Dr. Majolo and his student observed eight females with infants from two troops; "Flat face" and "Large". They found that four mothers from "Flat face" troop self-suckle themselves for a brief moment when their infant is still alive, possibly to improve milk flow when the infant change from one nipple to the other. All four of these females lost their infant due to predation or some unknown cause. They then observed these females self-suckling in bouts up to two minutes. Self-suckling was never observed between four females in "Large" troop before and after the death of an infant (only one monkey lost its infant in this troop).
Barbary macaque females from "Flat face" troop. Photo from BBC.
Majolo and McFarland think that self-suckling in Barbary macaque is cultural although they don't know why such behavior exists. It might be a response to make up for the energy they had invested in producing milk, help relieve engorged breasts, to help boost the females' immune system or even an emotional response to losing an infant.
"In humans and other species, breast-feeding reduces the stress through the release of prolactin. It is therefore possible that the self-suckling functions to reduce the stress generated by the loss of the infant ... It is interesting that we observed self-suckling in just one troop and not the other. This may indicate that self-suckling is a sort of cultural behavior. We will have to wait to see if self-suckling is consistently displayed by females in the same troop and not in the others", said Dr. Majolo.
Majolo, B. McFarland, R. 2009. Brief communication: Self-suckling in Barbary macaque (Macaca sylvanus) mothers before and after the death of their infant. American Journal of Physical Anthropology 140(2): 381-383. [10.1002/ajpa.21125]
Walker, M. 2009. Grieving monkeys drink own milk. BBC. Retrieved October 5, 2009, from http://news.bbc.co.uk/earth/hi/earth_news/newsid_8287000/8287774.stm