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Sexual imprinting in redheads

We may disagree on who's most attractive, but most of us prefer to mate with a member of our own species. But that can be tricky for redheads. Redhead mothers often lay their eggs -- I'm talking about ducks, of course -- in canvasback nests, saving themselves the trouble of raising the chicks. But what if chicks raised by canvasbacks end up thinking of themselves as canvasbacks? Or, however, they may think of themselves, what if their earlier experience makes them prefer canvasbacks as mates? They might end up mating with the wrong species.

This apparently doesn't happen very often in nature, but why not? One possibility is that redheads are genetically programmed to prefer redheads, even if they grow up with canvasbacks. After all, brush turkeys grow up to prefer mating with brush turkeys (below), even though they're incubated in compost piles managed mostly by males. Michael Sorenson, Mark Hauber, and Scott Derrickson, the authors of this week's paper, "Sexual imprinting misguides species recognition in a facultative interspecific brood parasite", published in Proceedings of the Royal Society, set out to test this hypothesis for redheads.
Brush turkey and brush turkey mound in Australia. Photo by Ford Denison.

They raised male redhead ducks in nests with either three female redheads or three female canvasback as foster sisters. As a control, they also included a male canvasback chick. They reasoned that, because canvasbacks almost never lay their eggs in redhead nests, canvasbacks that mate with the same species they grew up with would be mating with their own species. For redheads, though, this same behavior would often lead to mating with the wrong species. So they expected redheads, but not canvasbacks, to have a genetic preference for their own species, too strong to be overcome by early experience.

That's not what they found, however. Males of each species directed their courtship almost exclusively towards the species they were raised with as chicks. (This must have been based on their foster sisters, because none of them were raised by mother ducks.)

Based on these experimental results, you might expect lots of redhead-canvasback hybrids in the wild, or at least lots of mixed-species pairs (if hybrid chicks have low survival). But such mixed marriages appear to be rare.

The authors suggest some possible explanations. In the wild, most parasitised nests would have female redhead chicks as well as males. It would have been interesting to include a treatment with equal numbers of females of each species in each nest.

The preferences of females for their own species are also likely to be important. In the wild, their are usually several males per female. (I've noticed this in mallards and assumed it was because females on nests are more susceptible to predators, but I don't know if this is the real reason.) In these experiments, however, there were similar numbers of males and females, so females may have been less choosy.

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