Females protect offspring from infanticide by forcing males to compete through sperm instead of violence

Latest research shows the females of some mammal species will have many mates to ensure unclear paternity, so that males can't resort to killing their rival's offspring for fear of killing their own. This forces males to evolve to compete through sperm quantity, leading to ever-larger testicles. Scientists find that as testis size increases, infanticide disappears. Once sperm competition has become so intense that no male can be certain of his own paternity, infanticide disappears - Dieter Lukas Previous research has shown that infanticide by males is widespread in many mammal species, but most commonly occurs in those species where females live in social groups dominated by one or a few males. Outsiders will fight dominant males for access to females. When a rival male takes over a group, they will kill the infants of previously dominant males to render the females 'sexually receptive' again, so that they can sire their own offspring. This may be the main cause of infant mortality in some species, such as Chacma baboons.