a r i n g p r o je c t , th e c a r e g i v e r s w h o
a c c o m p a n i e d P a n b a n i s h a a n d P a n p a n z e e t h r o u g h o u t t h e d a y r e c o r d e d v i r t u a l ly a ll in s t a n c e s o f
s y m b o l p r o d u c t io n a n d s y m b o l a n d s p e e c h c o m p r e h e n s i o n b y e a c h s u b j ec t. : T h is w a s d o n e i n
o r d e r t o p r o v i d e a n e x h a u s t i v e r e c o r d o f a l l l a n g u a g e - r e l a t e d b e h a v i o r p r o d u c e d b y t h e a p e s
t h a t w o u l d b e a m e n a b l e t o s e v e r a l t y p e s o f a n a l y s e s r e l e v a n t t o d if f e r e n t a s p e c t s o f l a n g u a g e
a c q u i s it i o n . T h e A p p e n d i x p r o v i d e s e x a m p l e s o f n o t e s c o l l e c te d w i t h t h e s u b j e c ts d u r i n g t h e
s tudy pe r i od .
T h e d a i l y d a t a p r o v i d e d b y t h e a p e s a r e s i m i l a r t o t h o s e o f d i a r y s t u d i e s w i t h h u m a n
c h i l d r e n . E a c h e n t r y w a s h a n d w r i t t e n i n a n o t e b o o k a s s o o n a s p o s s i b l e a f t e r i t s o c c u r r e n c e
a l o n g w i t h c o d e s r e p r e s e n t i n g c o m m u n i c a t i v e f u n c t i o n a n d b e h a v i o r a l c o n c o r d a n c e o r
r e s p o n s e , a s w e l l a s c o n t e x t u a l n o t e s d e s c r i b i n g t h e c o n t e x t o f t h e u t t er a n c e o r r e c e p t i v e
b e h a v i o r . T h e e n t r i e s w e r e l a t e r e n t e r e d o n t o a c o m p u t e r d a t a b a s e , f r o m w h i c h t h e r e c e p t i v e
d a t a r e p o r t e d i n t h is m a n u s c r i p t w e r e e x t r a c te d .
T h e r e c e p t i v e e n t r i e s r e c o r d e d i n t h i s w a y r e p r e s e n t i n s t a n c e s d u r i n g d a i l y i n t e r a c t i o n i n
w h i c h a s u b j e c t s c o m p r e h e n s i o n o f c a r e g i v e r s u t t e r an c e s w a s e v i d e n t . T h e e x c h a n g e s
o c c u r r e d w i t h i n c o m m o n d a i l y r o u t i n e s a n d t h u s w e r e o f t e n c o n t e x t u a l l y a p p r o p r i a t e .
C a r e g i v e r s w e r e , h o w e v e r , i n s t r u c t e d t o r e c o r d o n l y i n s t a n c e s o f c o m p r e h e n s i o n t h a t d i d n o t
a p p e a r d e t e r m i n e d o r b o u n d b y e x t r a l i n g u i st i c c u e s i n c l u d i n g s e tt in g , g e s t u r e o r g l a n c e . T h u s ,
f o r e x a m p l e , a r e q u e s t o f g e t t he s o a p w o u l d n o t b e r e c o r d e d i f t h e u tt e r a n c e w a s
a c c o m p a n i e d b y t h e a d u l t s p o i n t i n g o r l o o k i n g t o w a r d t h e s o a p .
A l s o , b e c a u s e m o s t o f t h e a p e s c a r e g i v e r s h a d w o r k e d w i t h o t h e r a p e s i n s im i l a r c o n d i t io n s
a n d / o r h a d w o r k e d c o n s i s t e n t l y w i t h P a n b a n i s h a a n d P a n p a n z e e t h r o u g h o u t t h e s t u d y p e r i o d ,
t h e y w e r e a w a r e o f r e s p o n s e s t h a t w e r e m a d e f r e q u e n t l y a n d b e c a m e r o u ti n i z ed , a n d d i d n o t
r e c o r d t h e s e b e h a v i o r s a s e x a m p l e s o f c o m p r e h e n s i o n . I n f o r m a t i o n g u i d i n g t h e r e s p o n s e h a d
t o b e j u d g e d a s b e i n g g a i n e d f r o m t h e w o r d s p r e s e n t e d , o r f r o m t e x t u a l r a th e r t h a n c o n t e x t u a l
i n f o r m a t i o n . F o r e x a m p l e , an a p p r o p r i a t e r e s p o n s e t o U s e th e t o w e l a f t e r t a k i n g a b a t h
w o u l d n o t b e r e c o r d e d i f t h a t is e x a c t l y w h a t o c c u r r e d a t t h at s a m e t i m e e v e r y d a y a n y w a y .
H o w e v e r , t h e s a m e r e q u e s t a n d r e s p o n s e u s e d i n a n o v e l p la y s i tu a t io n , f o r e x a m p l e , w o u l d b e
r e c o r d e d .
I n i ti a ll y , f r o m t h e s t a r t o f t h e s u b j e c t s p a r t ic i p a t io n i n e a r ly J a n u a r y t h r o u g h O c t o b e r 1 9 8 6,
c a r e g i v e r s r e c o r d e d o b s e r v a t i o n s o f t h e i n f a n t s b e h a v i o r s t h a t t h e y j u d g e d t o b e i n t e re s t in g o r
s i g n if ic a n t . T h e s e n o t e s w e r e n o t s y s t e m a t i c , b u t t h e r e w e r e s e v e r a l re f e r e n c e s t o t h e s u b j e c t s
c o m m u n i c a t i v e b e h a v i o r t h a t p r o v i d e d a g o o d p o r t r a y a l o f t h e i r e m e r g i n g c o m m u n i c a t i v e
c o m p e t e n c i e s . S t a r t i n g in l a t e O c t o b e r 1 98 6, d a i l y r e c e p t i v e d a t a w e r e c o l l e c t e d s y s t e m a t i c a l l y
f o r b o t h s u b j e c t s t h r o u g h O c t o b e r 1 9 8 9 . T h e d a t a s e t r e p o r t e d i n t h i s m a n u s c r i p t t h u s
r e p r e s e n t s, a s c o m p l e t e l y a s p o s s i b l e , th e c o m p l e t e c o r p u s o f d a i ly d a t a c o l l e c t e d d u r i n g t h i s
3 - y e a r p e r i o d , f r o m t h e f i r s t s y s t e m a t i c r e c o r d i n g o f c l e a r i n s t a n c e s o f c o m p r e h e n s i o n w h e n
t h e s u b j e c t s w e r e l e s s t h a n 1 y e a r o f a g e t h r o u g h t h e e n d o f t h e s t u d y w h e n t h e a p e s w e r e
n e a r l y 4 y e a r s o l d . A s u b s e t o f t h e s e d a t a w a s s e l e c t e d f o r t h e m o r e d e t a i l e d a n a l y s i s o f
s e n t e n c e t y p e s r e p o r t e d i n th i s m a n u s c r i p t ; t h e s e w e r e c o l l e c t e d f r o m J a n u a r y t h r o u g h O c t o b e r
1 9 89 . T h i s p e r i o d r e p r e s e n t s th e e n d o f t h e c o - r e a r in g p r o j e c t, t hu s t h e s u b j e c t s c o m p e t e n c i e s
w o u l d b e a s h i g h l y d e v e l o p e d a s p o s s i b le . T h e i n t e r v a l a l s o c o i n c i d e s w i t h t h e p e r i o d o f f o r m a l
t e s t i n g o f s p o k e n E n g l i s h a n d l e x i g r a m c o m p r e h e n s i o n r e p o r t e d b e l o w .
