effects of pre-weaning exposure to a maze on stress ... · underlying neural correlates. however...
TRANSCRIPT
Effects of pre-weaning exposure to a maze on stress
responses in pigs at weaning and on subsequent
performance in spatial and fear-related tests
Janice M. Siegford a,*, Gloria Rucker a, Adroaldo J. Zanella b
a Animal Behavior and Welfare Group, Department of Animal Science, 1287 Anthony Hall,
Michigan State University, East Lansing, MI 48824, USAb Department of Production Animal Clinical Sciences, Norwegian School of Veterinary Sciences,
Oslo, Norway
Accepted 2 March 2007
Available online 17 May 2007
Abstract
Learning, memory and regulation of the stress response are mediated by the hippocampus. Biologically
relevant hippocampal-dependent tasks that develop and integrate cognitive processing of this region may
not be available to piglets in some current production systems. Additionally, weaning piglets at less than two
weeks of age may make them particularly vulnerable to disruptions during cognitive development. We
measured the effects of a spatial maze task (MT) on the stress response of piglets at weaning (12 days of age)
and subsequent fear response (at 7 weeks of age). Twenty-seven pigs from four litters were assigned to one
of three treatments: maze task (MT), isolation control (IC), or control with sow (SC), then combined into
same sex groups with each treatment represented. Each group was tested four times per day from 5–11 days
of age. MT piglets navigated the MT in order to return to the home farrowing crate containing the sow and
litter. IC piglets were isolated for the same length of time taken for the MT piglet in their group to navigate
the MT. SC piglets controlled for handling and were returned to sow as MT entered the MT. Saliva was
collected immediately pre- and post-MT on day 11 to measure cortisol concentrations, which were lower
pre-MT (F(1, 8) = 5.65, P = 0.04). Weaning at 12 days of age increased cortisol concentrations 2 h post-
weaning (F(4, 75) = 5.67, P < 0.001). When exposed to a modified Morris water maze (MWM), a
significant interaction of sex and treatment was found with MT males and IC females faster at solving
the MWM than male IC pigs (x2(2) = 9.14, P = 0.01). Lower cortisol concentrations were seen pre-water
maze versus post-water maze (F(1, 19) = 27.62, P < 0.001). At 50 days of age, fear response of pigs was
examined using three fear-related tests (open field test (OFT), novel object test (NOT), and human approach
www.elsevier.com/locate/applanim
Applied Animal Behaviour Science 110 (2008) 189–202
* Corresponding author at: 1287C Anthony Hall, Michigan State University, East Lansing, MI 48824, USA.
Tel.: +517 432 8212; fax: +517 353 1699.
E-mail address: [email protected] (J.M. Siegford).
0168-1591/$ – see front matter # 2007 Published by Elsevier B.V.
doi:10.1016/j.applanim.2007.03.022
test (HAT), each consisting of 1 m acclimation and 4 m testing. In the HAT, MT animals had a tendency to
touch the unfamiliar person more quickly (F(2, 19) = 2.51, P = 0.10), and more times than animals in other
groups (F(2, 49) = 6.31, P = 0.008). MT may result in less fear of novel persons and ameliorate cognitive
deficits in male pigs, suggesting benefits of exposing young pigs to environments requiring spatial learning.
# 2007 Published by Elsevier B.V.
Keywords: Early weaning; Stress; Learning; Memory; Fear; Salivary cortisol
1. Introduction
The weaning of pigs in intensive production systems is considered to be a stressful event that
result in impairments to their health and welfare including altered behaviors such as increased
aggression and stereotypies, immunosuppression, decreased food intake, and suppressed growth
(Fraser et al., 1994; Hyun et al., 1998; Robert et al., 1999). Further, there is evidence to indicate
that premature weaning causes distress in pigs, resulting in prolonged vocalization, restless
activity, and, in some cases, long-term behavioral changes (Gonyou et al., 1998; Weary et al.,
1999). Although, commercially housed pigs are, in general, weaned at an earlier age (3–4 weeks)
than they would be under natural conditions (�3–4 months, Newberry and Wood-Gush, 1988;
Jensen and Recen, 1989), separation distress and frustration of suckling motivation seem more
pronounced in piglets weaned before two weeks of age, which show slower development of
normal eating behavior and higher frequency of anomalous and aggressive behaviors compared
with piglets weaned after three weeks of age (Gonyou et al., 1998; Weary et al., 1999; Worobec
et al., 1999; Hohenshell et al., 2000).
