The animal kingdom is full of extremely weird mating behaviors. Some flatworms can reproduce by having sex with their own heads. The male honeybee's penis is pulled from its body during sex, disemboweling it. Now, several male sand martin birds have been filmed having sex with another male. Although homosexual behavior in the animal kingdom is fairly common, this example is bizarre – the second male had been dead for some time.

According to the study, published in the journal Ornithological Science, three separate sand martins (Riparia riparia) were seen trying to repeatedly copulate with a dead male lying face down on the ground. Obviously, there was no immediate reproductive gain from this.

There are, however, various animals that engage in homosexual behavior for other, less direct means of improving their mating chances with those of the opposite sex. Small male cuttlefish, for example, disguise themselves as female and groom other, stronger males; if a female cuttlefish turns up to mate with the stronger male, she can also choose to mate with the disguised male as well. The gamma male stag beetle tricks other males into wasting their sperm by copulating with them, taking them out of the competition.

This particular act of homosexual necrophilia has nevertheless left the researchers slightly baffled. It’s only the second paper describing this type of behavior in birds, although there are 30 confirmed reported cases of this unusual display. In this case, the researchers think that the homosexual part of the equation was down to a case of mistaken identity.

Male and female sand martins look the same, and the way this particular expired male was lying on the ground must have looked, at least to the three randy males, like a female presenting herself. This indicates that posture is likely to be a trigger in arousing the males’ sex drives.

It’s also likely that the birds were not actively seeking out dead members of their species to mate with either. Kees Moeliker, a researcher at the Natural History Museum in Rotterdam, the Netherlands, published the first description of homosexual necrophilia in birds; he told New Scientist that the necrophilic part of this behavior was definitely “a mistake.”

Saying that, necrophilia in the animal kingdom is also not unheard of. Male tegu lizards have been caught in the act of mating with a dead female long after it had begun to decay. Ducks, sea lions, penguins, and pigeons have also been seen committing the grim act, although there appears to be no reproductive benefit at all for the males in doing this.

The one known exception to this is the Amazonian frog, Rhinella proboscidea. In a process called “explosive breeding,” a huge crowd of males will attempt to mate with a far smaller number of females. This frequently results in the death of a female by drowning, but this is no obstacle for the stubborn male frogs: they squeeze the eggs out of the dead females’ bodies and fertilize them externally.