Yawn contagion in domestic pigs (Sus scrofa)

Abstract

Contrary to spontaneous yawning-an ancient phenomenon common to vertebrates-contagious yawning (elicited by others' yawns) has been found only in highly social species and may reflect an emotional inter-individual connection. We investigated yawn contagion in the domestic pig, Sus scrofa. Owing to the complex socio-emotional and cognitive abilities of Sus scrofa, we posited that yawn contagion could be present in this species (Prediction 1) and influenced by individual/social factors (Prediction 2). In June-November 2018, on 104 semi-free ranging adolescent/adult pigs, 224 videos were recorded for video analysis on yawning. Kinship information was refined via genetic analyses. Statistical elaboration was conducted via GLMMs and non-parametric/randomization/cross-tabulation tests. We found yawn contagion in Sus scrofa, as it was more likely that pigs yawned when perceiving rather than not perceiving (yawning/control condition) others' yawns (response peak in the first out of three minutes). Yawn contagion was more likely: (1) in response to males' yawns; (2) as the age increased; (3) within short distance (1 m); (4) between full siblings, with no significant association between kinship and distance. The influence of kinship suggests that-as also hypothesized for Homo sapiens-yawn contagion might be linked with emotional communication and possibly contagion.

Figure 1

Part of a yawning sequence in Sus scrofa (Large White breed). Photo: Ivan Norscia (Camera: Panasonic Lumix DMC-FZ60).

Figure 2

Error bars (mean, 95% confidence interval) showing the occurrence of yawning response (Y axis) as a function of the sight condition (X axis): "YES yawning perception"—the subject yawns after that another subject has yawned within its visual range (yawn condition); "NO yawning perception"—the subject yawns without perceiving a previous fellow's yawn (control condition). The variable has a significant main effect on the yawning response (results of Model₁ included in Table 1). CI confidence interval.

Figure 3

Error bars (mean, 95% confidence interval) showing the occurrence of the yawning response (Y axis) as a function of the number of additional yawns emitted by subjects other than the yawn emitted by the last trigger (X axis). The variable has a significant main effect on the yawning response (results of Model₁ included in Table 1), with the response being highest when one or more additional yawn had been emitted compared to no additional yawn (Tukey post-hoc test with Bonferroni correction; no additional yawn versus other conditions: 0.001 < p ≤ 0.045. CI confidence interval.

Figure 4

Box-plot showing the frequency of contagious yawning (Y axis) as a function of the minutes elapsing after the triggering stimulus (X axis). The difference across minutes is significant (Friedman’s test; N = 19, χ² = 19.279, df = 2, p < 0.001), with yawn contagion being significantly higher in the first minute than in other minutes (pairwise comparisons via Bonferroni–Dunn post-hoc test; first versus other minutes, 0.001 ≤ p ≤ 0.045). Solid horizontal lines indicate medians, length of the boxes corresponds to inter-quartile range and thin horizontal lines indicate minimum and maximum of the observed values.

Figure 5

Error bars (mean, 95% confidence interval) showing the occurrence of yawn contagion (Y axis) as a function of social factors (X axis): kinship (a) and inter-individual distance (b). Both variables have a significant main effect on yawn contagion (results of Model₂ included in Table 2), which was highest (a) between full siblings (Tukey post-hoc test with Bonferroni correction, full siblings versus other kinship: 0.031 ≤ p ≤ 0.033; Table 2) and (b) when the distance was within 1 m (Tukey post-hoc test with Bonferroni correction: within 1 m versus other distances: p < 0.05; Table 2). No significant association between the two nominal fixed factors kinship and distance was detected (Goodman and Kruskal’s lambda test: λ = 0.010, T = 0.200, p = 0.841). CI confidence interval.

Figure 6

Error bars (mean, 95% confidence interval) showing the occurrence of yawn contagion (Y axis) as a function of individual factors (X axis): sex of the trigger (a) and age of the responder (b). Both variables have a significant main effect on yawn contagion (results of Model₂ included in Table 2) with (a) yawn contagion being highest when the trigger was a male and (b) linearly increasing with age (bivariate correlation via randomization: n = 13, r = 0.600, p = 0.031). CI confidence interval.