Thermoregulation in the wild boar (Sus scrofa)

Abstract

The wild boar (Sus scrofa) originates from warm islands but now inhabits large areas of the world, with Antarctica as the only continent not inhabited by this species. One might be tempted to think that its wide distribution results from increasing environmental temperatures. However, any effect of temperature is only indirect: Abundant availability of critical food resources can fully compensate the negative effects of cold winters on population growth. Here, we asked if temperature as a habitat factor is unimportant compared with other habitat indices, simply because wild boars are excellent thermoregulators. We found that the thermoneutral zone in summer was approximately 6-24 °C. In winter, the thermoneutral zone was lowered to 0-7 °C. The estimated increase in the heart rate and energy expenditure in the cold was less than 30% per 10 °C temperature decline. This relatively small increase of energy expenditure during cold exposure places the wild boar in the realm of arctic animals, such as the polar bear, whereas tropical mammals raise their energy expenditure several fold. The response of wild boars to high T_a was weak across all seasons. In the heat, wild boars avoid close contact to conspecifics and particularly use wallowing in mud or other wet substrates to cool and prevent hyperthermia. Wild boars also rely on daily cycles, especially of rhythms in subcutaneous temperature that enables them to cheaply build large core-shell gradients, which serve to lower heat loss. We argue it is predominantly this ability which allowed wild boars to inhabit most climatically diverse areas in the world.

Keywords: Core–shell; Heart rate; Mass; Subcutaneous temperature; TNZ; Thermoregulation.

Fig. 1

Location of the implantation of loggers on wild boar. Each female investigated had a least two and up to 5 loggers (heart and temperature) implanted

Fig. 2

Heart rate as a function of T_a in wild boar during winter (a December–May) and summer (b June–November). Dots are original heart rates. The thermoneutral zone in summer was approximately 6–24 °C (vertical lines). In winter, the thermoneutral zone was approximately 0–7 °C (vertical solid gray lines). Red regression lines how increases below thermoneutrality in summer, blue lines depict winter. Linear regressions were computed using several minimum heart rates per animal and degree centigrade

Fig. 3

Intraperitoneal temperature as a function of T_a in wild boar. Temperature loggers measured T_b at 12 min intervals and were either sutured to the intraperitoneal wall (la) or free floating (found next to the liver, B). GAM models (red lines) indicated that intraperitoneal temperatures decreased with T_a (floating: edf (wiggliness) = 8.91, residual df = 8.99, F = 867.9, P < 0.0001***). Partial residual plots corrected for the influence of random differences between individuals. For loggers fixed to the wall, model parameters were (edf (wiggliness) = 8.92, residual df = 8.99, F = 1030, P < 0.0001***). All models were also corrected for initial autocorrelation (> 0.9)

Fig. 4

Subcutaneous temperature as a function of T_a in wild boar. Temperature loggers measured T_b at 12 min intervals and were either fixed to the lateral neck (a) next to the sternum (b). GAM models (blue lines) indicated that subcutaneous temperatures decreased with T_a (neck: edf (wiggliness) = 8.85, residual df = 8.99, F = 6281, P < 0.0001***; sternum: edf (wiggliness) = 8.54, residual df = 8.94, F = 1285, P < 0.0001***). Partial residual plots corrected for the effects of random differences between individuals. All models were also corrected for initial autocorrelation (> 0.9)

Fig. 5

Hourly means of heart rate (blue), ear-tag movements (red; scale not shown) as well as subcutaneous T_b (brown) as function of daytime in female wild boar under natural conditions in Austria. Further, ambient (purple) and black-bulb temperatures (black) are shown. The SEM on the mean ear-tag movement and the heat rates indicates the variation among 13 animals, the number of time points of each animal varies. This graph is meant to illustrate the daily rhythm of each variable