have led to allometric relationships (change in fruit
shape as a function of size; Mazer and Wheelwright
1993). Both within and among individuals of Ocotea
tenera, among the species of Lauraceae at Monteverde,
and among a sample of 167 bird-dispersed plant spe-
cies in diverse families, larger fruits are more elon-
gated, whereas smaller ones tend to be more spheri-
cal (Fig. 8.5). The same relationship also exists for
seeds, suggesting that the evolution of large fruit or
seed size is accomplished by constraining diameter
to facilitate consumption by birds while allowing
length to increase.
Manipulation of disperser behavior and physiology by
plants. Plant-frugivore interactions in Monteverde
also concern how plants enhance their reproductive
success by influencing the behavior or physiology of
fruit-eating animals.
Crop size The plant characteristic most com-
monly examined for its effect on disperser behavior
has been fruit crop size (Howe and De Steven 1979,
Howe and Vande Kerckhove 1979, 1981, Moore and
Willson 1982, Stapanian 1982, Davidar and Morton
1986). Large crops of ripe fruits should attract greater
numbers of visually oriented dispersers such as birds,
which disperse more seeds over a broader area. How-
ever, extremely large crops of ripe fruits may enable
fruit-eating animals to remain within or near the
crown of a fruiting plant for long periods of time and
hence disperse the seeds ineffectively. These conflict-
ing selection pressures have led to the prediction that
intermediate crop sizes would yield the highest repro-
ductive success (Howe and Estabrook 1977).
Studies in Monteverde lend mixed support to this
view. In eight species of Lauraceae, visit frequencies
by large frugivorous birds were significantly higher
at the four species that produced large crops of small
fruits than at the four species producing smaller crops
of large fruits (Wheelwright 1991). Because smaller
fruits can be swallowed by a greater variety of bird
species (Wheelwright 1985b), it is unclear whether the
pattern observed derives from crop size, fruit size, or
both. Similar comparisons within species, where the
range in fruit size is smaller, should allow for a more
robust test of the hypothesis. Fruit size can vary con-
siderably even within species: mean fruit diameter in
46 Ocotea tenera plants varied from 1.55 to 2.25 cm
(Wheelwright 1993). Variation in fruit size within
species can also have consequences for reproductive
success; in O. tenera, the proportion of a plant's fruit
crop that was successfully removed by birds (rather
than damaged by seed predators or ignored) was posi-
tively correlated with its mean fruit diameter (Wheel-
wright 1993).
Wheelwright (1991) also failed to find a positive
relationship between crop size and visit duration pre-
dicted by some authors. Instead, birds generally spent
only short periods of time (half of all visits were <4
min) in trees of all eight species of Lauraceae, regard-
less of crop size. Some quetzals remained in trees forFigure 8.5. Seed size variation in Ocotea tenera. Photograph by Nathaniel Wheelwright.261 Plant-Animal Interactions