Hanging Out in the Heliconias

Bats are usually sociable animals. Since large caves are relatively rare, when bats find one, literally millions of individuals can pack themselves into a single one, often comprising several different species. But the shortage of suitably large roosts means that many cave-dwelling bats will roost elsewhere including, in human-dominated landscapes, inside attics or other manmade spaces. Furthermore, of course, many bats don't live in caves at all, spending the day roosting in trees or other suitable sources of shelter.

One might think that these tree-dwelling bats would be less sociable than those forced to cram themselves into caves. But, while it's true that they certainly don't tend to form colonies numbering in the millions, this doesn't mean that they live on their own. In some species, the males do in fact do this, following a pattern that's often seen in other social mammals, while the females gather together in maternity roots or breeding colonies. But very few bat species are entirely solitary, with both sexes living alone and, when bats do gather together, the exact structure of their social groups varies considerably between species.

In some species, groups appear to have very little structure or coordination; the bats are simply living close together. Others form long-lasting harems dominated by a small number of males, others are centred on a group of breeding females, with a large number of males and non-breeding females living on the periphery to coordinate defence. Others employ what are termed "fission-fusion" groups, which means that the group as a whole may be long-lasting, but individual members come and go all the time - this is a pattern that's more commonly associated with primates. A few even rise to the level of quite complex societies with variable degrees of relatedness and mutual cooperation that we still haven't fully gotten to the bottom of.

Indeed, it's worth noting that bat social behaviour has been studied rather less than that of other species of mammals, such as primates, herd animals, and rodents. This is largely because it isn't that easy to do, but the end result is the real story is likely to be even more complex and varied than we already know. But, whatever method they use, how do bats living in structured groups, rather than mere aggregations, coordinate their behaviour?

A significant part of this almost certainly comes down to the sort of olfactory clues that are difficult for us to follow. Bats often have specialised scent glands, and their urine and saliva may also contain chemicals that others can sniff out and interpret. But, because these are bats, and necessarily have excellent hearing, it's also safe to say that auditory signals are at least as important and that bats do communicate with one another through sound.

One species of bat that has been comparatively well-studied in this regard is the otherwise obscure Spix's disc-winged bat (Thyroptera tricolor). This is one of the species that forms stable, long-lasting groups and this is significant because they refuse to stay in any given roosting site for longer than 24 hours. The roosting sites in question are large furled leaves, which it hangs underneath... although. just to be different from other bats, it does so with its head facing upwards and its feet hanging down. And we know that, in order to maintain these groups from day to day, the bats call out to one another and respond, letting their group-mates know when they have found a suitable roost and then being able to identify each other from the individual sound of their calls.

But that's just one species, and others may - indeed, almost certainly will - do things differently. A recent study therefore chose to look at a different species that also has an unusual social structure that requires communication and coordination between group members. This is the Honduran white bat (Ectophylla alba).

In many ways, this is quite an unusual bat, another illustration of the great variety of the group. To start with, there's its appearance. It has, as its name would suggest, white fur, which very few bat species do. Furthermore, the hairless areas on its nose, lips, and ears are bright yellow due to an apparently unique ability to deposit carotenoid pigments in its skin. This actually seems to help camouflage it and may also be a sexual signal.

The bat lives in eastern Honduras, Nicaragua, and Costa Rica, where it inhabits lowland tropical forests and eats nothing but figs. (In fact, unless it's really hungry, there seems to be just one species of fig tree it's willing to feed on. Fortunately, this being the tropics, it's in fruit year-round). Significantly, however, it's one of a small number of bats that make tents.

Honduran white bats roost beneath the large leaves of heliconia plants, relatives of the banana that are often seen today in ornamental gardens. The bats cut along part of the leaf so that it folds over to make a tent, and then huddle underneath through the day to shelter from the rain and hide from predators. Such behaviour is not unique among bats (although it is rare) but these particular bats have a different social system from other tent-making species.

The usual pattern is that individual males construct the nests and attract a group of females to roost with them, forming a harem that gives them obvious advantages in mating. But the white bats don't do this. Instead, they live in egalitarian mixed-sex groups, typically with five or six members each. Both sexes share in the work of constructing the tents, which can take some time to complete. Each group has several tents throughout the forest, which can last for up to six weeks before needing to be replaced, and they use different ones on different nights, perhaps so as not to attract predators to a regular location.

Interestingly, while the groups are stable and continue with the same members for long periods of time, the members of those groups do not appear to be related. Essentially, they are hanging out with their friends rather than living with their closest blood relations.

Watching a group of these bats in their tents over the course of several days, the researchers constructed what's called an ethogram. This is a list of the different behaviours that the animals engaged in, how common they were, and whether they were performed by males, females, pups, or some combination of the three. Most of these behaviours are largely what you'd expect. Since they were at home, and, by definition, not out looking for figs, and it wasn't the breeding season, most of the time the bats were just resting, looking about, cleaning themselves, fidgeting, or looking after their young. 

A couple of behaviours stood out, however. Every now and then one of the males would move up to another bat and gently chew on the fur between their shoulder blades. This didn't appear to annoy the bat being chewed upon, but the fact that it's such a specific area makes it unlikely that this is grooming of some kind, to remove parasites or whatever, as many primates do. More likely, the researchers suggest, this is a bonding act, marking the subject with scented saliva that can then be detected. In this way, the bats may help to keep their little group together, identifying friend from stranger.

This behaviour had been seen before, although it had not previously been appreciated that the males did it to each other, as well as to females (which has some implications for its purpose). The other unusual behaviour was entirely new, consisting of a young bat picking up a leaf and, so far as could be determined, simply playing with it for fun. Other species of bat have been observed playing before, so this isn't unique, but they are often playing socially with one another - wrestling, chasing, and so on - not with an object. In this case, an obvious possibility is that playing with the leaf is a forerunner of the bat constructing leaf tents later in life.

By making recordings of the bats, the researchers were also able to distinguish ten different kinds of call that they made, not counting the echolocation calls that they use while flying. This is a reasonably complex vocal repertoire, although, again, not that remarkable for bats. In many cases, it wasn't possible to determine what the calls 'meant' to the bats, since they didn't seem to be doing anything else specific at the time. On the other hand, some calls were clearly used between mothers and their young, often prior to nursing, and so obviously have a meaning specific to that.

There was also a specific call that male bats used while performing the fur-chewing behaviour, possibly to calm down the subject and indicate generally friendly intentions. The bats had two types of 'contact call', used when leaving or departing the roost. Again, these seem likely to be social in nature, perhaps helping to bond the group together or let bats of other groups know that the roost is already taken. Out of over 100 calls recorded, only two appeared to be at all aggressive or stressed and one of those was after a bat had just been bitten by a mosquito and was understandably aggrieved.

At least when they're at home, sheltered from the weather in their self-made tents, it seems that Honduran white bats have more time for socialising than stressing.

[Photo by Scott Olmstead, from Wikimedia Commons.]