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Infanticide among mountain gorillas

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After 50 years of closely studying mountain gorillas, Fossey Fund staff and scientists have observed and learned many things about their behavior. One of the more difficult things to observe, though natural, is the not-uncommon occurrence of “infanticide.”

Infanticide can occur when a female gorilla transfers into a new group with an infant who was born in her previous group, or during an interaction in which an outside silverback interacts with a group in attempts to acquire females. Dian Fossey documented a number of cases during her years studying the mountain gorillas, and found it both difficult to accept yet understandable as a breeding strategy.

When a female with an infant transfers to a new group, the dominant silverback in the new group often seems to know that the offspring is not his own and in an attempt to get the female to be ready to breed again, kills the infant. This then makes the mother ready to mate with him and further his own genetic line.

Our staff have observed this many times, and studies show that the rate of infanticide among the mountain gorillas can be nearly 40 percent, though it goes up and down according to how stable the gorillas groups are at any given time.

Recent occurrences

Unfortunately, two infanticides were observed this week in the gorilla groups we monitor daily. On June 10, a 2-year-old infant in Musilikale’s group was killed. Our trackers did not see the interaction but evidence suggests it was a lone silverback, who also left the mother, Musilikale and several other group members with injuries. The next day, the infant of mother Kurinda was killed by the silverback in her new group. She had just recently joined this group after the dispersal of her former group, when its silverback, Giraneza, died.

Another female from Giraneza’s group – Pasika – is traveling alone with her infant, avoiding other groups and lone silverbacks, seemingly aware of the dangers to her infant. Infanticide was also the cause of the death of the gorilla twins that were born last year to mother Isaro.

Isaro and the twins

What we know

“Infant mortality is generally fairly high among mountain gorillas, at around xx percent, and a lot of this is due to infanticide,” says Dr. Tara Stoinski, Fossey Fund president & CEO/chief scientist.  Since dominant silverbacks are focused on protecting their groups, there is less incentive for them to protect infants who do not carry their genes, Stoinski adds. Therefore, it’s in their interest to have breeding opportunities with the mother as soon as possible, which isn’t possible while she still has a young infant. Unfortunately, the killing of the infant brings the mother back into estrous sooner and the dominant silverback can then take his opportunity to add more of his own offspring to the group.

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Infanticide is not unique to gorillas and has been documented in other primates, as well as carnivores, birds and other types of species.