As it may be impossible to eliminate all weaning stress from a production environment,
procedures that reduce stress or increase the ability of pigs to cope with stress are needed. The
effects of several types of early life enrichment procedures on health, welfare and performance
measures have been examined in pigs, including neonatal handling (Hemsworth et al., 1986;
Hemsworth and Barnett, 1992; Day et al., 2002), brief maternal separation (Weaver et al., 2000),
and environmental enrichment (Beattie et al., 1996, 2000; De Jong et al., 1998, 2000; Day et al.,
2002). Results from these studies suggest a sensitive period in postnatal development exists in
pigs when environmental manipulations can cause long lasting changes in behavior, and likely in
underlying neural correlates. However, the most effective type of environmental manipulation is
yet to be determined. Some procedures may be beneficial in part to pigs, but not entirely. For
example, neonatal stroking appears to reduce fear in pigs up to 24 weeks later (Hemsworth et al.,
1986), however, neonatal handling which involves maternal separation causes reduction in
weight gain and evidence of dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis
(Weaver et al., 2000). Further, if handling treatments are performed after the neonatal period,
they may result in pigs that are harder to handle (Day et al., 2002).
Enrichment that involves the hippocampus may prove beneficial as the hippocampus mediates
many forms of learning and memory, including short-term and spatial memory, (e.g., Castro
et al., 1989; Maaswinkel et al., 1997) and plays a prominent role in regulation of the stress
response (e.g., Broom and Zanella, 2004; Fuchs and Flugge, 2003; Kaiser et al., 2003; McEwen,
2001). The high degree of developmental plasticity within the hippocampus may also make
enrichment involving this brain region beneficial (see Brunson et al., 2003; Mirescu et al., 2004).
In rats, reduced anxiety and emotionality and improved performance on spatial tasks have been
observed when complex environmental enrichment that requires learning and memory for
J.M. Siegford et al. / Applied Animal Behaviour Science 110 (2008) 189–202190
locations of resources is provided early in life (Fernandez-Teruel et al., 1997; Imanaka et al.,
2006; Iuvone et al., 1996; Pereira et al., 2006).
Providing young pigs with a complex environment that requires spatial learning and memory
while allowing them to perform biologically relevant behaviors may be an enrichment strategy
that simultaneously stimulates cognitive development and an adaptive stress–response (Cox and
Cooper, 2001). However, in intensive productive settings, particularly in indoor confinement
system, pre-weaning pigs are not given the opportunity to engage in many natural behaviors such
as rooting, interacting with piglets of other litters, searching for food, or remembering the
location of the nest. Research examining appropriate environmental enrichment for pigs has
highlighted the importance of choosing enrichment that encourages performance of foraging and
exploration (Van de Weerd et al., 2003). However, to date, no research has examined the effect of
providing piglets with forms of enrichment that require forms of learning and memory beneficial
to development of adaptive stress responses (Pryce et al., 2005).
The aim of the current study is to assess whether performance of a maze task, designed to
require spatial learning and memory, can reduce the stress associated with early-weaning in
piglets. We hypothesized that pigs that performed a hippocampal-dependent task prior to
weaning would show immediate and long-term reduced stress-related physiological and
behavioral consequences of early weaning and demonstrate improved cognitive ability through
faster learning of a second spatial task.
