Hot-headed peckers: thermographic changes during aggression among juvenile pheasants (Phasianus colchicus)

Abstract

In group-living vertebrates, dominance status often covaries with physiological measurements (e.g. glucocorticoid levels), but it is unclear how dominance is linked to dynamic changes in physiological state over a shorter, behavioural timescale. In this observational study, we recorded spontaneous aggression among captive juvenile pheasants (Phasianus colchicus) alongside infrared thermographic measurements of their external temperature, a non-invasive technique previously used to examine stress responses in non-social contexts, where peripheral blood is redirected towards the body core. We found low but highly significant repeatability in maximum head temperature, suggesting individually consistent thermal profiles, and some indication of lower head temperatures in more active behavioural states (e.g. walking compared to resting). These individual differences were partly associated with sex, females being cooler on average than males, but unrelated to body size. During pairwise aggressive encounters, we observed a non-monotonic temperature change, with head temperature dropping rapidly immediately prior to an attack and increasing rapidly afterwards, before returning to baseline levels. This nonlinear pattern was similar for birds in aggressor and recipient roles, but aggressors were slightly hotter on average. Our findings show that aggressive interactions induce rapid temperature changes in dominants and subordinates alike, and highlight infrared thermography as a promising tool for investigating the physiological basis of pecking orders in galliforms. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Keywords: aggression; dominance hierarchy; infrared thermography; pecking order; social defeat; stress-induced hyperthermia.

Figure 1.

Infrared thermographic image taken from FLIR Tools, showing an aggressive encounter between two juvenile pheasants (middle right of the image). The scale at the top indicates the colour coding of temperatures, while the red, upwards-pointing triangles automatically pinpoint the maximum temperature within a selection box drawn manually around each pheasant's head. Here, the aggressor on the left (Bx1), with a maximum head temperature of 37.5°C, has just delivered an aggressive peck to the recipient on the right (Bx2), who has a maximum head temperature of 35.7°C. The background reference temperature is 26.5°C, taken from a white plastic drinker filled with water (Sp1), suspended above the pen floor (top left of image). (Online version in colour.)

Figure 2.

Boxplots showing the distribution (median, interquartile range and outliers) of maximum head temperatures (averaged within individuals) of male and female captive juvenile pheasants engaged in different behavioural activities. (Online version in colour.)

Figure 3.

Change in head temperatures during an aggressive encounter between captive juvenile pheasants in aggressor and recipient roles. Lines are conditional smooths with 95% uncertainty intervals, generated from a GAMM using the package brms [79]. Boxplots show the distribution (median, interquartile range and outliers) of the times (averaged within individuals within encounters) at which the minimum (bottom) and maximum (top) temperatures occurred and their observed values (right). (Online version in colour.)