Accelerated Development

Risk of infanticide quickens monkeys' development.

 

Colobus monkeys with black-and-white coats

Stephanie Fox

The adult white-thighed colobus monkey, Colobus vellerosus, is a medium-sized, tree-dwelling primate with a distinct black-and-white coat. Infants, however, are born snow-white and transition to a gray coat before finally acquiring the adult coloring. But not all young monkeys transition through these “natal coat” colors at the same rate. Why do some individuals within this species develop faster than others?

A number of factors can influence how quickly infants develop, including predation risk and feeding competition. Another possible factor is infanticide pressure: in colobus monkeys, as in several other animal populations, adult males will kill the young of other males to induce females to mate with them. A research team led by Iulia Badescu, now an evolutionary anthropology doctoral candidate at the University of Toronto, and primatologist Pascale Sicotte at the University of Calgary, explored whether each of these factors might modulate the rate of colobus monkey development.

The researchers analyzed eight years of coat-color observations of nine colobus monkey groups living in central Ghana’s Boabeng-Fiema Monkey Sanctuary. The groups varied in size, and either had a single dominant male with multiple females or several males among many females.

Overall, Badescu and her team found that male infants developed faster than females. While predation or feeding competition could not yield this difference, infanticide could: young males pose potential future sexual competition to adult males, and are therefore more frequent targets of infanticide. Furthermore, young monkeys in unstable groups with multiple adult males generally grew faster. Risk of infanticide is higher in such multi-male groups, where power struggles are common.

Together, these results indicate that the threat of infanticide, rather than predation pressure or feeding competition, affects the rate at which colobus monkeys develop. Indeed, young colobus monkeys with black-and-white coats are less likely to be killed. According to Badescu, “a male may not try to kill an older infant because the mother may already be available to mate with him [while] finishing up the care on her previous offspring.” Exactly how development is hastened in young monkeys with higher infanticide risk remains unknown, though Badescu suspects a mix of maternal care and infant stress levels may be at play. (Animal Behaviour)