2. Methods
2.1. Animals
Twenty-seven piglets of both sexes from four litters were assigned to one of three treatments: maze task
(MT, n = 9), isolation control (IC, n = 9), or control with sow (SC, n = 9) such that each litter contained at
least one complete group of all three treatments per sex. Piglets were left undisturbed until postnatal day
(PND) 5 to allow for management practices and neonatal interventions such as ear notching, tail docking and
castration. Males were castrated on PND 1 following normal farm operating procedure. Teat order was noted
for all animals. Weights were collected and monitored from PND 5–21, then weekly until PND 49. All
protocols were approved by the MSU All-University Committee on Animal Use and Care.
2.2. Enrichment using a maze task
The MT piglets were exposed to the maze task (MT, Fig. 1) four times per day from PND 5–11 in a pen
adjacent to their farrowing crate. Prior to training, all of the animals in the group were removed from the sow
for 30 s. After this period, the SC animal, which controlled for the effect of handling, was returned to the
sow, the IC animal was isolated in a sectioned-off area of the home farrowing crate, to control for isolation in
a familiar environment, and the MT animal was placed in the maze. Piglets were trained in the maze by
gradually increasing the difficulty of the task as the MT animal succeeded at each training step. At the start
of training, all panels were removed and the door to the home farrowing crate (marked ‘finish’) was kept
open. The piglet was placed in the maze near the door to the sow. Once the piglet learned to go through this
door to reach the sow and the litter, the door was gradually lowered over successive trials until the piglet was
pushing through a closed door to reach the sow. The panels of the maze were then introduced one at a time,
initially with the correct door held open and gradually lowered as the piglet learned to navigate the maze.
The final maze structure consisted of three panels with three doors per panel, but only one door that would
open to allow passage of the piglet. Once the piglet had completed the full maze, the order of the panels was
changed so they must learn a new maze. The piglet’s motivation to rejoin the sow and litter were used as
motivation to solve the maze. The time that the MT animals spent in the maze (maximum of 5 min) was
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matched by the time the IC animal spent in isolation. In each test, when the MT piglet went through the
‘finish’ door, the panel separating the IC piglet from the sow and litter was removed to ensure that the MT
and IC piglets were isolated from the sow for the same amount of time. The number of trials taken to learn
the full task and time taken to solve each step were recorded. Pre- and post-maze saliva collection occurred
on PND 11 to measure cortisol concentrations.
2.3. Weaning
The piglets were early-weaned by litter on PND 12 at 1400 with saliva collection for cortisol
measurement occurring 2 h pre-weaning, immediately pre-weaning, 2 h post-weaning, 16 h post-weaning,
and 24 h post-weaning. The pens were fitted with drinkers and self-feeders, allowing access to unlimited
water and feed throughout the experimental period. Environmental conditions and management procedures,
as well as the care and feed provided for the early-weaned pigs, were carried out in accordance with standard
procedures used by staff at the Swine Teaching and Research Center at Michigan State University.
2.4. Spatial testing using the morris water maze
On PND 14, pigs from all treatments were individually tested in a water maze spatial memory task
(Laughlin and Zanella, 2003), according to the protocol described below, to examine spatial learning, which
is considered to be dependent on the hippocampus. The test was carried out in an inflatable swimming pool
(3.6 m diameter), marked into eight equal sectors, filled with opaque water to a depth of 50 cm. Non-toxic
green food coloring was used to make the water opaque. The temperature of the water was maintained with a
pool heater throughout the test period at 38–39 8C, within the thermoneutral zone of pigs at this age (Curtis
and Backstrom, 1986). A trial consisted of five exposures to the maze each separated by 10 min intervals
(thus each trial lasted about 50 min) at PND 14. Prior to testing, a platform was positioned in a randomly
chosen sector of the pool, about 30 cm from the edge. During each exposure, the pig was released from the
same randomly assigned sector of the pool and allowed to swim freely until the platform was located. Piglets
had to swim in order to remain afloat, therefore hidden observers were always on hand in the event that a
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Fig. 1. Diagram illustrating the MT maze created in a farrowing crate as well as the IC isolation area. Each panel in the
maze has three doors, however only one door opens to allow the piglet to pass through.
