Guanacaste Tree Seed-Swallowing by Costa Rican Range Horses

Guanacaste tree fruits (Leguminosae: Enterolobium cyclocarpum) were fed by hand to seven range horses (Equidae: Equus caballus) in Santa Rosa National Park, Guanacaste Province, Costa Rica. Guanacaste seeds are large and heavy (0.2—0.8 mL and 300—1000 mg) and easily perceived by the horse. The seeds swallowed ranged from 25.1% to 61.5% for different horses. Of the seven horses, four swallowed the same percentage of the seeds in a fruit, over a range in size from five to 18 seeds per fruit. The other three horses departed slightly from a constant percentage of seeds swallowed per fruit. A guanacaste fruit is constructed such that the ratio of fruit to seed stays about the same over a wide range of numbers of seeds per fruit, and therefore this response by the horses is not startling. Why seeds are spit out is discussed. While seed spitting undoubtedly lowers the quality of horses as guanacaste seed dispersal agents, the high frequency of seed spitting does not mean that the horse is necessarily a "poor" dispersal agent.