Differential Seed Survival and Passage Rates in Cows and Horses, Surrogate Pleistocene Dispersal Agents

Abstract

Two female Costa Rican range cattle were fed 961 and 1407 large hard dormant seeds of the guanacaste tree Enterolobium cyclocarpum. Of the 823 and 1111 hard dormant seeds defecated, 66 and 86% had emerged by the end of the fifth day and 87 and 96% had emerged by the end of the tenth day. By day 10, three range horses had defecated only 45, 50 and 71 % of the hard guanacaste seeds they would defecate. Compared with that of the cows, the daily distribution of seeds defecated by the horses had a proportionately lower peak, was proportionately much more skewed to the right and contained many days on which no seeds were defecated. The cows killed a maximum of 14-21 % of the seeds that they swallowed while the horses killed 44-83%. A lower proportion of the seeds defecated by the cows were soft (dead or alive) than was the case with the horses, one cow did not defecate heavier seeds at a different rate than it defecated lighter seeds, and one cow produced highly variable numbers of seeds per dung pile each day. Given the working hypothesis that the large caecum of the horse selectively takes large seeds out of the flow of digest a and later puts them back into it in pulses as it cleans the caecum, I hypothesize that the differences between the cow and horse in the manner of defecating guanacaste seeds is due to the much smaller caecum of the cow not acting in this manner. Additionally, a horse chews and sorts its food more carefully at first intake than does a cow with respect to large hard objects; this may be in part due to the danger to a horse of a caecum obstructed by such objects.