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U t t e r a n c e s d i r e c t e d t o t h e s u b j e c t s c o n s i s te d e i t h e r o f E n g l i s h o n l y , o r o f E n g l i s h w i t h
l e x i g r a m a c c o m p a n i m e n t . 3 A l l o f t h e u t t e r a n c e s e n t e r e d i n t h e d a t a b a s e w e r e i n c l u d e d i n t h e
i n it ia l , g e n e r a l a n a l y s i s f o r t h e t w o s u b j e c ts . F o r t h e m o r e d e t a i l e d a n a l y s i s o f s e n t e n c e t y p e s
( t h e 1 9 8 9 d a t a ), o n l y t h o s e e n t r ie s t h a t c o n s i s te d o f m u l t i w o r d u t t e r a n c e s a n d w e r e a m e n a b l e
t o s y n t a c t ic a n a l y s i s w e r e i n c l u d e d . T h e s e u t t e r a n c e s c o u l d b e c a t e g o r i z e d b y s e m a n t i c c o n t e n t
a n d w e r e c o d e d a c c o r d i n g ly ( e .g . A c t i o n - O b j e c t , O b j e c t - A c t i o n - L o c a t i o n , a n d s o o n ).
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t y p e o f e v a l u a t i o n w a s c o n d u c t e d d u r i n g t h e l a s t y e a r o f t h e s t u d y p e ri o d . T h i s c o n s i s t e d o f
c o n t r o l l e d t e s t s i t u a t i o n s i n w h i c h s u b j e c t s w e r e a s k e d t o i d e n t i f y a s p o k e n E n g l i s h w o r d b y
s e l e c t in g a m o n g t h r e e l e x i g r a m s p r e s e n t e d in u n i q u e a r r a y s .
T h e s e a s s e s s m e n t s p r o v i d e d a d e c o n t e x t u a l i z e d s i tu a t i o n t h a t c o u l d b e u s e d t o l i t e ra l ly t e s t
o r p r o b e l in k s b e t w e e n s p o k e n w o r d a n d g r a p h i c s y m b o l th a t th e s u b j e c t s h a d f o r m e d i n th e
n a t u r a l la n g u a g e - l e a r n i n g e n v i r o n m e n t d i s c u s s e d e a r l ie r . T h e a p e s w e r e r e q u i r e d t o g e n e r a l i z e
k n o w l e d g e g a i n e d i n d a i l y s o c i a l in t e r a c ti o n s t o v e r y d i f f e r e n t c ir c u m s t a n c e s .
T h e s e a s s e s s m e n t s c a n b e l i k e n e d t o a m u l t i p l e - c h o i c e v o c a b u l a r y t e st . T h e r e s p o n s e s t i m u l i
c o n s i s t e d o f t h e t a r g e t p l u s t w o r a n d o m l y s e l e c t e d a l t e r n a ti v e s . I n b o t h c o n d i t i o n s , a s t a n d a r d
p r o c e d u r e w a s f o l l o w e d ; t h e s u b j e c t s h a d b e c o m e f a m i l i a r w i t h t h i s t e s t s i t u a t i o n i n s i m i l a r
a s s e s s m e n t s o f s p o k e n E n g l i s h a n d p h o t o g r a p h i c s t i m u l i t h e p r e v i o u s y e a r . F o r e a c h t r i a l , E
g a i n e d t h e s u b j e c t ' s a t t e n t io n , t h e n p r e s e n t e d t h e s t i m u l u s w o r d e i t h e r v e r b a l l y o r b y p l a y i n g
a n a u d i o c a s s e t te . E t h e n o p e n e d a t e s t b o o k l e t h e l d b e t w e e n h e r s e l f a n d t h e s u b j e c t s o t h at o n l y
the sub j ec t cou ld s ee t he t h r ee a l t e rna t i ve s p l aced w i th in ( s ee F ig . 1 ) ; E was b l i nd t o t he
p o s i t i o n o f t h e t a r g e t a n d e a c h a l t e r n a ti v e . T h e s u b j e c t w a s t h e n a s k e d t o p o i n t t o o n e o f th e
t h r e e l e x i g r a m s i n t h e t e s t b o o k l e t .
A f t e r t h e s u b j e c t ' s r e s p o n s e , E l o o k e d t o s e e w h e r e S h a d p o i n t e d , j u d g e d w h e t h e r t h e
r e s p o n s e w a s c o r r e c t , a n d p r o v i d e d a p p r o p r i a t e v e r b a l f e e d b a c k ( e . g . ' T h a t ' s r i g h t ' o r ' N o ,
t h a t ' s n o t i t. S o r r y . ' ) p e r h a p s a c c o m p a n i e d b y a h u g . I f S p o i n t e d t o m u l t i p l e s y m b o l s , E a s k e d
' W h i c h o n e ? ' a n d a s k e d f o r a c l e a r , si n g l e r e s p o n s e b e f o r e l o o k i n g a n d j u d g i n g t h e re s p o n s e .
C o n t i n u e d m u l t ip l e r e s p o n s e s w e r e c o u n t e d a s i n c o r r e ct . S u b j e c t s w e r e u s u a l l y a l l o w e d t o e a t
o r p l a y q u i e t ly i n b e t w e e n t ri a ls r e g a r d l e s s o f p e r f o r m a n c e .
Videotaped records.
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b e f o r e n o t e s w e r e t a k e n s y s t e m a t i c a l l y ) a n d c o n f i r m t h e c o u r s e o f th e s u b j e c t s ' e a r l y
c o m m u n i c a t i v e d e v e lo p m e n t , v i d e o t a p e s f r o m t h e p e r i o d o f M a y t h r o u g h D e c e m b e r 1 9 86
w e r e r e v i e w e d . T h i r t y - s e v e n ta p e s , ty p i c a l l y r e c o r d e d w e e k l y o r b i w e e k l y a n d e a c h c o n t a i n i n g
1 - 2 h o u r s o f t h e s u b j e c t s' a c ti v it y , w e r e e x a m i n e d f o r e v id e n c e o f e m e r g i n g s p e e c h
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T h e f i r s t r e c o r d e d i n s t a n c e s o f r e s p o n s e s t o l a n g u a g e i n e a c h s u b j e c t - - a l b e i t h e a v i l y
s u p p o r t ed b y c o n t e x t a n d t o n e o f v o i c e - - i n v o l v e d r e s p o n s e s t o tw o t y p e s o f re g u l a to r y
u t te r a n c es a t a p p r o x i m a t e l y 6 - 7 m o n t h s o f a ge . T h e s e u tt e ra n c e s w e r e ' n o ' a n d ' c o m e h e r e '.
B o t h w e r e i m p o r t a n t i n e s t a b l i s h i n g b e h a v i o r a l l i m i t s f o r t h e y o u n g a p e s a n d w e r e u s e d
f r e q u e n t l y t h r o u g h o u t t h e d a y . W i t h b o t h d e m a n d s , a c l e a r r e s p o n s e a c r o s s s i tu a t io n s w a s
e x p e c t e d b y t h e a d u l t , a n d i f th a t r e s p o n s e d i d n o t o c c u r t h e re w e r e d i r e c t b e h a v i o r a l
c o n s e q u e n c e s r e l e v a n t t o t h e s i tu a t i o n a t h a n d .
