On the interplay between hypothermia and reproduction in a high arctic ungulate

Abstract

For free-ranging animals living in seasonal environments, hypometabolism (lowered metabolic rate) and hypothermia (lowered body temperature) can be effective physiological strategies to conserve energy when forage resources are low. to what extent such strategies are adopted by large mammals living under extreme conditions, as those encountered in the high Arctic, is largely unknown, especially for species where the gestation period overlaps with the period of lowest resource availability (i.e. winter). Here we investigated for the first time the level to which high arctic muskoxen (Ovibos moschatus) adopt hypothermia and tested the hypothesis that individual plasticity in the use of hypothermia depends on reproductive status. We measured core body temperature over most of the gestation period in both free-ranging muskox females in Greenland and captive female muskoxen in Alaska. We found divergent overwintering strategies according to reproductive status, where pregnant females maintained stable body temperatures during winter, while non-pregnant females exhibited a temporary decrease in their winter body temperature. these results show that muskox females use hypothermia during periods of resource scarcity, but also that the use of this strategy may be limited to non-reproducing females. Our findings suggest a trade-off between metabolically-driven energy conservation during winter and sustaining foetal growth, which may also apply to other large herbivores living in highly seasonal environments elsewhere.

Quantifying energetic and fitness consequences of seasonal heterothermy in an Arctic ungulate

“…A recent study on muskoxen in northeast Greenland found that nonpregnant adult females exhibited overwinter heterothermy, measured as declining throughout winter, whereas pregnant females maintained relatively stable over the same time period and region (Schmidt et al., 2020). Together with previous reports of lower organ weights, energy expenditure, and reduced maintenance needs during winter (Adamczewski et al., 1997; Lawler & White, 1997), it is evident that muskoxen adopt physiological strategies to conserve energy during harsh winter months.…”

“…We used the data on from Schmidt et al. (2020) to model the potential energetics and fitness consequences of seasonal heterothermy in muskoxen. For simplicity, we grouped animals into two categories that represent diverging seasonal patterns of , namely normotherms (relatively stable ; winter mean 38.2°C) and heterotherms (reduced overwinter ; overwinter mean 37.4°C) (Figure 1a).…”

“…These correspond to ‘survived, birth (wild)’ (normotherms, n = 3) and ‘survived, no birth (wild)’ (heterotherms, n = 2) animals in Schmidt et al. (2020). We acknowledge that both groups of animals are characterized by temporal variability in and thus could be considered along a heterothermy continuum (Boyles et al., 2013), but we use these simplified groupings herein to represent broad seasonal thermoregulatory patterns.…”

“…Our experimental design was focused on the systematic paired comparison of normothermic and heterothermic overwinter profiles as they relate to individual fitness. The model targeted the period of the year most influential to muskox population dynamics, namely the end of summer to early spring (i.e., overwinter period) when has been observed to fluctuate most between normothermic and heterothermic animals (Schmidt et al., 2020). The model used a daily time step over the simulation period of October to May, which covered the period of embryonic arrest, active gestation, and birth as well as the period with highest snow accumulations and least amount of forage (Figure 1a).…”

“…Because of opposing energetic demands, heterothermy and reproduction in some mammals may be mutually exclusive processes, leading to reproductive periods in spring and summer followed by periods of heterothermy in autumn and winter, with no temporal overlap in the two states (McAllan & Geiser, 2014). For many northern species, for example, mountain goats, arctic voles and lemmings, wolverines, reindeer, and muskox (Arnold et al., 2018; Bronson, 2009; Schmidt et al., 2020; Thiel et al., 2019), costly reproductive processes of gestation and lactation overlap with the food‐restricted winter season, when energetic stress is already at its greatest due to high thermoregulatory costs and low food availability. These winter conditions have been shown to be a major determinant of body condition, reproduction, and population dynamics in mammals, such as northern ungulates, living in harsh environments (Gaillard et al., 2000; Helle & Kojola, 2008; Schmidt et al., 2015), highlighting the potential impact of seasonal environments on animal fitness.…”