species, males are observed to groom females for longer durations and even allow females to not reciprocate. Since the mating demands of males are greater than those of offered by females, the females use social grooming as a method to choose mates and males use it to incite mating. Finally, kin selection is not as important a factor as friendship or mate preference when choosing a grooming mate as previously thought. In the 2018 Phelps et al. captive study on chimpanzees, it was seen that the animals remembered interactions that were 'successful' or 'unsuccessful' and used that as a basis to choose grooming mates; they chose grooming mates based on who would reciprocate rather than who would not. More importantly, if the delay between two chimpanzees grooming each other is very little, then the chimpanzees tend to 'time match', i.e. the second groomer grooms the first for the same amount of time that he/she was groomed. This 'episodic memory' requires a demanding amount of cognitive function and emotional recognition, and has been tested experimentally with respect to food preferences, where apes chose between tasty perishable and non-tasty non-perishable food at shorter and longer delays respectively after trying the food. Hence, apes can distinguish between different events that occurred at different times. Enforcing hierarchy and social structure In general, social grooming is an activity that is directed up hierarchy, i.e. a lower ranking individual grooms a higher ranking individual in the group. In meerkats, social grooming has been shown to carry the role of maintaining relationships that increase fitness. In this system, researchers have observed that dominant males receive more grooming while grooming others less, thereby indicating that less dominant males groom more dominant individuals to maintain relationships. In a study conducted on rhesus monkeys, it was seen that more dominant group members were 'stroked' more than they were 'picked at' when getting groomed, as compared to lower ranking group members. From a utilitarian standpoint, stroking is a less effective technique for grooming than picking, but it is construed as being a more affectionate gesture. Hence, grooming a higher ranking individual could be conducted in order to placate a potential aggressor and reduce tension. Moreover, individuals closer in rank tend to groom each other more reciprocally than individuals further apart in rank. Grooming networks in black crested gibbons have been proven to contribute greater social cohesion and stability. Groups of gibbons