Predator-Prey Arms Races

The constant competition between predators and prey animals is a major stimulus to evolution, sometimes called an evolutionary "arms race." Predators adapt to prey populations, and the prey populations adapt to the innovations of a predator.

What "arms race" occurs in evolution? What are examples involving bats and their prey?

Bats have a very effective system for locating insects, using a high-frequency sonar that bounces off insects and provides directional information. Bats are also excellent flyers capable of turning on a dime. Insects that cannot escape the bats are more likely to be eaten. Insects that develop a genetic mutation reducing the chance of bat predation are more likely to survive and reproduce. For example, the Noctuid moth has a simple three-neuron auditory system tuned to the frequency of the bat's sonar. When the neurons fire impulses (indicating that a bat is nearby) this triggers a motor program that causes the moth to dive into the grass, where it is safe from bats.

The praying mantis-itself a predator for many smaller insects-is just prey to bats. So the praying mantis evolved a special ear in the middle of its thorax, tuned to the exact frequency of the sound emitted by bats. When stimulated, the receptor triggers a "sudden full extension of its forelegs" and contraction of the abdomen leading to "an abrupt and dramatic deviation in its flight path" (Yager & Hoy, 1986). This helps the mantis avoid being eaten.

How do some bats overcome the defense strategies?

As prey species evolve successful defenses, predator species evolve better techniques for hunting. Some bats employ unusual echo frequencies, so they overcome specialized defense systems tuned to the usual frequency of bat sonar. Some bats use non-echo hunting strategies, so they can catch insects that have defenses only against the echolocation strategy. The predator-prey competition stimulates innovation on both sides.

What was Smith's experiment with hand-reared motmots, and what does it illustrate?

Animals commonly have built-in circuits to trigger avoidance of stimuli indicating danger. For example, a Central American bird called the motmot preys on snakes, but it must avoid the poisonous coral snake. Coral snakes have a distinctive color pattern consisting of red, yellow and black rings, so motmots are born with a fear of this pattern. Smith (1975) reared some motmots by hand, to insure they never saw a coral snake. She then exposed them to a series of models-wooden tubes painted rings or stripes of various colors. She reported:

The motmots had no hesitation in pecking at plain red, yellow, green, blue or unpainted wooden models. However, the bird's initial reaction to the solid yellow and red ring model was one of avoidance: all flew up to the opposite corner of the cage and in many cases gave alarm notes. (p.759)


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