The natural world is sometimes cruel and heartless, and infanticide isn’t as rare as you might think. Polar bears, for example, sometimes turn on and eat the young cubs of others. Now, a new piece of research has highlighted the plight of young male black-and-white colobus monkeys, who are sometimes hounded and murdered by older males looking to control the reproductive females in the area.
The study, published in the journal Animal Behavior, reveals that young males are somehow induced to growing quicker, in order to ward off the aggressive, killer intentions of any approaching, suspicious males. Those that grow faster tend to survive into adulthood, whereas those that don’t are likely to be wiped out.
Black-and-white colobus monkeys are a group of Old World primates containing at least five different species. Generally speaking, they live in territorial troops of around nine individuals, consisting of a single adult male with several females and their offspring. When baby monkeys are born, they’re completely white, but their coat becomes gray within a few weeks. Between two and five months, it becomes a mixture of black and white.
For this new study, the team of researchers wanted to know why there was such a variation in when babies attain their “adult” black and white coating. Essentially, they wanted to uncover why different babies matured at different rates.
A baby colobus with its white coat. If this is a male, it’ll be at risk from being killed by an aggressive older male. Stephanie Fox
In order to investigate this, they spent eight years following nine separate groups of ursine colobus monkeys in the wild. These monkeys typically face many threats, including from other predators, habitat destruction, and poachers hunting for bushmeat, but the young also have to contend with dangerous males within their own species: Groups with multiple males are more likely to lead to infanticide.
“Typically, an adult male kills an infant sired by another male so that he can mate with the mother and sire his own infants with her,” Iulia Bădescu, a doctoral researcher in evolutionary anthropology at the University of Toronto and lead author of the study, said in a statement. When an older male attempts this, he seeks out the most vulnerable male babies rather than the perhaps more independent juveniles.
The researchers noted – by carefully observing the development of their coat coloration – that baby males in infanticide-prone groups grew into their adulthood quicker than those who were in less under threat groups. Infant females, who are generally at a far lower risk of being killed in this way, grew at a noticeably slower rate within these at-risk groups.
Their analysis ruled out both pressure from predation or from a sudden change in food resources, meaning that the threat of infanticide actually seemed to cause baby males to rapidly grow. This means that the mother must be rearing the male babies under threat in such a way that they do mature faster.
When the mother senses the threat of infanticide is upon her young – perhaps through social or hormonal cues from the aggressive male – she may experience an increase in stress hormones. These will filter down through her milk, which when taken up by the male babies may provoke them to develop into adults quicker than they normally would. The researchers think that the infants’ own stress hormones may also help accelerate their growth.