Société nationale de protection de la nature et d'acclimatation de France, Paris (FRA)
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 vertebrales.
Frugivorous mammals apparently need two different kinds
of fruit in their diet, sorne rich in sugar and others rich in fat.
Trees producing these two categories of fruit produce their crop
successively, so that the necess ary nutrients are made available
throughout the year.
A definite decrease in fruit production takes place in our
study are ajust 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 shawn by an increased mortality rate of pouch
young