Soil-Eating by Grey Parrots in Cameroon: an answer to mineral deficiencies or toxins in the diet?

by Carolyn Sandra Bentley (Wells)

A Thesis Submitted to the Department of Biochemistry

In Partial Fulfillment of the Bachelor’s Degree



The University of Arizona

May 1999



I would like to begin by thanking Dr. Irene Pepperberg for offering the class “Do Animals Think?” my freshman year. Not only was it a wonderful practicum for the critical eye required in scientific study, but it introduced me to the interesting work conducted in her lab. Through independent study with the infamous (and stubborn) Alex, cute Grif, and crazy Kyo, I met Diana May to whom I am very grateful for providing a working hypothesis to and the chance to accompany her on her second trip to Central Africa to study these “soil-eating Greys”. And, of course, without the financial support of Irene, the UBRP program (with Carol Bender’s permission–thank you!), and the Honors Center, none of this would have been possible. A big thank you again goes to Irene and Diana for their support and for editing and revising those dreaded grant proposals.

I would like to thank the Cameroon Ministry of Scientific and Technical Research for permission to conduct this study and to Leonard Usongo, Tim Davenport, and Asonganyi of WWF-Cameroon for their logistical assistance, including transportation and plant identification. I am also most grateful to Drs. James Riley, Carolos Martinez del Rio, Jamie Gilardi, and Bill Mahaney for their assistance. Funding for myself was provided by the University of Arizona Undergraduate Biology Research Program and by grants from the University of Arizona Honors Center, to Diana May from American Museum of Natural History (Frank M. Chapman Award for Ornithology), WWF-U.S. Species Action Fund and Grey Parrot Program (grant number AC52), and to Irene Pepperberg from the National Science Foundation (grant number IBN-9603803).


Geophagy, or soil-eating, is widespread among vertebrates, including humans. Proposed hypotheses for uses of soil consumption are as a digestive aid, mineral supplementation, buffer of gastrointestinal pH, detoxifying agent, and gastrointestinal cytoprotection. By studying various species that engage in geophagy, we can gain better understanding of this behavior. May (1996) observed that Grey parrots (Psittacus erithacus) in a Congo basin forest engage in extensive ground foraging, possibly ingesting soil as well as plants. That the birds expose themselves to aerial predators during such foraging suggest this behavior has an adaptive function. We subsequently observed Grey parrot feeding behavior in Lobeke Reserve, southeastern Cameroon. We found evidence of geophagy and plant ingestion and collected representative soil and plant samples to analyze nutrient availability and other properties. Although Gilardi (1996) found that Peruvian parrot populations most likely consume clay as a neutralizing agent for toxic seeds in their diet, the high sand versus clay content of soil collected in Cameroon suggests that parrots foraging at our study site consume soil for alternative reasons. Soil analysis supports the soil’s possible role as a digestive aid and in mineral supplementation.

…the remainder is available upon request…