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Does the smell of earth tempt you to eat it? Well, many primates do it!

When the raindrops touch the parched earth, the ‘earthy’ fragrance that emanates tempts many to eat the soil. The act of ‘geophagy’, or ‘eating the earth’, is not uncommon; many toddlers tend to do it even today. This behaviour is found not only in humans but in many primates and other vertebrates. In a recent study, researchers from the National Institute of Advanced Studies (NIAS), Bengaluru, Kyoto University, Japan, University of Colorado, and the Northwestern University, USA, have documented this behaviour among other primates. The study, published in the American Journal of Physical Anthropology, also tries to understand the causes of this strange preference.

Although there is no consensus on the underlying reasons for geophagy in animals, scientists have proposed various hypotheses. One of them is the ‘protection hypothesis’, which states that eating earth is an adaptive behaviour that protects an animal from different toxins, parasites and pathogens. These toxins may be present in plants eaten for food. The ‘supplementation hypothesis’, on the other hand, proposes that geophagy helps in gaining essential elements absent in the diet. A third hypothesis suggests that geophagy could be a non-adaptive behaviour and there is no benefit from eating earth.

The researchers of the current study systematically reviewed previous studies on geophagy in non-human primates and tried to validate the above hypotheses. They conclude that geophagy is a prevalent behaviour among non-human primates.

“We identified 287 accounts of geophagy among 136 species, adding 79 new primate species to the list of those considered in prior reviews”, say the authors about their findings.

So, why do non-human primates indulge in mud-eating? “Geophagy seems to be an adaptive behaviour among non-human primates. We found moderate support for the protection hypothesis, as well as modest evidence for the supplementation hypothesis”, say the authors. They confess that they cannot, at this point, pinpoint how exactly geophagy benefits non-human primates as the available data are irregular in quality and limited in quantity. “We would expect that the motivation does not have to be exclusively mineral supplementation or protection—and maybe both”, they add. They suggest that factors like the type of available soil, the diet of an animal and its physiological needs may play a role.

There are many implications of understanding this bizarre behaviour of eating soil, including conservation. “Understanding the causes of geophagy is important because it could offer insight into conservation initiatives needed to help animals cope with increasing dietary challenges due to habitat destruction, climate change, and the concomitant interspecies competition for finite dietary resources”, the authors say. They also hope such studies can help in maintaining the health of captive animals.

“Given the plausibility of geophagy for maintaining the health of both wild and captive populations, we urge further study and conservation of geophagy sites”, they conclude.