Bats building bonds

It's cosy in here

ZOOLOGY is a field in which you have to put your boots on and go out in search of the data. Sometimes, though, you can answer questions by adding up what has already been found. So it was with Maria Sagot and Richard Stevens, of Louisiana State University in Baton Rouge. They were interested in the habit certain groups of bats have of building collective tents in which to shelter, using tree leaves. They thought that there might be a link between how socially integrated a bat colony is and the nature of its tents. Indeed, there is. But the details of the link surprised them.

Twenty-two species of bat are known to be tentmakers. Some stick together for long periods, but for others the association is transient. Likewise, some tents last for a long time whereas others fall to pieces rapidly. Presumably, these things are correlated: a group stays together because its dwelling is built to last.

Ms Sagot and Dr Stevens were able to gather relevant data from previous studies on 15 of the 22 species. As they report in Biotropica, ten of these 15 build their tents from palm leaves and other sturdy plant materials. Such tents often survive for more than a year. The other five species usually have only herbaceous plants available. They end up building tents that fall apart after less than two months. Socially, only four of the 15 species are known to form groups that last for more than a year. These, it might be expected, would be among the favoured builders-from-palm.

But they are not. To their surprise, Ms Sagot and Dr Stevens found that all four of the most sociable species are among those forced to use herbaceous plants, and thus to build ramshackle homes. Longevity of dwelling-place, then, does not promote longevity of society. Indeed, the opposite seems to be true. The need to build a new place to live every few weeks trumps familiar domestic comfort as a promoter of social cohesion.

It is possible that only collaborative species are able to get away with the nomadic life imposed by having to use herbaceous plants as building materials. But that seems less likely than the alternative. For a start, the four collaborative species are unrelated, suggesting that a propensity for cohesion has evolved independently. More prosaically any animal, regardless of how social it is, would presumably prefer not to have to move house regularly. That there are no highly social bats which build long-lasting tents suggests it is the instability of the dwelling that is driving the evolution of the behaviour. As in people, so in bats: adversity promotes solidarity.