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F i g . 1 . P a n b a n i s h a c o m p l e t e s a v o c a b u l a r y t r i a l b y a ) f i r st l i s t e n i n g t o t h e s p o k e n w o r d p r e s e n t e d v i a
h e a d p o h o n e s , a n d b ) p o i n t i n g to o n e o f t h e a v a i la b l e l e x i g r a m s a f t e r E l i ft s t h e s c re e n . P a n b a n i s h a
k e e p s h e r f i n g e r o n t h e l e x i g r a m u n t i l E c a n s e e a n d r e s p o n d t o h e r c h o i c e .
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B y A u g u s t 1 9 8 6 it w a s c l e a r th a t c a r e g i v e r s e x p e c t e d P a n b a n i s h a a n d P a n p a n z e e t o r e s p o n d
a p p r o p r i at e l y t o b o t h n o a n d c o m e h e r e , a n d th a t n o n - c o m p l i a n c e w a s c o n s id e r e d d ef ia n c e .
O b s e r v a t i o n s t h a t t h e a p e s l o o k e d a t t h e s p e a k e r a n d s o m e t i m e s v o c a l i z e d , m o v e d a w a y , o r
e n g a g e d i n d i s p l a y b e h a v i o r w h i l e d o i n g s o , s u p p o r t e d th e c a r e g i v e r s c o n c l u s io n s .
F r o m t h i s t i m e o n , P a n b a n i s h a a n d P a n p a n z e e s t a r t e d e m i t t i n g b e h a v i o r s r e l a t e d t o t h e
g r a p h i c s y m b o l s u s e d a r o u n d t h e m . T h e y o f t e n h e l d t h e i r f a c e s v e r y c l o s e a s t h e s y m b o l w a s
b e i n g t o u c h e d b y a n a d u l t. B e t w e e n 6 a n d 8 m o n t h s o f a g e , e a c h i n fa n t w e n t t h r o u g h a p e r i o d
d u r i n g w h i c h t h e y f r e q u e n t l y t o u c h e d a s y m b o l i m m e d i a t e l y a f t e r a c a r e g i v e r h a d t o u c h e d it.
S u c h b e h a v i o r s s u g g e s t e d t h a t t h e a p e s w e r e s t a rt i n g t o a t te n d v e r y c l o s e l y to c o m m u n i c a t i v e
u t t e rances , ev en i f t hey d id no t ye t demo ns t r a t e t ha t t hey un de r s tood exa c t l y wha t w as be ing s a id .
O c c a s i o n a l r e s p o n s e s t o n o n - r e g u l a t o r y w o r d s b e g a n t o a p p e a r w h e n t h e s u b j e c t s w e r e 8 - 9
m o n t h s o f a g e . O n s e v e r a l o c c a s i o n s d u r i n g t h i s p e ri o d , P a n b a n i s h a e m i t t e d a v o c a l ( e .g . f o o d -
b a r k ) o r b e h a v i o r a l r e s p o n s e t o m i l k o r g o , s o m e t i m e s r e s p o n d i n g to th e u se o f t h e l e x i g r a m
a n d s o m e t i m e s t o t h e s p o k e n E n g l i s h a l o n e .
P a n p a n z e e , s l i g h tl y y o u n g e r t h a n th e b o n o b o , c o n t in u e d t o a p p e a r m o r e d e p e n d e n t o n
c o n t e x t u a l s u p p o r t t h a n P a n b a n i s h a d u r i n g t h i s t i m e , s o m e t i m e s c h a n g i n g h e r b e h a v i o r i n
r e s p o n s e t o n e w i n f o r m a t i o n r e g a r d i n g f a m i l i a r r o u t in e s . T h e d a i l y n o t e s f r o m 6 A u g u s t 1 9 8 6
c o n t a i n t h e f o l l o w i n g d e s c r i p t io n : A s w e v i s i t [t he ] d o g s , P a n z e e s e e m s to a n t ic i p a t e w h e n I
t o u c h O P E N , o v e r a n d o v e r , t h a t w e a r e g o i n g t o l e t t h e m o u t a s w e h a v e t h e l a s t t w o d a y s .
H e r h a i r [ p i lo e r e c ts ] a n d s h e i m m e d i a t e l y [ r u ns ] a n d [ h o ld s ] t ig h t l y to K e l l y . ( P a n p a n z e e - -
k n o w n a s P a n z e e t o h e r c a r e g i v e r s - - w a s j u s t g e tt in g to k n o w t h e d o g s a t t h is t im e a n d w a s
appa ren t l y s t i l l a f r a id o f t hem. )
S y m b o l i c m a r k e r s w e r e r e g u l a r l y i n s e r t e d i n t o t h e o n g o i n g s t r e a m o f i n t e r a c t i o n w i t h
P a n b a n i s h a a n d P a n p a n z e e , s i g n a l i n g i m p o r t a n t t r a n s i t i o n p o i n t s i n t h e a c t i v i t y . T h e u s e o f
s y m b o l s a s m a r k e r s t o a n n o u n c e p a r t s o f ro u t i n e s h a d a p r e d i c t i v e v a l u e a n d l e t t h e i n fa n t a p e s
a n t i c i p a t e u p c o m i n g e v e n t s . T h i s a l l o w e d t h e i n d i v i d u a l t o m a k e a p p r o p r i a t e r e s p o n s e s ( s u c h
a s h o l d i n g a n a d u l t t i g h tl y ) a n d e x e r t s o m e c o n t r o l o v e r w h a t h a p p e n e d t o h er . C o n t e x t u a l
c u e s - - t h e t e m p o r a l a n d p h y s i c a l s e tt in g , t h e c a r e g i v e r s p o s t u re , g e s t u re s a n d v o c a l
i n t o n a t i o n - - m a y h a v e s u p p o r t ed t h e i n f a n t s i n te r p re t at io n o f th e m e s s a g e t o s o m e d e g r e e, b u t
c e r t a i n l y d i d n o t e t e rm i n e it. T h e c a r e g i v e r s t o p ic e a s i l y c o u l d h a v e b e e n a n o t h e r a c t i v it y o r
o b j e c t a p p r o p r i a t e t o t h e s a m e s e t t in g .
D u r i n g t h e l a s t f e w m o n t h s o f 1 9 8 6 c h a n g e s i n th e b e h a v i o r o f b o t h c a r e g i v e r s a n d t h e a p e s
t h e m s e l v e s b e c a m e a p p a re n t . P a n b a n i sh a a n d P a n p a n z e e n o w e n g a g e d i n n o t ic e a b l y im p r o v e d
v i s u a l a t te n t i o n t o t h e a d u l t s b e h a v i o r a s t h e c a r e g i v e r p r o c e e d e d t h r o u g h t h e s e q u e n t i a l s t e p s
i n v o l v e d i n t a l k i n g a b o u t a n d c a r r y i n g o u t r o u t i n e s . J o i n t a t t e n t i o n a l a c t i v i t i e s - - i n c l u d i n g
l o o k in g a t o b je c t s a n d e v e n p ic t u re b o o k s - - b e c a m e m o r e c o m m o n t h a n i n e a r li e r m o n t h s , w i t h
t h e i n f a n t a p e s o f t e n t a k i n g a n a c t i v e r o l l ( b y p o i n t i n g t o p h o t o s i n a b o o k o r m o v i n g a
c a r e g i v e r s h a n d t o w a r d a t o y , f o r e x a m p l e ) .
F o r t h e i r p a rt , c a r e g i v e r s b e c a m e m o r e d e m a n d i n g o f t h e s u b j e ct s , w a i t i n g u n ti l t h e y h a d t h e
s u b j e c t s f u ll a tt e n t io n b e f o r e p r o c e e d i n g w i t h c o m m u n i c a t i o n s o r a c ti v i ti e s o r a s k i n g th e
sub j ec t s t o f u l f il l pa r t s o f the rou t i ne s t hem se lves . I f t he sub j ec t s had d i f f i cu l t y do in g so ,
c a r e g i v e r s p r o v i d e d j u s t e n o u g h g u i d a n c e t o a c h i e v e t h e g o a l a t h a n d .
