A mouse that eats yogurt daily, left, is slimmer and shinier than his brother. (Courtesy Susan Erdman)

Scientists studying the power of probiotics to fight obesity got more than they bargained for: Not only does yogurt make mice slimmer; it also makes them sexier.

Studies in humans suggest eating yogurt may help stave off age-related weight gain. But Massa­chusetts Institute of Technology researchers Eric Alm and Susan Erdman wanted to know why.

"Maybe it has to do with the healthy bacteria that live in our guts," said Alm, an evolutionary biologist, explaining how there are 10 times more bacteria in the body than human cells. "Maybe probiotics in the yogurt have something to do with the effects on weight."

To test the theory, Alm and Erdman fed one group of mice a normal mouse diet and another group the same diet with a mouse-sized serving of vanilla yogurt.

"One of the first things we noticed was their fur coat," said Erdman, assistant director of comparative medicine at MIT. "It was so thick and shiny; shockingly shiny."

But shiny fur wasn't the only thing that set the yogurt-eating mice apart from their siblings: They were also slimmer, and the males had "swagger."

"We knew there was something different in the males, but we weren't sure what it was at first," Erdman said. "You know when someone's at the top of their game, how they carry themselves differently? Well, imagine that in a mouse."

A lab technician would soon discover what was giving these males their sexy strut.

"She noticed their testicles were protruding out really far," Erdman said.

It turns out their testicles were 5 percent bigger than those of their non-yogurt eating counterparts, and 15 percent bigger than those of mice on a diet designed to mimic "junk food" in humans. And in this case, bigger was better.

"Almost everything about the fertility of those males is enhanced," Erdman said, explaining how yogurt-eating males mated faster and produced more offspring. "There were legitimate physiological differences in males fed probiotics, not just the extra sexiness."

And let's not forget the ladies. Female mice that ate yogurt were even shinier than the males, and tended to be better moms to their larger litters.

"We think it's the probiotics in the yogurt," Alm said. "We think those organisms are somehow directly interacting with the mice to produce these effects."

Although the study is ongoing, the fluky findings could have implications for human fertility and weight control, not to mention hair health.

"When I saw those fur coats, I thought about adding more yogurt to my diet," Erdman said.