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

Abstract

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.