I t w a s a t t h is t i m e t h a t P a n b a n i s h a s a n d P a n p a n z e e s r e c e p t i v e s k il ls b e g a n t o b r o a d e n . B y
t h e e n d o f J a n u a r y 1 9 8 7, o n e o r b o t h i n f a n t s h a d a p p r o p r i a t e l y r e s p o n d e d a t l e a s t o n c e t o
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' c h a s e ' , ' b i t e ' , ' o r a n g s ' ( o r a n g u t a n s ) , ' t i c k l e ' , ' o p e n ' , ' b y e ' , ' b u g ' , ' g r a p e ' , ' K e l l y ' , ' t - r o o m ' ,
' r u b b e r b a n d ' , a n d ' w a t e r ' . T h e s e r e s p o n s e s a l l o c c u r r e d w i t h i n s o m e c o n t e x t , n a t u r a l l y , b u t
c a r e g i v e r s j u d g e d n o n e t o b e d e t e r m i n e d o r b o u n d b y t h e c o n t e x t i n w h i c h t h e y o c c u r r e d .
A f t e r t h i s p o i n t , b o t h s u b j e c t s c o n t i n u e d t o r e s p o n d a p p r o p r i a t e l y t o a g r e a t e r v a r i e t y o f
u t t e r a n c e s , w i t h i n c r e a s i n g l y g r e a t e r d i s s o c i a t i o n f r o m c o n t e x t u a l s u p p o r t . H o w e v e r , a s w i l l
b e c o m e e v i d e n t in s u b s e q u e n t s e c t io n s o f t h is m a n u s c r i p t , t h e b o n o b o P a n b a n i s h a c o n s i s t e n t l y
d e m o n s t r a t e d g r e a t e r r e c e p t i v e u n d e r s ta n d i n g o f s y m b o l s a n d s p e e c h th a n d i d t h e c h i m p a n z e e
Panp anze e , bo th w i th in t he con t ex t o f da i l y ac t i v i t y and i n con t ro l l ed t e s t s i tua t i ons . The
c h i m p a n z e e n e v e r t h e l e s s d i d e v i d e n c e c o m p r e h e n s i o n o f s e v e r a l w o r d s a n d s e n t e n c e s u s i n g
t h e d i f fe r e n t m e a n s o f a s s e s s m e n t .
C o m p r e h e n s i o n o f c o m p l e x u t t e ra n c e s i n d a i l y i n te r a c t io n
D u r i n g t h e 3 - y e a r p e r i o d i n w h i c h u t t e r a n c e s w e r e r e c o r d e d , t h e b o n o b o P a n b a n i s h a
r e s p o n d e d a p p r o p r i a t e ly t o 2 6 16 o f 2 8 5 2 t o ta l r e c o r d e d u t te r a n c e s (9 2 ) w h i l e t h e c h i m p a n z e e
P a n p a n z e e r e s p o n d e d a p p r o p r i a t e l y t o 1 5 9 6 ( 8 1 ) o f t h e 1 8 25 u t t e r a n c e s t h a t w e r e d i r e c te d t o
h e r a n d r e c o r d e d . A s T a b l e 1 s h o w s , t h e a c c u r a c y r a t e f o r e a c h s u b j e c t r e m a i n e d c o n s i s t e n t
o v e r th e 3 - y e a r p e r io d . T h i s s u g g e s t s t h a t a t s o m e l e v e l t h e c a r e g i v e r s w e r e s e n s i t i v e to t h e
c h a n g i n g i n d i v id u a l c o m p e t e n c i e s o f t h e s u b j e c ts a n d m a d e r e q u e s t s o f th e m a c c o r d i n g l y .
P a n b a n i s h a r e c e i v e d a p p r o x i m a t e l y t w i ce a s m a n y s e n te n c e s a s P a n p a n z e e d u r i n g e a c h o f
t he f i rs t 2 yea r s o f t he s t udy ; th i s a lso sugges t s t ha t the ca r eg ive r s ' exp ec t e d ' m ore o f t he bono bo
t h a n o f th e c h i m p a n z e e . D e s p i t e th e i n c r e a s e d d e m a n d s , t h e b o n o b o c o n s i s t e n t l y r e s p o n d e d
wi th g r e a t e r a ccu racy t han d id he r ch im pan zee co un t e rp a r t ()~2 [1, 4655 ] = 121 .52 , p < 0 .01 ) . '
Table 1. Utterances presented to Panbanisha and Panpanzee 1986-1989
Number correct Total number given Percentage correct
Panbanisha
1986-1987 671 719 93
1988 1268 1406 90
1989 663 712 93
Total 2602 2852 92
Panpanzee
1986-1987 298 365 82
1988 593 718 83
1989 578 735 79
Total 1469 1818 81
B e c a u s e t h e d i f f e r e n c e b e t w e e n t h e s u b j e c t s ' c o m p e t e n c i e s w a s a p p a r e n t , a s u b s e t o f
s en t ences co l l e c t ed i n 1989 was ana lyzed i n f u r t he r de t a i l i n o rde r t o l oca t e pos s ib l e sou rce s
o f d i s s i m i l a r i t y . S e n t e n c e s w e r e c o d e d w i t h r e g a r d t o t h e n a t u r e a n d n u m b e r o f ' k e y t e r m s '
i nc luded . T hes e cons i s t ed o f a c t ion , agen t , ob j ec t , r e c ip i en t , and l oca t i on t e rm s tha t t he
s u b j e c t s w o u l d n e e d t o u n d e r s ta n d i n o r d e r to r e s p o n d a p p r o p r i a t e ly . A t o t al o f 7 1 3 u t t e r a n c es
w e r e r e c o r d e d f o r P a n b a n i s h a f r o m J a n u a r y 1 9 89 t h r o u g h O c t o b e r 1 98 9. O f t h e s e, 5 1 3 m e t th e
c r i t e r i a f o r i nc lu s ion i n t he cu r r en t ana ly s i s ( i . e . en t i r e u t t e r ance w i th mu l t i p l e ' k ey t e rms '
r e c o r d e d , s p e c i f i c a c t io n r e s p o n s e r e q u i r e d , s p e e c h a n d l e x i g r a m i n p u t o n l y ) . O f th e 7 3 8
u t t e r a n c e s p r e s e n t e d t o P a n p a n z e e d u r i n g t h e s a m e p e r i o d 4 7 7 m e t t h e c r i t e r i a a n d w e r e
s i m i l a r l y a n a l y z e d . M o s t o f t h e u t t e r a n c e s t h a t w e r e d r o p p e d w e r e r e c o r d e d a s s i n g l e - t e r m
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OMPREHENSION INPAN 3
u t t e r an c e s s u c h a s ' t i c k l e ' o r ' b o w l ' . B e c a u s e m o r e o f t h e s e w e r e r e c o r d e d f o r P a n p a n z e e , h e r
d a t a s e t w a s i n f l u e n c e d s l i g h t ly m o r e t h a n P a n b a n i s h a ' s b y t h e i r e x c l u s i o n f r o m t h e a n a l y s is .
D u p l i c a t e ( in b o t h w o r d c o n t e n t a n d m o d a l i t y ) u t te r a n c e s w e r e a l s o d r o p p e d f r o m t h e a n a l y s is .
