Saturday, 20 June 2009

Year of the Gorilla 5: The dark side of gorillas

Gorillas. Peaceful, placid vegetarians, wanting only to be left alone for a photo op with David Attenborough. It is quite impossible that these gentle giants could have a nasty streak.

Yeah. Right.

Actually, all the great apes have a violent streak. Humans of course are the most creative in this respect, but all our close relatives can be killers too. Chimpanzees go in for tribal warfare, orang-utans go for sexual assault and many wild bonobos are missing fingers from fights.

With gorillas, violence takes the form of infanticide. This is not just an occasional accident, but a deliberately pursued policy of attacks on infants, especially by solitary silverbacks, with the goal of persuading females to join new groups.

The rationale behind the attacks seems to go like this:

1. A female with an infant is unlikely to leave the group she is in unless the silverback dies.

2. Silverbacks are hard to kill, especially if they have a group of breeding females to help them.

3. If a female loses an infant, she is likely to abandon the silverback who failed to protect her baby.

4. Even if the male whose group she joins is the one who killed her baby in the first place

Even if a female without a baby joins a new male, if she gives birth soon after joining, there is a good chance that the silverback of the group may kill her baby, despite what are usually vigorous attempts at defence.

This kind of behaviour is widespread in many animals with a reproductive system of a single (or small number) of males and a larger group of females, especially when raising a newborn to independence takes a large part of an adults life. By killing unrelated infants, the male increases the chance of his own offspring being raised before he is replaced in his turn.

So far at least, infanticide has mostly been observed in Mountain gorillas, where infanticide causes about 37% of infant deaths (total deaths in the first year of life run at about 34%). Putting these two numbers together, it suggests that 1 in 10 baby Mountain gorillas are killed by their own kind before they are one year old.

In Western gorillas, infanticide (so far) seems to be less common. This is probably because, as mentioned in a previous post, Western gorillas seem to live in extended clans of related male family groups, and any unattached males are likely to be related to any infants they encounter.

The heavy poaching that Mountain gorillas have experienced has probably made matters worse. Silverbacks are the first to be targeted by poachers, and the result has been a breakdown of gorilla society where unrelated males, fleeing poachers, have moved into areas where they are unknown and any infants are not related to them.

All this has implications for gorilla conservation, especially the release of male orphans into the wild. Release of human-raised or captive bred animals is problematic in any case, because of the risk of exposure of wild animals to human diseases caught in captivity and the difficulty in teaching true independence to an animal which has not had the opportunity to learn how to survive from its mother. Releasing even successfully rehabilitated males into a social environment where they do not know the males in the wild groups they encounter might well do more harm than good, resulting in increased attacks on infants by the new males and a negative impact on the wild numbers.

In future posts, I will discuss how orphaned gorillas are cared for and what can be done to give them at least near-normal lives.
Picture - Salome and Komale, Bristol Zoo gorillas

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