Since Bruce Bagemihl‘s wonderful book Biological Exuberance (and Ricky Gervais’s stage adaptation of it), there’s no denying the fact that homosexuality is everywhere in the animal kingdom. Not as the occasional embarassing mistake, but often as a standard part of the sexual repertoire. Clearly, homosexuality has evolved in many species, but the question is, how and why? Since evolution works via success in spreading one’s genes, how could a behaviour that is not primarily geared towards leaving offspring, be advantageous?
The crucial word here is “directly”. Perhaps in most cases homosexuality itself is not *directly* advantageous, but it may be indirectly so. One example of this has just been published in the journal Biology Letters. Katharina Engel and colleagues of the University of Ulm, Germany, studied the burying beetle Nicrophorus vespilloides. This is a common beetle in Europe and Asia, known for its habit of interring dead animals (small birds or rodents, usually) and then tending the buried cadaver as food for their young. (In fact, they are one of the few kinds on insect where the parents care for the young larvae till they are grown up.)
In the scramble around a newly-discovered corpse, where many beetles will vie for the right to take possession of the corpse, male beetles face great risks but also great opportunities: if a male manages to outcompete rivals *and* fertilize the female that eventually is going to lay its eggs on the dead mouse, it will have reached its reproductive goal. And, as Engel and colleagues discovered, whether these males will engage in any gayness, will depend on how frantic the scramble around the mouse is.
They conducted two experiments.
In the first one, they placed males in cages with zero, one, or three females. After several weeks, they introduced such males to other males to see whether they would fancy them. The beetles that had been housed with females would never mount another male, but the males that had been housed in celibacy, would be more so inclined. So, the researchers say, males that “think” females are few and far between, won’t let a potential female get away, even if the female is actually a male. In other words, they can no longer afford to check first.
In the second experiment, they kept males in isolation for 60 days and then gave some a dead mouse and some not. Then, they introduced a male “lover”. As it turned out, the males-with-a-mouse were less likely to try to mate with the new male than the males-without-a-mouse. The explanation in this case is that the other male will be more inclined to fight the starry-eyed male if there’s food around that’s worth fighting for. So in this case, mounting a potential female and running the risk that it turns out to be a male, is a wise choice only if there’s little risk that the male (if in fact it turns out to be one) will bite back.
A (heterosexual) copulation in the burying beetle Nicrophorus vespilloides (male on top). Photo by Heiko Bellmann (from the Biology Letters paper–supplementary information)
In conclusion, in male burying beetles, homosexuality seems to be an option born out of the risk of missing out on a heterosexual encounter with a real female.
Stay tuned for more discoveries from the realm of biological exuberance.