T h e r e m a i n i n g 4 8 3 u n i q u e s e n t e n c e s p r e s e n t e d to P a n b a n i s h a a n d t h e 3 81 u n i q u e s e n t e n c e s
p r e s e n t e d t o P a n p a n z e e w e r e t h e n a n a l y z e d b y i n p u t m o d a l i t y ( E n g l i s h o n l y v s E n g l i s h p l u s
l e x i g r a m ) a n d s e n t e n c e t y p e ( e . g . a c t i o n - o b j e c t , o b j e c t - a c t i o n - l o c a t i o n ) . T h i s w a s d o n e t o
d e t e r m i n e w h e t h e r w e c o u l d i d e n ti f y s o m e o f t h e d i ff e re n t c o m p o n e n t s o f l a n g u a g e
c o m p r e h e n s i o n t h a t a f f e c t e d th e p e r f o r m a n c e o f e a c h s u b j e c t.
Input modal i ty . O f t h e 4 8 3 u n i q u e u t t e r a n c e s p r e s e n t e d t o P a n b a n i s h a , 3 6 2 ( 7 5 ) u s e d
l e x i g r a m p l u s E n g l i s h i n p u t , w h i l e 1 21 w e r e i n s p o k e n E n g l i s h o n l y . T h r e e h u n d r e d a n d
e i g h t e e n ( 8 3 ) o f P a n p a n z e e ' s 38 1 s e n t e n c e s w e r e in E n g l i sh a n d le x i g r a m ; o n l y 6 2 w e r e
p r e s e n t e d i n E n g l i s h o n l y ) T h u s , t h e c a r e g i v e r s g a v e P a n p a n z e e a l o w e r p r o p o r t i o n o f
s e n t e n c e s i n E n g l i s h o n l y ( Z2 [ 1 , 8 6 3 ] = 9 . 7 2, p < 0 . 0 1 ). I n s o d o i n g t h e y p l a c e d f e w e r d e m a n d s
o n h e r s p e e c h c o m p r e h e n s i o n s k i l l s a n d p r o v i d e d r e l a t i v e l y m o r e l e x i c a l a s s i s t a n c e w i t h t h e
v i s u o g r ap h i c s y m b o l s .
N e v e r t h e l e s s , P a n b a n i s h a a g a i n a p p r o p r i a t e l y r e s p o n d e d t o r e q u e s t s f o r a c t i o n m o r e o f t e n
t h a n d i d P a n p a n z e e , r e g a r d l e s s o f w h e t h e r u t t e r a n c e s w e r e p r e s e n t e d i n E n g l i s h a l o n e o r
E n g l is h w i th l e x i g r a m a c c o m p a n i m e n t . R e s u l t s b y i n p u t m o d a l i ty a r e s u m m a r i z e d i n T a b l e 2 .
P a n b a n i s h a r e s p o n d e d a p p r o p r i a t e l y o n 9 0 o f E n g l i s h o n l y s e n t e n c e s a n d o n 9 5 o f E n g l i s h
p l u s l e x i g r a m s e n t e n c e s . P a n p a n z e e r e s p o n d e d a p p r o p r i a t e l y o n 7 8 o f E n g l i s h o n l y
s e n t e n c e s a n d 7 3 o f E n g l i s h p lu s l e x i g r a m s en t e n c e s. T h u s , g i v e n th a t c a r e g i v e r s w e r e
sens i t i ve t o t he capab i l i t i e s o f t he sub j ec t s and ad ju s t ed t he i r i npu t a pr ior i e a c h s u b j e c t
r e s p o n d e d e q u a l l y w e l l t o E n g l i s h s e n t e n c e s w i t h o r w i t h o u t l e x i g r a m s u p p l e m e n t s . T h e
b o n o b o , h o w e v e r , h a d o n l y a f e w i n a p p r o p r i a t e r e s p o n s e s w h i l e t h e c h i m p a n z e e h a d d i f fi c u l ty
w i t h a b o u t o n e o u t o f e v e r y f o u r r e q u e s t s m a d e o f h e r .
Tab le 2. Comprehensionof different input m odalities
N um ber c o r r e c t N u m b e recorded Percentageorrect
Panbanisha
Eng lish on ly 109 121 90
English + lexigram 342 362 94
Panpanzee
English only 49 62 79
English + lexigram 229 318 72
Note: one utterance directed to Panpanzee was presented via lexigram only, with no speech
(she w as partially correct; see text).
R e c e p t i v e l a n g u a g e c o m p e t e n c y r e f le c t s th e i n t e r p l a y o f s e v e r a l s e n s o r y , c o g n i t i v e , a n d
m o t o r s y s t e m s . T h e d i f f e r e n t i a l r a t e s o f a p p r o p r i a t e r e s p o n s e s b e t w e e n s u b j e c t s c o u l d a r i s e
f r o m s e v e r a l fa c to r s. F o r e x a m p l e , P a n p a n z e e m a y s i m p l y n o t h a v e u n d e rs t o o d a s m a n y w o r d s
a s t h e b o n o b o , h e r re s p o n s e s m a y h a v e b e e n m o r e c o n t e x tu a l ly b o u n d t h an P a n b a n i s h a ' s , s h e
m a y h a v e h a d t r o u b l e w i t h e x t r a c t i n g i m p o r t a n t i n f o r m a t i o n f r o m l o n g c o m p l e x s t r i n g s o f
w o r d s , o r s h e m a y h a v e b e c o m e d i s t r a c t e d m o r e e a s i l y a n d t h u s w o u l d n o t a t t e n d c l o s e l y t o
the u t t e r ances d i r ec t ed t o he r .
A l t h o u g h h e r c a r e g i v e r s d i d c o n s i d e r P a n p a n z e e t h e m o r e d i s t r a c ti b l e o f th e t w o s u b j e c t s ,
t h i s t ype o f a t t en t i ona l d i f f e r en ce d id no t l end i t s e l f t o d i r ec t ana ly s i s w i th t h i s da t a ba se . O the r
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1 32 K A R E N E . B R A K K E a nd E . S U E S A V A G E R U M B A U G H
factors, however, were exa min ed by analyz ing this group of sentences from 1989 as well as
more structured vocabulary tests administered throughout the study period.
Sentence type and complexity
In order to determine whether sentence type or complexity
affected Panpanzee's responses, caregiver utterances were categorized according to
grammatical form. The sentence types along with correct, incorrect, and partially correct
responses for each subject are presented in Table 3. Percentage of sentences given in each
syntactic category is depicted in Fig. 2; the percentage correct r esponse for each sentence type
is sho wn in Fig. 3.
Table 3. Sentencecomprehensionby grammatical form January 1989-October 1989
Panbanisha Panpanzee
Sentence type* Ct I PC NR C I PC NR
Two-term
AO 221 13 3 136 33 6 5
OA 4 5
OL 19 1 19 1 2
AL 42 1 21 2 2
AR 13 1 8
Three-term
AOL 77 4 2 2 37 16 9 3
OAL 7 10 6
ALO 14 9 2 1
AOR 10 1 1 5 1 1
ARO 12 1 12 6 1 1
ARL 6 2 1
Other 2__6 2 1__33
Totals 451 23 7 2 278 69 25 9
483 Total sentences 384 Total sentences
* 'Key term' codes: A = Action, O = Object, L = Location, R = Recipient.
t Responsecodes: C = Correct response, I = Incorrect response, PC = Partially correct response,
NR = No response/ignore.
