The New Neotropical Companion

the fungus they cultivate. Many tannins retard microbes

and pathogens, and therefore presence of tannin may

affect rates of decomposition and nutrient cycling.

Cyanogenic Glycosides

Many species of tropical plants contain compounds

called cyanogenic glycosides, which consist of cyanide

(a potentially deadly compound) linked with a sugar

molecule. When combined with enzymes from either

the plant or an herbivore’s digestive system, the sugar is

released, leaving hydrogen cyanide.

Cyanogenic glycosides, as well as alkaloids, tannins,

and other defense compounds, are well represented

in passionflowers (Passiflora spp.). Very few insect

herbivores feed upon passionflower leaves or stems. The

defense compounds apparently do act to discourage

most herbivores, though not all. The caterpillars of

Heliconius butterflies feed on Passiflora species. This

group of lepidopterans has evolved resistance to the

Passiflora defense compounds. Much will be said of

them later in this chapter.

Terpenoids

Terpenoids are a complex group of fat- soluble

compounds in plants. Some are used in the synthesis of

compounds that may mimic insect growth hormones

(preventing rather than promoting growth of the

insect); others are modified into cardiac glycosides,

which are well- known medicinal chemicals, such

as digitalis, that act to stimulate the heart. Some

terpenoids discourage both insects and fungi. One

terpenoid in particular, caryophyllene epoxide, has

been shown to repel the fungus garden (leaf- cutter)

ant Atta cephalotes from clipping leaves of Hymenaea

courbaril. This terpenoid was shown to be toxic to the

fungus that the ants culture. In a survey by Stephen

Hubbell and colleagues of 42 plant species from a Costa

Rican dry forest, 75% contained terpenoids, steroids,

and waxes that repelled leaf- cutter ants.

Toxic Amino Acids

Some tropical plants, especially members of the

legume, or bean and pea, family (Fabaceae), contain

amino acids that do not build protein but instead

interfere with normal protein synthesis. Canavanine,

for example, mimics the essential amino acid arginine.

Perhaps the best known of the toxic amino acids is L-

DOPA, a strong hallucinogen. Both canavanine and L-

DOPA are concentrated in the seeds of some tropical

plants. In general, the major function of nonprotein

amino acids, at least in legumes, seems to be to

discourage herbivores.

Taken together, the groups of potential defense

compounds just described present an image of a

tropical forest that suggests a chemical complexity

every bit as impressive as its structural complexity and

equally important.

Other Types of Plant Defenses

We drive on a defensive adaptation of tropical plants—

namely, rubber. The Rubber Tree (Hevea brasiliensis),

which can reach heights of 36.5 m (120 ft), is one of

many tropical trees that produce latex, resins, and

gums, substances that render the trees less edible.

Rubber Tree sap is a milky suspension in watery

liquid contained in ducts just below the bark, external

to the cambium and phloem. It congeals upon exposure

to air and may aid in closing wounds to the plant,

protection against microbial invasion, and hindering

herbivores. Latex is present in plants of many families

(Euphorbiaceae, Moraceae, Apocynaceae, Caricaceae,

Sapotaceae, and others), a case of a convergent

defensive adaptation among distantly related species

of tropical plants. The Chicle Tree (Manilkara zapota),

which grows in rain forests throughout Central

America, produces latex called chicle from which

chewing gum is made.

Some insect herbivores have adapted to latex defense.

One caterpillar species clips leaves of Papaya (Carica

papaya) in such a way as to cause the defensive latex to

flow away from where the insect is feeding.

Tropical trees may be spiny, thorny, or have leaves

coated with diminutive “beds of nails,” called trichomes,

that literally impale caterpillars. Experiments have

shown that sharply toothed leaf edges reduce caterpillar

grazing. When teeth are experimentally removed,

caterpillars inflict much greater damage to the leaf.

Many palm species have spines lining the lower trunk

(see plate 3- 34). Some palms also have long, sharp

spines on the undersides of leaf midribs. Wood of

many tropical trees is hard, a possible adaptation to

discourage termites and wood- decaying fungi.

chapter 11 evolutionary arms races: more coevolution, more complexity 191