only mature leaves are available, the monkeys eat just
a little on one tree and then move to a different tree,
minimizing their exposure to various arrays of defense
compounds. Sometimes they eat only the leafstalk,
or petiole, ignoring the blade; the petiole has lower
alkaloid content.
Ecologist Katherine Milton showed that protein and
fiber content are also important factors affecting leaf
choice in howler monkeys. Fiber, as noted earlier in the
chapter, makes leaves difficult to digest. Monkeys prefer
to eat young leaves, which have less fiber. Protein is
also proportionally higher in young leaves. The greater
the ratio of protein to fiber, the more desirable the leaf
is to howlers.
Howlers have long intestinal systems, especially the
hindgut. It takes food up to 20 hours to pass through
a howler’s digestive system. Geoffroy’s Spider Monkeys
(Ateles geoffroyi) eat mostly fruits, which, because they
contain more protein and fewer defense compounds
than leaves, are much easier to digest (plate 11- 41). It
takes food only about 4.4 hours to move through the
shorter gut of a spider monkey. Howlers, with their
long hindguts, are able to more efficiently digest leaves,
coping to a reasonable degree with both the high fiber
and the defense compounds.
Plants vs. Butterflies
To plant species, butterflies, with their complete
metamorphosis, are evolutionary examples of Dr.
Jekyll and Mr. Hyde. As adult sexually reproducing
insects, butterflies play the role of benevolent Dr. Jekyll,
dispersing pollen essential to the plants’ reproduction.
As voracious caterpillars they become malevolent Mr.
Hyde, devouring leaf tissue and reducing plant fitness.
How do these scenarios play out in tropical ecology?
Various lepidopterans in temperate and tropical
areas have strong affinities for feeding on specific
plant families. Caterpillars are much more strongly
affiliated with particular plants than butterflies, their
“adult” form. Butterflies feed on nectar, aiding in
pollen dispersion, and their interactions with plants
are fundamentally mutualistic. They tend to feed on a
wider range of plants than larval lepidopterans, which
have by natural selection evolved defenses against
specific plant defense compounds. Caterpillars are
voracious herbivores, and being folivores, they harm
plants. Of course they encounter plant defenses,
including defense compounds, as they chew on leaves.
Since different families of plants produce different
combinations of defense compounds, natural selection
has acted on caterpillars in such a way that various
caterpillar species have evolved tolerance for defense
compounds associated with different and specific plant
families. In other words, they have tended to specialize.Heliconid Butterflies and PassionflowersA note of clarification to the reader: Please be aware that
Heliconius butterflies have no coevolutionary relation-
ship to plants in the genus Heliconia. The similarity in
their names is coincidental. Adult heliconid butterflies
may feed on Heliconia nectar, but they feed on nectar
of many other kinds of flowering plants too. Heliconid
butterflies, as will be explained below, are associated as
caterpillars only with passionflowers, genus Passiflora.
The names Heliconia and Heliconius are often points of
confusion among visitors to the Neotropics.Heliconius butterflies and their kin in the subfamily
Heliconiinae, collectively known as heliconids and
commonly called the longwing butterflies, are a diverse
and colorful group of multiple genera and species, almost
all of which are Neotropical (plate 11- 42). They belong to
the brush- footed butterfly family (Nymphalidae), which
numbers nearly 3,000 species globally. The Heliconiinae
is represented by about 50 species, which have many
local races throughout tropical America. Only three
species of longwings, Heliconius charithonia, H. erato,
and Dryas iulia, regularly reach the United States,
ranging through southern Texas and the Southeast,
especially southern Florida.
It is important to point out that caterpillars,
the larval stage of lepidopterans, as well as adult
lepidopterans (the butterflies and moths) are heavily
preyed upon, particularly by birds (plate 11- 43). This
dynamic of nature sets up a potential evolutionary
arms race between lepidopterans and birds that plays
out constantly in the tropics.
Heliconid adults feed on a variety of plant species,
but the caterpillars feed virtually exclusively on species
of Passiflora, or passionflower (family Passifloraceae),
a common vine genus numbering approximately 500
species, a few of which reach North America (plate
11- 44). Like heliconids, passionflowers are largely
Neotropical. Very few herbivores other than heliconid
caterpillars dine on passionflower vines, and another
name for heliconids is “passionflower butterflies.” Most
passionflowers contain a smorgasbord of cyanogenic196 chapter 11 evolutionary arms races: more coevolution, more complexity