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The sex lives of Yukon squirrels were the subject of a recent study, which found that promiscuity among females wasn't genetic.

The list of philanderers who can blame genetics is getting shorter: Female squirrels are the latest to lose an evolutionary excuse for sleeping around.

The sexual appetite of males is largely forgiven because of a need to ensure they produce as many offspring as possible. But biologists have long been baffled by why female squirrels are just as frisky. Promiscuity, after all, doesn't always make much sense: It uses up a lot of energy, and there's the potential spread of sexually transmitted disease.

Now, researchers at the University of Guelph have discovered the secret to the promiscuous sex lives of female squirrels - it has everything to do with opportunity and nothing to do with genetics. The study found that the female offspring and relatives of a squirrel that had fewer partners did not share that characteristic, which means that the species would not evolve to be less promiscuous even if it were advantageous to do so.

So squirrels, and perhaps other animals, may be well adapted to their environment now, but the absence of genetic variation means that they are limited in their ability to adjust to changes.

"Evolution by natural section, in general, does a really good job of allowing organisms to excel at solving the ecological problems that they face. But there are going to be some circumstances where their underlying genetics places some limitation on what they're able to achieve," said Andrew McAdam, one of the researchers and an assistant professor in the department of integrative biology.

In the study, released online recently in the Royal Society Journal Biology Letters, researchers followed 108 mating chases involving 85 female red squirrels in Yukon over four years. The female squirrel goes into heat for a day each year. She leaves a scent beforehand to let males know she's ready, and on the day itself, she encourages them to give chase.

Prof. McAdam, along with graduate student Eryn McFarlane and a team of researchers from across Canada, analyzed the data from a Yukon squirrel camp. They found that female squirrels mated with one to 14 partners, and even if a mother mated with only one male, that behaviour didn't appear in her offspring. The researchers found that even if the costs outweigh the benefits, the mating strategies of female squirrels don't appear to evolve to lower levels of promiscuity.

"Regardless of whether the trait is costly or beneficial, it's not going to change due to natural selection in this population. They're going to keep doing what they're doing," Ms. McFarlane said.

"In a lot of cases, we expect species and animals to be perfectly adapted to their environment. But they won't be if there's no genetic variation."

Root Gorelick, an associate professor in biology at Carleton University, said the study's findings are significant because they signal that not all behaviour is adaptive, and therefore animals and organisms cannot necessarily evolve out of sometimes destructive behaviour. He added that the study hints about the fact that traits may be correlated with one another, such as female promiscuity being correlated with male promiscuity.

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