Times of plenty spell big trouble in paradise as 'hippie monkeys' turn into killers

Published on Friday 17 July 2009 20:17

IN THE Atlantic Forest on the east coast of Brazil live a bunch of furry, buff-coloured monkeys who munch leaves and hug each other a lot. The muriqui, also called the woolly spider monkey, is known as the world's most egalitarian and peaceful primate species.

But is it really? The cuddly reputation of this "hippie monkey" has had a battering after a gang of six were seen savagely attacking an old male, leaving him to bleed to death.

The observations, which were published in the American Journal of Primatology, demonstrate how animals' behaviour can change dramatically given slight changes in ecology.

The muriqui's peaceful reputation comes from the long-studied northern population. These monkeys live in disturbed forest where the main food source is leaves, but because leaves are everywhere the monkeys can munch them side by side without fighting.

And when it comes to sex, the monkeys are as polite as they are about food. When a female becomes sexually receptive, males queue up patiently for their turn to mate. But this doesn't mean there is no competition – instead of fighting, the guys get their sperm to do the running.

Muriquis have the largest testes relative to body weight of any primate, and they produce copious amounts of sperm. The male with the most virile sperm will be the one to impregnate the female.

It all sounds very civilised, but why did primatologist Mauricio Talebi, of the Federal University of Sao-Paulo-Diadema, witness a gruesome and savage attack by the monkeys at his study site?

Dr Talebi has been studying the behaviour of muriquis in the southern Atlantic Forest for several years, and in this area the monkeys live in more natural conditions than they do in the north, because the forest is untouched by human development.

There is a lot more fruit available to this population, the preferred food of the muriqui, and, because of this, the monkey group splits up into small units to feed during the day, since fruit grows in widely dispersed small clumps.

This makes it more difficult for males to monitor the whereabouts of sexually receptive females, leading to more general tension in these monkeys compared with those in the north, although fights are still very rare.

But observations one January morning showed that these monkeys, given the right circumstances, are capable of behaviour as violent and bloody as any other species.

On the morning in question, Dr Talebi and two of his co-workers contacted the study group at dawn. Suddenly, a commotion of barking began and the researchers witnessed seven of the adult males huddled together upside-down in a hanging embrace – muriquis can suspend themselves using their strong prehensile tails.

But this was no affectionate cuddle; after several minutes, six males simultaneously attacked the seventh, savagely biting him on his front, back, face and genitals while he bled profusely. One male held him up by his tail while the others attacked, and even a female with a clinging infant came to join in, biting the victim's testicles.

Eventually, he was dropped from high up in the canopy, and died on the ground after nearly an hour.

The researchers suggest the motivation for the attack was to reduce competition among males for mates; sperm competition isn't always enough. In spite of the fact the ratio of males to females is about 1:1, many females nursed infants or were pregnant, meaning the number of sexually receptive females was low, and it would benefit males to get rid of unwanted competitors.

That makes sense, but the question arises: why did the gang attack? Such behaviour is very rare in primates, and while the big baboon or macaque males, for example, fight viciously over females, it's generally every man for himself.

It seems the sort of attack the muriqui males indulged in could perhaps have been predicted from their social organisation. For a start, they live in what's known as a fission-fusion system, where the main group of monkeys split up into small groups to travel and feed. This means it is possible for gangs of males to come upon others travelling alone, where to attack him would pose little risk to themselves.

Secondly, while in most primate species the females stay in the group where they were born, and band together with their female kin to defend feeding territories against neighbours, in muriquis it's the males who stay at home and the females who leave for pastures new. This means males grow up with strong trusting relationships with their brothers and other male relatives, and it's this familiarity that allows them to form strong alliances against other males.

Male gang attacks have been seen only in a handful of primate species: chimpanzees, red colobus monkeys and the muriquis' close relative, the spider monkey, and all share this rare, male bonded social system.

The idea groups of male allies band together against outsiders could explain why it was an old male muriqui who was targeted in the attack. According to Professor Phyllis Lee, of Stirling University, a co-author on the paper, old males are likely to find it difficult to be part of the in-crowd. She says: "If you become a somewhat peripheral older male, unfamiliar to everyone, they have less of a relationship with you and therefore less of a reason to tolerate you, and then it may be that you are targeted."

If the idea of an ostracised, weak individual being attacked by a crowd seems familiar, it's not surprising, as this is also the way humans operate. In most human societies, it is usual for men to stay in the same area as their relatives and form bonds with familiar others. And gang violence is rife in our species, where men attack those in smaller, more vulnerable groups.

And it is this male philopatry that is likely to be at the root of human warfare, according to Filippo Aureli, of Liverpool John Moores University. Dr Aureli has witnessed gang male killing in spider monkeys in Mexico, as well as male parties patrolling community boundaries, probably on the lookout for foreign intruders. Such behaviour has also been seen in chimpanzees. When an interloper is detected, he may be dispatched in a gruesome and ferocious manner.

And there are obvious parallels to be drawn with human male behaviour.

"Primitive forms of warfare are very likely to have been coalitions of males going on raids into neighbouring communities," Dr Aureli says. "And the bonds that link them have been built up through the fact that they grow up together, do things together and trust one another."