Nature is fighting back in response to female promiscuity by producing a biological 'sperm' chastity belt, say US scientists.
A sticky solution to sperm wars
Semen becomes more sticky to act as a plug, thereby preventing sperm from competitors impregnating females who sleep around, they found.
The Howard Hughes Medical Institute team, along with colleagues from Chicago and Kansas City, studied humans, monkeys and gorillas.
The findings appear in Nature Genetics.
The researchers examined the semen of 12 different species of primates.
In species where the females were most promiscuous, the males had developed several strategies to ensure they would be the male most likely to father any offspring and pass on their genes.
As well as having larger testicles and producing more sperm, the semen was more sticky.
Chimpanzees, for example, which are a promiscuous species, had more advanced evolution of a gene controlling the stickiness of semen than gorillas, which tend to be monogamous and stay faithful to their partner for life.
Humans were midway between, suggesting that while women are nothing like as promiscuous as chimps, neither are they as faithful as gorillas.
Lead researcher Dr Bruce Lahn said: "The idea is that in species with promiscuous females, there's more selective pressure for the male to make his semen more competitive.
"It's similar to the pressures of a competitive marketplace.
"In such a marketplace, competitors have to constantly change their products to make them better, to give them an edge over their rivals - whereas, in a monopoly, there's no incentive to change."
The gene in question is called SEMG2 and controls the production of a component of semen, called semen coagulum, that acts like a glue.
In the most extreme cases, its effects were so pronounced that the semen became a solid plug - effectively a chastity belt.
Dr Allan Pacey, Senior Lecturer in Andrology at Sheffield University and member of the British Fertility Society, said: "We know that genes controlling reproductive processes are some of the most rapidly evolving in the animal kingdom.
"Clearly, successful conception is not solely about the quality or quantity of the sperm inseminated - subtle aspects of semen biochemistry, or aspects of the physiology of the female reproductive tract, are just as important.
"Sperm competition is an evolutionary arms race, with males developing increasingly sophisticated strategies to tip the balance towards successful conception and at the same time hinder the chances of rival males."