New paper: Can light-induced synchronized arousals help bats survive white-nose syndrome?

Almost 20 years ago, a cold-loving fungus was transmitted from caves in Europe to upstate New York, unleashing one of the worst wildlife disease outbreaks ever recorded. White-nose syndrome is named after the white fungus growing on the faces of hibernating bats (image below). As the fungus spread across North America, hibernating bats died by the thousands, then millions. In a single winter, a hibernation site with thousands of bats might see over 90% of those bats die, and some sites were extirpated completely.

White-nose syndrome dominated the conversation among bat biologists. We struggled to do something, anything, but conservationists don’t have a good track record of dealing with invasive diseases. We watched helplessly as bat populations declined as the wave of infected sites spread out across North America. Each winter, new hibernation sites were struck and bat carcasses were found littering the ground. Although white-nose syndrome has not driven any species extinct, some of the most common bats that I used to watch as a kid on summer nights, like the little brown bat, are now very rare and declining enough to be considered for the endangered species list. Several other formerly common bat species, such as the northern long-eared bat, are now endangered species. For non-bat people to understand what this is like, imagine if Gray Squirrels or your favorite songbird almost completely disappeared over just a few years.

By 2015, the invasive fungus had arrived at one particular hibernation site of little brown bats (Myotis lucifugus) inside a dam in Michigan, called Tippy Dam (see below). While little brown bats predictably died in massive numbers throughout Michigan and the Midwest, the bats at Tippy Dam survived. The fungus was found at the dam in 2015 but somehow, for some reason, the bats were fine. In fact, the population increased in size over the next two years from about 21,000 to over 27,000. It seemed like a miracle. Nobody knows for sure why the bats at Tippy Dam were protected from white-nose syndrome. However, the dam was being studied by Professor Allen Kurta, an expert on North American bats at Eastern Michigan University. One interesting hypothesis that Kurta wanted to test was that these bats might have survived because they aroused together.

To understand this idea, first you need to know that white-nose syndrome kills bats by causing serious skin infections that wake the bats up from hibernation repeatedly over the winter. During these arousals, the infected bats use up their energy reserves, wasting away and dying before food is available in the spring.

Arousals are energetically expensive in part because when individual bats arouse from hibernation, they are either surrounded by the cold cave air or by the cold bodies of other hibernating bats. As they awaken, their heat diffuses into their surroundings, and they must burn their valuable fat stores to warm themselves. It’s cold to wake up alone.

Note that hibernating bats do not save energy by hibernating in clusters; they don’t lose heat because their bodies are already cold (that’s the point of hibernation). However, the bats could benefit from arousing in clusters. If hibernating bats aroused together, then instead of waking up to the cold air, a bat could be surrounded by bodies that are warm or at least re-warming. This ‘social thermoregulation’ would feel like waking up surrounded by warm puppies rather than alone on a slab of rock.

In theory infected individual bats could save energy by arousing in clusters. But how could hibernating bats ‘plan’ to arouse together at the same time? It might be difficult because hibernating bats lack a circadian rhythm. Why? Because internal clocks require calibration with light cues and a hibernation site in a cave is completely dark.

However, the space where bats hibernate in Tippy Dam has a small amount of light, shining through small ventilation holes. One possible mechanism of synchronized arousal is that this cue of daylight might cause the hibernating bats to maintain a circadian rhythm.

My PhD student Haley Gmutza tested this idea during her Masters thesis with Al Kurta. She used high-resolution thermal cameras to monitor arousals at Tippy Dam. A sample of her results for two time periods are below. The colored distributions show how many bats aroused at each hour of the day (the dashed line is roughly sunset, after which we might expect to see a change in arousal events). If bats aroused randomly (like the patterns on the left) the arousal events would be spread uniformly over time. Instead, what Haley found is that hibernating bats tended to wake up right after sunset. Before sunset, arousals were rare (patterns on the right). They kept a circadian rhythm consistently day after day over the whole winter.

The synchrony above caused most observed arousals (>68%) to occur within clusters of bats, meaning that the individuals benefited from social thermoregulation. These findings are consistent with the hypothesis that light-induced synchronized arousals contribute to the unprecedented absence of mass mortality from white-nose syndrome at Tippy Dam. A mathematical model could estimate the exact energy savings of increased social thermoregulation under different assumptions, but based on what is known about the energetics of arousal and social thermoregulation, the energy savings are likely to be significant.

The design of Tippy Dam might possess the necessary temperature and humidity properties of a good natural hibernation site, while also having a bit of light. If having that tiny beam of light keeps the hibernating bats on a circadian rhythm, it could help them by synchronizing their arousals, but this synchrony might not be possible in natural hibernation sites because the bats’ internal clocks inevitably drift apart inside the complete darkness of a cave or mine.

Is this hypothesis true? Future work is still needed to evaluate it. There is still much uncertainty about why the bats at Tippy Dam survive exposure to the fungus that causes white-nose syndrome. However, if this hypothesis does turn out to be correct (if light-induced synchrony causes social thermoregulation that protects bats from white-nose syndrome), then it might be possible to use light to maintain a circadian rhythm in bats as a tool for reducing mortality from white-nose syndrome in bats.

The more general point is that studying populations that have been largely unaffected by white-nose syndrome, like Tippy Dam, could prove useful in developing future mitigation strategies for protecting the remaining populations of these imperiled bats.

This paper is published in PLOS One.

Here’s a great video of little brown bats arousing from hibernation.

A few more thoughts…

As someone interested in the evolution of cooperation, I would also like to learn more about traits for huddling and social thermoregulation– the “snuggle for survival”. I’m interested in the noticeable variation in how often bats hibernate in clusters (across and within species). The costs and benefits of clustering during hibernation are not completely clear, but the variation we see is likely to reflect tradeoffs between different selective pressures over the evolutionary history of each bat species. These topics also get at the fundamental question of why some species live in groups and others do not.

I’m also fascinated by the weird possibility that a mutually beneficial behavior (in this case synchronized arousals) might have been constrained by the lack of light in the natural setting of a cave, but is now possible in this new human-built environment. It’s like the opposite of an “ecological trap”.

We are changing the world rapidly. We can no longer assume that biological traits exist in the environments in which they evolved, because those environments are gone. This stark fact also reminds me of how the artificial environments that we build for ourselves so clearly shape the expression of our own evolved social traits, our empathy, kindness, and desire for connection. We don’t even understand what things we can change about ourselves and what things are truly aspects of human nature. We are highly social primates building a zoo around ourselves without a full understanding of our species’ complex needs. Sadly, it might mean that the world will make less and less sense to our evolved minds.

Like the bats finding sanctuary in Tippy Dam, I hope we humans also manage to discover newly formed islands of safety amidst the sea of emerging threats. We face interconnected global threats– climate change, biodiversity loss, pandemics, demographic changes, and social conflicts. The opportunities for cooperation are there if only we could synchronize. I put my hopes in our growing capacity for science, reason, and compassion.