updated 10/6/2003 10:46:12 AM ET 2003-10-06T14:46:12

What’s the best way to keep from being eaten: Keep mustering new defenses, or create a single overwhelming one and warn potential attackers that they’ll be sorry?

BOTH APPROACHES SEEM to work, according to new research. Some plants and beetles adapt to one another by evolving new attack and defense strategies, while poisonous frogs develop bright colors to warn predators that biting them can be a fatal error.

The development of these traits is described in a pair of papers in this week’s online issue of Proceedings of the National Academy of Sciences.

Normally animals tend to develop coloring that hides them from predators, but certain frogs have taken the opposite approach, evolving bright colors that make them more conspicuous.

Juan Carlos Santos and colleagues at the University of Texas, Austin, report that the frogs appear to have developed bright coloring at the same time they were specializing in a diet of ants, termites and other insects that provided toxins that accumulate in the frogs bodies, making them poisonous to predators.

The bright color serves as a warning that the frog is poisonous, tastes terrible or is generally not worth the effort — something predators quickly learn if they sample the frogs.

Some other species, such as Monarch butterflies, have similar color warnings.

David C. Cannatella, a member of Santos’ team, said scientists thought the development occurred only once in a species’ development because the poisons are so complex. But the team studied the animal’s DNA to map when various traits were acquired and found that the poisons and bright coloring had evolved several times in various varieties of frogs.

In a separate paper in PNAS, Judith X. Becerra of the University of Arizona, Tucson, reports on the evolving relationship between plant-eating beetles and their target plants as the two evolved defense and attack strategies through the centuries.

The plants and beetles forced each other to continue adapting as they alternated strategies in a process called coevolution. Becerra says her study, by dating the ploy and counterploy between specific species, provided the first direct evidence of synchronous changes.

Becerra studied Bursera, a type of flowering plant common in Mexico, and Blepharida, a beetle that feeds on Bursera.

She worked out a timeline for the development of various traits in the plants and beetles over 112 million years and found the two locked in a series of defense and attack steps.

Some species of Bursera, for example, evolved the trait of holding toxic resins under pressure so that when a leaf is damaged, such as by being chomped by a beetle, the resin would squirt out. Besides being toxic the resins solidify in the air, potentially encasing small insects.

That was followed by the development of species of beetles that learned to cut leaf veins, releasing the pressure, before beginning to eat the leaves.

Other species of the plants then evolved a series of complex toxic chemicals to repel insects, followed by the development of beetles that could safely ingest those chemicals.

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