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Previous studies revealed that promiscuity alters DNA and boosts immunity in female mice.  Now, new research reveals that having more sex also helps mothers protect their babies from getting killed.

Researchers have long known that mice can obtain genetic benefits when females mate with multiple mates.  However, until recently, researchers were unable to determine the conditions under which females will voluntarily mate with multiple mates.

In a new study, scientists conducted a series of experiments in which female house mice could mate freely with one or two males while not in danger of sexual coercion by a mate.

The findings revealed more evidence for the infanticide avoidance explanation. Researchers explain that males that have a chance of reproducing with a female are less likely to kill her young. 

Because virgin males are known to more likely to kill their offspring, female mice tend to mate with multiple virgin males to reduce the danger of infanticide. The study found that less promiscuity was observed when female mice were exposed to more experienced males. Researchers said these findings suggest that females can tell whether a male is experienced or not by detecting differences in males' scent markings.

The study also revealed that females were more likely to mate with multiple males when they produced very similar levels of scent markers. On the other hand, females tended to mate with a single male when they are able to detect a significant difference in the males' scent.  Researchers said uniqueness of scent markings also appears to be another factor that influences females' mating decisions.

Scientists also found that litter sired by more than one male were larger than single-sired litters.  However, this was only true when there was fierce competition among males.

"Our results shed some new light on questions about the sexual behavior of mice, but we still don´t have all the answers", says lead author Kerstin Thonhauser. "We need further studies to understand why litters are larger when there is intense competition between males. Another interesting question that has remained unresolved so far is how multiple paternity affects the fitness of the young."

The findings are published in the journal Behavioral Ecology and Sociobiology.