Throughout our days in the field with the monkeys we often see them in this position. Monkeys are famous for their grooming behaviour and it is something they take the time to do no matter the time of year or the weather conditions. Experimental studies have shown that grooming in monkeys reduces anxiety and also provides hygienic benefits in terms of dirt and parasite removal. It is also a factor in contributing to their social bonds, especially with kin. 

Best wishes

Rich