Comparison of first- and last-born pigs revealed no effect of the birth process on acquisition and reversal of the cognitive holeboard task

Abstract

Hyperprolific pig breeds produce particularly large litters. Birth order within these litters, reflecting the duration of the expulsion phase of farrowing, may be associated with an increased risk of birth complications that may affect the later functioning of the pig. In this study, piglets from 12 litters were weighed at birth. The first-born (FB) and the last-born piglet, that went through the longest expulsion period, were identified and selected per litter. These pigs were tested in a cognitive holeboard (HB) task, which allows simultaneous assessment of spatial working memory (WM) and reference memory (RM) performance. In the HB, the pigs learned to find hidden rewards (M&M's®) in 4 of 16 holes distributed in a 4 × 4 matrix. After task acquisition, a new pattern of baited holes was presented (reversal). Salivary cortisol samples were collected before and after the first reversal trial. We hypothesized that the FB piglets would be less likely to have their health and fitness compromised by the birthing process than the LB piglets. Also, the LB pigs would have lower birth weights, poorer memory performance in the holeboard, and be more stressed by the change to a new pattern of baited holes, as indicated by a more pronounced increase in salivary cortisol concentrations. Birth weights of FB and LB piglets did not differ, but mean birth weights decreased with increasing litter size. Acquisition of the WM and RM components of the HB task was similar in FB and LB pigs. When confronted with a reversed configuration of baited holes, FB and LB pigs showed an initial decrease in both WM and RM performance that was unaffected by birth order. Both groups reached high levels of performance on both memory components in the reversal phase. Switching to the reversed configuration of baited holes had no effect on salivary cortisol, i.e. it may not have stressed the pigs. The inter-visit interval, i.e., the average duration per hole visit, which is thought to reflect a pig's motivation to search for food rewards, was not differentially affected by birth order but was highly variable, presumably reflecting the influence of external conditions during training, most likely ambient temperature.