piglet began sinking. This was not the case for any piglets in this experiment. Successful location of the
platform was defined as the pig having at least three feet on the platform in a ‘stable’ position for 10 s. If the
animal failed to locate the platform after 120 s, the experimenter guided the pig to the platform and allowed
it to stand there for 10 s. For pigs that did not reach the platform before time ended, 120 s was used as their
latency. Saliva samples were collected before and after the water maze to determine salivary cortisol
concentrations for the different treatments and sexes. The latencies of pigs to reach the platform were
recorded. Average latencies to reach the platform were calculated using times for exposures 2–5 and used to
examine differences by treatment and sex.
2.5. Fear testing
To determine if the MT has long lasting effects on pigs’ response to novel and/or fearful situations, the fear
response was assessed by measuring behavioral and physiological outcomes using an open field test (Vierin
and Bouissou, 2002). Each pig was tested once per day, from PND 49–51, in a 4 m � 5 m arena (Fig. 2) in the
following situations: (1) open field test (OFT): a pig was placed alone in the arena for 5 min. The first minute
was considered an acclimation period and the last 4 min were the test. (2) Novel object test (NOT): A basketball
was lowered into the arena using a rope after the 1 min acclimation period and the responses of the pig recorded
for 4 min after the ball was introduced. (3) Human approach test (HAT): a person unknown to the pig entered
the arena after the 1 min acclimation period. The person sat silent and immobile in a chair (seat height = 45 cm)
for 4 min. The order that all pigs were exposed to the fear-related tests was: (1) OFT, (2) NOT, and (3) HAT.
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Fig. 2. Diagram of the open field arena used to examine the fear response of piglets at PND 49–51. Positions of the
basketball and person during NOT and HAT tests respectively are illustrated. The 6 inner squares represent the center of
the open field and the 14 outer squares were scored as periphery. Hatched squares represent those considered proximate to
the ball in NOT while light gray squares indicate those considered proximate to the unfamiliar person in HAT.
All testing occurred between 08:30 and 11:30 and pigs were tested in a random order each day. Saliva
samples were taken immediately before and after testing each day to examine cortisol levels. Tests were
video recorded and later analyzed to examine time spent in the center and periphery of the OFT, as time
spent in the center of an open area may provide an indication of fear of novelty in pigs (Andersen et al., 2000;
Jensen et al., 1995). Time spent in proximity to the ball or person, latency to approach and touch the ball or
person, and number of times the ball or person was touched were analyzed for NOT and HAT, respectively.
2.6. Saliva collection and testing
Saliva was collected on all occasions by placing the pigs in pairs or threes into a weigh cart. A 30 cm piece of
soft, unflavored dental floss was tied around a small piece of cotton gauze. The gauze was inserted into the pig’s
mouth and held in place using the floss until the gauze was saturated. After collection, samples were placed in
14 ml conical tubes on ice. Samples were centrifuged at 3500 rpm for 5 min to remove saliva from the gauze
then stored in polypropylene tubes at �20 8C until assayed. Cortisol concentrations were determined with
radioimmunoassay kit (Coat-a-Count, Diagnostic Products Corporation, Los Angeles, CA) with a sensitivity
of 0.2 mg/dl, following the protocol. Intra- and interassay CV’s were 6 and 9%, respectively. Using saliva to
detect changes in cortisol concentrations in pigs has been demonstrated previously (Cooper et al., 1989).
2.7. Data analysis
Data were analyzed using SAS1 (version 9, Statistical Analysis Systems Institute, Cary, NC). Unless
indicated, data met assumptions of normality and were not transformed. Teat order was analyzed using the
glimmix macro in SAS with a multinomial distribution to handle ordered data (as the fore teats provide better
nutrition than hind teats (Kim et al., 2000) and including treatment, sex and their interactions. The mixed
models (Proc Mixed) used to analyze time taken to complete the MT and number of training steps repeated in
the MT used sex as the independent variable. Pig (litter � sex � treatment) was included as a random effect.
