Strategies in Herbivory by Mammals: The Role of Plant Secondary Compounds
W. J. FREELAND AND DANIEL H. JANZEN
Department of Zoology, University of Michigan, Ann Arbor, Michigan 48104
Plants display a vast array of traits that give them some degree of protection from herbivores. One of the most conspicuous traits is the production of chemicals that injure herbivores. Each plant and plant population produces a relatively distinct set of defensive chemicals (so-called secondary compounds), and these chemical defenses affect different animals in different ways. Study of the effects of such chemical diversity on herbivores has been confined largely to animals small enough to spend their life, or an important developmental stage, on an individual plant or plant population (e.g., Dixson 1970; Ehrlich and Raven 1964; Feeny 1968a, 1969; Janzen 1969, 1971, 1973a, 1973b; Krieger et al. 1971; Rehr, Bell, et al. 1973; Rehr, Feeny, and Janzen 1973; Rehr, Janzen, and Feeny 1973; Root 1973; Rothschild 1972; Southwood 1972; van Emden 1972). Here we explore some effects of plant secondary compounds on feeding traits of animals that normally take food from many plants and plant populations during their lifetime. Our discussion is confined to mammals, but herbivorous birds, lizards, fish, and generalist insects such as leaf-cutter ants and some orthopterans should be amenable to analogous analysis.
Our intent is not to review the toxic effects of various secondary compounds or the biochemistry of particular detoxification mechanisms (e.g., Fowden et al. 1967; La Du et al. 1972; Levin 1971; McLean 1970; Parke 1968; Robinson 1968; Williams 1959). With respect to mammalian herbivores, we focus on dosage effects, the importance of "toxic" foods, the two primary types of detoxification, and detoxification capacities. From this we generate a set of hypotheses about feeding strategies of mammalian herbivores and evaluate them in terms of knowledge of mammalian feeding behavior.