effects of pre-weaning exposure to a maze on stress ... · underlying neural correlates. however...

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

mailto:[email protected]

http://dx.doi.org/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

J.M. Siegford et al. / Applied Animal Behaviour Science 110 (2008) 189–202 191

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


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.


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).


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


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


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


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


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.


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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