Animal Defenses against Predators
Some animals that feed on plants rich in secondary compounds
receive an extra benefit.
When the caterpillars of
monarch butterflies feed on plants of the milkweed family,
they do not break down the cardiac glycosides that protect
these plants from herbivores.
Instead, the caterpillars concentrate
and store the cardiac glycosides in fat bodies; they
then pass them through the chrysalis stage to the adult and
even to the eggs of the next generation. The incorporation
of cardiac glycosides thus protects all stages of the monarch
life cycle from predators. A bird that eats a monarch butterfly
quickly regurgitates it (figure 25.9) and in the future
avoids the conspicuous orange-and-black pattern that characterizes
the adult monarch. Some birds, however, appear
to have acquired the ability to tolerate the protective chemicals.
These birds eat the monarchs.