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

body temperature) could be a mechanism of water economy in desert conditions, as body cooling down by evaporation through the skin and respiratory tract requires a considerable amount of water that might compromise survival [73]. This reveals how behavioural response to heat stress can have complex physiological implications. Climate warming implications on deer body size Climatic models suggest with medium confidence that in the Northern Hemisphere the period 1983–2012 was the warmest of the last 1400 years. It is believe that this was the product of an uptake of energy by the climate system due to a positive total radiative forcing, being its largest driver an increase in atmospheric CO2 since 1750 [1]. Under different scenarios climate mod- els predict an increase in land surface temperature of 1–2˚C for the period 2046–2065, and between 1 and 3.7˚C for the period 2081–2100 [1]. This climatic pattern is projected in the form of warmer and/or fewer cold days and nights, and warmer and/or more frequent hot days and nights. A similar warming pattern has been described for the Iberian peninsula, annual mean temperature has increased in 0.10˚C/decade for the period 1850–2005, annual warming was more affected by the increase in autumn and winter temperatures than those in spring and summer, and there were higher rates of change in maximum than in minimum temperatures [74]. Implications of Iberian peninsula warming during this period and that pre- dicted on models at global scale [1], together with results of our study, suggest that body size in

red deer might have been decreasing and will continue to do so. This is, however, a very sim- plistic projection, as climate warming might act on deer body growth in different directions. For example, it has been claimed that increasing atmospheric CO2, because of human GHG emissions, promotes photosynthesis and so plant growth, which in turn increases plant bio- mass for the use of large herbivores. On the other hand, it is argued that plant growth is not constrained by atmospheric CO2 availability but nitrogen input from soil [75], which highlight the complexity of assessing how global warming might affect body size in deer. Our findings have interesting ecological implications on body size geographical clines. Bergmann´s rule establishes that in cold climates large animals benefit by having a lower sur- face area to volume ratio than smaller animals, because they radiate less heat per unit of mass and so spend less energy to maintain homeothermy, the opposite being true in warmer envi- ronments [76]. There is evidence to support this rule in endotherms [77, 78] although there are alternative explanations to this phenomenon [79, 80]. Our results suggest that heat stress in warm environments penalises body growth during lactation. If body growth is not compen- sated for later in life, this could contribute to explain an intra-specific cline in body mass increase from warm to cold environments. Our results indicate that under heat stress conditions, red deer male size is more negatively

affected than female size, whether this would have long-term adaptive consequences in body- size remains unknown, as we commented above, food and other compensatory factors might affect body size [81]. However, we could hypothesise that if warming conditions hamper male´s growth more than female´s growth, this could have implications in polygynous mating systems, which are mainly based on sexual differences in body size [82]. Among the implica- tions are (i) changes in parental investment between sexes [83], (ii) reduction of sexual differ- ences in mortality age-dependent [66], (iii) reduction in spatial sexual segregation [84], and (iv) increase in trophic niche sexual overlap [85, 86]. Implications of heat stress on deer farming Special effort in developing mechanisms to mitigate heat stress has been undertaken in dairy cattle, breeds with high metabolic rate, because heat stress has a negative effect on intake and PLOS ONE Heat stress and calf growth in red deer PLOS ONE | https://doi.org/10.1371/journal.pone.0233809 June 1, 2020 20 / 26