Winter Energetics of Female Indiana Bats Myotis sodalis

Abstract

Understanding physiological limits and environmental optima is critical to developing protection strategies for endangered and threatened species. One theory to explain the decline in endangered Indiana bat Myotis sodalis populations involves increasing cave temperatures in winter hibernacula. Altered cave temperatures can raise metabolism and cause more arousals in torpid bats, both of which use more fat reserves. In addition, fluctuations in cave temperatures may cause additional arousals. Our objectives were to quantify the effect of temperature and fluctuations thereof on torpid metabolism and arousal frequency in this species. Female Indiana bats (n=36) were collected from caves just before hibernation, maintained in an environmental chamber that simulated hibernacula conditions, and had skin temperature recorded every 30 min throughout the winter. One environmental chamber containing bats (n=12) was sequentially set at 8°, 6°, and 4°C over the winter. The second chamber containing bats (n=12) experienced the same mean temperatures, but temperature fluctuated ±2°C on a regular basis. Torpor bouts were longest at 4°C and were not affected by temperature fluctuations. However, the temperature fluctuations appeared to cause longer arousals. Other bats (n=12) were individually placed in metabolic chambers to calculate oxygen consumption during torpor and during arousals. Torpid metabolism was affected by temperature; at 9°C, it was higher than at 7° or 5°C. Metabolism during arousals was not different among temperature treatments, but rates were almost 200 times higher than torpid metabolic rates. We calculated a winter energy budget and, from the energetic perspective, determined an optimum hibernation temperature (3°-6°C) for female Indiana bats. These findings suggest that hibernacula that provide these conditions deserve extra protection, although other factors in addition to energetics may play a role in temperature preferences.

Temperature driven hibernation site use in the Western barbastelle Barbastella barbastellus (Schreber, 1774)

“…For example, for Myotis lucifugus, their model indicates that for a winter length of 193 days, hibernation should take place between 0 and 12 °C, with a minimum energy consumption at 2 °C. Similar conclusions were obtained for Myotis sodalis albeit with a narrower temperature range 39 . This may render hibernating species particularly vulnerable to changing winter climatic conditions as ambient winter temperatures modify the microclimatic conditions within sites, potentially raising hibernacula temperatures above optimal hibernation temperatures and negatively impacting crucial life history phases such as hibernation 40 .…”

Cold truths: how winter drives responses of terrestrial organisms to climate change

“…). For example, hibernating bats ( Myotis sodalis and M. lucifugus ) expend more energy when hibernaculum temperatures are above an optimal low temperature (Humphries et al , ; Day & Tomasi, ), dormant hatchling turtles ( Chrysemis picta ) consume more energetic reserves and emerge in poorer physiological condition after warm winters (Muir et al , ), and energy drain means that goldenrod gall flies ( Eurosta solidaginis ) that overwinter in relatively warm subnivean microclimates have reduced survival and fecundity compared to conspecifics overwintering in exposed, colder sites, because their metabolic rates increase exponentially with temperature (Irwin & Lee, ). All else being equal, this energy drain is likely to be more pronounced in organisms with high baseline metabolic costs, and less pronounced in organisms, such as diapausing butterfly pupae, that substantially suppress their metabolism during winter (Table ; e.g.…”

Optimal hibernation theory

“…Collectively, this suggests the optimal expression of hibernation falls at some intermediate level for any individual not nearing starvation. Even after widespread recognition that hibernation is driven by far more than a need to conserve energy, it is not uncommon for physiologists to refer to conditions that minimise energy expenditure as 'optimal' (Day & Tomasi 2014). Humphries et al's (2003b) seminal work provides a thorough review of the costs and benefits of hibernation built on foundational work that came before and jumpstarted research into understanding the expression of hibernation under natural conditions.…”