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

those mothers bearing a female. Mothers weight increased with mother age (p = 0.023) at a lower rate when the mother was bearing a male than a female, with this only corroborated by AIC model selection basis (Fig 5). The model indicated that mother weight responded to phys- ical environment variables in a more complex fashion than the response of calf growth to the same set of physical variables. Mother weight responded in a quadratic fashion to air tempera- ture (p = 0.002), while the response to humidity and wind speed was positive and almost linear (p < 0.001, Figs 6 and 7). Mother weight response to solar radiation followed a similar pattern to that described for THIWS model above, that is, mother weight increased within solar radia- tion values between 5 and 24 MJ�m-2 and decreased for higher values of solar radiation (p < 0.001, Fig 7). In general, the physical environment variables had a more detrimental effect on mothers that bore a male than on mothers bearing a female (Table 6, Figs 6 and 7). Discussion The results of our study clearly indicate that heat stress reduces the rate of growth of red deer calf, and this seems mainly due to the direct effect that physical environment variables have on offspring energy budget across lactation. It is unlikely attributable to the effect that heat stress may have on mother condition. This conclusion is based on the fact that the two heat stress indices (THI, THIWS), mean air temperature and wind speed had all negative effects on calf

growth rate, while heat stress indices, air temperature, wind speed and solar radiation had less clear effects on mother weight during lactation as the response was not linear in some variables. The results suggest that adult hinds had energy and/or behavioural resources to offset the heat stress at which they were exposed during lactation, as compared with calves that were not capable of completely offsetting the energy expense required to maintain homeothermy with- out diverting energy resources from growth. Despite this, our models indicate that mother weight suffered at heat stress THIWS >107, mean air temperature >16˚C and relative humid- ity <60%, and increased with wind speed. Physical environment affected calves´growth depending of maternal traits. The growth of offspring males was more negatively affected by heat stress indices (THIWS) and physical environment variables than the growth of offspring females. Older females produced offspring with faster rates of growth, and after the effect of mother age was controlled for, mother weight had a negative effect on the growth rate of calf. Interestingly, THIWS indicated that males off- spring were more sensitive to heat stress than females, supporting the prediction that the sex with the highest energy requirements is the one most affected by a demanding physical envi- ronment, supporting Hypothesis 1. Table 6. (Continued) model 0 model 1 model 2 model 3 model 4 model 5 model 6 model 7 model 8 model 9 model 10 final model k 3 3 3 3 3 3 3 3 3 3 3 3

ΔAIC 157.81 161.91 133.28 130.11 125.50 103.21 48.87 8.19 0.86 0.89 0.60 0 weights AIC 1.77E- 35 2.27E- 36 3.74E- 30 1.83E- 29 1.83E- 28 1.27E- 23 8.01E- 12 0.01 0.21 0.21 0.24 0.33 Kullback– Leibler 0.11 0.99 0.83 0.90 0.99 1 1 0.97 0.49 0.53 0.57 — T: daily mean temperature (˚C); Hr: relative air humidity (%); SR: solar radiation (MJ�m-2); W: wind speed (m�s-1). Female is the sex level of reference. Other acronyms as in Tables 2 and 5. https://doi.org/10.1371/journal.pone.0233809.t006 PLOS ONE Heat stress and calf growth in red deer PLOS ONE | https://doi.org/10.1371/journal.pone.0233809 June 1, 2020 15 / 26