SPIDERS are known for many things. Sociability is not one of them. Most spiders are more likely to try to eat their neighbours than befriend them. Given that there are at least 43,678 species of the critters, though, it is not too surprising that a few have overcome their natural grumpiness and teamed up to form societies. So far, about two dozen such social spiders have been identified. And among them, something really strange has just been found. For one type of spider society turns out to involve two different but closely related species. It is as though anthropologists had discovered villages populated both by human beings and chimpanzees.

This was discovered by a team led by Lena Grinsted of Aarhus University in Denmark. They were studying a social species of spider called Chikunia nigra, living near Beratan Lake in Bali. Later, as they looked in more detail at their specimens, they realised its genes and genitalia revealed that it was actually two species, according to their findings just published in Naturwissenschaften.

Exactly what the spiders get out of being social is not clear. They do not hunt together. One explanation may be that the colony is acting like a giant crèche.

Ms Grinsted discovered this possibility by experiment. First, she identified 19 females who were looking after recently hatched young, and another 20 who had eggs. In each case she introduced an intruder, in the form of a spider from the same colony. Both mothers and mothers-to-be were surprisingly tolerant of what would, in most spider species, be a serious threat. Only 40% of the time did they attempt to chase the intruder away, or bite it.

Ms Grinsted then took another 40 spiders and swapped some of their broods (though always to a female from the same colony). The upshot, she found, was that a female was as likely to look after and guard another’s brood as she was her own.

Which is intriguing, but not all that extraordinary in social groups which are composed of closely related individuals. Except that Ms Grinsted now knows that this cannot always be the case for her spiders, since two different species are involved. The species in question are pretty similar, which would seem to rule out another common cause of collaboration: that different creatures bring different adaptations to the party, thus dividing the labour of staying alive into specialisms.

Because Ms Grinsted did not know at the time of her experiment that two species were involved, she cannot be sure how many of the fosterings she induced were cross-specific. The two species seem more or less equally abundant, so the chances are it was about half of them. If colony members are acting as foster mothers in the wild (which has yet to be established), something most odd is going on. Altruism is not a concept often associated with spiders. Xenophilic altruism is truly bizarre.