Eliana Zuckerman and Hannah Rodgers, students at Oberlin College, Oberlin, Ohio, came to Phana to carry out their “winter term project”. They were ‘looking for a way to learn about primate behavior and contribute to environmental education’.
After spending a little time getting to know the monkeys, they settled on a study of female grooming as a means of identifying high-ranking females.
Here is their study:
Social Rank and Grooming in Female Macaca fascicularis (Long-Tailed Macaques)
Eliana Zuckerman and Hannah Rodgers
Phana Macaque Project, Don Chao Poo Forest, Phana, Thailand
This study examined dominance in wild female long-tailed macaques through grooming and antagonism concerning access to food. In long-tailed macaque troops, grooming is an important social behavior that signifies social status. Access to scarce food can induce antagonistic encounters. The Don Chao Poo Forest in Phana, Thailand is home to over 1,000 long-tailed macaques. Data was collected by identifying a pair of female monkeys grooming, tossing rice in front of them, and labeling one monkey as dominant based on the successive behaviors. Our data supported the hypothesis that the monkey being groomed is more likely to win aggressive encounters and to access the food.
Long-tailed macaques (Macaca fascicularis) have the third largest geographic range among primates. They adapt well to the presence of humans and are considered sacred at some Buddhist temples, though they have also become problematic in some areas due to aggression and transmission of diseases. They are the most frequently observed primate in Thailand (Malaivijitnond and Hamada, 2008).
Long-tailed macaques organize into social groups with multiple males and multiple females. Their social interactions are based on strong despotic and nepotistic dominance hierarchies. Their highly complex social life allows them to maintain relationships and assess the ranks of other monkeys (Karimullah and Anuar, 2011). Some evidence shows that higher-ranking females are groomed more than subordinate females, who spend more time grooming others (Hambali et al., 2012).
Don Chao Poo park, created in 1973, is located 200 meters away from the village of Phana in the north east of Thailand. The park is home to a population of about 800 long-tailed macaques (do Carmo Jorge, 2013). Don Chao Poo is a protected forest of about 1 sq km that is a sanctuary to multiple troopes of long-tailed macaques. These macaques are accustomed human interaction as they have been filmed and researched frequently over the past 5 years. The members of the village and the visitors to the forest frequently feed monkeys grains, fruits, and vegetables (Whiting, 2015).
Long-tailed macaques were observed in open areas of the Don Chao Poo Forest, generally in the morning (8-10:30AM) or evening (5-6 PM), as monkeys were most likely to be grooming at these times. Two observers recorded 100 trials over 10 days between January 11 and January 24, 2016.
Focal animals were chosen by randomly picking a pair of grooming female monkeys. Infants and young juveniles were not picked. The time, general location, and noticeable descriptors were recorded for the monkey grooming (B) and the monkey being groomed (A). Next, an observer tossed about 5g of uncooked rice directly in front of the monkeys. The other observer recorded which animal began eating first and noted other interactions until either the rice was finished or one/both of the monkeys left. Either inconclusive, A dominant, or B dominant was noted for each trial.
The following situations were noted as inconclusive:
- Neither or both monkeys eat the rice, and are not scared away.
- Other monkeys scare away both A and B.
- One monkey is scared away, but later returns and eats.
- Both monkeys eat the rice, then one monkey leaves for an unknown reason.
The following situations were categorized as A dominant over B:
- A chases B away from the rice and eats.
- A eats the rice. B leaves or moves away without eating rice.
- Other monkeys come up and scare B away from the rice but not A.
The same situations were used to categorize B over A. Observers stopped recording trials if they were followed by other monkeys who chased away focal animals. Videos were taken of some trials to provide examples of each categorization.
Statistical significance was tested using a binomial distribution test only considering the conclusive trials, and was evaluated at the .05 alpha level.
This study found that the monkey being groomed (A) displayed dominant behaviors more often than the monkey grooming (B). The difference was statistically significant (p=.004). Out of the conclusive trials, A was noted as dominant 73% of the time and B was noted as dominant 27% of the time (Figure 1).
|First to begin eating
Differences between A and B, listed in increasing p-value.
This data supports the hypothesis that monkeys who are groomed more display more dominant behaviors. Though the difference between A and B for monkeys with infants was also statistically significant, the sample size (9 monkeys) is too small to generalize beyond this study. This study investigates how social dominance effects aggression and access to food in macaque societies, which could help in finding ways to minimize human/macaque conflict. This is particularly important in areas where long-tailed macaques regularly take or are given food from humans, such as in the Don Chao Poo forest.
Potential problems with this study could arise in observer bias and variations in the testing environment. Trials were observed at different times of day and among different troops. Some monkey pairs may have been tested more than once, and some pairs were more visible and more likely to be chosen as focal animals. Some data was taken after other people had fed the monkeys. Monkeys may have been full and therefore less likely to fight for or eat the rice, regardless of social rank.
Though every effort was made to only categorize unambiguous trials as “A” or “B,” some trials may have been miscategorized. Pre-study practice observing monkeys could help observers better determine what behaviors indicate dominance. Additionally, many trials categorized as “inconclusive” may have included dominant behavior too subtle for observers to recognize. Future studies could first mark individual monkeys, then note their grooming and feeding behaviors over a longer period of time. As humans and macaques have increased contact, new methods may be needed to ensure that humans and macaques can coexist without eradicating macaque populations or exposing humans to disease.
Do Carmo Jorge, F., 2013. Preliminary study of time-budget, home-range and diet of a long-tailed macaque troop in Phana, Thailand. Phana Macaque Project, 1-11.
Hambali, K., Ismail, A., Md-Zain, B. M., 2012. Daily activity budget of long-tailed macaques (Macaca fascicularis) in Kuala Selangor Nature Park. International Journal of Basic & Applied Sciences 12, 47-52.
Karimullah, Anuar, S., 2011. Social organization and mating system of Macaca Fascicularis (long tailed macaques). International Journal of Biology 3, 23-31.
Malaivijitnond, S., Hamada, Y., 2008. Current situation and status of long-tailed macaques (Macaca fascicularis) in Thailand. The Natural History Journal of Chulalongkorn University 8, 185-204.
Whiting, L., 2015. Thai Monkey Forest, https://thaimonkeyforest.wordpress.com/ (January 24, 2016).