a reappraisal of the reproductive ecology of arvicanthis in africa
TRANSCRIPT
Afr. J . Ecol. 1991, Volume29, pages 17-27
A reappraisal of the reproductive ecology of Arvicanthis in Africa
MARTIN F I S H E R Department oJBiology, Sultan Qaboos University. PO Box 32486 Al Khod, Muscat, Sultanate of Oman
Summary
Arvicanthis (Rodentia, Muridae) is widespread in tropical Africa, and its ecology has been studied in both East and West Africa under various climatic regimes. In order to take an overall look at the reproductive ecology of Arvicanthis in Africa, data were extracted from the literature on the climate and reproductive ecology and weight of Arvicanthis at eight localities. Relationships between aspects of reproductive ecology and climate were explored statistically.
The length of the breeding season of Arvicanthis shows a strong, non-linear relationship with the number of ‘humid’ months in a year (as determined from a standard climograrn), but weight of adult males and females, number of implanted embryos and proportion of reproductively active females do not appear to be correlated with climate. The available evidence does not support the hypothesis that populations of Arvicanthis in hotter-drier environments are more r-selected, and that populations in cooler-wetter localities are more K-selected. The necessity for consistent interpretations of r-K theory and standardization of measures of ‘stress’ and ‘predictability’ is discussed.
Key words: Arvicanthis, reproduction, Africa, r/K-selection
Rbume
Arvicanthis (Rodent&, Muridae) est rtpandu en Afrique tropicale et on a etudie son kcologie en Afrique de 1’Est et de 1’Ouest sous diffkrents regimes climatiques. Pour obtenir une vue gCnCrale de 1’Ccologie de la reproduction d’hvicanthis, en Afrique, nous avons repris des donnkes dans la litterature sur le climat, la repro- duction et le poids, en huit endroits. On a ktudit les relations statistiques entre la reproduction et le climat.
La duree de la saison de reproduction d’Arvicanthis montre une relation Ctroite non linCaire avec le nombre des mois humides par an, (On l’a determine avec un clirnogramme standard), rnais le poids des adultes miles et femelles, le nombre d’embryons implantb et la proportion de femelles qui participent A la repro- duction ne semblent pas lies au climat. Les chiffres ne confirment pas I’hypothese colon laquelle les populations d’drvicanthis sont plus r-selectionnbes, ni que les populations d’emplacements plus froids et plus humides sont plus K-stlectionnCes. On discute la nkcessitb d’interprktations fiables de la thkorie r-K et de la standard- isation de mesures de ‘stress’ et de ‘probabilit6‘.
18 M . Fisher
Introduction Arvicanthis (Rodentia, Muridae) is widespread in tropical Africa. Population levels are generally low, but there are occasional large increases, during which the animal can be a serious pest of agriculture (Delany, 1986). The biology of the genus has been studied in Senegal (Poulet & Poupon, 1978), Nigeria (Rabiu & Fisher, 1989), Sudan (Ghobrial & Hodieb, 1982; Happold, 1966), Ethiopia (Muller, 1977) and in various parts of East Africa (Delany, 1986; Delany & Kansiimeruhanga, 1970; Delany & Monro, 1985a,b, 1986; Delany & Roberts, 1978; Neal, 1981; Senzota, 1982; Taylor & Green, 1976). Various classifications are available for the genus Arvicanthis, some recognizing only one species (Rosevear, 1969) and others recognizing up to five species (Delany & Monro, 1986, and references therein). Most of the populations studied have been referred to as Arvicanthis niloticus (Desmarest).
