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Monkey eat, monkey sick: Macaques throw up surprising data
http://education.guardian.co.uk/higher/research/improbable/story/0,,2053163,00.html
by Marc Abrahams
Tuesday April 10, 2007
The Guardian

"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."

(Thanks to Eduardo B Ottoni for bringing this to my attention.)

· Marc Abrahams is editor of the bimonthly 
magazine Annals of Improbable Research and 
organiser of the Ig Nobel Prize