Testing post-weaning food motivation in low and normal birth weight pigs in a runway and operant conditioning task

Low birth weight (LBW) pigs face more welfare challenges than their normal birth weight (NBW) siblings. Understanding the underlying mechanisms of cognitive and learning abilities in these pigs may help to improve their welfare. Early competition in life over resources, combined with the higher need for nutrient intake, make it likely that LBW pigs have a higher motivation for food than NBW pigs. This study aimed to compare the motivation to obtain food rewards between LBW and NBW pigs, using variable numbers of rewards in two separate tasks; a runway and an operant conditioning task (the nose wheel task). Ten pairs of littermates were used. From each litter, one low birth weight piglet (mean birth weight±SEM: 854±33g) and one normal birth weight piglet (1332±53g) was selected. Pigs were tested in the runway task at 12 weeks of age and the operant nose wheel task at 19 weeks of age. Both tasks consisted of a baseline phase (two rewards), a high reward phase (eight rewards) and an extinction phase (no rewards). Statistical analyses using mixed models showed that NBW animals left the start box faster than LBW animals in the high reward phase in the runway task. However, their run time in this phase was not shorter and no other birth weight effects were found in any other phase or measure in this task nor in the nose wheel task. All animals decreased their run time in the runway task between the baseline phase and high reward phase, and increased their run time in the extinction phase (p<0.05). Likewise, in the nose wheel task, all animals reached a higher number of total rewards gained and spent a lower percentage of time away from the feeder in the high reward phase compared to the baseline phase (p<0.05). Additionally, they showed a decrease in motivation during the extinction phase (p<0.05). Our results indicate that there is no difference in motivation to obtain food rewards between LBW and NBW pigs. However, both the results of the runway and the nose wheel task show a phase effect between the baseline, high reward and the extinction phase. This is in accordance with the underlying theory that animals have a higher motivation for resources that are more desired. Therefore, we show that both tasks are sensitive enough to measure motivation for food rewards in pigs, and are consequently useful to study factors influencing pig motivation.