Les mammifères frugivores arboricoles nocturnes d’une forêt guyanaise : inter-relations plantes-animaux

Abstract

The relationships between a community of nine nocturnal frugivorous mammal species and the plants on which they feed were studied during 14 consecutive months in a secondary forest near Cayenne, French Guiana. Two major trophic groups can be defined : 1.-The seed eaters, which open unripe fruits to eat their seeds. All are rodents : Coendou prehensilis (average adult weight 4 000 g), Echimys armatus (400 g) and Oryzomys concolor (35 g). 2.-The pulp eaters, which specialize in ripe fruits. They eat the pulp and swallow most of the seeds which are disseminated with their feces. Five of them are marsupials : Didelphis marsupialis (1 000 g), Philander opossum (400 g), Caluromys philander (300 g), Marmosa cinerea (80 g) and Marmosa murina (45 g). The sixth species is a Procyonid Carnivore : Potos flavus (3 000 g) . These differing types of feeding strategies are discussed in relation to the morphology of the teeth and digestive tract of the species. Species with a similar diet and comparable body size generally live in different forest layers. The population density and biomass of these nocturnal frugivorous mammals were estimated on the basis of direct counts along forest trails at night, trapping results and radio-tracking. Out of the 127 plant species listed, 26 play a major role as a food source for the mammals studied. The trunk diameter (DBH) of the 13 most common species was measured, and the distribution of these trees mapped in the 8.5 ha study area. These 13 tree species account for approximately half of the total basal area, which is considered here as an index of standing crop biomass. The same 13 species account for 25 % of the total number of individual trees. Fruit production was measured during 13 months by weighing all fallen fruits along 1 200 m of trails. Fruit production and consumption of the same species were also estimated by the use of fruit collectors located under fruiting trees. Special attention was given to the phenological cycles of trees. Three types of fruiting cycles could thus be related to three different foraging strategies of the sympatric frugivorous mammals : 1.-Species with a low fruit production spread over an extended time period. Only a few fruit ripen at the same time, but fruiting takes place almost all year long. Such trees are early- or late-pionneer species ; their fruits have tiny seeds which are mainly eaten by small vertebrates which scatter them more or less homogeneously. 2.-Species with synchronous, but irregular cycles of fruit production. Such trees have rather large seeds which are very attractive to seed-eating rodents. These animals even destroy part of the fruit crop before ripening. The massive, irregular, and unpredictable fruiting cycles of these species may reduce seed predation by rodents. The few trees belonging to this category which do not bear fruit synchronously with their conspecifics may have their crop totally destroyed by rodents. 3.-Species with synchronous and regular cycles of fruit production. Such trees have rather large seeds, which are ingested by large frugivorous vertebrates and spread through their feces. Chemical or physical deterrents are generally present in such fruits before ripening. The tree species belonging to this category have successive and partly overlapping periods of fruit production. In this way, food is made available for pulp-eaters during most of the year. This pattern of fructification is presumably the end-result of a long lasting mutual interaction between plants and seed-scattering vertebrates. Frugivorous mammals apparently need two different kinds of fruit in their diet, some rich in sugar and others rich in fat. Trees producing these two categories of fruit produce their crop successively, so that the necessary nutrients are made available throughout the year. A definite decrease in fruit production takes place in our study area just before the beginning of the dry season. The marsupial pulp-eaters have adapted to this temporary food shortage by storing fat during the season of plenty. However, their production of young is strongly affected by this seasonal food shortage, as shown by an increased mortality rate of pouch youngs.