tapirs, deer, and primates (plate 10- 4). Fruits provide
a calorie- rich, nontoxic, and relatively easily acquired
resource (it certainly does not run away).
There are downsides to a diet of fruit. Protein is often
in low quantity, thus an all- fruit diet, while rich in
calories, may be nutritionally deficient and need to be
augmented in some way with animal food having more
concentrated protein. Fruiting patterns vary, often
significantly, both in time and space. Recall that tree
species may be widely separated in tropical forests. This
means that frugivorous animal species routinely must
travel widely to find a suitable fruiting tree. A fruiting
fig tree presents a bonanza, but fruiting figs may be few
and far between. This is one reason a tree burgeoning
with fruit can be a bonanza for the naturalist. Just stand
there and enjoy the continuous visitations, as birds and
mammals come to feed at the tree. Seasonal changes
in fruit abundance occur throughout the tropics, and
some montane frugivores such as the Resplendent
Quetzal (Pharomachrus mocinno; chapter 15) undergo
regular seasonal migration to lower elevations in
search of favored fruits.Fruit as an Adaptation
The evolutionary function of fruit is to advertise itself to
animals so that it will be consumed. In the evolutionary
sense, fruits “want” to be eaten. The nutritious pulp of
the fruit is a bribe to an animal. The seeds contained
within the fruit pass through the alimentary system of
the animal (or are regurgitated) and, because the animal
is mobile, are deposited away from the parent plant. And
that turns out to be very good for the plant (plate 10- 5).
Seed dispersal is an essential adaptation. The worst
place a seed can fall is in the shadow of its parent
tree. Doing so sets up a severe competition for light,
water, and soil nutrients with an already established
conspecific, the “mother” tree. Ecologists recognize
what is called a seed shadow effect, in which the optimal
distance for a seed to be is well away from the parent
tree but not so distant as to find itself in an unsuitable
habitat or where cross- pollination becomes unlikely.
Seeds that are distant, like seeds that are close, have
reduced likelihood of success. Therefore the real
function of a fruit consumer from the evolutionary
standpoint of the plant is to take the precious seeds
some distance away and drop them where they have a
good opportunity to germinate.Fruits therefore are anatomical adaptations for seed
dispersal. Ideally the animal derives nutrition from the
fruit but also disperses seeds; thus, the relationship
between animal and plant is fundamentally
mutualistic— both parties win. The animal provides
mobility, a contribution not unlike that in animal-
vectored pollination (discussed below). Thus it is to
the plant’s ultimate advantage to invest energy to make
fruit, and it is to the animal’s immediate advantage to
eat the fruit. But it does not always work out that way.
Some species digest the seed as well as the pulp of the
fruit or else injure the seed, and it does not germinate.
Animals that destroy or fail to disperse seeds are not
useful to the plant and represent an evolutionary cost
to the fruiting plant (plate 10- 6).
Fruit comes at a cost to the plant. Lots of carbohydrate
and fat typically is incorporated in fruit. The plant has
to pay the animals to disperse the seeds, so to speak.
But not all tropical plants produce expensive animal-
dispersed fruit. Some rain forest canopy trees, vines,
and epiphytes utilize wind or water for dispersal of
seeds. Wind dispersal is most common at the canopy
level or in open tropical deciduous forests, where
leaf drop can help facilitate wind movement of seeds.
However, in dense interior rain forests where wind is
attenuated, animals are essential for seed dispersal.
Plants have little evolutionary choice but to bribe them
accordingly.How Fruit Drives Evolutionary
Patterns in BirdsOrnithologists David Snow and Eugene Morton have
independently committed many years of fieldwork that
revealed how a diet heavy in fruit results in evolutionary
consequences to some groups of birds. Fruit is temporally
and spatially a patchy resource, meaning that it may be
abundant on a given tree but, as mentioned above, trees
laden with mature edible fruits may be widely spaced
in the forest. For much of the year, a fruiting tree may
be barren of fruit. Such a resource distribution selects
for social behavior rather than individual territoriality.
Flocks of avian frugivores can locate fruiting trees
more efficiently than solitary birds, and there is little
disadvantage to being part of a flock once the fruit is
located, since there is usually more than enough fruit
for each individual. Even if not, it is extremely difficultchapter 10 tropical intimacy: mutualism and coevolution 157