Baby monkeys grow faster to avoid infanticide: study

Some baby monkeys develop faster than others in the same population to avoid being killed by adult males, a new study suggests. The research looked at infant development in wild ursine colobus monkeys.

"Infanticide occurs in many animals, including carnivores like lions and bears, rodents like mice, and in primates," said Iulia Badescu, a PhD candidate at the University of Toronto.

Black-and-white colobus includes several species of medium-sized monkeys found throughout equatorial Africa. Colobus babies are born pure white and their coat colour changes to grey after a few weeks before turning black-and-white between two and five months.

The researchers were intrigued by the fact that infants varied in the age at which their coats became grey, and then black and white. They also realised that these colour transitions were helpful to track the development of the infants, in a non-intrusive fashion.

Earlier research at the study site, Boabeng-Fiema Monkey Sanctuary in Ghana, established that some scenarios are more likely to lead to infanticide by males. Groups with multiple males, for example, have more instances of infanticide.

The team observed nine groups of ursine colobus monkeys in the wild over a period of eight years (2007 to 2014). "We found that infants facing a greater risk of infanticide developed faster than infants facing lesser risk," said Professor Pascale Sicotte from the University of Calgary.

However, the researchers suggest that infants and their mothers have more avenues to avoid infanticide and ensure survival than previously believed. They speculate that the mothers may invest more energy in their infants in a short time span, which might accelerate their growth.

When a male enters the group and starts behaving aggressively towards infants, he might focus on the younger, more vulnerable infants rather than those close to being independent juveniles whose mothers are probably starting to ovulate soon anyway.

The researchers also found that infant males developed faster than females. "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," said Badescu.

The researchers also considered other possibilities to explain faster development, including access to food. They found, however, that variation in group size, which would lead to more competition for food in larger groups, did not influence infant development. The study was published in the journal Animal Behaviour.