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Hunting Virtual Moths, Blue Jays Offer Eye on Evolution

Bijal P. Trivedi
National Geographic Today
February 7, 2002
 
Experiments with live blue jays and virtual moths on a computer screen
have shown researchers how moths evolve patterns of camouflage on their
wings.

Alan Bond and Alan Kamil, both of the University of
Nebraska in Lincoln, trained blue jays to "hunt" and peck at virtual
moths on a computer screen. If the birds found a moth, they were
rewarded with a food pellet. If they were unable to find a moth, they
were trained to peck at a green dot that would initiate a new trial.



In about half of the trials, a moth was randomly inserted into the displays. The background of the displays looked like TV static or "cold gray oatmeal," while the digital moths were tiny images in black, white, and gray, Bond explained.

It has been hypothesized since the 1950s that moths with more common wing patterns and coloring will be recognized and preyed on more frequently. This gives moths with rarer wing patterns a survival advantage so they can breed and increase their numbers.

After many generations, however, the situation will flip. Moths with the more common wing pattern become rarer as blue jays single them out for prey; moths with rarer patterns become dominant and thus the new prey of choice.

This idea had never been fully tested until Bond and Kamil set out to study the evolutionary consequences of the blue jays' hunting strategy on a population of 200 virtual moths.

Virtual Breeding

Every day, each of four blue jays examined more than 400 screens searching for moths. When a moth was found and pecked by a blue jay, it was eliminated from the lineup—equivalent to the loss of that moth and its ability to breed and contribute to the gene pool.

When all the blue jays had completed their trials for the day, the results were entered into an algorithm that "bred" the surviving moths and generated a new population of 200 moths with the slightly different wing patterns that had evolved. This new population was then shown to the four blue jays the next day.

After 100 generations, the virtual moths had evolved a diverse array of wing patterns. The designs blended more into the background, just as a moth on tree bark is often hard to detect.

"A broad range of wing patterns developed, but the patterns also evolved to become more cryptic so that the moths were more difficult to find," said Bond, co-author of a report on the study published February 7 in the journal Nature.

The diversity in the range of wing patterns almost doubled and the designs became about 30 percent more cryptic, said Bond.

Strategy for Evasion

Bond said the study is one of the first experiments to show how the psychology of the predator—the hunting strategies of the blue jay—can affect the diversity of wing patterns in nature and the complexity of the camouflage.

"People have the impression that animals of the same species all look the same, but they show tremendous variation just like humans," said evolutionary biologist John Endler of the University of California at Santa Barbara. An important reason for this variation in appearance is to evade predators.

Diversity in appearance is common throughout the insect world. Grasshoppers, stick insects, praying mantids, and moths all show tremendous diversity in appearance—different coloration and patterns—within the same species.

The extraordinary variation in moths' appearance—"almost no two are alike," said Bond—also occurs because among moths themselves, appearance is irrelevant. "Moths don't look at one another, they detect each other by smell," said Bond. So altering appearances does not affect their sex lives and opportunities for breeding.

After about 30 generations in the experiment with blue jay and virtual moths, the moths were too well camouflaged for Bond to find. "The entire screen just looked like gravel," he said.

"But the blue jays were amazing," he added. Even as the moths became harder to detect, the blue jays were about 80 percent accurate in detecting the moths throughout the entire trial, said Bond.

Blue jays have a rather straightforward predatory strategy. They focus on one specific feature of a wing design, called a searching image—perhaps a pair of whitish dots. They then hunt all moths with this feature until that particular design is rare and the birds get hungry. Moths without the particular image the blue jays are seeking that day avoid detection. This strategy helps the birds compensate as wing patterns become more cryptic.

"This is a really neat computer simulation of how things work in the wild," said Endler.

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