Are there merits to munching mud? Some monkeys seem to go out of their way to add it to their standard diet of leaves, fruits and insects. In Amazonian Peru, at least, one primate species seems to use mud medicinally, possibly to prevent stomach upsets before they even begin.
Why some monkeys eat mud has been much debated, with the main options being to kill parasites, as a mineral supplement or to cure stomach upsets.
“Many previous reports involved just a few sightings, or come from accidental encounters,” explains Dara Adams at the Ohio State University in Columbus, who led the study. “We were really focused on answering this question, and that seems to have made the difference.”
The team studied Rylands’ bald-faced saki monkey (Pithecia rylandsi), a rainforest canopy specialist. With thick grey fur, it has a similar shaggy appearance and size to a Maine Coon cat. The sakis’ treetop lifestyle means they did not get their mud from the ground, but from the nest casings of tree-living termites.
“In 1125 hours, we recorded 76 feeding bouts at 26 termite mounds,” says team member Jennifer Rehg, from Southern Illinois University Edwardsville. “They ate mound casing – they weren’t focusing on the termites. They even ate inactive mounds.”
But why termite mounds? Enter Mrinalini Watsa from Washington University in St Louis, Missouri, who conducted a detailed analysis of the mud the sakis were eating.
“The important thing is that this isn’t just any mud, it’s termite-processed mud,” she explains. “Compared to topsoil, it has a higher carbon and clay content.”
Watsa adds that the mud can also absorb cations – positively charged particles – so could mop up potentially toxic metal ones from other substances in the monkeys’ diet. Furthermore, the mud is rich in organic carbon and kaolin clays, both of which excel at absorbing tannins and other plant toxins.
“This was key,” says Adams. “A large part of a saki’s diet is seeds from unripe fruit, and these are packed with toxic chemicals. Combined, the cation-capacity, clay and carbon absorb many of these, allowing these monkeys to have this unusual diet.”
The region’s other major seed eaters – macaws and parrots – fly to clay-rich soils on riverside cliffs for stomach-calming intestinal mudbaths. Without that option, the sakis have found a solution closer to their treetop homes.