"Researchers have given little consideration to vomiting in non-human primates." Quite so. A new report called Vomiting in Wild Bonnet Macaques points that out, and tries to remedy the deficiency.
Elizabeth Johnson, Eric Hill and Matthew Cooper published their study in the International Journal of Primatology. Johnson is at Oglethorpe University in Atlanta, Georgia. Hill is at Arizona State University, and Cooper at Georgia State University.
They start with a fond look back at the work of earlier experts. The consensus view, they say, is that vomiting "is a theoretically complex behaviour that to date lacks a comprehensive explanation".
Johnson, Hill and Cooper spent time with macaques, carefully noting when each individual animal vomited and whether it then reingested (for that is the technical term) whatever came up. All told, the scientists compiled "both quantitative and qualitative data on observations of 163 instances of vomiting from two groups of bonnet macaques in southern India". They used this data to "establish a conservative rate of vomiting in free-ranging macaques".
The rate is 0.0042 vomits per individual per hour. That's the conservatively high estimate, using data gathered by watching macaques who live near a temple on Chamundi Hill, a forested outcrop near Mysore in Karnataka. But it is not the whole story. Another group of macaques lives in the Indira Gandhi Wildlife Sanctuary, in Anaimalai Hills, Tamil Nadu. These forest-dwellers vomit at a different rate from their temple cousins: 0.0028 vomits per macaque per hour.
The scientists observed closely and keenly. Here is a typical passage from their report: "Only one adult female in the forest showed interest in another macaque's vomit; she twice smelled the mouth of an adult female. During observations at the temple, we saw 20 different individuals show interest in another's vomit on 21 occasions. Ten of the individuals were successful in eating some of it on 11 occasions. Of the individuals that ate or tasted another monkey's vomit, two were adult females, two were adult males, three were juvenile females, and three were infants."
The study builds to a thrilling conclusion. The researchers explain what, to them, is a central mystery about vomiting in wild bonnet macaques. Why, they ask, don't the macaques simply vomit and walk away? Why do they immediately "reingest" the vomit?
Earlier scientists seem not to have noticed this mystery or, if they did notice, to have offered a good explanation.
The key, according to Johnson, Hill and Cooper, lies in a simple fact. Macaques have spacious pouches in their cheeks. Johnson, Hill and Cooper apply some logic. "We suggest that the tendency to hoard food in their cheek pouches explains why they reingested the vomit."
The study concludes with a modest statement: "Our data offer insight into a normal, but largely ignored, behaviour of cercopithecines."