Understanding WNS resistance mechanisms in big brown bats

Every winter, many of Canada’s insect-eating bats perform a nearly unbelievable feat of endurance – survival without eating for up to 8 months. They do this by accumulating several grams of fat in fall, then use long bouts of torpor (periods of reduced body temperature) to save energy during winter. Torpor is ideal for saving energy, but in the past decade has become a liability for bat species affected by an invasive fungal disease – white-nose syndrome (WNS). The fungus invades the skin of bats’ wings causing them to arouse from torpor too frequently, use their fat stores too quickly and die from starvation before spring. My research addresses mechanisms underlying expression of torpor and hibernation in bats and aims to understand why some species are less susceptible to WNS than others.

As an undergraduate honours student, I worked with Dr. Craig Willis at the University of Winnipeg to understand pre-hibernation feeding behaviour, during the crucial fall fattening period, of little brown bats (Myotis lucifugus) – a species endangered as a result of WNS. For my M.Sc., I have continued with Dr. Willis studying hibernation, however, this time in big brown bats (Eptesicus fuscus), a species much less susceptible to WNS. Big brown bats can tolerate much drier hibernation conditions than little brown bats, which could help explain their WNS resistance. My overall aim is to understand how this species maintain water balance across such a broad range of humidity. For my thesis, I used a captive colony to test the hypothesis that hibernating big brown bats rely on behavioural flexibility to maintain water balance in dry conditions. I housed bats in two temperature- and humidity-controlled incubators set at either 50% or 98% relative humidity and allowed them to hibernate for 110 days. During hibernation I quantified each bats frequency of arousals from torpor (which for other species increase in dry conditions) and drinking behaviour. As expected, bats in the dry treatment drank more often during arousals but, surprisingly, did not arouse from torpor more often than bats in humid conditions.

Bats are among the most gregarious of mammals and often huddle in tightly packed groups during hibernation. Therefore, I also tested a second hypothesis that huddling mitigates water loss in dry conditions. I used open-flow respirometry to measure metabolic rate and evaporative water loss of solitary bats or groups of 5 individuals huddling but found no effect of huddling on either metabolic rate or evaporative water loss. Overall, my results suggest that behavioural and physiological flexibility play a role in allowing this species to survive hibernation in a range of environments which could be important for understanding resistance mechanisms of big brown bats to WNS.

Submitted by Kristina Muise, MSc candidate at the University of Winnipeg

This featured story was included in our Bat Monthly Chauves-souris Mensuelles newsletter of April 2020. You can find the full newsletter here.