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In a Nicobar long-tailed macaque troop, does rank determine who grooms whom?

The Nicobar long-tailed macaque is a species of monkeys endemic to three islands of the Andaman and Nicobar archipelago. Found in troops that consist of an alpha male and female, there is a system of hierarchy, as is seen in all group living animals. While adults are further categorised as beta individuals and subordinates, juveniles and infants also form a part of the troop. In many social animals, like the macaques, the ranking of an individual determines the role they play on other activities within the group. In a recent study, researchers have investigated if an individual’s rank has any effect on how the members groom each other.

The study, published in the journal Primates, was carried out by researchers from the Sálim Ali Centre for Ornithology and Natural History, Bharathiar University, University of Mysore, Manipal Academy of Higher Education, Jawaharlal Nehru Centre for Advanced Scientific Research, and the University of Tennessee, Knoxville. It was carried out in the Great Nicobar Island and was funded by the Department of Science and Technology (DST) and the Science and Engineering Research Board (SERB).

Long-tailed macaques, also known as crab-eating macaques, are the second most widespread macaque species after the rhesus macaques. They are further divided into ten subspecies based on the type of their habitat and geographical location. One such subspecies is the Nicobar long-tailed macaque (Macaca fascicularis umbrosus) found in the Great Nicobar, Little Nicobar, and Katchal islands. Although the 2004 tsunami had a disastrous effect on the numbers of these monkeys, recent census studies show that they are now on the road to recovery. On the IUCN Red List, they are classified as ‘vulnerable’.

Social grooming in primates includes cleaning their bodies and maintaining hygiene. It is a way to bond and maintain a social structure. Previous studies on some species of monkeys have shown that some individuals groom others eyeing some benefits. These benefits may not be necessarily comparable to the efforts they put in grooming. Involving in social grooming reduces the time an individual spends on other vital activities like being vigilant against predators. However, some individuals would incur such costs to help others related to them by blood.

In the current study, which is the first to investigate grooming behaviour among the Nicobar long-tailed macaques, looked into three aspects. First, the patterns of sex-specific grooming among these monkeys, which includes female-female grooming and male-male grooming. Second, the effect of authority, troop structure, and the ability of an individual to control resources of grooming. Finally, the relationship between grooming and individuals’ tolerance of each other.

According to the biological market theory, an evolutionary concept that explains cooperation, individuals offer services to those who have more resources like food and better support during fights. Hence, the researchers of the study hypothesised that females would groom strong, superior females with higher rank, as they hold more access to food resources. Among males, however, the hierarchy depends on who has the most access to fertile females in the troop. Thus, the researchers assumed that subordinate males would groom the stronger, more dominating ones.

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