Male infanticide among mammals is quite common, and infants of either sex can be killed. But a new study of spider monkeys has found that, in this species, only the male infants are targeted. What's more, the infants were all from the monkeys' own social groups.

Over 119 species of mammal are known to commit infanticide, including about 35 primate species. But in spider monkeys infanticide has only been previously recorded five times.

A new long-term field project has now recorded this behaviour again in three separate suspected incidents. The researchers now believe that it may be more common than was originally thought.

The report in Primate Journal proposes that males commit infanticide due to sexual competition between the males in the group. If there are too many new males, the competition would only get worse.

The study also suggests that their observations could now explain the female-biased sex ratios that spider monkeys groups are known to have.

Instances of infanticide are extremely difficult to capture and require detailed and long-term field studies. Researchers need to know how each animal relates to the rest of the group to assess the significance of family ties.

A male was observed being violent towards a mother with a young baby

In the current research, Sarah Alvarez from the Complutense University of Madrid in Spain and colleagues studied three different species of spider monkey in Ecuador, Colombia and Belize: Ateles belzebuth, A. hybridus and A. geoffroyi. These groups were all used to the presence of researchers and all animals were known to the team.

In one case, a male was observed being violent towards a mother, named Kuao, with a young baby. Although no aggression was observed towards the child at that time, seven days later he was found crying on the forest floor with many cuts and open wounds. A day later he died from his injuries. The killer was never identified.

Nine months later, Kuao was spotted with a new infant, suggesting she conceived shortly after the infanticide.

In another instance, a baby spider monkey was left for dead after a violent attack. The mother and one of the other males had injuries, suggesting a fight had taken place. Another female adult was then seen hugging a motherless child.

This baby was later saved by the research team and reunited with his mother. If they had not intervened, they say, his injuries were so severe he would have died.

The researchers say that attacks from other predators "cannot be discounted" but that the wounds they observed were consistent with other spider monkey attacks.

Infanticide also opens up the female to mating again. A female spider monkey typically only gives birth every three years, but this is reduced to 9-10 months if her child dies.

That is in line with existing theories about infanticide. In another recent study published in Science, researchers looked at the evolutionary reasons why males kill infants, in a comprehensive report assessing 260 mammal species, including the known 119 species this occurs in. The team concluded that in most cases, male infanticide occurs when males have to compete to reproduce