Looking through the most recent literature on white-nose syndrome isn’t exactly reassuring, although it does suggest that progress is being made in understanding how the fungus affects bats and what conditions may help bats survive infection. As of last year, at least 19 US states and 4 Canadian provinces contained infected bats (Blehert 2012) and three additional species had been infected: gray bat, southeastern bat, and cave bat (Foley et al. 2011).
“Bat survey in Ohio”- because these little brown bats roost in such large numbers, the chance of passing along infection is high.
Little brown bats continue to be particularly hard hit, and we are developing a better picture of how and why the fungus is so devastating for that species. It has been suggested, based on studies of both bats and ground squirrels, that dehydration is the trigger for awakening during hibernation (Willis et al. 2011), and wings are critical moderators of water loss in bats. Wings damaged by the fungus may lead to greater water loss and therefore more time spent out of torpor during the winter. Little brown bats do exhibit more evaporative water loss than other species, which could be why they are so susceptible to the fungus. And some behaviors of the bats themselves may lead to infection and mortality. Prior to hibernation, little brown bats swarm and breed, so individuals carrying the fungal spores have the opportunity to infect others (Hallam & Federico 2012). This could be a particular issue for males who spend the summer using colder roosts and may therefore harbor the fungus between winters (see below for more on the temperature problem). It is also possible that males are more likely to succumb to white-nose syndrome: the torpor optimization hypothesis states that individuals with greater energy reserves should reduce their time in torpor (so that they are less vulnerable to predation and other threats) unless those energy reserves are vital to spring survival or reproduction (Jonasson & Willis 2011)- since female little brown bats need extra energy for springtime pregnancy, they spend more of the winter in torpor and should therefore have more reserves in late winter when white-nose syndrome is most prevalent. Females should have a better chance of surviving infection. Surviving the winter may not be enough, though. Infected females with the energy to survive to spring may not have enough energy to reproduce, so individuals that did not survive can’t be replaced by a new generation. And since males may be particularly susceptible, this could lead to massive imbalances in the sex ratio of colonies, with a variety of consequences including decreased genetic diversity and lower reproductive success.
Is there any way to counteract the fungus? Possibly, although there are still quite a few unanswered questions about this. Laboratory investigation of G. destructans suggests that optimal growth occurs between 12.5 and 15.8 °C and no growth takes place in temperatures above 19.8 °C (Verant et al. 2012)- it could be possible that colder temperatures in hibernacula would slow the development of the fungus, and it may also be possible that bats which experience warmer temperatures during the summer may be able to rid themselves of infections (which is why male little brown bats in colder summer roosts could harbor the fungus). On the other side of the temperature issue, since surviving the winter is so energy-dependent, some researchers have suggested that, if infected bats had access to warmer areas when they woke up from torpor, they might be able to conserve energy and have a better chance of surviving the winter (Boyles & Willis 2010)- however, general warming of hibernacula would increase the rate of water loss for the colony, plus it would be complicated to create artificially warmer sections of a cave, so this idea is tricky. Certainly the question is how to make sure that infected bats have enough energy to survive the winter- and comparison of mortality of bats across the Northeast and in Europe suggests that longer winters spent in drier hibernacula lead to more bat deaths (Puechmaille et al. 2011; Flory et al. 2012), so increasing cave humidity in some areas could be helpful. Boyles & Willis (2010) felt that making food available in caves for bats that came out of torpor might be helpful, but wasn’t a practical solution. Hallam & Federico (2012) suggested that a boost in energy reserves right before hibernation might be the key and advocated planting vegetation near hibernacula that would attract foraging moths to give the bats a little help at the end of the summer.
To a certain extent, scientists are still trying to determine exactly what the fungus is doing to bats as it develops through the winter, but, given how quickly it has spread and how devastating it has been to some species, there isn’t really an opportunity to sit back and wait for lots of data before putting a response plan in place. Bats need help now, and not just from scientists trying to understand the energetics of the infection- we all have a part to play, whether that’s in prevention or support or recovery. For my final post of the month, I’ll look at the ways that we can get involved in bat conservation in the face of white-nose syndrome.
Works cited:
Blehert, DS. 2012. Fungal disease and the developing story of bat white-nose syndrome. PLoS Pathogens 8(7): e1002779.
Boyles, JG and CKR Willis. 2010. Could localized warm areas inside cold caves reduce mortality of hibernating bats affected by white-nose syndrome? Frontiers in Ecology and the Environment 8: 92-98.
Foley, J, Clifford, D, Castle, K, Cryan, P, and RS Ostfeld. 2012. Investigating and managing the rapid emergence of white-nose syndrome, a novel, fatal, infectious disease of hibernating bats. Conservation Biology 25: 223-231.
Flory, AR, Kumar, S, Stohlgren, TJ, and PM Cryan. 2012. Environmental conditions associated with bat white-nose syndrome mortality in the north-eastern United States. Journal of Applied Ecology 49: 680-689.
Hallam, TG and P Federico. 2012. The panzootic white-nose syndrome: an environmentally constrained disease? Transboundary and Emerging Diseases 59: 269-278.
Jonasson, KA and CKR Willis. 2011. Changes in body condition of hibernating bats supports the thrifty female hypothesis and predict consequences for populations with white-nose syndrome. PLoS ONE 6(6): e21061.
Puechmaille, SJ, Wibbelt, G, Korn, V, Fuller, H, Forget, F, et al. 2011. Pan-European distribution of white-nose syndrome fungus (Geomyces destructans) not associated with mass mortality. PLoS ONE 6(4): e19167.
Verant, ML, Boyles, JG, Waldrep Jr, W, Wibbelt, G, and DS Blehert. 2012. Temperature-dependent growth of Geomyces destructans, the fungus that causes bat white-nose syndrome. PLoS ONE 7(9): e46280.
Willis, CKR, Menzies, AK, Boyles, JG, and MS Wojciechowski. 2011. Evaporative water loss is a plausible explanation for mortality of bats from white-nose syndrome. Integrative and Comparative Biology 51: 364-373.