WHEN fish evolved placentas, it may have shifted how they pick sex partners. It seems placentas made females more promiscuous, and males less competitive.
The placenta allows mothers to feed developing young via their blood, instead of stuffing lots of nutrients into each egg.
David Reznick of the University of California, Riverside, and his colleagues looked at 150 species of poeciliid fish. Most poeciliids give birth to live young, but only some have placentas.
Reznick found that the males of species with placentas were less colourful and did not dance when courting females, compared to species without placentas. For their part, females with placentas racked up more mates (Nature, DOI: 10.1038/nature13451).
From a female's point of view, having a placenta means she does not have to invest lots of resources to make each egg, so can mate more. In turn, males do not need to show off to females. But their embryos may compete with others for food from the placenta.
Other placental animals might behave similarly, says Ingo Schlupp of the University of Oklahoma in Norman.
This article appeared in print under the headline "Placentas changed fish sexual politics"
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