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

milk production, suppresses their immune and endocrine system, enhancing disease suscepti- bility [87]. Cooling systems based on providing shades, improving ventilation, water sprinklers and evaporation enhancers have been demonstrated to be effective in reducing breath rates by -16 up to -38 breaths/min in cows [33, 34, 35], but less clear is their effect on hormonal stress indices of chronic stress, such as triiodothyronine and cortisol [33]. A simplistic way of assessing the energy costs associated to losses in calf body mass due to heat stress, is by estimating the corresponding energy value of the milk a calf needs to produce changes in body growth associated to values of heat stress index between 0.1 and 0.9 quantiles through the first 143 days of lactation (Fig 2), which is 324 MJ, with a mean of 1126 kJ�d-1 (cal- culated using allometric equation in Robbins et al [88], Energy kcal�d-1 = 226 Wkg0.837). This is an important amount of energy to justify the application of measures to minimize heat stress in captive deer. Spiers and collaborators [58] found in dairy cattle that increasing air tempera- ture 10˚C above thermoneutral conditions (20˚C) increased rectal temperature by 1.5˚C, respi- ration rate from 60 to 90 breath min-1, and across 4 days milk yield dropped from 35 kg�d-1 to 26 kg�d-1 and dry matter intake from 24 kg�d-1 to 12 kg�d-1. Our study indicates that despite the experimental arena offering cooling opportunities they were not enough to completely offset the effects of heat stress in calf growth. The results justify

applying management measures to minimise heat stress in red deer, such as providing shades, good ventilation, baths and water sprinklers, as it is recommended in cattle dairy farms [59]. This is especially advisable in deer farms located in hot environments and where animals do not have free access to waterbodies for bathing, nor plenty of tree cover or enough ground that offers deer a variety of physical environments across which thermal stress could be minimised. Actual climatic data and projections of mean maximum temperature into 2100 in Spain (www.aemet.es) suggest that, north-west areas are those that rarely reach heat stress conditions at present and also less chance of reaching those in the future. Under this scenario it seems advisable to locate deer farms in those areas, which has the additional benefit of having some of the best grazing conditions in Spain [89], and so having to rely less on feed supplementation making deer production more sustainable. It is advisable to deer managers to consider the provision of thermal cover to reduce ther- mal stress in deer, which also depends on habitat quality and food availability [14]. Moen [90] pointed out that in fawns and does of white-tailed deer (Odocoileus virginianus) under the thermal stress conditions of his experiment, the energy balance predicted should not cause a negative balance as long as food supply was sufficient and the deer could seek shelter. Similarly, it has been highlighted in wild red deer the importance of appropriate food supply and the use

of behaviour as means to compensate for potential heat stress [91]. This highlights the impor- tance that food energy supply has when assessing the impact of heat stress on the animal’s energy budget and consequently in the animal performance. Supporting information S1 Fig. Frequency distribution of monitoring events across 143 days of lactation in the study period (2000–2018); mean: Continuous vertical line; median = dashed vertical line. (JPEG) S2 Fig. Frequency distribution of days between monitoring events across the study (2000– 2018); mean: Vertical line. (JPEG) S3 Fig. Supplementary material. Actual calf weight (kg) versus calf age (d) across the first 143 days of life in males and females of Iberian red deer. Grey triangle: male calf; magenta circle: PLOS ONE Heat stress and calf growth in red deer PLOS ONE | https://doi.org/10.1371/journal.pone.0233809 June 1, 2020 21 / 26