Alkaloids and Herbivorous Vertebrates

While most insects which feed on plants are monophagous or oligopha-gous, i.e., they feed only on one or a few species of plants, the vertebrates are usually polyphagous. There are, of course, some exceptions, such as the Australian koala, but the majority of herbivorous mammals feed on a broad spectrum of plants. For 65 million years, the only truly herbivorous vertebrates have been the mammals. The Mesozoic reptiles disappeared following the mesophytic flora. Birds, while they feed on seeds and berries, seldom do so on leaves, and they frequently use insects, in addition to plant parts, as a food source (Emlen, 1971). While a single plant can be a host for hundreds of insect larvae, hundreds of plants comprise a daily menu for a 

In North America in the western ranges the situation is similar. Keeler (1969, 1975) describes the hazards of alkaloid toxicoses (see p. 110). He estimates that perhaps tens of thousands of livestock deaths occur yearly in the western United States from toxic plant ingestion and that five or six times that many cases of moderate toxicoses occur. 

It is noteworthy that domesticated animals much more frequently commit mistakes than wild ones. In unpublished experiments of ours, white laboratory mice and freshly captured meadow voles Microtus arvalis) were given a choice of cakes, some of which had added gramine. The white mice preferred the alkaloid-free cakes but from time to time also ate these with the alkaloid. The meadow voles never touched the poisoned food. 

The introduction of some new species of grasses into North America and Australia caused losses in animals. The grass species involved were Phalaris tuberosa and P. arundinacea, both accumulating indole derivatives. P. tuberosa contains predominantly A,A-dimethyltryptamine, 5-methoxy-Ar,AT-dimethyltryptamine, and bufotenine, while P. arundinacea contains mainly gramine and hordenine. The tall fescue, Festuca arundinacea, introduced mostly into Missouri, the Dakotas, and Minnesota has other types of alkaloids, loline and perloline. Although the level of these 

In summary, the insects probably have had a great influence on the establishment of the alkaloid character in some plant families, and the herbivorous mammals are responsible for preserving this character furthermore, even if the alkaloid, due to simultaneous evolution of insects, can no longer defend the plant from specialized vermin, it is still preventing predation by unadapted herbivores. 

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