A group of Little Brown Bats "cuddling" during hibernation in a cave in Manitoba, photo provided by Dr. Craig Willis
What type of roosts do Little Brown Bats prefer to use when building up their fall fat reserves?
Little Brown Bats (Myotis lucifugus) are one of six bat species native to Manitoba, and one of three species that hibernate here in winter, using caves in limestone karst. The human introduction of a fungal pathogen (Pseudogymnoascus destructans or Pd) from Eurasia brought the disease White Nose Syndrome (WNS) to hibernation habitats of North American bats. It was first discovered in a cave in New York State in 2006, and since then has spread across North America, causing drastic declines in the populations of multiple bat species, including the Little Brown Bat here in Manitoba.
White Nose Syndrome disrupts the bats’ hibernation process, causing them to arouse three times more often during hibernation than healthy bats. Each arousal reduces a bat’s energy reserves so that more frequent arousals lead to the premature depletion of their fat reserves and high rates of death in infected bats.
Little Brown Bats are one of the species most heavily affected by WNS, with declines of up to 90% in populations. Manitoba’s largest known bat hibernaculum, the St. Georges Bat Cave in the Lake St. Georges Cave Ecological Reserve, has seen declines of 75-80% in its pre-WNS population of over 10,000 bats. The species is now listed as Endangered in Manitoba and Canada. Some bats over-wintering in the St. Georges Bat Cave travel hundreds of kilometres to other parts of Manitoba and the Prairies in the spring and summer, making their survival important to bat populations and ecosystem health throughout the province.
Since the Pd invasion, bats in some populations now store more pre-hibernation fat than bats did before WNS was introduced. Energetic models have suggested that larger fall fat reserves can reduce mortality caused by WNS and help bats survive the winter. Yet physiological and behavioural mechanisms used by Little Brown Bats to build fat reserves in the fall are not well-understood.
Research to understand hibernation energetics of Little Brown Bats has noted the role of torpor in conserving energy reserves for overwinter survival. While Little Brown Bats in the Manitoba Interlake have some of the longest, deepest torpor bouts of any hibernator, their use of torpor and links between torpor and habitat selection in the fall before hibernation are completely unknown.
Bats spend the fall swarming, roosting and building fat reserves for hibernation. It is important to understand how bats use their habitat to build the fat reserves they need to survive winter and WNS, in order to identify and conserve critical fall habitat for bats.
To address these issues, 40 adult female Little Brown Bats were captured and radio-tagged with temperature-sensitive radio-transmitters near the St. Georges Bat Cave during the fall fattening period in 2023. The Nature Manitoba Native Habitat Grant was used to purchase some of these transmitters. Bats were monitored to locate fall day-roosts, record roost characteristics (i.e., tree or cave), and record the bats’ skin daytime temperature as well as their skin temperature while roosting. General weather conditions in the study area and cave were also recorded.
The data were used to test two hypotheses:
1) Little Brown Bats use torpor while roosting during pre-hibernation period to minimize energy costs and help build fat reserves; and
2) Fall torpor expression, fat reserves, and commuting costs influence habitat selection prior to hibernation.
Preliminary research findings show some evidence to support the first hypothesis. The researchers are now further analyzing skin temperature data and studying links between torpor expression, weather conditions and preferences for different types of roosts (tree cavity vs cave), as well as quantifying the particular features of tree roosts.
More emphasis is being placed on conservation measures that might help bats build greater fat reserves for hibernation. Understanding the habitats and behaviours that help bats build these reserves is key to being able to create effective conservation measures, such as the types of habitats that should be preserved near hibernacula, or “set-back distances” around hibernacula that should be protected from industry and development, to help support the recovery of bat populations.
(Above summary is based on the Spring 2024 Project Report by Pranav Sadana and Craig K.R. Willis.)