No. 81: May-Jun 1992
Many biologists are convinced that apes, bears, cats, and dogs eat plants -- many of them obviously distasteful -- in order to medicate themselves for diseases and parasites. What also seems likely, according to K. Strier, of the University of Wisconsin, Madison, is that some monkeys regulate their fertility by the judicious consumption of certain plants. Going even farther, K. Glander, Duke University, suggests that howler monkeys control the sex of their offspring through their diets.
Glander divides howler monkey females into three groups. In the first are the high-ranking females that predominantly produce male offspring. This 'male-offspring' strategy favors these females because the males they produce tend to become dominant adults that will pass on more of the females' genes than would female offspring, who are limited in the number of infants they can engender in comparison to the males. Similar optimization strategies, according to Glander, induce middleranking females to produce mainly female progeny, and low-ranking females to birth almost all males.
These howler monkeys seem to control the sex of their offspring pharmologically by selecting certain plants to eat. These plants, in turn, control the electrical conditions in the females' reproductive tracts to either attract or repel sperm carrying the male Y-chromosomes, which are thought to carry different electrical charges than the X-carrying sperm! (Lewin, Roger; "What Monkeys Chew to Choose Their Children's Sex," New Scientist, p. 15, February 22, 1992. Also: Gibbons, Ann; "Plants of the Apes," Science, 255:921, 1992.)
Comments. It will take much more research to validate these startling assertions. We also have to ask how these instincts (or conscious, calculated strategies?) evolved. Since so many of the medicinal plants are distasteful, why would the monkeys eat them in the first place and thus learn, instinctively or consciously, their value in advancing the prospects for their genes?
Reference. The medicinal use of plants by mammals (more common than generally supposed) is cataloged in BMB21 in our catalog: Biological Anomalies: Mammals I. Ordering information here.