Strategies in Herbivory by Mammals: The Role of Plant Secondary Compounds

Abstract

Large herbivores must select food from a wide variety of plant parts, species, and strains. These differ in nutritional value (protein, carbohydrate, etc.), toughness, spinosity, etc. Even greater differences are found in types and concentrations of secondary compounds. Every plant produces its own set of secondary chemical compounds, which to a great extent are unique to it or its species. Ingestion of natural concentrations of these compounds can lead to either death or severe physiological impairment. The ubiquitous nature of these compounds would make herbivory impossible unless animals had mechanisms for degrading and excreting them. An animal displaying no obvious symptoms of poisoning is not free of the problem of ridding itself of toxic compounds; if it is eating plants, it almost certainly has this problem. Herbivores are capable of detoxifying and eliminating secondary compounds. Limitations of these mechanisms force mammalian herbivores to consume a variety of plant foods at any one time, to treat new foods with caution, to ingest small amounts on the first encounter, and to sample food continuously. Selection of foods is based on learning in response to adverse internal physiological effects, and herbivores probably cannot predict these from the smell or taste of new foods. Herbivores prefer to eat familiar foods and can seek out and consume foods that rectify specific nutritional deficiencies induced by detoxification. They should prefer to feed on foods that con…