Condensed Tannins in Fruits Eaten by Chimpanzees

Abstract

Condensed tannins were assayed in the unripe fruit flesh of 37 species collected in Gombe National Park, Tanzania. These fruits are known to be eaten by chimpanzees when ripe, but are rarely or never eaten when unripe. The median concentration of condensed tannin was high enough (1.6% wet weight) to suggest that tannins significantly reduce the palatability of many wild unripe fruits. Tannin levels did not vary systematically with plant growth form, fruit morphology or fruit size, but the fruits which were eaten most often when ripe had higher tannin levels when unripe than those less commonly eaten. FRUGIVOROUS ANIMALS NORMALLY EAT RIPE FRUITS in preference to unripe fruits. As a result, they disperse seeds which would be destroyed or less viable if the fruit were consumed while unripe (McKey 1975). Why are immature fruits not eaten? First, they may have less available energy, protein and other nutrients than ripe fruits (Foster 1977). Second, they can be protected by toxins such as alkaloids, or by digestion-inhibiting compounds such as tannins and lignins (McKey 1979, Herrera 1982). Species differences in nutritional content have been found to influence the attractiveness of ripe fruits to frugivores (Howe and Vande Kerckhove 1980), but comparatively little is known about the concentrations or effects of secondary compounds in unripe wild fruits. However, tannins have been proposed as a protective agent of particular importance Janzen 1978, Herrera 1982), partly because many domestic species such as bananas, grapes, guavas, persimmons and pomaceous fruits have high levels of condensed tannin in unripe fruit flesh (Goldstein and Swain 1963, Hulme 1971). Tannins render the flesh unpalatable because they precipitate salivary mucoprotein and thereby cause astringency Joslyn and Goldstein 1964), and they interfere with digestion by reducing protein availability in the gut (Swain 1979). Many wild tropical plants are known to have high levels of condensed tannins in bark, leaves and seeds (Gartlan et cal. 1980), so tannins may be present in fruits also. Furthermore, tannins seem appropriate for protecting fruit because they are easily rendered biologically inactive. In ripening domestic fruits, they form large polymers or complexes with pectins, thereby losing their astringency. As a result, the fruit becomes more palatable (Goldstein and Swain 1963). We therefore investigated the levels of condensed tannins in a set of African wild fruits whose seeds are normally dispersed by frugivores. In order to relate condensed tannin concentrations to frugivore feeding behavior, we analysed fruits in Gombe National Park, Tanzania, where the diets of chimpanzees, Pan troglodytes, had been studied previously. In Gombe, chimpanzees spent 59.4 percent of their feeding time eating fruits (Wrangham 1977). Other frugivores present included primates (blue monkey Cercopithecus mitis, redtail monkey C. ascanius, baboons Papio anubis, red colobus monkey Colobus badius), bats (straw-colored fruit bat Eidolon helvum), and hornbills (Tockus spp.). MATERIALS AND METHODS All fleshy or arillate fruits which could be found were collected in Gombe National Park in August 1978 (middry season) and December 1978 (early wet season). The Park lies on the eastern shore of Lake Tanganyika (4?40'S, 29?38'E) in a strip of mountainous country between 772 m and 1500 m altitude, with an annual rainfall of 145171 cm. Evergreen forest, characterised by a well-developed upper canopy at about 30 m and abundant vine cover, occurs in valley bottoms and other well-watered sites. Valley sides support semi-deciduous forest, and upland areas include both a single-canopy miombo woodland and open grassland (Clutton-Brock and Gillett 1979). Ripe fruits were classified as those of terminal size, texture and color; unripe fruits were smaller, harder, and/ or colored differently. The proanthocyanidin assay for condensed tannins was performed within 24 hours of collection on fruit flesh from which stalk, skin and seeds had been removed. Further samples of fruit flesh with, where possible, separated seeds, were preserved in 100 percent methanol for laboratory analysis. This method of preservation was used because dried ripe fruit are susceptible to fungal attack during prolonged storage. BIOTROPICA 15(3): 217-222 1983 217 This content downloaded from 207.46.13.178 on Wed, 15 Jun 2016 06:47:33 UTC All use subject to http://about.jstor.org/terms Samples of fruit flesh were air-dried to constant weight to give an estimate of average moisture content in im-