In general Arvicanthis breeds during the rainy season, except at high altitudes in Ethiopia, where it breeds after the rains (Muller, 1977). Breeding begins shortly after the onset of the rains in east African populations, but just before the rains in Nigeria (Rabiu & Fisher, 1989). Neal (1981) compared the reproductive biology and life-history strategy of Arvicanthis in five localities: two in west Uganda, one in central Kenya, one in Ethiopia (Muller, 1977) and one in west Kenya (Taylor & Green, 1976). Variations in the ranking of body size, litter size, reproductive rate, growth rate and age at first reproduction between localities were found to be related to a crude ranking based on the environmental ‘variability’ or ‘stress’ of each location. In environments regarded by Neal (198 1) as more variable or stress- ful (more arid, greater temperature variations, disruption of food supply by agri- culture) individuals were smaller and had higher potential growth rates, in general agreement with the theory of r/K-selection (MacArthur & Wilson, 1967; Pianka, 1970). Similarly, Fleming (1974) studied two species of Costa Rican heteromyid rodents, and found that Liomys salvini, which inhabits deciduous, seasonal tropi- cal forest, displays more r-selected characteristics than Heteromys desmarestianus, which inhabits wet, relatively non-seasonal tropical forests. However, popu- lations of Arvicanthis in semi-arid areas of West Africa which would normally be regarded as ‘stressful’, have both a high average weight and large litter sizes (Poulet & Poupon, 1978; Rabiu & Fisher, 1989), in apparent contradiction to Neal’s assessment of the animals’ reproductive strategy.
In order to take an overall look at the reproductive ecology of Arvicanthis in Africa, 1 have extracted data from the literature on the climate and reproductive ecology and weight of Arvicanthis at eight African localities. Since the taxonomy of the genus Arvicanthis is still in debate (Delany & Monro, 1986), these data may represent several species within the genus Arvicanthis, populations or subspecies of A . niloticus, or a mixture of both species and subspecies. Since the breeding of Arvicanthis is modified when supplemental food is supplied (Taylor & Green, 1976), I did not include data on the breeding of Arvicanthis on permanently cultivated land in the Nile valley in Sudan (Ghobrial & Hodieb, 1982).
In this paper I explore the relationships between size and reproduction in Arvicanthis, and the influence of climate on size and reproductive ecology. The analysis gives an overall picture of certain aspects of the reproductive ecology of Arvicanthis in Africa. 1 also test the hypothesis of Neal (1981) that Arvicanthis exhibits r-selected characteristics (lower weight, larger litter sizes) in more variable or stressful localities, and K-selected characteristics (greater body weight, smaller
Reproductive ecology of Arvicanthis 19
litter sizes) in less variable, less stressful localities, and I discuss this with reference to developments of the theory of r- and K-selection (Boyce, 1984; Greenslade, 1983; Grime, 1977; Southwood, 1977, 1988; Stearns, 1977; Taylor, Aarssen & Loehle, 1990).
Methods Data extractedjrom the literature The data on Arvicanthis that could be obtained for most of the sites were the mean number of implanted embryos, the length of the female breeding season, the mini- mum weight at which females become reproductively active, the proportion of adult females in a year that are reproductively active and the mean weight of both adult males and females. Data were obtained for twelve months at eight localities, and for two years at one of them, giving nine one-year data sets in all (Table 1).
The number of implanted embryos was used rather than litter size because the former is more readily available in the literature. Litter size is normally lower than the number ofimplanted embryos (Neal, 1981; Rabiu & Fisher, 1989). I am making the assumption that litter size as a proportion of the number of implanted embryos is the same for all localities. The number of months in which females were pregnant or lactating was used as the ‘basic’ definition of length of breeding season, since it is the most reliable evidence of breeding. Where this was not available the assessment of length of breeding season was matched as closely as possible to this definition.
Mean annual temperature, which for most of the sites is the mean of the mean annual minimum and maximum, was not available for three localities: Kitale (Taylor & Green, 1978), Nakuru (Delany & Monro, 1986) and Semien National Park (Muller, 1977). Since Kitale and Nakuru are climatically similar to, and at approximately the same altitude as Nairobi, the mean annual temperature of Nairobi was substituted. The mean annual temperature of Addis Ababa for 23 years (White, 1983) was substituted for Semien National Park. Since Addis Ababa is at 2440 m and the site is at 3700 m the mean temperature used is probably an overestimate. Neal (1981) did not give climatic data for one of his two Ugandan sites (Crater Track). However, since Crater Track is only 10 km from the site at which climate was recorded (Mweya), and both sites are at much the same altitude, I have used the climatic data at Mweya for both localities.
For each locality the number of ‘humid’ and ‘dry’ months were calculated. Months in which the rainfall curve falls below the temperature curve on a standard climogram have been empirically established to be relatively dry, and months in which the rainfall curve rises above the temperature curve are relatively humid (White, 1983). Since the humid and dry months in a year total twelve, either, but not both, can be used as a climatic variable. I arbitrarily used the proportion of humid months in a year.
