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Why do certain crows have sex with corpses?

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Crows are extremely social birds. They are known to assemble noisily around the bodies of their dead comrades. Some of them at times go even further: they copulate with the corpses. 

Crows are not the only species to do so: ducks and dolphins have also been caught having sex with dead members of their own species. But to what extent is this behaviour common between species? Two researchers who study American crows (Corvus brachyrhynchos) recently carried out a study aiming to observe and document the practice of copulating with cadavers. One of the study’s main objectives was to determine why, and how frequently, this occurs.

Numerous studies have documented crows’ intelligence, their ability to solve problems using tools, and their capacity for remembering the faces of humans that threaten them. Other research has demonstrated evidence of crows’ social behaviour, noting that certain crows react to the sight of their dead.

Kaeli Swift, PhD student in environmental science and forestry in the University of Washington (United States), along with John Marzluff, specialist in fauna and co-author of the new study, documented a crow “funeral” in 2015, in which unusual sexual activity was observed for the first time. The researchers were in fact in the middle of analysing the birds’ vocal responses to seeing a dead crow, which indicates a potential threat to living crows. They noticed something that had never before been observed: a crow approached the cadaver and started copulating with it.

But if a dead crow is a danger sign, why would a live crow want to get near it to have sex with it? “Getting into close contact with a dead member of the same species could expose them to disease, parasites or scavengers”, wrote Kaeli Swift on her blog. For this new study, the researchers carried out a series of experiments in four cities, testing 308 pairs of wild crows. They exposed the birds to strategically placed stuffed crows – and to other stuffed animals such as pigeons and squirrels – in order to see whether the crows’ responses were specific to their own species or not.

The researchers noticed that the birds were more likely to give out a warning cry when the cadaver was a crow, particularly if it was in an exaggerated posture, and not in a natural position. The birds approached the dead crows around 25% of the time, but only 4% of them initiated sexual activity, which suggests that this is not a common behaviour. What’s more, the crows that mounted the dead birds often presented with aggressive behaviours as well as a sexual response.

It is possible that stress built up over the mating season, combined with the sight of a dead crow, simply confused certain crows. As a result, they react to cadavers with both aggression and sexual behaviour. However, further research will be necessary in order to say with certainty what drives certain crows to react in this way.

You can find all of the details of this study in the journal Philosophical Transactions of the Royal Society B.

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