Effects of birth weight and gender on spatial learning and memory in pigs

In current pig farming, selection for large litter sizes has led to an increased number of low birth
weight (LBW) piglets being born and surviving to slaughter age. In humans, LBW is a risk factor
for various cognitive deficits. In pigs, the consequences of LBW are less well known. Cognitive
deficits may influence animal welfare if they result in an impaired ability to cope with housing
and rearing conditions. Previous studies have provided contradictory results when comparing
cognitive performance of normal birth weight (NBW) and LBW piglets in a spatial holeboard
task. It is possible that gender acted as a confounding factor on these results, as both males and
females were tested without using gender as a factor in statistical analysis. Therefore, this study
compared cognitive performance of LBW and NBW piglets of both sexes. Forty piglets from 15
available litters were selected (as NBW-LBW sibling pairs of the same gender). This resulted in
twenty NBW piglets (1,446±49 g) and twenty LBW piglets (810±24 g), each group consisting of
ten males and ten females. They were moved to the research facility at approximately 4 weeks of
age and housed in groups of 10 in mixed sex groups, but separated by birth weight. All piglets
were housed in pens enriched with straw. The piglets were trained to search for the locations
of four hidden food rewards (chocolate M&M’s) in the spatial holeboard, an open arena
containing a 4×4 matrix of potentially baited holes. The holeboard allows for the simultaneous
assessment of multiple cognitive variables, such as working memory (assessed by how many
revisits a pig makes during a trial) and reference memory (assessed by how well a pig learns
its rewarded configuration). All piglets received 44 acquisition trials. Testing in the holeboard
lasted until the pigs were approximately 3 months of age. Effects of birth weight and gender
were analysed using a mixed model ANOVA. LBW piglets showed an impaired reference
memory performance (F=5.16; df=1,360; P=0.024) compared to NBW piglets throughout the
acquisition phase. No effect of birth weight was found on working memory scores. Birth weight
did not influence the latency to visit the first hole of a trial, indicating comparable motivation
to perform the task for LBW and NBW piglets. However, LBW piglets were on average slower
to find their first reward (F=8.54;df=1,360; P=0.004) than NBW piglets. Gender did not affect
any of these measures. Together, these results confirm our hypothesis that LBW causes cognitive
deficits in pigs. Furthermore, male and female piglets show comparable cognitive performance
in the acquisition of the holeboard task. This suggests the negative cognitive effects of LBW
are not gender-specific.