Animal Arms Race

Animal Arms Race

A type of arms race appears to be developing among many species of animal life, particularly the insects. Such a race demands a rapid evolution of improved hunting—defense tactics for all groups of organisms involved.

The monarch butterfly has developed a slick evolutionary trick that shields it from voracious birds such as the blue jay. In the past monarchs were memorable morsels for the blue jay, but in recent years, probably stemming from the butterflies' diet of milkweed, they sequester in their body packets of poison capable of sending a naïve predator into violent fits of retching. Any blue jay swallowing a monarch butterfly in this day and age is almost immediately taken ill. When recovered, the bird will avoid the monarch like the plague. Of course this doesn't do the original swallowed butterfly any good, but its sacrifice helps preserve the race. And because the blue jay will subsequently avoid any bug that even remotely resembles the butterfly, other insects are protected as well.

The evolving traits of protective body poisons apparently extend to other lowly forms of life. For years scientists assumed that invertebrates such as insects were of too limited intelligence to learn from feeding experiences. Recent experimentation indicates that this is definitely not so, at least for the praying mantis. One research entomologist, May Berenbaum, fed snacks of milkweed bugs to a type of mantis and watched as the predator regurgitated its meal. The mantis very plainly refused a second helping and went so far as to haul away the foul—tasting bug offered as dessert. The conclusion was definite, as the scientist observed: "Obviously they're not so stupid. They will reject noxious prey. They not only perceive differences between edible and inedible bugs, but they learn to generalize."

To test her theory further, Berenbaum and a graduate student raised milkweed bugs on two different diets. One group was fed a diet of milkweed, a plant containing a high concentration of cardenolides, a chemical known to be toxic to many animals. A second group of identical bugs was fed a special diet of sunflower seeds. The results were illuminating.

A mantis encountering milkweed bugs for the first time showed no reluctance in pursuing, capturing, and devouring the unfortunate insects. Those bugs fed sunflower seeds received the same treatment and were quickly converted into dinner. However, as was expected, the mantises that had eaten milkweed—fed bugs became violently sick and quickly gave up their meal. It was then very noticeable that the sadder but wiser predators learned to avoid and reject even harmless bugs that had been painted by the scientists to resemble milkweed—fed bugs.

The evolution of animal protective techniques has expanded in such a manner that insects can even compete for survival with higher forms of life. An excellent example in the growing competition for survival is one involving the bat.

The bat emits sounds at frequencies of up to 230,000 vibrations per second, well above the human range of hearing. The bat's built—in sonar system permits it to read echoes of these sounds, which bounce off intended victims, and the hunter can thereby judge the location of the prey as well as the speed and direction in which it is moving.

Certain species of moths have developed methods of countering the bat's sonar system. Their own hearing organs are, depending on the species, located on various parts of their bodies and are used as a built—in early warning system tuned to the bat's high—pitched cries. The element of surprise is thereby defeated, and the forewarned moth is often able to escape.

Extreme refinement seems to be developing in the tiger moth. This species has developed a way to jam the bat's sonar system by producing a clicking sound that actually mimics the bat's echo location cry. Thus the information relayed to the bat is false and causes it to swerve off course and miss its prey.

Admittedly research into the learning process of lower forms of life is only in the infant stage. But clearly such learning processes may be an important selective force that has permitted some insect species to survive through the ages. Most recent research has shown that creatures ranging from scorpions to spiders can also be trained to avoid distasteful prey, much to the benefit of the prey.

From the book: 
Our Fascinating Earth