Statistical analyses Initially, Pearson product-moment correlation coefficients were calculated in order to explore the relationships between the biological and climatic variables. To deter- mine whether correlations between the weight and reproductive variables of females are influenced by climate, partial correlation coefficients, controlling for correlation with the number of humid months, were then calculated. The influence of climate on weight and reproduction was further explored using regression analysis. SYSTAT (Wilkinson, 1987) was used for all statistical analyses.
h)
0 F
Tab
le 1.
Dat
a ext
ract
ed fr
om th
e lite
ratu
re o
n: (a
) the
geo
grap
hy a
nd cl
imat
e of
eigh
t Afr
ican
loca
litie
s whe
re A
rvic
anfh
is ha
s bee
n st
udie
d, an
d (b
) the
wei
ght a
nd re
prod
uctiv
e ec
olog
y of A
rvic
anfh
is a
t the
se lo
calit
ies.
Stan
dard
erro
rs ar
e gw
en in
pare
nthe
ses i
f ava
ilabl
e. T
he cl
imat
ic d
ata w
as ta
ken
from
the s
ourc
es gi
ven
in (b
) unl
ess o
ther
wis
e not
ed
(a) G
eogr
aphy
and
clim
ate
Loca
lity
Ann
ual
Mea
n an
nual
N
umbe
r of
Alti
tude
Ti
me
rain
fall
tem
pera
ture
hu
mid
mon
ths
(m)
Latit
ude
Long
itude
pe
riod
(mm
) ("
C)
in y
ear
Cra
ter T
rack
, Uga
nda"
Fe
te-O
le, S
eneg
al
Kita
le, K
enya
K
ano,
Nig
eria
K
ano,
Nig
eria
M
wey
a, U
gand
a N
akur
u, K
enya
R
ojw
ero,
Ken
ya
Sem
ien
Nat
iona
l Par
k. E
thio
pia
1050
10
0 19
00
470
470
950
I920
65
0 37
00
o"o6
's 2 15
"N
1"O
I'N
12"0
3'N
lT
03"
O"1
I'S
0" 17
's o"
1 I'N
x
13"
N
2Y54
E
x 15
"W
35"O
I'E
8"32
'E
8"32
'E
29"5
4'E
36" 1
OE
38=l
OE
x38"
E
Apr
'65-
Mar
'66
Jan-
Dec
'76
Jan-
Dec
'70
Dec
'84-
Nov
'85
Apr
'6SM
af66
A
pr'8
ILM
ar'8
2 Ju
1'74-
Jun'7
5 A
pr'7
2-M
ar'7
3
Dec
'83-
NO
~'84
-
338
1082
46
3 65
3 81
2 999
543
1200
-
29
1 8b
28
27
24
1 8b
28
16'
-
2 10 3 4 9 8 4 8
Th
e cl
imat
ic d
ata
for M
wey
a was
used fo
r thi
s site
(see
text
for d
etai
ls).
hTem
pera
ture
data
is fo
r Nai
robi
(se
e tex
t for
det
ails
). 'T
empe
ratu
re d
ata
is fo
r Add
is A
baba
(see t
ext f
or d
etai
ls).
(b) B
iolo
gica
l dat
a of
Arv
ican
fhis
Loca
lity
Leng
th o
f M
inim
um w
eigh
t fe
mal
e M
ean
of f
emal
es
bree
ding
nu
mbe
r of
M
ean
adul
t Pr
opor
tion
at fi
rst
Mea
n ad
ult
seas
on
impl
ante
d fe
mal
e of
fem
ales
re
prod
uctio
n m
ale
(mon
ths)
em
broy
s w
eigh
t (9)
br
eedi
ng
(g)
wei
ght (
g)
Lite
ratu
re
sour
ce
Cra
ter T
rack
, Uga
nda
Fete
-Ole
. Sen
egal
K
itale
, Ken
ya
Kan
o, N
iger
ia (1
984)
Kan
o, N
iger
ia (1
985)
Mw
eya,
Uga
nda
Nak
uru,
Ken
ya
Roj
wer
o, K
enya
Se
mie
n N
atio
nal P
ark,
Eth
iopi
a
10”
3.67
* -
9
Y
6.00
7‘
8.14
7‘ 6.
