Thermal climate‐linked variation in annual survival rate of hibernating rodents: shorter winter dormancy and lower survival in warmer climates

Abstract

1. In seasonal climates, many animals, including a wide range of small mammals, are physiologically capable of prolonged dormancy (~hibernation), when foraging-related activity ceases entirely for part the year. Low metabolic rates while dormant minimise energy and water requirements, but the behavioural state of inactivity also reduces exposure to mortality risks, especially predation. 2. Thermal effects on activity underlie spatial patterns in annual survival rate of ectothermic animals. We hypothesised that, because local thermal conditions affect the duration of mammalian hibernation, positive effects of dormancy on survival could also underlie a negative relationship between spatial variation in thermal climate and annual survival rate among populations of hibernating rodents. 3. We applied mixed models to test for effects of local thermal climate on collated data of hibernation duration and annual survival rate among populations of hibernating and a representative sample of non-hibernating rodent species. 4. Our analyses revealed strong negative effects of mean annual temperature on hibernation duration and adult annual survival rate within hibernating species (b: À8Á6 day and À5Á1% per 1°C) but no effect in non-hibernating rodent species. This thermal climate-linked pattern in annual survival rate seems to be unique among mammals. A parsimonious explanation lies in the large increase in monthly survival rates during hibernation compared to activity. 5. The current spatial pattern suggests ongoing climate warming might reduce annual survival rates of hibernating rodents by shortening their hibernation season. Decreased annual survival because of increased activity in warmer climates with shorter winter seasons is an unappreciated mechanism leading to impacts of global warming on animal populations in temperate climates.

Modelling mammalian energetics: the heterothermy problem

“…If an animal increases its T b, rather than shouldering the costs of increased T a through energetically costly means of defending a set T b, some of the costs of rising T a may be overestimated. Conversely, in an animal attempting to reduce energy and water usage through torpor, higher T a can reduce potential savings [64,155]. In contrast to species that use torpor opportunistically, strict hibernators that are less flexible in their physiological response are likely to face negative consequences when surface temperatures rise.…”

“…In contrast to species that use torpor opportunistically, strict hibernators that are less flexible in their physiological response are likely to face negative consequences when surface temperatures rise. Recent studies have indicated that warmer winter temperatures lead to more frequent arousals during hibernation periods [155][156][157], imposing the risk for small seasonal hibernators to deplete fat reserves before the end of hibernation [158]. Therefore, to be able to accurately predict mammalian responses to climate change, we need to incorporate a level of predictability in heterothermic responses.…”

Nocturnal torpor by superb fairy-wrens: a key mechanism for reducing winter daily energy expenditure

“…The most effective physiological energy-saving mechanism available to small endotherms is the use of temporary, controlled reductions in body temperature (T b ) and resting metabolism [9,10]-a state known as torpor [9]. Torpor is employed by a wide-range of small mammals and birds [11] and can provide large energy-savings that have wide-ranging ecological consequences, including compensating energetically for reductions in foraging intake because of predation risk [12] and interspecific competition [13], or energy costs of migration [14], and increasing population survival rates [15,16] and reducing the risk of extinction [17,18].…”

Denning in brown bears

“…An increase in winter temperatures would have a negative effect on reproductive success and cub survival after den exit in brown bear populations: energy demands of hibernating mammals would increase with higher winter temperature, due to the increase of energetic costs of torpor (Albrecht et al, 2017;Humphries, Thomas, & Speakman, 2002;Post & Forchhammer, 2008;Turbill & Prior, 2016).…”