Males can drive species decline

Too much aggressive sex can kill off lady lizards.

In common lizards, females are victims of sexual aggression by males. Credit: © Jean-François Le Galliard

Having too many males around can be bad news for lizards. Scientists have found that an excess of males can cause a small population of several dozen lizards to shrink because females are subjected to more male aggression during mating attempts, which reduces their survival and fertility.

If the finding applies generally, removing excess males could be a useful tactic to save small, isolated populations of endangered species, the scientists say.

Male aggression during sex occurs in many species. The male red-sided garter snake (Thamnophis sirtalis parietalis), for example, sometimes suffocates his partner during copulation. But this is the first study showing the effects this can have on population size, says Xavier Lambin, a behavioural ecologist at the University of Aberdeen who is not connected to the study. "Such effects have been speculated but not demonstrated before," Lambin says.

Rough sex

The researchers monitored the reproduction and survival of two groups of common lizards (Lacerta vivipara) kept in enclosures in a meadow that were covered by nets to stop the lizards being picked off by birds.

In one group, 78% of the adults were females, whereas, in the second group, the same proportion were males. After a year, the population with excess females had grown from 73 to 118. In contrast, the population with excess males had shrunk to just 35, and contained more males than before; females in the male-dominated group died four times more often and also had three or four instead of the usual five young per year.

The researchers speculate that the females died of stress caused by continuous male copulation attempts. The females housed with more males were subjected to more bouts of aggressive sex, says Jean-François Le Galliard, who led the research while at the Higher Teacher Training School in Paris, France, and at the University of Oslo, Norway.

Driven by desire

Male lizards normally bite females during copulation, he adds, leaving a patch of skin missing from their back. Surviving females in the male-dominated group of lizards had three times as much missing skin as their counterparts in the other group. The researchers have published their research in the Proceedings of the National Academy of Sciences¹.

The researchers calculate that because an excess of males led to greater female mortality, it would eventually drive the population to extinction. This may mean that certain rare species whose sex ratios are already skewed may find it harder to recover if males show aggression to females.

Female lizard. In common lizards, females are victims of sexual aggression by males. Credit: © Jean-François Le Galliard

Le Galliard says conservationists are already removing males to save some populations. For example, males of the endangered Hawaiian monk seal (Monachus schauinslandi) have a strong libido and a tendency to fight and kill females, reducing their species' chances of survival.

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Proceedings of the National Academy of Sciences