The distribution of sentence types was similar for the two subjects. Approximately half of
all sentences given were in Act ion- Obje ct form, such as ' GIV E BOW L' or 'Slap the ball'
(lexigrams are indicated by all-caps). Another 17 for each subject were Act ion -Ob jec t-
Location, for example, ' Take the orange to the group room.' Most other sentences comprised
two- or three-term combina tions of action, object, location, and recipient. Sente nce types that
were presented only rarely, such as 'O bje ct- Loc ati on- Act ion ' or four-term utterances such as
'Action-Recipient-Object-Location ' , are represented in the 'Other' category.
Panbani sha responded appropriately to at least 92 of each sentence type (or group of
related sentence types): her performance did not appear affected by the grammatical form or
lengt h of the utterance. In fact, the low num ber of totally inappropri ate responses made by the
bonob o precluded any log -li near analysis of the effect of sentence type because expected cell
values could not be raised to appropriate levels, even when different sentence types were
combined.
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C O M P R E H E N S I O N I N
P N
33
26
2 0 0
1 8 0
1 0 0
6 0
0
m
b
e
r
R
e
o
o
r
d
e
d
A O O L A L A R A O L O A L A L O A R O A O R
O t h e r
t t e r ance
Type
P a n b a n l e h a ~ P a n p a n z e e
Fig. 2. Total number of sentences that were presented to Panbanisha and Panpanzee that fit the criteria
for inclusion see text). These sentences comprised the 1989 subset of utterances. The proportional
distribution of the different sentence types was very similar for the two subjects.
P
e
r
o
@
n
t
C
o
r
r
e
O
t
1 0 0
8 0
6 0
4 0
2 0
0
A O O L A L A R A O L O A L A L O A R O A O R T o ta l
t t e r ance Type
P a n b a n l a h a ~ P a n p a n z e e
Fig. 3. Percentage of appropriate responses made to the different sentence types by the two subjects.
Panbanisha responded well to all grammatical categories, but Panpanzee had significantly more
difficulty with three-term sentences and Action-Object utterances than with the other two-term
categories.
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134 KAREN E. BRAKKE and E. SUE SAVAGE-RUMBAUGH
Panpanzee, on the other hand, performed extremely well in response to some sentence types
but had many errors with others. Her percentage correct ranged from 57
(Action-Object-Location) to 90 (Action-Recipient). In order to determine the significance
of differences between sentence types, Panpanzee' s data were submitted to log- linear analysis.
Because of the low number of sentences included in some categories, some sentence types
were grouped together in order to facilitate this analysis. By reducing the number of
categories, expected values for the remaining cells were raised to levels considered acceptable
for such analysis (Tabachnick and Fidell, 1989). The groupings were made based on types and
number of key terms included and are presented in Table 4. The 'other' category was dropped
from this analysis because it contained a disjoint group of sentences and its inclusion as a
category was meaningless.
Table 4. Pan panzee s performance by sentence type: log-li near analysis results
Sentence Observed Expected Adjusted
types frequency frequency residual
Correct responses
AO + OA 141.00 134,55 1.522
OL 19.00 16,00 1.482
AL + AR 29.00 24,00 2.050
OAL + ALO 19.00 20,36 -0.6 02
AOR + ARO 17.00 19,64 -1. 184
AOL + ARL 39.00 49.45 -3.1 58
Other responses
AO + OA 44.00 50.45 -1.5 22
OL 3.00 6,00 -1.4 82
AL + AR 4.00 9.00 -2.050
OAL + ALO 9.00 7,64 0.602
AOR + ARO 10.00 7,36 1.184
ARL + AOL 29.00 18,55 3.158
Likel ihood ratio chi-square = 16.97638, df = 5, p = 0.005.
Log- linear analysis supports the conclusion that the chimpanzee' s performance varied as a
function of sentence type (G2 [5, 381] = 16.98, p < 0.01). Review of adjusted residuals
indicates that she responded relatively well to Action-Location and Action-Recipient
utterances (e.g. 'GO SCRUBBY-PINE-NOOK' or 'WASH KAREN') and had particular
difficulty with the cluster of Action-Object-Location and Action-Recipient-Location
utterances (almost wholly attributable to AOL errors, e.g. 'CARRY the STRING to the
GROUP ROOM') No other adjusted residuals were significant, although the direction of the
residuals for correct responses to all two-term sentence types were positive while the residuals
for all three-term categories were negative.
Three of the four sentence types that Panpanzee had the most difficulty with (as indexed by
percentage correct) were three-term sentences. Even if the action term of such an utterance is
relatively simple, such as 'get' or 'give', there still remain at least two additional terms that
must be remembered and integrated as the action is carried out. Panpanzee's response patterns
indicate that she may have had difficulty with one or more of the steps required to remember
and/or coordinate the multiple terms in her responses. A review of the contextual notes
accompanying the entries reveals that Panpanzee often acted on the appropriate object but took
it to the wrong location or performed an inappropriate but familiar action on that object. For
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COMPREHENSION IN P N 135
e x a m p l e , a f t e r re t r i e v in g a s w e e t p o t a t o f r o m t h e r e f r i g e r a t o r P a n p a n z e e c a r r ie d i t to t h e s i n k
a n d w a s h e d i t ( a s is o f t e n d o n e ) i n s t e a d o f p u tt i n g i t i n th e m i c r o w a v e o v e n a s r e q u e s te d . T h i s
p a t t e r n u s u a l l y d i d n o t o c c u r i f P a n p a n z e e a l r e a d y p o s s e s s e d t h e o b j e c t i n q u e s t io n o r i f i t w a s
i m m e d i a t e l y a v a i l a b l e ; s h e t h e n c o m p l e t e d t h e r e q u e s t a p p r o p r i a t e l y . H e r b e h a v i o r s u g g e s t e d
d i f f ic u l t y in r e m e m b e r i n g o r r e l a ti n g a l l o f th e c o m p o n e n t s o f th e u t t e ra n c e . I t s e e m e d t h a t i f
t h e l i n g u i s t i c i n f o r m a t i o n b e c a m e t o o c o m p l i c a t e d , P a n p a n z e e f e l l b a c k o n h e r e x t a n t
k n o w l e d g e o f ty p i c a l r o u ti n e s t o c o m p l e t e w h a t s h e t h o u g h t w a s a n a p p r o p r i a t e a c ti o n . In o t h e r
w o r d s , w h e n c h a l l e n g e d , c o n t e x t u a l i n f o r m a t i o n o v e r r o d e v e r b a l o r t e x t u a l i n f o r m a t i o n f o r
P a n p a n z e e .
T h e s e d i f f i c u l t i e s d o n o t a p p e a r t o b e r e s p o n s i b l e f o r t h e c h i m p a n z e e ' s e r r o r s i n t h e
A c t i o n - O b j e c t c a t e g o r y . M o s t o f t h e s e r e q u e s t s w e r e s y n t a c t i c a l l y r e l a t i v e l y s i m p l e . I n s t e a d ,
t h e h i g h n u m b e r a n d v a r i e t y o f s u c h s e n t e n c e s p r e s e n t e d i n d i c a te s t h a t s o m e o t h e r f a c t o r s u c h
a s v o c a b u l a r y s i z e m a y h a v e c o n t r i b u t e d t o h e r e r r o r s . D a t a f r o m c o n t r o l l e d t e s t s o f s i n g l e -
w o r d c o m p r e h e n s i o n , r e p o r t e d b e l o w , a r e c o n s i s te n t w i t h t h e r e su l t s o f t h e s e n t e n c e a n a l y s i s
a n d i n d i c a t e t h a t v o c a b u l a r y s i z e m a y i n d e e d h a v e c o n t r i b u t e d t o h e r l o w e r l e v e l o f a c c u r a c y
i n r e s p o n d i n g t o u t t e ra n c e s m a d e d u r i n g r e g u l a r d a i l y i n te r a c ti o n .