67
12d
4.62
(k0
.21)
* -
Y
8d
5.57
(k0
.24)
8‘
4.
88 (
k0.2
3)
74
92
66
(k3.
0)
102
101
86
(kl.
4)
70
49 (* 1.
4)
89
(kl.
8)
0.43
0.52
0.
35
-* 0.48
0.32
-* 0.80
0.
16
60
63
45
63
70
60
40
35
68
75
100 89
101
110 98
85
52
97
Nea
l (I9
8 1 )
Poul
et &
Pou
pon
(197
8)
Tayl
or &
Gre
en (1
978)
R
abiu
& F
ishe
r (19
89) a
nd
Rab
iu &
Fis
her (
unpu
b.)
Rab
iu &
Fis
her (
1989
) and
R
abiu
& F
ishe
r (un
pub.
) N
eal (
1981
) D
elan
y &
Mon
ro (1
985
Nea
l (19
81)
Mul
ler (
1977
) 9
~ ~
~ ~~
~
~~
~ ~
~ ~
~ ~
~ ~
2.
% *D
ata
not a
vaila
ble
in li
tera
ture
; “ba
sed
on N
eal (
1981
). p.
177
; bba
sed o
n Po
ulet
& P
oupo
n (1
978)
, p. 1
92; ‘
base
d on
num
ber
of fe
mal
es p
regn
ant/l
acta
ting;
dba
sed
on
CE
r,
num
ber o
f fem
ales
pre
gnan
t; ‘b
ased
on
Del
any
& M
onro
(l98
6), p
. 94
and
Fig.
7; ‘
base
d on
num
ber o
f mon
ths i
n w
hich
birt
hs o
ccur
red.
e
22 M . Fisher
Results The correlation coefficients between all biological and climatic variables are given in Table 2. Since a large number ofcorrelation coefficients were calculated the usual method of determining significance cannot be used, and because the data set is small there are insufficient degrees of freedom for the calculation of reliable Bonferroni-adjusted probabilities (Wilkinson, 1987). The matrix of correlation coefficients was therefore used only in a comparative sense: to indicate which climatic variables have a greater influence on weight and reproduction, and to indicate possible relationships between the biological variables.
The three climatic variables (rainfall, temperature and number of humid months) have high correlation coefficients with each other. For investigating the effect of climate on weight and reproduction only the number of humid months in the year was used, since, firstly, it is jointly determined by both rainfall and tem- perature and is therefore an ‘index’ of both, and, secondly, it has higher correlation coefficients with five out of the six biological variables than either rainfall or temperature.
Partial correlation coefficients between the biological variables, controlled for correlation with the number of humid months, are given in Table 3 . The correlation coefficients of both minimum weight at first reproduction and proportion of females reproductively active with mean female weight remain high, indicating that the relationships of the two variables with female weight (Fig. 1 ) are unaffected by climate. However, all but one of the other correlation coefficients are reduced, suggesting that any effect that climate has on female weight, number of embryos and length of breeding season is probably direct and not via intercorrelations between the biological variables.
The regressions of female weight and number of embryos with humid months are not significant (Fig. 2). Male weight was not regressed on number of humid months (Fig. 2) since the correlation coefficient was close to zero (Table 2). The relationship between length of breeding season and number of humid months (Fig. 2) is better fitted by a hyperbolic equation than by linear regression. The hyperbolic equation has corrected R2 of 0.712, as against an adjusted R2 of 0.629 for linear regression.
