Heat stress reduces growth rate of red deer calf: Climate warming implications

Behaviour as mitigation mechanism of heat stress As we commented above changes in behaviour is one of the animal responses to mitigate heat stress. Heat stress can be offset by active using the weather-sheltering environment provided by thermal cover, for example forest cover. Cook et al [52], using an experimental design in which elk were exposed to winter and summer conditions under different forest cover envi- ronments, demonstrated that body condition was not affected by reduced wind speed, elevated nocturnal temperature in winter, and shading from solar radiation in summer, but dense for- est cover in winter had a negative effect on deer condition, presumably because of reduction in solar radiation flux. Migratory movement is other behavioural mechanisms to mitigate heat stress. In northern populations of red deer, summer and especially winter habitats increase their suitability to deer because of reducing snow cover in climate warming scenarios [65]. Similarly, in Mediterranean environments, it should be expected a reduction in summer habi- tat suitability because of heat stress increase, under this scenario, altitudinal migration is a plausible strategy of heat stress mitigation. However, for many populations of Iberian red deer this might be a challenge. A large number of Iberian red deer of the southern Spain popula- tions live in fenced hunting estates [66] that precludes long distance and altitudinal move- ments. Besides, most of these populations are under unsustainable management that relies on

food supplementation during mid and late summer, when plants quality and availability is insufficient to maintain high densities of deer [67, 68]. Over-crowded deer populations aimed to hunting eco-tourism have a negative impact on forest renewal, which reduces natural shade availability in these populations. Though no much canopy area is necessary to provide enough shade for a large number of deer, small canopy area concentrates deer on the limited shade spots, increasing aggregation, which promotes diseases [69], and concentrates the impact of herbivory worsening plant regeneration [70]. Reduction in the density of deer in fenced hunt- ing estates could be a solution to achieve sustainable deer populations under climate warming scenarios, but whether this reduction in deer numbers could maintain these hunting estates economically profitable is arguable [71]. Iberian red deer populations from the north of Spain, which almost entirely dwell on open estates and large public grounds, are likely be in the most favourable conditions to cope with climate warming. These deer populations are harvested at sustainable rates and receive no food supplementation year round. Their densities are fitted to the natural resources available, many of them spread across mountain ranges that enable con- siderable altitudinal seasonal migration, so they can adapt their spatial movement to plant resources availability and heat stress. Behavioural response to changes in the physical environment can be fast and flexible. Our

experimental plots were fitted with water sprinklers during summer, showering under water sprinklers was an unusual behaviour in mothers and offspring, mud bathing was frequently observed in calves but less in mothers. Parker and Robbins [18] described that panting is the primary means of heat dissipation in hot environments in mule deer (Odocoileus hemionus) and elk (Cervus elaphus), however, this was not a behaviour that we observed in our experi- mental animals. The most frequent behaviour, to mitigate heat stress observed in mothers and calves in our experiment during the hottest hours of the day, was to reduce activity by lying down under shade. Studies on skin temperature in Odocoileus hemionus suggest that prefer- ence for shadow beds to avoid direct solar radiation may be instrumental to reduce thermal stress at environmental temperatures that exceeded 25˚C [72]. In desert extreme hot summer conditions, Arabian oryx (Oryx leucoryx) laid down in shade in the morning, shortly before air temperature exceeded its core body temperature (38.5˚C), and remained in shadow beds until evening, when body temperature exceeded air temperature [73]. In the same study it has been suggested that body heat accumulation (temporal higher core body temperature than mean PLOS ONE Heat stress and calf growth in red deer PLOS ONE | https://doi.org/10.1371/journal.pone.0233809 June 1, 2020 19 / 26