This is largely because most bats eat insects. In temperate climates, insects aren't around much during the winter; in many cases they've died off and left their eggs to hatch the following spring when the weather has improved. Any animal that relies heavily on eating insects to survive has to deal with this scarcity somehow. Insect eating birds, such as swallows and thrushes, cope with the problem by migrating in the winter, heading to warmer climes where insects are still common. There are some bats that do exactly this - for example, the hoary bat migrates annually between Canada and the southern US - but more commonly, they deal with the absence of food by hibernating.
So it comes as no surprise to discover that the activity around bat caves declines dramatically during the winter. But, in many cases, there is a significant rise in activity during the autumn, with bats forming large swarms during which they fly in and out of the caves, chase each other, and generally seem to be quite busy. But, if anything, while you're preparing to hibernate you want to cut down on your daily activity. By lazing about and doing as little as possible other than find food, you'll get fat, and a hibernating animal really needs to build up fat reserves to avoid starving through the winter. So the bats must have a pretty good reason for this swarming behaviour, and the obvious one (as it so often is) is sex.
But there might be more to it than that.
Townsend's big-eared bat (Corynorhinus townsendii) lives throughout the western USA, Canada, and Mexico, as well as in smaller populations as far east as Virginia. They belong to the vesper bat family, which is the largest and most widespread of all the many families of bats, with over three hundred known species, including perhaps the best known British bat, the common pipistrelle.
Vesper bats Free-tailed Funnel-eared
& Pallid bats bats
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| | | bats, etc.
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Townsend's big-eared bats give birth in the spring, when the females cluster into maternity colonies until the young are old enough to fly. Males and females then live somewhat apart for much of the summer, until taking part in the great swarms of later summer and early autumn. Finally, having put on about 25% of their body weight as fat (the equivalent of around 30 or 40 pounds for a human of average height), they huddle close together to hibernate through the winter.
A survey of 158 abandoned uranium mines in Colorado looked at how the bats used these artificial "caves" throughout the year. In addition to the Townsend's big-eared bats, eight other species shared the caves - something not at all uncommon where bats are concerned - and it is worth noting that many of these swarmed at around the same time, prior to their own hibernation. The researchers didn't look at the maternity colonies, for fear of disturbing them, but they did shed some light on swarming and hibernation.
There is no doubt that bats mate when they are swarming, although it has also been suggested that the swarms might help young bats, about to hibernate for the first time, become familiar with the lay out of the caves.
You might think, incidentally, that hibernating while pregnant doesn't sound a particularly good idea, since shutting down most of your bodily functions surely isn't very conducive to nourishing a rapidly growing infant. If so, you'd be right. In fact, like some other mammals, female Townsend's big-eared bats store their partner's sperm inside their bodies right through the winter - so, although they mate in the autumn, they don't actually become pregnant until they wake up in the spring.
What was significant, however, was that only around 40% of the mines in use by the bats were used both for swarming and hibernation. That is, over half the time, the bats chose different sites for these two activities. This would certainly count against the theory that young bats are learning the cave lay-outs, but also suggests that what makes a good hibernation site doesn't necessarily also make a good swarming site, and that the bats are looking for some specific sort of environment that not every cave has.
The key difference between the mines used solely for swarming, and those used only for hibernating, was that the latter tended to be colder. Mines and caves tend to have a fairly stable temperature, regardless of the weather outside, but it is worth noting that those 40% of mines that were used for both activities tended to be the ones with the most airflow. In other words, cold winter air entered those caves more frequently, making them cooler when the bats hibernated than when they swarmed, earlier in the year.
Why should the temperature make a difference? Temperature is especially important to small mammals, such as bats, because they can more easily lose body heat than large animals, and need to work harder to keep their body temperature up. In hibernating animals, low temperatures lower their activity, conserving energy and reducing the number of calories they burn. For bats about to hibernate, this could be very useful, allowing them to build up fat reserves in between feeding. On the other hand, it is much easier for the bats to wake up again at warmer temperatures, so that they can go out and find more food. So, during the swarming time, when the bats are preparing to hibernate, they want to strike some sort of balance, and find a cave with just the right temperature.
Once they actually are hibernating, these priorities might change. Like most hibernating animals, Townsend's big-eared bats don't literally spend the entire winter in a semi-coma. They do wake up from time to time, especially if some of the winter nights are unusually mild. But waking up burns a lot of their energy, so they probably don't want to do it too often - finding a cold cave could help them stay asleep for longer, and might be a something of a benefit to them.
This suggests that swarming isn't just about mating - although that's clearly a major part of it. It's also about preparing for hibernation; going out regularly for food, and then returning to the cave for a nap to conserve energy and build up fat. Or at least it is for the females, because it turns out that males actually lose weight during swarming, while the females pile on the ounces.
It has to be said that the males haven't been doing an awful lot during the summer, while the females have been busy raising their young, providing them with milk and so on. So it could be that the males have already put on the fat they need, and are free to spend the swarming season looking for mates. The females have to wait until later before making their preparations, and, in any event, can afford to sit about waiting for the males to come to them.
But it's also true that, if a male bat dies during hibernation, it's no great loss to the species - he's already mated with several females by that point. But a female has to survive in order to get pregnant and then raise her young, so having those fat reserves is more important to her in the grand scheme of species survival.
Of course, this may not be true for all species of bat, or even for this one species in all climates. Some may well have other considerations to take into account. But, at least for these particular bats in Colorado, swarming seems to be about the males rushing around looking for sexual partners, while the females patiently get themselves ready for the work of the following year...
[Picture from Wikimedia Commons]
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