Table 5. Single-word test results
Number of words meeting Number of trials
Total 75 criterion correct
words
Condition tested Panban Panzee Panban Panzee
English to lexigram 217 179 79 603/733 365/795
(82 ) (36 ) (82 ) (46 )
Audiotaped English 76 71 39 224/247 171/277
to lexigram (93 ) (51 ) (91 ) (62 )
English to lexigram 8 (Pb) 7 18 22125 61/78
(retest) 23 (Pz) (88 ) (78 ) (88 ) (78 )
Tests of single w ord comprehension
A s s h e d i d w i t h t h e u t t e r a n c e s p r o v i d e d i n n a t u r a l s e t t i n g s , t h e b o n o b o P a n b a n i s h a
p e r f o r m e d c o n s i s t e n t l y w e l l o n t h e b l i n d r e c e p t i v e v o c a b u l a r y t e s t . P a n p a n z e e h a d m o r e
d i ff ic u l ty a l t h o u g h s h e p e r f o r m e d w e l l a b o v e c h a n c e l e v e l s a n d h e r v o c a b u l a r y a p p e a r e d t o b e
i n c r e a s i n g i n s i z e t h r o u g h o u t t h e s t u d y . T h e r e s u l t s o f th e t w o t e s t c o n d i t i o n s a n d t h e a g e s o f
t h e s u b j e c ts d u r i n g e a c h a r e p r e s e n t e d in T a b l e 5 a n d a r e s u m m a r i z e d b e l o w .
English to lexigram.
E n g l i s h t o l e x i g r a m t e s t i n g b e g a n i n J a n u a r y 1 9 8 9 . A t o t a l o f 2 1 7
s y m b o l s w e r e t es t ed w i t h P a n b a n i s h a a n d P a n p a n z e e . T h e b o n o b o P a n b a n i s h a w a s c o r r e c t o n
8 2 o f t h e t ri a ls p r e s e n t e d t o h e r a n d m e t t h e 7 5 c r i te r i o n ( a t l e a s t t h r e e o u t o f f o u r c o r r e c t )
o n 1 7 9 o f t h e w o r d s ; s h e w a s 1 0 0 c o r r e c t ( a t l e a s t t h r e e o f th r e e c o r re c t ) o n 1 4 0 o f t h e se .
P a n p a n z e e , t h e c h i m p a n z e e , s e l e c te d t h e c o r r e c t l e x i g r a m o n o n l y 4 6 o f t h e t ri a ls , m e e t i n g
c r i te r i o n o n 7 9 o f t h e s y m b o l s . S h e w a s 1 0 0 c o r r e c t o n 3 3 o f t h e s e. T a b l e 6 l is ts th e w o r d s
i n e a c h s u b j e c t ' s r e c e p t i v e v o c a b u l a r y a s d e t e r m i n e d b y t h i s m e a n s .
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Table 6. English --~ lexigram receptive vocabularies, 1989
Panbanisha
Apple Groom Nail
Austin Group Room No
Backpack Gully Gushers Noise
Bad Hamburger Noodles
Ball Hammer Observation Room
Balloon Happy Oil
Banana Hat Onion
Bedroom Hide Open
Bite Hilltop Orange
Blackberries Honeysuckle Orange drink
Blanket Hot Orange juice
Blueberries Hotdog Orangutan
Book Hug Outdoors
Bowl Hurt Paint
Bubble Ice Panbanisha
Bug In Panzee
Bunny Jello Paper
Butt Jelly Peaches
Butter Juice Peanut
Cabinet Kanzi Peas
Camper Cabin Karen Perrier
Can opener Keepaway Phone
Car Kelly Pillow
Carrot Key Pineapple
Carry Kiwi Plastic bag
Cereal Knife Play
Chase Koolaid Popsicle
Cheese Krista Potato
Cherry Lana Privet berry
Chow Lemon Quiet
Clay Lemonade Rain
Clover Lever Raisin
Coffee Light Refrigerator
Coke Lighter River
Cold Liz Rubber band
Collar Log Cabin Salt
Colony Room Lookout Point Sandpile
Crisscross Comers Mary Ann Scare
Dog Matata Scrubby Pine Nook
Egg Melon Sherman
Fire Middle Test Room Shirt
Flatrock Midway Shop
Go Milk Shot
Good Money Slap
Goodbye Monster Sleep
Gorilla Mouth Snake
Grab Mushroom Soap
Grapes Mushroom Trail Sour cream
Spin Surprise Treehouse
Spoon Sweet potato Turtle
Staff Office T-room TV
Stethoscope Throw Umbrella
Stick Tickle Vacuum
Straw Tomato Velvet plant
Strawberries Toolroom Vitamin
String Toothbrush Wash
Sue Toothpaste Water
Sue s Gate Towel Wipes
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S u e s O f f i c e T o y Y e s
Sugar Tr as h Y ogu r t
Sugarcane Tree
P a n p a n z e e
A p p l e I c e R a i s i n
A us t i n Je anni ne R o c k
B a d K i w i S a l t
B a l l o o n K o o l - a i d S c a r e
Banana Lana Shirt
Bi t e L i ght e r S n a k e
Blackberries M M ' s S o a p
Blueberries M elon Sparkler
B o o k M i d d l e T e s t R o o m S t ic k
B u g M i d w a y S tr aw
Bu nny Mus hr oom St r aw ber r y
C h a s e N o S t r i n g
C h ow O bs e r vati on R oo m Sugar
C l ove r O i l Sugar c ane
C o f f e e O n i o n S w e e t p o ta to
C o k e O p e n T i c k l e
C r i s s c r os s C om e r s O r ange Tom at o
D o g O r ange j u i c e Toot hbr ush
Fire Perrier W ater
Fo ot Pineapple Yogurt
G o o d b y e P l a y
G r o o m P o p s i c l e
H i d e P o t a t o
H o n e y s u c k l e P r i v e t b e r r y
H ur t Q ui e t
T h i s t e s t p r o v i d e d t h e c l e a r es t d e m o n s t r a t i o n th a t P a n b a n i s h a ' s r e c e p t i v e v o c a b u l a r y w a s
m o r e e x t e n s i v e t h a n P a n p a n z e e ' s . I t is e q u a l l y i m p o r ta n t , th o u g h , t o n o t e t h at t h e 3 - y e a r - o l d
c h i m p a n z e e e x h i b i te d c o m p r e h e n s i o n o f n e a r ly 8 0 s p o k e n w o r d s u n d e r c o n t r ol le d ,
d e c o n t e x t u a l i z e d t e st c o n d i t i o n s , a p e r f o r m a n c e t h a t f a r e x c e e d s t h at o f o t h e r Pan trogl odytes
s i m i l a r ly t es t e d ( S a v a g e - R u m b a u g h , 1 9 8 6 ). T e s t s w i t h S h e r m a n a n d A u s t in , e v e n a s a d u l ts ,
y i e l d e d o n l y c h a n c e p e r f o r m a n c e i n r e s p o n s e t o a n y s p o k e n w o r d s ( C e r u t t i e t aL 1 9 9 3 ) .