Mixed models analyzing weight at weaning and percent of body weight lost at weaning included sex and
treatment as independent variables and considered interactions between these variables. Pig (lit-
ter � sex � treatment) was included as a random effect. Mixed models examining changes in concentration
of salivary cortisol before and after the MT, surrounding weaning, and before and after the WM incorporated a
repeated measures statement with time as the repeated measure and pig as the subject. Time, sex and treatment
were included in these models as independent variables and interactions between these variables were
considered as well. Latency to escape the water maze was analyzed with a survival analysis using Proc Lifereg
in SAS. Latencies of 120 s, indicating that the pig did not solve the task, were censored in the analysis. Mixed
models examining continuous behavioral variables in OFT, NOT, and HAT (i.e., percent of time spent in a
location or in proximity to the person or object, latency to approach the person or object, and latency to touch
the ball or person) included sex and treatment as independent variables. Percentage data was transformed by
taking the arcsine of the square root to meet assumptions of normality. The discrete response variables in NOT
and HAT (i.e., number of times the pig touched the ball or person) were analyzed in SAS with a glimmix macro
and a Poisson distribution using models that included sex, treatment and their interactions. Tukey–Kramer tests
were used to perform multiple comparisons on the least squared means when significant effects were observed
for variables with more than two levels. P < 0.05 was considered significant while p-values between 0.05 and
0.1 represented a tendency toward significance. Data are presented as least squares means� standard error of
the means (S.E.M.). Error bars in graphs are S.E.M.
3. Results
Teat order of animals was not significantly affected by sex, treatment or an interaction of the
two, suggesting that there was no bias in the nutrition received by piglets of varying treatments or
sexes prior to weaning (P > 0.05 in all cases).
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There was no significant effect of sex on how quickly MT pigs completed each step of the MT
(F(1, 7) = 0.44, P = 0.53; females: 63.0 � 8.8 s versus males: 71.8 � 9.8 s). There was also no
significant effect of sex on the number of training steps MT pigs needed to repeat as they learned
to navigate the MT (F(1, 7) = 0.780, P = 0.07; females: 0.6 � 0.3 steps repeated versus males:
0.5 � 0.3 steps repeated). On the final day of the MT, salivary cortisol was sampled in pigs of all
treatments immediately before and after the task. Cortisol concentrations were significantly
lower before MT (F(1, 8) = 5.65, P = 0.04), but no effects were observed between treatments or
in the interaction of treatment and time (Fig. 3). The time effect appears to be largely due to
increased cortisol concentrations in IC animals, which were isolated while the MT animals
performed the MT. MT animals and SC animals, which were the handling control, did not have
marked increases in cortisol concentrations.
Average weight at weaning was 4.21 � 0.30 kg and average percent of body weight lost at
weaning was 4.3 � 0.9%. There were no significant effects of sex, treatment or the interaction of
J.M. Siegford et al. / Applied Animal Behaviour Science 110 (2008) 189–202 195
Fig. 3. Salivary cortisol concentrations before and after the final MT on PND 11 for piglets of all treatment groups.
Different lower case letters indicate significant difference between pre-MT and post-MT cortisol concentrations
(P < 0.05).
Fig. 4. Salivary cortisol concentrations surrounding weaning on PND 12 for piglets of all treatment groups. Cortisol
concentrations were significantly affected by time (P < 0.001) with higher concentrations seen 2 h pre-weaning and 2 h
post-weaning compared to all other time points. Different lower case letters indicate a significant difference between
cortisol levels 2 h pre-weaning, 2 h post-weaning and all other time points (P < 0.05).
sex and treatment on weight at weaning or percentage of body weight lost following weaning
(P > 0.05 in both cases).
