INSECTS REVEAL UNUSUAL BEHAVIORS

by Whit Gibbons


July 29, 2007


Cicada killers may not have the political solution to the U.S. immigration problems, but they definitely demonstrate that the powerful territorial instincts of many animals are more efficiently managed when boundaries are distinct.

Cicada killers are giant wasps native to the United States. A clever study conducted on the species several years ago by Perri K. Eason and Gary A. Cobbs of Northeast Louisiana University and Kristin G. Trinca of the University of Louisville provided evidence to support the idea that marking boundaries is the key to preventing trespassing. Their findings with an insect also provide further evidence that any animal species can have fascinating behavior.

Adults of both sexes of the black-and-yellow solitary wasps reach lengths of more than an inch and look like they would deliver a serious sting but are generally harmless to humans. Rather than living together in a nest, each female deposits her eggs in the ground on a full-grown cicada that she has paralyzed and carried to that spot for the developing larvae to feed on.

Female cicada killers spend most of their time looking for cicadas. The males spend their efforts preparing to meet and mate with females. Adult males emerge from the ground in late summer before the females and set up mating territories, which they defend against other males. The territories are established in areas where female cicada killers will also be emerging from the ground. Upon emergence, the females normally mate with a territorial male, and presumably the closest one.

Male cicada killers confront other males who enter their territory. Using intimidation as their primary weapon, males will sometimes even defend their territory from other animals, including people who enter the area. Despite their imposing appearance, the males have no stingers, so if a male cicada killer chases you simply stop and enjoy its antics. Or if you prefer such sport, run, and it will chase you.

The researchers conducted field experiments with cicada killers to confirm previous anecdotal reports that naturally occurring landmarks are used by some species to define the territorial boundaries. To test the importance of visual landmarks in territorial behavior, the investigators caught individual males and marked them by painting unique patterns of colored dots on their bodies. After releasing the insects in a flat, grassy lawn with no obvious landmarks, they determined by field observations the size and shape of each defended territory, generally occupied by a single male that was identifiable by its paint pattern.

The researchers then placed thirty, three-foot-long wooden dowels horizontally on the lawn in a random pattern to serve as tangible landmarks in the otherwise homogeneous habitat. Upon remapping the territories of the wasps, the investigators found that by the next day 42 territories had been redefined within the study area, with the dowels being used as boundaries. And no male's territory crossed a dowel into the territory of another male, indicating that these landmarks were observed as the boundaries by males on either side.

Further observation revealed an interesting comparison among wasps that were defending boundaries where a dowel could be used as a landmark on two sides and where no clear landmark was available on the other two sides. These males spent more than six times as long during the day defending the sides with no landmarks as they spent defending the sides where dowels were present and could be used as landmarks.

In the experiment, dowels represented natural landmarks. One conclusion offered by the investigators was that the use of obvious natural landmarks to define territorial boundaries could have evolved because of the reduction in costs of territorial defense. That is, establishing a territory in a homogeneous habitat is not energy efficient for a cicada killer because of the constant patrolling and vigilance necessary. When the boundaries are distinct, the wasp can be more resourceful.

Our national problems of illegal immigration are in essence a territoriality issue but more politically complex. Our boundaries are already distinct. Thus, immigration is readily defined. Our national leaders, who seem either unwilling or incapable of solving the problem, apparently are grappling more with what should be viewed as illegal, something cicada killers have already resolved.



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