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Complex food-processing techniques by gorillas, chimpanzees, and orangutans have allowed comparisons of complex hierarchical cognition between great apes and humans. Here, we analyse preliminary observations of free-ranging long-tailed macaques (Macaca fascicularis) (n = 3) in Thailand processing Opuntia sp. cactus fruits. From our observations, we suggest that there is potential to extend the analyses of hierarchical cognition to Old World monkeys. We found that the macaques used six behavioural sequences to obtain Opuntia fruits, remove irritant hairs from the skin of the fruits, and break open, and consume the fruits, each a unique combination of 17 action elements. Removing irritant hairs involved abrading fruits on a sand or rock substrate, and washing fruit in water. The behavioural sequences that macaques use to process Opuntia potentially show features of hierarchical organisation described in the leaf-processing behaviours of great apes. Our observations highlight the need for closer study of complex food-processing behaviour in monkeys to better understand the organisational capacities involved.

We collected data on grooming, proximity, and aggression in long-tailed macaques (Macaca fascicularis) in Kalimantan, Indonesia. We used this data to study how grooming influenced a receiver's (B) behavior towards the bout's initiator (A). In our first analysis, post-grooming samples were collected after A groomed B. These were compared to matched-control samples of similar conditions but A had not previously groomed B. This comparison was performed on 26 individuals (16 female, 3 male, 7 immature) and tested whether A's initial act of grooming increased the pair's time in proximity and the amount of time B groomed A. We also tested if A's grooming decreased B's aggression towards A per time in proximity. Rates of B--A aggression per time in proximity with A for 39 individuals (18 female, 5 male, 16 immature) were compared between post-grooming and focal sample data. Finally, we studied 248 grooming bouts to test if the first two grooming episodes were time matched. We assessed the influence of age, sex, rank and inferred kinship on time matching, and controlled for individual variation and tendency to groom using a general linear mixed model. Our results showed that A--B grooming acted to increase B--A grooming and the pair's proximity, while lowering B--A aggression. Despite these effects, episodes in grooming bouts were generally not matched, except weakly among similar partners (i.e., female pairs and immature pairs). Grooming imbalance was greatest across age-sex class (i.e., male-female and adult-immature pairs). In similar pairs, grooming duration was skewed in favor of high-ranking individuals. We conclude grooming established tolerance and increased the likelihood that grooming reciprocation would occur, but grooming durations were not typically matched within bouts. Lack of time matching may be the result of grooming that is performed to coordinate interchanges of other social services.