Discussion Several authors have made qualitative observations on the relationship between climate and the breeding season of Arvicanthis, which has generally been observed to breed during the rainy season, and for a longer period in wetter, cooler localities (Delany, 1986; Neal, 198 1 ; Rabiu & Fisher, 1989; Taylor & Green, 1978). Figure 3 quantifies this relationship and provides a predictive equation for the length of the breeding season of Arvicanthis in Africa. The breeding season of Arvicanthis does not always coincide with the humid months: in Ethiopia Arvicanthis breeds immediately after the rainy season (Muller, 1977), and in northern Nigeria breed- ing begins shortly before the onset of the rains (Rabiu & Fisher, 1989). Neverthe- less, there appears to be a strong relationship between the number of humid months and length of the breeding season, presumably via the effect of the number of humid months on plant growth. Similarly, the length of the breeding season increases when supplemental food is provided for wild Arvicanthis (Taylor &
Tab
le 2
. Pea
rson
pro
duct
-mom
ent c
orre
latio
n co
effic
ient
s bet
wee
n le
ngth
of b
reed
ing
seas
on, m
ean
num
ber o
f im
plan
ted
embr
yos,
mea
n ad
ult f
emal
e wei
ght,
prop
ortio
n of
fe
mal
es in
repr
oduc
tive
cond
ition
, min
imum
wei
ght a
t whi
ch fe
mal
es b
ecom
e re
prod
uctiv
ely
activ
e an
d m
ean
adul
t mal
e w
eigh
t of A
rvic
anfh
is, a
nd a
nnua
l rai
nfal
l. m
ean
annu
al te
mpe
ratu
re a
nd n
umbe
r ofh
umid
mon
ths
in a
yea
r at e
ight
Afr
ican
loca
litie
s and
for t
wo
year
s at o
ne o
fthe
loca
litie
s. Ir
rele
vant
cor
rela
tions
hav
e be
en o
mitt
ed. n
= 9
for a
ll co
mpa
riso
ns e
xcep
t for
thos
e in
volv
ing
mea
n nu
mbe
r of i
mpl
ante
d em
bryo
s and
pro
port
ion
of fe
mal
es in
repr
oduc
tive
cond
ition
, for
whi
ch n
= 7
Leng
th o
f fe
mal
e M
ean
Min
imum
wei
ght
of fe
mal
es
Mea
n N
umbe
rof
bree
ding
nu
mbe
r of
Mea
n ad
ult
Prop
ortio
n at
firs
t M
ean
adul
t A
nnua
l an
nual
hu
mid
se
ason
im
plan
ted
fem
ale
of fe
mal
es
repr
oduc
tion
mal
e ra
infa
ll te
mpe
ratu
re
mon
ths
("C
) in
ayea
r (m
onth
s)
embr
yos
wei
ght (
g)
bree
ding
* (g
) w
eigh
t (g)
(m
m)
Leng
th o
f fem
ale b
reed
ing
seas
on
(mon
ths)
M
ean
num
ber o
f im
plan
ted
embr
yos
Mea
n ad
ult f
emal
e wei
ght (
g)
Prop
ortio
n of
fem
ales
bre
edin
g*
Min
imum
wei
ght o
f fem
ales
at fi
rst
repr
oduc
tion
(g)
Mea
n ad
ult m
ale
wei
ght (
g)
Ann
ual r
ainf
all (
mm
) M
ean
annu
al te
mpe
ratu
re ("
C)
Num
ber o
f hum
id m
onth
s in
a y
ear
1 .Ooo
- 0.
709
-0.3
73
-0.1
81
- 0.
222
0.53
9 - 0.
440
0.82
2
1 .O
oo
F
0402
1 .O
oo
0.08
8 -0
.770
I 4
00
0.91
0 -0
.827
0.
074
z 2. e rp
rp
hi
1 .O
oo
2
-0.7
18
-0.3
74
-0.3
44
-0.2
04
-0.1
12
0.86
6 -0
.821
1.
Ooo
.q
1 .om
1 .O
oo
-0.1
23
-0.0
04
0.17
8 0,
020
-0.9
72
-
-
-
0,88
9
5 - 0.
508
- 0.
237
- 04
82
0.46
2 0.
239
0.48
7 I ~
Ooo
> s.
*Arc
sine
tran
sfor
med
. 0
-3 h,
w
24 M . Fisher
Table 3. Partial correlations between length of breeding season. mean number of implanted embryos, mean weight of adult females. minimum weight at which females become reproductively active and proportion of females in reproductivecondition of Arvicunfhis, given their correlation with the number of humid months in a year. n = 9 for all comparisons except those involving mean number of implanted embryos and proportion of females in reproductive condition, for which n = 7
Length of Mean Minimum weight breeding number of Mean adult Proportion of of females at
season implanted female females first (months) embryos weight (g) breeding* reproduction (g)
-
Length of breeding season
Mean number of implanted -0.378 1 .ooo
Mean adult female weight (g) -0.070 0.337 I .om
breeding* 0.118 -0.243 -0'916 I ,000
at first reproduction (9) -0'097 0.062 0.9 I9 -0.897 I ~OOO
I .ooo (months)
embryos
Proportion of females
Minimum weight of females
*Arcsine transformed.
