Male orangutans don't commit infanticide. Why?

Photo credit: Zac Mills, the Wildlife Collective

Infanticide is a common reproductive strategy throughout the animal kingdom. It is seen in many species, from lions to gorillas to dolphins. By killing an unrelated infant, an adult male forces the mother to stop lactating and return to a fertile state much sooner. This allows him to sire his own offspring, ensuring his genes are passed on.

Yet, despite over 30 years of observation in Sumatra and Borneo, there are almost no reported cases of infanticide among wild orangutans. Orangutans have a dominant male (like gorillas), and compete for access to females (like chimpanzees). So, why is infanticide so rare? For this behaviour to be beneficial three conditions must be met.

The three conditions are: the male must not be the father, the female must return to fertility sooner, and the male must have future access to her. These conditions are rarely met in wild orangutans. Firstly, the dominant male does not “monopolise” access to females. Although females show a preference for flanged males, paternity is never certain.

Next, a female orangutan’s return to fertility may depend more on fruit availability than on lactation. During periods of low fruit availability, females experience high energetic stress. When a female’s “energetic condition” is poor, she typically cannot conceive, regardless of whether she is lactating or not. So, infanticide may not actually speed up fertility.

Lastly, orangutans are semi-solitary, so there is no guarantee that the male will meet the female again during ovulation. This means that the cost of infanticide outweighs the reproductive benefits. Because orangutans are so slow to reproduce, every infant counts. We must understand their complex social systems in order to protect wild orangutans.

Note: There has been one suspected instance of infanticide in wild orangutans. You can read more about this here: Possible Male Infanticide in Wild Orangutans and a Re-evaluation of Infanticide Risk – PMC