There was a significant effect of time on concentrations of cortisol found in saliva of pigs near
weaning (F(4, 75) = 5.67, P < 0.001, Fig. 4). No effects of treatment, sex or interactions between
factors were observed (P > 0.05 in all cases). Cortisol concentrations 2 h post-weaning were
significantly greater than those at all other times except 2 h pre-weaning. Cortisol concentrations
2 h pre-weaning were similar to those 2 h post-weaning. Cortisol concentrations immediately
pre-weaning, 16 h post-weaning, and 24 h post-weaning were similar.
The interaction between sex and treatment had a significant effect on the latency of pigs to
escape the MWM (x2(2) = 9.14, P = 0.01, Fig. 5). Male MT pigs and female IC pigs learned to
find the platform significantly more quickly than male IC pigs. There was a significant effect of
time on salivary cortisol concentrations surrounding exposure to the water maze, with lower
levels immediately before the test compared with levels after (F(1, 19) = 27.62, P < 0.001;
0.55 � 0.1 mg/dl versus 1.10 � 0.1 mg/dl, respectively). There were no significant effects of sex,
treatment or their interactions on salivary cortisol concentrations (P > 0.05 in all cases).
Treatment did not affect the amount of time pigs spent in the center or periphery of the open
field during the OFT (P > 0.05 in both cases). No differences were seen in amount of time pigs
spent in areas of the test arena, including in proximity to the person or object in the HAT and
J.M. Siegford et al. / Applied Animal Behaviour Science 110 (2008) 189–202196
Fig. 5. Average latencies for piglets of all treatment groups to escape the spatial water maze on PND 14. IC males were
slower at escaping the water maze compared to MT males and IC females (P = 0.01). Different lower case letters indicate
significant difference between groups (P < 0.05).
Fig. 6. Latency of pigs to touch the unfamiliar person in HAT. MT animals tended to touch the person more quickly than
animals in other groups (P = 0.10).
NOT, respectively (P > 0.05 for all). No differences were seen in latency to approach the object
or person, latency to touch the object or in the number of times the object was touched (P > 0.05
in all cases). In the HAT, MT animals had a tendency to touch the unfamiliar person more quickly
than animals in other groups (F(2, 19) = 2.51, P = 0.10; Fig. 6). Treatment had a significant effect
on the number of times piglets touched the unfamiliar person in HAT (F(2, 19) = 5.45, P = 0.013)
with pigs in the MT group touching the unfamiliar person twice as often as IC pigs and three
times as often as SC pigs (Fig. 7). Weights at PND 49 were not affected by sex, treatment group,
or an interaction between the factors (P > 0.05 in all cases; MT: 12.51 � 0.79 kg, IC:
11.01 � 0.50 kg, and SC: 11.47 � 0.52 kg).
4. Discussion
The use of a maze task in piglets for a week before early weaning at PND 12 resulted in
decreased fear of unfamiliar persons at seven weeks of age compared to animals receiving similar
handling alone or similar handling plus equal lengths of isolation. The benefits of the pre-
weaning maze task may be due to its requirement for both physical and cognitive activity during a
developmentally sensitive time period in pigs (Hemsworth et al., 1986; Hemsworth and Barnett,
1992). Many forms of learning and memory, including short-term and spatial memory, are
mediated by the brain region known as the hippocampus (e.g., Castro et al., 1989; Maaswinkel
et al., 1997). In addition, the hippocampus plays a prominent role in regulation of the stress
response (e.g., Broom and Zanella, 2004; Fuchs and Flugge, 2003; Kaiser et al., 2003; McEwen,
2001). Early life experiences affect development of cognitive processes, including the stress
response later in life, perhaps due in part to the high degree of developmental plasticity within the
hippocampus (see Brunson et al., 2003, Mirescu et al., 2004). These experiences also can
enhance or disrupt the development of appropriate social skills (Cushings and Kramer, 2005).
