Digestive Seed Predation by a Costa Rican Baird's Tapir

The large ungerminated seeds of guanacaste (Enterolobium cyclocarpum) and carao trees (Cassia grandis), and plastic buttons, were fed to a captive adult male Costa Rican Baird's tapir (Tapirus bairdi) in artificial fruits. Seed survival during gastrointestinal passage and seed passage rate was studied. The tapir's digestive processes killed 78 percent of the guanacaste seeds and all of the carao seeds, despite their hard seed coats, apparently because they germinated and thereby become suspectible to digestive fluids. Guanacaste seeds that survived required an average of 15.3 days to make the trip through the tapir; the dead carao seeds required an average of 10.3 days and the two sizes of buttons required an average of 8.1 days (730 mg) and 10.3 days (360 mg) to make the passage. The tapir is clearly a seed predator of guanacaste seeds as well as a potential dispersal agent. Which of these functions is more important depends on the unknown behavioral interaction between the tree and the tapir. VERTEBRATE SEED PREDATORS KILL SEEDS by chipping them with a beak or incisors, breaking and grinding them with molars or a gizzard, and by direct chemical digestion. Very hard seeds and nuts, and especially legume seeds, are generally viewed as immune to chemical digestion. Hard seeds and nuts which are dispersed by complete passage through the digestive tract are often encountered in the dung of large herbivorous mammals. It is commonplace to assume that the number found in the dung approxi-mate the number of intact seeds that were swallowed. This assumption may be reasonable for certain species of seeds and animals, but here I show that it is not valid for Baird's tapir (Tapirus bairdi) and the large hard seeds of two legumes, guanacaste (Enterolobium cyclocarpum) and carao (Cassia grandis). MATERIALS AND METHODS I used the captive adult male Baird's tapir at Finca La Pacifica (7 km north of Caiias, Guanacaste Province, Costa Rica) for experiments. Born in late 1970 and acquired by Finca La Pacifica a few months later, it ranged free around the ranch house, eating milk, table scraps, and wild vegetation. At about two years of age, it was placed in a wood-fenced corral approximately 20 by 20 m in area. It was seven years old at the time of the experiments and suffered only occasional indispositions with tick infestations and foot fungal attacks, both of which were cleared up by treating with the chemicals used to cure horses and cows of the same diseases. It seemed well adjusted to corral life and did not display pacing or other behaviors associated with neurotic caged animals. Its diet, on which it appeared quite healthy, was kitchen refuse, table scraps, and copious supplies of weedy vegetation offered twice a day. It took food readily from the hand and was an eager participant in the feeding experiments described here which were conducted in the same period as were choice tests with foliage (Janzen 1981a); the regular diet was therefore supplemented with a greater species richness of foliage than usual, but not greater bulk or roughage. I have found that wild Baird's tapirs in Santa Rosa National Park (northwestern Guanacaste) defecate in water wherever possible, and the captive tapir did the same (and see Young 1906). The stream that flowed through its corral was diverted into a concrete-lined pool 20-40 cm deep with a plugged drain. Dung floated in the pool for a short time and then disintegrated through soaking and trampling. The seeds and buttons used in the experiments are all denser than water, with the result that they collected in the pool bottom. Once a day the plug was pulled and the pool contents passed through a 3 mm mesh screen that removed all particles large enough to be a button or seed fragment. The particular species of seeds used in the experiments (fig. 1) were chosen because 1) they were large enough to locate easily after passage through the animal but small enough to be swallowed readily, 2) they were hard enough to escape digestion, 3) one seed type (guanacaste) is found in livestock dung and is believed to have been dispersed this way by Pleistocene mammals (Janzen and Martin 1981), and 4) they are potentially dispersed by tapirs in the wild since the captive tapir, would eat the fruit of both species and both species occur in tapir habitat. The guanacaste seeds (Enterolobium cyclocarpum) weighed between 450 and 650 mg (specific gravity 1.34) and came from a single crop collected in March 1977 from below tree number 34 in Santa Rosa National Park. They were shelled by hand out of naturally fallen mature fruits and not touched by REPRODUCTIVE BOTANY 59-63 1981 59 This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 05:10:34 UTC All use subject to http://about.jstor.org/terms metal objects. When a sample of 300 living seeds was placed in water, 3 percent germinated immediately, and the remainder did not take up water for at least two months. When a subset of 100 of the ungerminated seeds was notched with a file and placed in water, all germinated within two days. Guanacaste seeds are hard and smooth and require about 70-200 kg direct pressure to break, depending on the angle of attack (Janzen and Higgins 1979). The carao seeds (Cassia grandis) were similar in weight, shape, texture, and appearance to the guanacaste seeds (fig. la and Janzen 1977) and were hand-shelled out of freshly fallen mature pods from the tree growing by the carpenter's shop at Finca La Pacifica. When a sample of 100 seeds was placed in water for two months, none germinated. After these seeds were notched with a file, all germinated within two days in water. 60 Janzen If guanacaste fruits were offered to the tapir, it chewed them thoroughly and spat out a variable percent of the seeds, swallowed others, and broke or scarified a few. It would not chew up entire carao fruit in its well-fed state. Therefore, to examine the fate of the seeds in a maximally controlled manner, I used artificial fruits to entice the tapir to swallow large numbers of seeds in one meal. Unpeeled ripe bananas were cut into 3 cm sections, and two seeds (or buttons, see below) were inserted in a slit in the pulp of each section (fig. Id). Since ripe banana was a favorite food of the tapir, the seeds were always swallowed (though an increase in seed numbers per 'fruit' resulted in seed spitting, Janzen 1981b). Rarely, the seeds were chewed lightly before swallowing, and, on five occasions, a guanacaste seed was broken. When a seed was broken, the break made a very loud noise, and these seeds were not counted in the tabulations reported here. The volume of seeds and buttons fed, about 200 cc, is a trivial portion of the daily diet of the tapir. There is no reason to believe that the seed-passage rate was influenced by the contents of the artificial fruits. It ate several kilos of bananas almost every meal, and the seeds were immediately separated from the slippery banana pulp by tongue, palate, and throat action. The seeds were collected from dung until no more seeds appeared for at least four days. In order to know if seeds which never reappeared had been digested or were simply delayed in passage, I also fed the tapir indigestible white plastic buttons along with the seeds. Some were broken in chewing, but all fragments of a quarter of a button or better were recovered from the dung. When a button broke, it could be heard distinctly. The particular buttons were chosen because their shape and size were similar to those of the seeds (fig. lc). They weighed 360 or 730 mg and had a specific gravity of 1.75. These experiments cannot be repeated exactly because the tapir died of a respiratory disease in August