A ud i o taped En g l i sh to lex ig ram. I n o r d e r t o f u r t h e r e n s u r e t h a t t h e p e r f o r m a n c e o f t h e s e
s u b j e c ts w a s n o t d e p e n d e n t o n f e a t u r e s o f t h e s it u a t io n s u c h a s h o w t h e ex p e r i m e n t e r w a s
s a y i n g t h e t ar g et w o r d , s e v e r a l w o r d s w e r e s u b s e q u e n t l y e v a l u a t e d u s i n g a u d i o t a p e d E n g l i s h
p r e s e n t e d o v e r h e a d p h o n e s . A g a i n , P a n b a n i s h a ' s p e r f o r m a n c e w a s c o n s i s t e n t w i t h e a rl ie r
t e s ts . S h e s e l e c t e d t h e c o r r e c t l e x i g r a m o n 9 1 o f th e t r ia l s a n d m e t c r i t e r io n o n 7 1 o f th e 7 6
w o r d s . P a n p a n z e e w a s c o r r e c t o n 6 1 o f t h e t ri al s, m e e t i n g c r it e ri o n o n 3 9 o f th e w o r d s i n t h e
t a p e d s t i m u l u s s e t .
B e c a u s e P a n p a n z e e ' s p e r f o r m a n c e i m p r o v e d i n t h e h e a d p h o n e c o n d i t i o n r e l a t i v e t o t h e
s p o k e n E n g l i s h t a s k , th e w o r d s t h a t e a c h a p e h a d f a i le d i n o n e c o n d i t io n a n d p a s s e d i n th e
o t h e r w e r e r e t es t e d w i t h o u t t h e h e a d p h o n e s i n S e p t e m b e r 1 9 8 9 ( i. e . th i s te s t w a s c o n d u c t e d
u n d e r t h e s a m e c o n d i t i o n s a s t h e la r g er E n g l i s h t o l e x i g r a m t e st s e t t h at b e g a n 9 m o n t h s
e a r li er ) . T h i s w a s d o n e i n o r d er t o d e t e r m i n e w h e t h e r P a n p a n z e e ' s i m p r o v e m e n t c o u l d b e
a t tr ib u t ed t o m a t u r a ti o n o r w h e t h e r t h e h e a d p h o n e p r e s e n t a ti o n a c t u a ll y g a v e P a n p a n z e e s o m e
a d v a n t a g e o v e r t h e ' l i v e - s p e e c h ' c o n d i t i o n b y f o c u s i n g h e r at te n t io n o n t h e t a s k m o r e
e f f e c t i v e l y .
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138 KAREN E. BRAKKE and E. SUE SAVA GE RUMB AUGH
I n t h is f i na l s ubs e t, Panb an i s ha was t e s t ed on e i gh t wor ds ( t h r ee o r f ou r tr ia l s pe r wo r d ) and
was co r r ec t on 22 o f 25 tr ia l s. Th e on l y s y m b o l on w h i ch s he f a i l ed to m e e t c r it e r i on was
f oo d . Panb an i s ha had a l s o m i s s ed t h is w or d i n t he i den t ica l cond i t i on ea r li e r bu t had p a s s ed
i t in t he au d i o t aped cond i t i on .
Pan panz ee was r e t e s t ed on 23 w or ds ; s he was c o r r ec t on 61 o f 78 t r ia l s and m e t c r i t e r ion on
18 o f t he wor ds . Four o f t he f i ve wor ds t ha t t he ch i m panzee f a i l ed t o i den t i f y a t t h i s t i m e
( b l a n k e t , g o o d , m u s h r o o m - t r a il , t u r t le ; t h e f if th w a s r a i n ) w e r e o n e s t h a t sh e h a d
m i s s ed i n t he f i rs t l i ve - s pe ech cond i t i on and had pa s s ed whe n l i s ten i ng t o headpho nes . The
da t a s ugges t t ha t bo t h s ub j ec t s we r e s t i l l expe r i enc i ng s om e vocabu l a r y g r owt h du r i ng t h i s
pe r i od , becaus e i n t h i s f i na l t e s t t hey each pe r f o r m ed be t t e r on m os t o f t he s ym bo l s t hey had
f a i l ed m o n t hs ea r l i e r und e r i den t i ca l cond i t i ons . How eve r , a f ew w or ds ( s uch a s r a i n ) m ay
h a v e b e e n o n t h e c u s p o f c o m p r e h e n s i o n a t t h is t im e , a n d m a y h a v e b e e n s u b j e c t t o
f l u c tu a t in g p e r f o r m a n c e a s e v i d e n c e d b y h e r m a s t e r y o n o n e o c c a s i o n a n d f a i lu r e a t a l a te r
date.
i s c u s s i o n
F o r t h e f i r s t t i m e , m e m b e r s o f t w o n o n - h u m a n s p e c i e s h a v e b e e n r a i s e d t o g e t h e r f r o m
i n f a n c y a n d p r o v i d e d w i th a n e n v i r o n m e n t th a t f o s te r s d e v e l o p m e n t o f n o n - s p e e c h l a n g u a g e
c o m p e t e n c y . T h i s a l lo w s d i r e c t c o m p a r i s o n s t o b e m a d e b e t w e e n m e m b e r s o f t h e s e s p ec i es
and s ugges t s a r ea s o f inqu i r y t ha t m ay p r ove f r u i t f u l f o r i nves t i ga t ion o f the l anguag e - l ea r n i ng
p r o c e s s i n h u m a n s .
The da t a s e t s r epo r t ed above , a l t hough co l l ec t ed i ndependen t l y u s i ng d i f f e r en t t e chn i ques ,
d e m o n s t r a t e r e m a r k a b l e c o n v e r g e n c e o f r e s u l t s . U n d o u b t e d l y , t h e b o n o b o h a d t h e g r e a t e r
r e c e p t i v e c o m p e t e n c y o f t h e t w o s u b j e c t s t h r o u g h o u t t h i s p e r i o d o f d e v e l o p m e n t . H e r
p e r f o r m a n c e w a s n o t a f f e c t e d b y r e m o v i n g p o t e n t ia l c o n t e x t u a l s u p p o r t o r b y c o m b i n i n g t h e
w o r d s s h e k n e w i n to c o m p l e x s e n t e n c e s. P a n b a n i s h a s p a t te r n o f r e c e p t i v e d e v e l o p m e n t ,
t o g e t h e r w i th t h a t re p o r t e d f o r M u l i k a ( S e v c i k an d S a v a g e - R u m b a u g h , 1 9 9 4) a n d f o r K a n z i a t
o l d e r a g e s ( S a v a g e - R u m b a u g h e t a l . 1986) , pa i n t s a po r t r a i t o f bon obo l anguage acqu i s i t ion
t ha t, i n cha r ac t e r , i s n o t un l i ke t ha t o f ch i l d r en p r i o r t o 21/~ yea r s o f age . A l t hough t he a pe s
v o c a b u l a r y s iz e a p p e a r s t o i n c r e a se m o r e s l o w l y t h an t h a t o f h u m a n c h i ld r e n , b o t h l e a rn b y
p a r t i c i p a t i n g i n r o u t i n e s w i t h l e x i c a l m a r k e r s u s e d b y a n a d u l t , a n d b o t h u n d e r g o
d e c o n t e x t u a l iz a t i o n o f s y m b o l c o m p r e h e n s i o n a s t h e y m a t u r e .
M o r e s u r p ri si n g t h a n t h e b o n o b o s h i g h l e v e l o f p e r f o r m a n c e w a s t h e c h i m p a n z e e s
d e m o n s t r a ti o n o f s p e e c h c o m p r e h e n s i o n . A l t h o u g h h e r l e v e l o f a c c u r a c y r e m a i n e d l o w e r th a n
P a n b a n i s h a s t h r o u g h o u t t h e s t u d y p e r i o d , P a n p a n z e e n e v e r th e l e s s d e m o n s t r a t e d s p e e c h
c