A new study shows that the trait of female promiscuity may have developed in order for female mammals to protect their offspring. 

Infanticide by rival males has long been observed in many mammal populations. When an outside male invades a group and chases off the reigning patriarch, the new male will attack the troop's infants. This renders the females "sexually receptive" to begin furthering the new male's genetic line.

But many female mammals have adopted a counter-trait: promiscuity. In species such as the mouse lemur, females will mate with as many males as possible. This creates hesitance among the new male: by committing infanticide, he could be killing one of his own.

"In species in which infanticide occurs, testicles size increases over generations, suggesting that females are more and more promiscuous to confuse paternity," said co-author Dieter Lukas of the University of Cambridge in Britain.