Behavioural responses of a large, heat‐sensitive mammal to climatic variation at multiple spatial scales

Abstract

Climate warming creates energetic challenges for endothermic species by increasing metabolic and hydric costs of thermoregulation. Although endotherms can invoke an array of behavioural and physiological strategies for maintaining homeostasis, the relative effectiveness of those strategies in a climate that is becoming both warmer and drier is not well understood. In accordance with the heat dissipation limit theory which suggests that allocation of energy to growth and reproduction by endotherms is constrained by the ability to dissipate heat, we expected that patterns of habitat use by large, heat‐sensitive mammals across multiple scales are critical for behavioural thermoregulation during periods of potential heat stress and that they must invest a large portion of time to maintain heat balance. To test our predictions, we evaluated mechanisms underpinning the effectiveness of bed sites for ameliorating daytime heat loads and potential heat stress across the landscape while accounting for other factors known to affect behaviour. We integrated detailed data on microclimate and animal attributes of moose Alces alces, into a biophysical model to quantify costs of thermoregulation at fine and coarse spatial scales. During summer, moose spent an average of 67.8% of daylight hours bedded, and selected bed sites and home ranges that reduced risk of experiencing heat stress. For most of the day, shade could effectively mitigate the risk of experiencing heat stress up to 10°C, but at warmer temperatures (up to 20°C) wet soil was necessary to maintain homeostasis via conductive heat loss. Consistent selection across spatial scales for locations that reduced heat load underscores the importance of the thermal environment as a driver of behaviour in this heat‐sensitive mammal. Moose in North America have long been characterized as riparian‐obligate species because of their dependence on woody plant species for food. Nevertheless, the importance of dissipating endogenous heat loads conductively through wet soil suggests riparian habitats also are critical thermal refuges for moose. Such refuges may be especially important in the face of a warming climate in which both high environmental temperatures and drier conditions will likely exacerbate limits to heat dissipation, especially for large, heat‐sensitive animals.

How can physiology best contribute to wildlife conservation in a warming world?

“…‘Niche Mapper’ is a tool developed for this purpose ( Kearney and Porter, 2016 ) and is freely available ( http://niche-mapper.com/ ). This biophysical niche modelling approach has been used very successfully to predict the efficacy of thermoregulation to buffer ectotherms from climate warming ( Kearney et al, 2009 ; Sunday et al, 2014 ), model behavioural responses of a large mammal (moose, Alces alces shirasi ) to climate variation ( Verzuh et al, 2023 ), assess heat stress in a vervet monkey ( Chlorocebus pygerythrus ) ( Mathewson et al, 2020 ) and assess the overwintering energetics of wood frogs ( Lithobates sylvaticus ) under climate warming ( Fitzpatrick et al, 2020 ), among many other applications. The strength of this biophysical niche modelling lies in the incorporation of specific physiological data, thereby linking environmental conditions explicitly to physiological responses ( Briscoe et al, 2023 ).…”

Increased summer temperature is associated with reduced calf mass of a circumpolar large mammal through direct thermoregulatory and indirect, food quality, pathways

“…Reduced food intake decreases thermogenesis due to digestion (Youngentob et al 2021 ). Moose have also been known to reduce travel (Thompson et al 2021 ) and increase resting at high ambient temperatures (Ditmer et al 2018 ) and to increase use of thermal shelters (e.g., mature coniferous forest, wet areas; Verzuh et al 2021 ; Verzuh et al 2022 ). The selection of some of these thermal shelters, mature coniferous forest particularly, may reduce access to forage, particularly at temperatures over 20 °C (Van Beest et al 2012 ; Verzuh et al 2021 , 2022 ).…”

“…Moose have also been known to reduce travel (Thompson et al 2021 ) and increase resting at high ambient temperatures (Ditmer et al 2018 ) and to increase use of thermal shelters (e.g., mature coniferous forest, wet areas; Verzuh et al 2021 ; Verzuh et al 2022 ). The selection of some of these thermal shelters, mature coniferous forest particularly, may reduce access to forage, particularly at temperatures over 20 °C (Van Beest et al 2012 ; Verzuh et al 2021 , 2022 ). Despite the potential foraging costs of behavioral thermoregulation, adult females that optimize use of foraging habitats and thermal shelters, based on ambient temperature, tend to gain more summer mass (Van Beest and Milner 2013 ).…”

Density‐dependent changes in elk resource selection over successional time scales following forest disturbance

“…Studies should further assess the response of large herbivores to forest treatments in contrast with areas left untreated, and examine the potential avoidance of treated areas during nonfeeding hours, given the need for structurally complex habitats for other requirements including safety cover and thermoregulation (Churchill, 1982; Hebblewhite et al, 2009; Spitz et al, 2018). In particular, in heat‐sensitive species the use of disturbed areas may come at a greater cost of thermoregulation, because reduced overhead canopy cover increases solar radiation and prevents heat dissipation (Verzuh et al, 2023). This may cause ungulates to forage in open‐canopy disturbances only during the nighttime hours during the summer months to avoid heat stress (Lamont et al, 2019).…”

“…Our objectives were to (1) use selection ratios (i.e., metrics describing whether a discrete resource is used more or less than its availability) and resource selection functions (RSFs) to clarify diel and (2) monthly patterns of selection for or against treated areas, (3) determine the duration of these effects, (4) assess whether the selection of treated areas was mediated by elk density, and (5) at the level of the harvest unit, determine whether prescribed burning or (6) the size of the treatment influenced the amount of use they received by elk. Because reductions in canopy cover associated with logging increase the quantity and quality of forage for elk (Cook et al, 2016), we hypothesized that elk would exhibit increased selection of areas that had undergone logging treatments and this would be (H1) stronger during the night than the day in summer months because of the need to avoid direct solar radiation that prevents heat dissipation (Ager et al, 2003; Lamont et al, 2019; Roberts et al, 2017; Verzuh et al, 2023); (H2) that the selection of logged areas would be strongest during midsummer when grasslands (the other available open habitat type in our study area) were senesced relative to early spring and late fall green‐up periods (Skovlin, 1967); and (H3) that the selection of logging treatments would persist for at least 10–20 years, because forage biomass and digestible energy typically remain high for this duration or sometimes longer following logging (Cook et al, 2016). Further, consistent with the ideal free distribution, we hypothesized (H4) that the areas of treated forest would not be enough to sustain all individuals in the high‐density elk population and that some individuals would, instead, be forced to utilize other vegetation types.…”