Dossiers Sexologiques

15 February 2008

Necrophilia and incest, hardly the St Valentine ‘s Day favourite topics, occur in humans and animals alike. Necrophilia, having sex with the dead, is one of the fetishes found among humans. But in 2003 the Ig Nobel prize for biology went to the person who first documented necrophilia in animals, C. W. Moeliker (published in Deinsea, 2001). Moeliker worked in Natural History Museum in Rotterdam when he heard a thud against his office window. When he went over to have a look at what caused the thud he saw a dead male duck on the ground next to the museum. The duck must have died as it collided with the window in full speed flight. Yet this duck wasn’t alone. Waddling around him was another male duck. Waddle he did not for long, because, as Moeliker reported, this other duck proceeded to rape the deceased duck for 75 minutes, taking only 2 brief breaks (to get some air I assume). So not only were both of the ducks male, one of them was dead and this still didn’t stop the eager copulation. Ducks are often observed chasing each other in aerial pursuits and the explanation for this event is that this was happening when one duck collided. The surprising thing is that the other duck still engaged in sex with the dead duck. Perhaps it didn’t realize the other duck was dead? This report came out only recently and so a few years ago the idea that necrophilia can happen in animals might have sounded ridiculous. Yet, the more we look the more diverse the sexual lives of animals we find. One activity that many animals still stray away from though is incest: breeding with their siblings. Although many insects will happily do it and some even have sex before they have left their mother’s body, most animals don’t do it because it can reduce the fitness of the offspring. Two mechanisms invoked are heterozygous advantage, the idea that more diverse alleles (copies of the same gene coming from either parent) the fitter the organism, and the fact that homozygous (having both alleles the same) organisms are more likely to end up with two deleterious copies of gene. These may cause so called inbreeding depression. In humans, for example, children of first cousins (consanguineous) have been found to be 4-5% more likely to die than non-consanguineous children. Indeed, in the two examples where non related children are raised together (Kibbutz children from Israel and Sim-pua marriages from Taiwan) they are less likely to get married than expected by chance (children raised together in Kibbutz) or are less likely to reproduce and more likely to be unfaithful (Sim-pua marriages). The explanation for these is that there is a sensitive period during which children accept whoever they are growing up with to be their sibling to reduce the possibility of incest and inbreeding later on in life (Westermarck Effect). We don’t necessarily need laws to prevent incest as this is a natural tendency already. Yet, avoidance of close relatives as reproductive partners may be outweighed by attraction to distant relatives with whom we interact socially. One idea for why the incest-preventing laws exist is to prevent relatives from accumulating power by marrying among each other and thus threatening the social status of the leaders. So it seems we have some evolved psychological mechanism of avoiding incest and the cultural component of incest laws (in religion and politics) may be due to political and economic rather than the biological and medical reasons.