Overnight Social Isolation in Pigs Decreases Salivary Cortisol but Does Not Impair Spatial Learning and Memory or Performance in a Decision-Making Task

Abstract

Pigs in modern farming practice may be exposed to a number of stressors, including social stressors such as mixing or isolation. This may potentially affect both cognitive abilities and stress physiology of the animals. We tested the hypothesis that overnight social isolation in pigs impairs performance in a cognitive holeboard (HB) task (Experiment 1) and the Pig Gambling Task (PGT) (Experiment 2), a decision-making task inspired by the Iowa Gambling Task. In addition, we tested the effect of overnight social isolation on salivary cortisol levels. A within-subjects approach was used in which performance in the two behavioral tasks and cortisol levels were first determined during normal social housing, followed by performance and cortisol levels after experiencing stress induced by overnight social isolation. A total of 19 female pigs with a birth weight closest to their respective litter average was selected from 10 different litters and placed in two pens after weaning. Following habituation, pigs were trained in the HB task, starting at 10 weeks of age. Then, the pigs were isolated overnight, five individuals per night, at 15, 16, and 17 weeks of age. Between these three isolations, social housing and training in the HB continued. Starting 6 weeks after the end of the HB experiment, at approximately 23 weeks of age, the pigs were trained in the PGT. The effects of overnight social isolation on performance in this task were assessed once, when the pigs were 25 weeks old. Salivary cortisol was measured from samples collected 15 min after the start of isolation and at the end of the isolation period and compared to baseline values collected before the start of social isolation. Our results did not confirm the hypothesis that isolation impaired HB performance and decision-making in the PGT. Unexpectedly, overnight social isolation decreased cortisol levels below baseline values, an effect that was not associated with changes in performance of the behavioral tasks. We hypothesized that the housing and testing conditions may have prepared the animals to cope efficiently with stress.

Keywords: Iowa gambling task; cortisol spatial learning and memory; decision-making task; holeboard; isolation stress; pig (Sus scrofa); reference memory; working memory.

Figure 1

The holeboard (left) and the apparatus for testing decision-making in pigs (the PGT: pig gambling task; right), side by side (Illustrations: Yorrit van der Staay).

Figure 2

Acquisition of the holeboard task and effects of three overnight isolations in 19 pigs. The means and SEM of the working and reference memory performance (A) and of the log₁₀ transformed latencies to first visit, inter-visit interval, and trial duration (B) are depicted. Blocks 1–10 represent 10 successive block means of four trials each, whereas pre 1, pre 2, and pre 3 represent block means of the two trials of the sessions before isolation, and post 1, post 2, post 3 represent the block means of the two trials of the sessions after isolation.

Figure 3

Effects of overnight social isolation on salivary cortisol. Saliva was collected on the last 3 days of the HB acquisition phase for determining baseline cortisol values. In three successive weeks (once per week), saliva was collected 15 min after the isolation started (at 15:15) and at 9:00 the next morning, for determining the effects of isolation on cortisol levels. For the HB experiment, the averages of the three baseline measurements and of the three measurements during isolations are depicted. For PGT, the averages of three baseline measurements are shown. Effects of social isolation were tested once in the PGT. Note that outliers detected by Grubbs’ test (http://graphpad.com/quickcalcs/grubbs1/) were excluded from statistical analysis.

Figure 4

In the decision-making task, a pig can choose between two sides, an advantageous, and a disadvantageous side (see also the PGT apparatus; Figure 1, right panel). Pigs should learn to respond to the advantageous side because in the long run, this choice yields the largest number of rewards (M&M’s^®).

Figure 5

Mean percentage (±SEM) of advantageous choices per block of 20 trials and during the session of 10 trials pre- and the session of 10 trials post-isolation across all pigs (A), and across pigs qualified as learners, non-learners, preferring the advantageous or disadvantageous side (B). Note that only two pigs preferred the disadvantageous side, from which one switched to the advantageous side after overnight social isolation.

Figure 6

Contingencies in the PGT. Pigs may choose (I) invariably either the left or right goal box (side preference, side bias; as long as the pig persists in selecting one side, it will not learn anything about the contingencies that are in effect on the other side), (II) the side that yields the larger reward (i.e., four M&M’s^®), (III) the side that yields reward with the highest probability in the long run, (IV) the side, which yields the lowest probability of punishment, i.e., non-reward in the long run, or, finally, (V) the side, which renders the largest number of M&M’s^® in the long run. The green and red bars against the gray background on top of the figure show the relative contrast between the advantageous and disadvantageous choices, depending on the contingency according to which the pig chooses. It is obvious that the contrast between the advantageous and disadvantageous choices is lowest with option (V).