There is evidence indicating that experiences during early development are long-lasting and they
may affect overall coping strategies of animals, including humans (see Pryce et al., 2005 for a
review). In rats, long lasting effects on the stress response have been observed with protocols
involving neonatal handling, environmental enrichment, and exercise (e.g., Meaney et al., 1991;
Paylor et al., 1992; Tang, 2001; Tong et al., 2001). These protocols not only result in more modest
increases of adrenocorticotrophic hormone (ACTH) and corticosterone in response to acute
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Fig. 7. Number of times pigs touched the unfamiliar person in HAT. Different lower case letters indicate a significant
difference between this group and the others (P < 0.05).
stressors but also result in hippocampal changes in ability to bind corticosterone (Meaney et al.,
1989) and altered gene expression (Tong et al., 2001). Conversely, protocols that socially isolate
young animals including pigs can sensitize the stress response to future stressors (Kanitz et al.,
2004; McCormick et al., 1998; Tuchscherer et al., 2006). This study represents the first attempt to
address the question of whether neonatal enrichment involving hippocampal activation in piglets
through neonatal exposure to a spatial task provides any long-lasting behavioral, physiological or
production-related consequences.
Though salivary cortisol concentrations increased following the maze task on PND 11, this
increase appears to be mainly the result of higher values in IC pigs after they were isolated during
testing period (Fig. 3). Pigs in the MT and SC groups did not appear to experience increased
cortisol concentrations following the test, suggesting that handling alone and performance of the
test did not cause an acute stress response. MT pigs may not have been stressed on the final day of
testing because they were both familiar with the test and able to control their return to the sow. IC
pigs, on the other hand, had no control over when they would return to the sow.
Requiring piglets to perform the MT four times a day, for a total time of about an hour, did not
negatively affect the performance of pigs at weaning. At weaning, the elevation of cortisol levels
seen across groups 2 h pre-weaning may be due to circadian rhythms of cortisol or because three
of the four litters had nursed right before samples were collected. Salivary cortisol
concentrations were similar between pigs of all treatment groups and sexes at weaning.
However, there was a tendency for cortisol concentrations in IC animals to remain elevated 16 h
after weaning. The social isolation experienced by these animals may have sensitized their
response to future separation from the sow or generally increased the responsivity of the HPA
axis to stressors.
Weaning weights were similar between treatments and sexes as was percentage of body
weight following weaning, indicating that maze task did not have a negative impact on
production measures. Weights of animals up to seven weeks of age were also similar between
treatment groups, suggesting that the neonatal experience of these animals did not impact their
growth long-term. However, it may be necessary to follow the weights of animals through the
growing and finishing periods to verify that this is the case. Weaver et al. (2000) observed pigs
through seven months and saw reduced weight gain in pigs that received neonatal handling. It is
important to note that in this case, handling of pigs mimicked protocols used in rats and mice,
which involve removing pups from the nest and separating them from the mother for a short
period. Thus, maternal separation may be a more appropriate description for that protocol
(Weaver et al., 2000), and as such, the protocol may have been more stressful than enriching.
In rats, improved performance on spatial tasks has been seen when animals are tested after
living in complex environmental enrichment from an early age that requires learning and
memory for locations of resources (Fernandez-Teruel et al., 1997; Imanaka et al., 2006; Iuvone
et al., 1996; Pereira et al., 2006). We hypothesized that providing young pigs with a complex
environment that required spatial learning and memory might have similar long lasting effects on
spatial learning ability in pigs.
