I’ve been trying to tune into developments with white-nose syndrome because it’s one of the worst emerging pathogens to hit North American wildlife in recent history. Ever since the first breakout in a New York cave in February 2006, this white fungus has killed off well more than a million bats from six different species. Sure, I know, there was some quibbling over whether the fungus was causal or secondary, but the bulk of the evidence seems to be pointing to it being causal. And for my story, a conservation biologist I interviewed at Bat Conservation International said, “We think the fungus is clearly the smoking gun,” so that cinched it up for me.
But I didn’t know, when I filed that story, about a research paper published March 5, 2009 in Frontiers in Ecology and the Environment, in which two authors offered experimental evidence based on computer models that artificially warming caves or mines throughout a bats hibernation period may thwart the disease process enough to quell the winter carnage. The paper says that by offering bats “thermal refugia” of 28C (about 82F) degrees, survival rates could be increased by up to 75 percent. This is premised upon the idea that bats are forced to burn through their fat stores too fast when infected by the Geomyces destructans fungus that causes WNS, because the skin infection itches and rouses them from their hibernating slumber. Being roused multiple times during hibernation then causes their metabolism to speed up, and they grow hungry due to running through their stored energy, forcing them to leave their caves too early and then starve for want of insects in mid- to late-winter.
Because G. destructans is a cold-loving fungus, the thermal refugia might also halt its growth on individuals during winter months. It is thought to be temperature limited at about 20C (68F), according to the paper’s authors. Because several different species of bats tend to use the same hibernation sites, called hibernacula, thermal refugia may also to help limit transmission between species. Starvation appears to the cause of death in bats affected by white-nose syndrome, but researchers are still trying to figure out how the fungus throws their energy budgets so off balance that they don’t make it through the winter. During hibernation, most bats alternate between long periods of torpor (where their metabolisms are very, very low) and brief periods of arousal (where their metabolism spikes). They write:
A normal hibernator spends over 99% of its time during winter in bouts of deep, prolonged torpor, during which [body temperature] falls close to ambient temperature (2–8 ̊C for cave-hibernating bats) and metabolic rate can be reduced to less than 1% of that during euthermic (ie non-torpid periods).
When the bats do become aroused, it may be from a few hours to a day or so. No one really knows why they do become aroused, but it appears to make up less than 1 percent of their winter time budget — but this 1 percent of their time can eat up 80 percent or more of their winter energy budget. This goes to show how energetically expensive these arousal sessions are. For bats in the northeast, this arousal means they have to produce internal heat to raise their body temperatures by as much as 35C (95F), and keep it raised for several hours. The authors write that the metabolic cost to do this can be up to 400 times that of staying in their torpor — a huge and costly energetic expenditure. So it’s easy to make the leap and see how, if a hibernating bat became infected with an itchy fungus like WNS, it could cause a bat to awake more than normal, perhaps to groom, and then quickly expend its fat reserves warming itself up — leading it to starve mid-winter. The authors write:
In the absence of a “cure” for WNS, there are only two management interventions that could prevent starvation in affected bats: (1) provide more energy to bats in the form of food or (2) reduce the amount of energy that bats must spend to survive hibernation.
The first option, via captive feeding or rehabilitation would be too costly given the scope of the problem, so the authors focused on the second one. Relying on records showing that aroused bats are often found in warmer parts of a thermal gradient in hibernacula, the authors hypothesized that offering small areas of comparatively warmer ambient temperatures in a hiberncula could lessen the energy expenditure on infected bats that are aroused often from their torpors. But they do caution that the theoretical thermal refugia should not alter the overall cold temperatures in the hibernacula, which are needed to maintain the energetic cost-savings of torpor.
They modeled survival in hypothetical populations of 1,000 little brown bats, and repeated the simulations 100 times per population. They experimented with the temperature being around 2C, but with the bats having the ability to travel short distances to thermal refugia ranging from 12C, 20C, or 28C. They did not incorporate flight costs into their models because they would apparently be negligible. They used the models to determine how much fat the bats would have left at the end of their hibernation given multiple arousals of 12, 18 and 24 hours (each modeled separately), and total torpor lengths of 200 hours. They assumed in the model that WNS would cause a decrease in torpor duration from 800 to 200 hours. (This assumption was based on a separate model they did to fit their model to real life mortality rate data, based on the assumption of the bats burning through their fat reserves due to increased metabolic demands from arousals.)
Their results show that when warmer thermal refugia are modeled, more bats survive. Regardless of whether they manipulated the total torpor length, or the arousal length, both variables resulted in bats having more fat reserves left over at the end of hibernation when thermal refugia were available to them. The authors write: “Mean mortality dropped to 63.0% in hibernacula with thermal refugia of 12 ̊C, 43.6% in hibernacula with refugia of 20 ̊C, and only 25.2% in hibernacula with refugia of 28 ̊C.” Put another way, the survival rate was up to 75 percent in hibernacula with the warmest thermal refugia.
I’m not clear how, exactly, spot-contained thermal refugia could be created, but it does seem to be an interesting idea. Definitely not a solution, but a stop-gap measure that sounds worth investigating to at least buy time while more research is poured into a longer term solution or intervention.
Are you a bat researcher, or are you familiar with this idea? If so, please let me know in the comments how this idea was received by those working to figure out WNS. Thanks.
Boyles, J., & Willis, C. (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 (2), 92-98 DOI: 10.1890/080187