Sexual selection refers to species' selection for traits that are attractive to the opposite sex, the tail of male peacocks being an iconic example.
This special type of natural selection enhances opportunities to mate. Biologists at the University of California, Riverside have now found that sexual selection and "placentation" — the formation of a placenta — are linked. Describing the life histories of more than 150 species of fish in the family Poeciliidae, the researchers found that species with placentas tend to have males that do not have bright coloration, ornamentation or courtship displays. They tend to be much smaller than the males of species without placentas. They also tend to be very well endowed, enabling males to sneak up on females to mate with them without the formality of courtship.
"It impresses me as being a bit like science fiction to say that male morphology and mating behavior and female preferences will be in any way governed by the female's mode of reproduction," said David Reznick, a distinguished professor of biology, whose lab led the research. "I would have thought that what was going on in the inside of the animal would be largely independent of what is going on on the outside."
All of 150 species Reznick's team described give birth to live young, but some of these species have the equivalent of a mammalian placenta. The researchers discovered that the placenta has evolved multiple times and that species vary considerably in how well their placentas have developed.
"This diversity is enabling us to address questions about how and why the placenta evolved and to learn something about the consequences of having one," said Bart J. A. Pollux, a former postdoctoral scholar in Reznick's lab, a member of the research team and the lead author on the research paper.