Research Reveals the Lethal Attacks of Critically Endangered Monkeys

The world’s first research on intergroup gang attacks in the critically endangered crested macaque has identified that this primate species engages in severe and lethal attacks against each other.

Gang attacks are described as instances in which multiple aggressors attack a single victim. It is amongst the most widespread forms of violence between human groups and has been documented in other social mammals, notably chimpanzees, hyenas, and meerkats.

Researchers at the University of Lincoln, UK, together with collaborators at Bogor Agricultural University (Indonesia), Liverpool John Moores University (UK), conducted the 13-year-long research project which analysed the behaviours of three different groups of macaques. The outcomes of the study have recently been published in the journal Animal Behaviour.

The groups of macaques were observed daily from dawn to dusk between 2006 and 2019 in the Tangkoko-Batuangus Nature Reserve in Indonesia.

The study compares the between-group aggressive behaviour in humans and other group-living primates – for example, multiple aggressors teaming up to attack, and often kill, an isolated enemy. The attacks showed similarities to those of chimpanzees and humans.

For example, researchers found that coalitionary bonds, hostility towards outgroup individuals and the ability to estimate numerical odds may trigger intergroup gang attacks – especially when the conditions favour an imbalance of power between victims and attackers.

The study also outlines that, in crested macaques, adult females are more likely than adult males to gang attack members of other groups – one reason given might be that of defending resources such as food.

It is also suggested that the primates can effectively estimate numerical odds when attacking a victim – with aggressor groups frequently outnumbering their target. The victims of attacks were mostly adult females or infants.

Dr Bonaventura Majolo, Professor of Social Evolution at the University of Lincoln, said “Our study suggests that lethal gang attacks may occur in species displaying coalitionary bonds and intergroup hostility. As such, lethal gang attacks may be more common in group-living animals than previously thought.

“Although gang attacks in crested macaques share some similarities with attacks in other species, including raids in chimpanzees and humans, the cognitive skills and mechanisms involved may differ between species.

“We hope these findings will be useful in comparative studies on how intergroup relationships vary depending on socioecological factors, for example resource abundance and social structure.”

Dr Laura Martínez-Íñigo , former PhD student at the University of Lincoln who led this research, said: “So far, species in which gang attacks between groups have been observed share some socioecological features such as territoriality, social organisation, cooperative breeding or coalitionary bonds. Often, they have groups that split (fission) and merge (fusion) flexibly throughout the day in subgroups of different sizes and compositions. This so-called fission-fusion social organisation was thought to be necessary for gang attacks between groups because it facilitates encounters in which one group might be much bigger than the other.

“However, the scarcity of data has made it challenging to determine whether one or all of these socioecological features is necessary and sufficient to drive the evolution of gang attacks.”

“Our findings show that intergroup gang attacks occur in species without fission-fusion social organisation and in fact may arise from widespread features of social animals.”

Dr Martínez- Íñigo is a Scientific Advisor in the Guinean branch of the Wild Chimpanzee Foundation, where her team are establishing the Moyen-Bafing National Park, located in the Fouta Djallon region.

The park is home to approximately 5,000 western chimpanzees (being the largest population known of the subspecies) as well as lions, leopards, hippopotamus, and buffalos.

The full study can be read here: https://lncn.ac/macaques