What is winter? Modelling spatial variation in bat host traits and hibernation and their implications for overwintering energetics

Abstract

White-nose syndrome (WNS) has decimated hibernating bat populations across eastern and central North America for over a decade. Disease severity is driven by the interaction between bat characteristics, the cold-loving fungal agent, and the hibernation environment. While we further improve hibernation energetics models, we have yet to examine how spatial heterogeneity in host traits is linked to survival in this disease system. Here we develop predictive spatial models of body mass for the little brown myotis (Myotis lucifugus) and reassess previous definitions of the duration of hibernation of this species. Using data from published literature, public databases, local experts, and our own fieldwork, we fit a series of generalized linear models with hypothesized abiotic drivers to create distribution-wide predictions of pre-hibernation body fat and hibernation duration. Our results provide improved estimations of hibernation duration and identify a scaling relationship between body mass and body fat; this relationship allows for the first continuous estimates of pre-hibernation body mass and fat across the species’ distribution. We used these results to inform a hibernation energetic model to create spatially-varying fat use estimates for M. lucifugus. These results predict that WNS mortality of newly and soon-to-be infected M. lucifugus populations in western North America may be comparable to the substantial die-off observed in eastern and central populations.

Temperature alone is insufficient to understand hibernation energetics

“…This may be a difference between species (the previous studies on the topic were on Myotis lucifugus), but we suspect our observation relates to differing reproductive constraints on males and females in different areas. The thrifty female studies (Czenze et al, 2017;Jonasson and Willis, 2011) were performed in Manitoba, Canada, where hibernation duration is estimated to be 199 days (Hranac et al, 2021). At high latitudes, environmental demands may approach the limits of physiological capacity, forcing females to be energetically conservative if they are to reproduce successfully.…”

A hybrid correlative‐mechanistic approach for modeling winter distributions of North American bat species

“…These estimates were based on an existing bioenergetic model of bat winter survivorship, recently updated and parameterised for western bat species. Full details are elsewhere (Haase et al, 2019; Hranac et al, 2021), but briefly, the model uses the hypothesised energetic requirements of bats in torpor to dynamically model torpor bouts for the duration of a predicted winter under specified hibernaculum conditions. For M .…”

“…Here, we integrated a bioenergetic model of bat hibernation (Haase et al, 2019; Hayman et al, 2016; Hranac et al, 2021) into a correlative species distribution modeling approach to predict winter distributions of bat species whose ranges extend into the West. The bioenergetic model makes species‐specific predictions of remaining fat stores and thus the likelihood of survival at the end of winter in a given location.…”

Projecting the compound effects of climate change and white-nose syndrome on North American bat species

“…These species, including Corynorhinus townsendii, Myotis californicus, M. lucifugus, M. velifer, and Perimyotis subflavus, were selected based on data availability and representation of diverse distributions and habitat requirements among hibernating bats. To estimate bats' probability of occurrence given exposure to P. destructans , we ran the spatial bioenergetic model described in Hranac et al (accepted;also see Haase et al 2019) to project winter survivorship from parameters capturing the influence of the hibernaculum environment (temperature and humidity) on fungal growth and the resulting impact of the fungus on bat hibernation physiology. To estimate bats' probability of occurrence given the additional impacts of climate change, we ran the bioenergetic model with the P. destructans growth parameters above as well as projected future climate parameters (winter duration and 'best available' temperatures, identified as the subterranean temperature closest to the species' preferred temperature as identified from published literature that was projected to be available in a given location; Fig.…”

“…We thank Nathan Justice, Eric Stofferahn, and Tony Chang for valuable technical support. Finally, we are indebted to the many individuals and organizations who generously provided raw data that made possible the studies supporting this paper, including subterranean microclimate data, bat species occurrence locations, hibernaculum immergence and emergence observations, and bat physiology records (see Haase et al 2019, McClure et al 2020, Hranac et al accepted, andMcClure et al accepted for further details).…”