The significant interaction of sex and treatment on MWM performance, however, suggests
that neonatal interventions and subsequent pre-weaning treatments may have interacted to
influence later cognitive abilities of pigs. Male IC pigs were slower at solving the MWM
compared to IC females and MT males. Neonatal pain causes long-lasting adverse behavioral and
cognitive changes that can be compounded by isolating animals or alleviated by providing
environmental enrichment (Anand and Scalzo, 2000; Bhutta and Anand, 2002; Li et al., 2005;
Imanaka et al., 2006). Males in this experiment were castrated without anesthesia at PND 1 and
J.M. Siegford et al. / Applied Animal Behaviour Science 110 (2008) 189–202198
had their ears notched and tails docked as females did. Castration in pigs causes pain that can
persist for up to 22 h, while pain-related behaviors associated with tail docking and ear notching
desist within minutes (Noonan et al., 1994; Weary et al., 1998; Taylor et al., 2001). Alternatively,
castration could have removed gonadal steroids in males important for the development of
hippocampal cells and performance in spatial tasks, both of which are altered by neonatal
castration in rodents (Roof, 1993; Isgor and Sengelaub, 2003). The testes of young pigs show a
peak in gonadal steroid production at 2–4 weeks of age, possibly important for neuronal
development (Schwarzenberger et al., 1993). Thus, neonatal castration coupled with social
isolation could have impaired the ability of IC males to learn a later spatial task, while exposure
to the MT may have reversed this effect in males (Imanaka et al., 2006). In contrast, IC females,
in the absence of earlier intense pain and gonadectomy, may have responded to social isolation
with improved cognitive performance the way young rats do (e.g., Fernandez-Teruel et al., 2002;
Meaney et al., 1991). Future experiments should examine the role of pain and gonadectomy on
behavior and cognition of pigs to elucidate the effect of neonatal interventions on subsequent
welfare.
Exposure of pigs to a maze task prior to weaning appears to reduce fear of unfamiliar persons
at seven weeks of age. MT pigs tended to touch the unfamiliar person more quickly and touched
the unfamiliar person significantly more times. Neither acute stress in the form of brief social
isolation or enrichment in the form handling of the animals during maze trials resulted in similar
changes in IC and SC animals in response to the unfamiliar person. Evidence in rodents suggests
that enrichment early in life may reduce fear of novelty and anxiety related behavior (Fernandez-
Teruel et al., 1997; Imanaka et al., 2006; Iuvone et al., 1996). Conversely, protocols that socially
isolate young animals including pigs can sensitize the stress response to future stressors, such as
exposure to an unfamiliar person (Imanaka et al., 2006; Kanitz et al., 2004; McCormick et al.,
1998; Tuchscherer et al., 2006). Thus, the SC pigs that did not receive neonatal enrichment and
the IC pigs that were socially isolated during the MT may have both had more fear of novel
person than MT pigs.
In all aspects of this study, testing a larger group of animals may yield more definitive
results as there was substantial individual variation in most measures. Repeating the study
using pigs weaned at 3–4 weeks of age, as is typical in most countries, might also yield more
dramatic findings. Pigs weaned younger than 2 weeks of age show cognitive impairments on
spatial and social tasks when they are tested following weaning combined with brief social
isolation while piglets weaned at 3 weeks of age do not (Laughlin and Zanella, 2003; Souza
and Zanella, 2004; Souza et al., 2004; Souza et al., this volume). Weaning at 3–4 weeks has
less impact on cognition and behavior (Gonyou et al., 1998; Weary et al., 1999; Worobec
et al., 1999; Hohenshell et al., 2000), therefore enrichment performed prior to weaning pigs
at later ages might have more profound impacts because weaning might be less likely to
disrupt any benefits resulting from the enrichment. Examination of the impact of the maze
task on brain measures in pigs would provide a more comprehensive view of possible
cognitive benefits to environmental enrichment that requires both physical and mental
exertion and may elucidate the importance of timing of both enrichment and weaning.
However, to date, the impact of early-environmental factors on brain organization has not
been investigated in depth in farm animal species. This research provides initial support for
and direction in enriching crates or housing in ways that require piglets to use more spatial
abilities to locate to resources, however, more needs to be learned on what environmental
features are most important and how long the enrichment must be provided in order to
maximize benefits.
J.M. Siegford et al. / Applied Animal Behaviour Science 110 (2008) 189–202 199
5. Conclusion
Exposing piglets before weaning to a spatial task results in less fear of novel persons,
suggesting benefits for neonatal opportunities that provide mental and physical exercise. Further,
provision of environmental enrichment of this sort to piglets appear not to negatively impact their
performance at weaning.
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