By Matt Walker
Editor, Earth News
"Flat face" troop macaques
Female monkeys in Morocco have been observed suckling themselves, drinking their own milk.
The behaviour, rarely recorded by scientists, may have been exaggerated by grief, as each monkey did it more often after the death of her infant.
By suckling their own milk, the female monkeys may be alleviating stress or boosting their immune systems, scientists speculate.
Whatever the cause, the behaviour appears to be culturally learnt.
Dr Bonaventura Majolo and his PhD student Richard McFarland noticed the unusual behaviour while studying wild barbary macaques (Macaca sylvanus) living in two troops in the Atlas Mountains of Morocco.
The barbary macaque is the only macaque species living outside of Asia.
Together with two field assistants, Dr Majolo and Mr McFarland were studying the behaviour of the troops, one dubbed "Flat face" and the other "Large" troop.
"We observed self-suckling by chance while we were with the monkeys collecting data for other projects," says Dr Majolo, who has published details of the observations in the American Journal of Physical Anthropology.
Death in the family
The scientists first saw four females within the "Flat-face" troop engage in self-suckling while their infants were still alive.
While each female was suckling her infant on one nipple, she would briefly place her other nipple in her own mouth, leaving it there for a second or two. She would usually then move her infant onto this second nipple.
The scientists suspect this behaviour helps stimulate or improve the flow of milk, helping the infant to feed.
However, soon after, all four females lost their infants. Three disappeared while one female macaque, named "Jessica", was seen carrying her dead infant for six hours before finally moving on.
Difficult times ahead
After the deaths of their offspring, the four female macaques engaged in much longer bouts of self-suckling, lasting up to two minutes at a time.
Jessica self-suckled for 106 straight days after her infant's death.
Self-suckling is an extremely unusual behaviour.
Before now, scientists have only published accounts of self-suckling in chimpanzees and feral goats.
"But no study has observed this behaviour with such frequency as in our report, or linked self-suckling to an infant's death," says Dr Majolo.
The behaviour appears to be cultural, as it only occurred in the "Flat face" troop. A female macaque which lost her infant in the "Large" troop at the same time didn't engage in similar behaviour.
The scientists can only speculate as to the cause.
They suggest that it could be a way for the females to recoup the energy they'd invested in producing the milk in the first place.
Or it could help relieve engorged breasts, or help boost the females' immune systems.
"The most interesting explanation, in my opinion," says Dr Majolo, views self-suckling as related to the emotional consequences of the loss of 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."
Dr Majolo now hopes to study the monkey troops again during their next mating season.
"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 behaviour."
"We will have to wait to see if self-suckling is consistently displayed by females in the same troop and not in the others."