0.6
0.2 - 04 1 1 I I I I
40 50 60 70 80 90 100 110
Weight of adult females (g)
Fig. 1. The relationship of (a) the minimum weight of reproductively active females and (b) proportion of females in reproductive condition in a year with mean adult female weight of Arvicunrhis Regression results: (a)adjusted R*=0.804, F=33.903, P=O.OOI (b)Adjusted R2=0.547, F=6.037, P=0.057.
Green, 1976). High protein foods such as seeds and insects are more abundant during the rainy season and at the beginning of the dry season, during which time they form a major component of the omnivorous diet of Arvicanthis (Delany & Monro, 1986; Neal, 1981; Rabiu & Fisher, 1989; Taylor & Green, 1976).
Reproductive ecology of Arvicanthis 25
90- - 0 E t 80-
g 70- E .’ 60-
-0
s 50
0
a
0
- 0
m U al .-
?! n
70
60
50
Lc 0
a - 0 0
-
- 0
f m
f
6-
5-
4-
40 I I 1 I 1 I
0
0
0 0
0
0
t 9 0
c 0
40‘ 1 I I I I 1
u) g n E m c 0
7
“0 2 4 6 8 10 12 Number of humid months in year
Fig. 2. The relationships of (a) mean adult male weight, (b) mean adult female weight, (c) length of annual breeding season and (d) mean number of implanted embryos of Arvicanlhis with the number of humid months in the year. Regression results for M (b) R’=0.007, F = 1,059, P=0,338 (c) for fit of hyperbolic curve (length of breeding season = 12.736* number of humid months/[2~812+number of humid months]) corrected R’=0.712. F=24593l. PiO.001 (d)adjusted Rz=0.418, F=5.315, P=0.069.
If populations of Arvicanthis conform to the general predictions of r/K theory, then correlations would be expected between environmental ‘stress’ (measured here as the number of humid months in a year) and adult weight and the number of embryos. The absence of such correlations contradicts the hypothesis (Neal, 198 1) that populations of Arvicanthis are more r-selected in hotter, drier localities and more K-selected in cooler-wetter places. Additionally, there is no indication that the climatically equable Kitale in western Kenya, which was classified as ‘stressful’ because of disruption of the animals’ food supply by agriculture (Neal, 1981), is any more ‘stressful’ than other climatically similar localities. That the reproductive ecology of Arvicanfhis does not fit in with r/K theory is not in itself startling: Stearns (1977) reviewed thirty-five case studies and found that only eighteen fitted the r/K scheme. The populations of Arvicanthis reviewed here are probably all r-selected sensu Pianka (1970).
26 M . Fisher
Models have been proposed in which various types of ‘stress’ and ‘predict- ability’, and their effects on life history strategies, are summarized in a small number of axes (Greenslade, 1977; Grime, 1977; Southwood, 1977, 1988; Taylor et a/., 1990). Whatever the merits of the various approaches (Boyce, 1984; Southwood, 1988; Taylor et al., 1990) consistency of interpretation of r- and K-selection (Boyce, 1984), and standardization of quantitative measures for the assessment of ‘stress’ and ‘predictability’ are required. I have used the number of humid months in the year as a simple measure of the environmental ‘stress’ on different populations of Arvicanthis, but other methods could be tried. The ideas of Colwell (1974; see also Weis & Schwartz, 1988) could be utilized to estimate the ‘predictability’ of periodic phenomena such as climate.
Acknowledgments I would like to thank the Department of Biology, Sultan Qaboos University for research facilities, Y. Yesilicay for statistical advice and D. Gardner, S. A. Ghazanfar and R. Victor for comments on the manuscript.
References BOYCE, M.S. (1984) Restitution of r- and K-selection as a model ofdensity-dependent natural selection. Ann.
COLWELL, R.K. (1974) Predictability, constancy, and contingency of periodic phenomena. Ecol. 55,
DELANY, M.J. (1986) Ecology ofsmall rodents in Africa. Mamm. Rev. 16,141.
arable plot near Kampala, Uganda. Rev. Zool. Bof . Afr. 81,417425.
niloticus (Rodentia: Muridae). Afr. J . Ecol. 23,121-131.
niloticus) in Kenya. J . Trop. Ecol. 1, 1 11-130.
