The rate at which infant monkeys develop is affected by their risk of infanticide, a new study suggests.
Wild ursine colobus monkeys (Colobus vellerosus) are found throughout equatorial Africa, and have a characteristic black and white furry coat. The colour of this coat changes as an infant matures, making it a reliable measure of development.
At first it is white, turning grey after just a few weeks, and becoming black and white within 2 to 5 months. But infant monkeys of the same population develop their coats at different rates. Why would this be?
Researchers from the University of Toronto and the University of Calgary set about to find out, taking into account group size, food availability and risk of infanticide.
Infanticide occurs when new males enter a group and kill the infants, an act that brings females back into oestrus, meaning that they are reproductively receptive. This allows infanticidal males to mate with the females more quickly, passing on their genes to offspring of their own. Infanticide tends to focus mainly on very new infants, whose mothers are furthest away from oestrus, or males, who, when fully grown, will become reproductive competitors.
“Infant males are at greater risk of infanticidal attacks because killing a male infant not only gives reproductive access to the mother, but also eliminates a future sexual rival for the infanticidal male and his future offspring,” explains Iulia Bădescu, the lead author of the study.
The researchers observed the behaviour of 9 different groups of monkeys over a period of 8 years. They found that factors such as group size, competition, and access to food had no effect on how their fur developed, whereas their risk of infanticide did.
However, despite this interesting result, it is still unclear exactly how mothers and infants determine whether they are at risk, and how they speed up development. “These are all good questions for future research,” says Bădescu.
Perhaps the mother becomes aware of the threat by interpreting the new male’s body language, or through hormonal cues. Scientists have suggested that females aware of a threat may produce higher concentrations of stress hormones, which are passed on to their young through lactation, triggering faster development. At the moment this is just theory, but further research should unravel the answers.
Iulia Bădescu, Eva C. Wikberg, Lisa J. MacDonald, Stephanie A. Fox, Josie V. Vayro, Angela Crotty, Pascale Sicotte. Infanticide pressure accelerates infant development in a wild primate. Animal Behaviour, 2016; 114: 231 DOI: 10.1016/j.anbehav.2016.02.013
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