Dispersal of Small Seeds by Big Herbivores: Foliage is the Fruit

Abstract

I hypothesize that, for a number of species of small-seeded herbaceous plants, a normal and selected for mode of seed dispersal was through consumption of the seeds by large herbivores while they were eating the foliage ofthe parent plants. I also hypothesize that the big herbivores ate the plants, at least in part, because of plant traits selected for through the value to the plants of having their seeds thus dispersed. If the vegetative portions of a herbaceous parent function ecologically as an attractive fruit as well as photosynthetic machinery, numerous complications are introduced into the study of the function of secondary compounds and other aspects of the biology of herbaceous plants. Here I introduce the hypothesis and discuss how large mammals disperse small seeds. It is commonplace to encounter herbaceous plant seeds in, and seedlings growing from, the dung of livestock (cows, horses, sheep) and the larger Old World herbivorous mammals in their native habitats (e.g., oryx, elephant, rhino, hippopotamus; Adams 1907; Brahmachary et al. 1974; Ridley 1930; Kurosaki and Iizumi 1960). Survival of weed seeds and pasture grass seeds during passage through livestock has been the subject of considerable experimentation and observation (Adams 1907; Wilson and Hennessy 1977; Courtney 1973; Milne 1915; Burton and Andrews 1948; Kurosaki and Iizumi 1960; Wicklow and Zak 1983; Ozer 1979; Sugawara and Iizumi 1960; Watkin and Clements 1978; Atkeson et al. 1934; Sarukhan 1974; Harmon and Keirn 1934; Beach 1909; Yamada and Kawaguchi 1972; Yamada et al. 1972; Takabayashi et al. 1979; Piggin 1978; Korsmo 1911; Oswald 1908; Dore and Raymond 1942; Ridley 1930). These studies show unambiguously that: (1) the small seeds of both herbaceous dicots and grasses consumed as part of grazed or fed fodder can survive the trip through the animal gut and germinate directly in the dung or soil where the dung was deposited. Wicklow and Zak (1983) have shown that grass seeds can survive the trip through pronghorn antelope (Antilocapra) as well as through cows and germinate from dung as much as 30 mo old, cow parsnip seeds can survive the trip through grizzly bears (Applegate et al. 1979), and wheat seeds can survive passage through emus (Davies 1978).