Kenya. J. Zool.. Lond. 209,85-103.
Carnegie Mus. Nut. Hist. 6,97-108.
Rev. Ecol. Syst. 15,427447.
1148-1 153.
DELANY, M.J. & KANSIMERUHANGA, W.D.K. (1970) Observations on the ecology Of rodents from a small
DELANY, M.J. & MONRO, R.H. (1985a) Growth and development of wild and captive Nile rats, Arvicanthis
DELANY, M.J. & MONRO, R.H. (1985b) Movement and spatial distribution of the Nile rat (Arvicanthis
DELANY, M.J. & MONRO, R.H. (1986) Population dynamics of Arvicanthis niloticus (Rodentia: Muridae) in
DELANY, M.J. & ROBERTS, C.J. (1978) Seasonal population changes in rodents in the Kenya Rift Valley. Bull.
FLEMING, T.H. (1974) The population ecology of two species of Costa Rican heteromyid rodents. J. Ecol. 55,
GHOBRIAL, L.I. & HODIEB, A.S.K. (1982) Seasonal variations in the breeding of the Nile rat (Arvicanthis
GREENSLADE, P.J.M. (1983) Adversity selection and the habitat templet. Am. Nut. 122,352-365. GRIME, J.P. (1977) Evidence for the existence of three primary strategies in plants and its relevance to
HAPPOLD, D.C.D. (1966) Breeding periods of rodents in the northern Sudan. Rev. Zool. Bof . Afr. 74,
MACARTHUR, R.H. &WILSON, E.O. (1967) The Theory of island Biogeography. Princeton University Press,
MULLER, J.P. (1977) Populationsokologie von Arvicanthis abyssinicus in der Grassteppe des Semien
NEAL, B.R. (1981) Reproductive biology of the unstriped grass rat, Arvicanthis in east Africa. Z. Saugetierk.
PIANKA, E.R. (1970) On r- and K-selection. Am. Nut. 104,592-597. POULET, A.R. & POUPON, H. (1978) L‘invasion d’Arvicanfhis niloticus dans le sahel senegalais en 19751976
RABIU, S. & FISHER, M. (1989) The breeding season and diet of Arvicanfhisin northern Nigeria. J . Trop. Ecol.
493-510.
niloticus). Mammalia 46,319-333.
ecological and evolutionary theory. Am. Naf. 111,I169-1194.
357-363.
Princeton, USA.
Mountains National Park (Aethiopen). Z . Saugetierk. 42,145-172.
46,174-189.
et ses consequences pour la strate ligneuse. La Terreet la Vie, 32,161-193.
5.375386.
Reproductive ecology of Arvicanthis 27
ROSEVEAR. D.R. (1969) The Rodents of West Africa. Publ. Brit. Mus. (Nut. His[ . ) No. 677, London. SENZOTA. R.B.M. (1982) The habitat and food habits of the grass rats (Arvicanrhis niloricus) in the Serengeti
SOUTHWOOD, T.R.E. (1977) Habitat, the templet for ecological strategies? J. Anim. Ecol. 46, 337-365. SOUTHWWD. T.R.E. (1988) Tactics, strategies and templets. Oikos 52,3-18. STEARNS, S.C. (1977) Theevolution oflife history traits. Ann. Rev. Ecol. Syst. 8, 145-171. TAYLOR, D.R.. AARSSEN. L.W. & LOEHLE, C. (1990) On the relationship between r/K selection and environ-
mental carrying capacity: a new habitat templet for plant life history strategies. Oikos 58,239-250. TAYLOR, K.D. & GREEN, M.G. (1976) The influence of rainfall on diet and reproduction in four African
rodent species. J . Zoo/.. Lond. 180,367-389. WEIS. I.M. & SCHWARTZ. S.S. (1988) The calculation and interpretation of climatic predictabilities. J .
Biogeog. 15,4 19429. WHITE, F. (1983) The Vegetation c$A.frica. UNESCO, Paris. WILKINSON, L. (1987) SYSTAT. SYSTAT Inc., Evanston, Illinois, USA,
National Park, Tanzania. Afr. J. Ecol. 20,241 252.
(Manuscript accepted 6 November 1990)