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Will fighting crickets unlock the mystery of human depression?
Cricket fighting has been a popular sport in China for centuries.
According to Chinese folklore, when two male crickets engage in combat, the loser will refuse to fight again unless he's shaken and tossed in the air by his trainer.
Now biologists have found a scientific explanation for what the ancient Chinese knew all along. These new insights into cricket behavior even may help unlock some of the mysteries about depression in human beings.
Hans A. Hofmann, a postdoctoral neurobiologist in the Department of Psychology, describes his findings in the Feb. 10 issue of the journal Nature.
"It is not fully understood what makes animals aggressive, although ancient Chinese gamblers, betting fortunes on the outcome of cricket fights, seemed to know a few tricks," writes Hofmann and co-author Paul A. Stevenson. "Inspired by their age-old wisdom, we found that we could completely restore aggressiveness in defeated subordinate crickets simply by activating their motor program for flying."
Hofmann and Stevenson conducted their cricket research at the University of Leipzig in Germany, where they built a miniature arena to stage wrestling matches between male Mediterranean field crickets (Gryllus bimaculatus De Geer).
They discovered that two aggressive males often engage in a six-stage, choreographed battle that starts out slowly then escalates into a fierce struggle (see diagram).
"The contestants initially fence with their antennae," write Hofmann and Stevenson, "and then display spread mandibles [jaws] which later interlock before the animals finally wrestle. The contest can be stopped at any level by one animal retreating."
After his defeat, the loser will avoid aggressive encounters with all other crickets and will refuse to fight for approximately 24 hours.
But cricket handlers can't wait an entire day. They want their bugs to get back in the ring immediately and take on other challengers.
How do you convince a vanquished cricket to start fighting again?After an exhaustive review of classical Chinese literature, Hofmann found the answer: a centuries-old technique that involves shaking a defeated cricket in your clasped hands, then throwing it in the air and catching it several times.
"When I described this to my colleagues," Hofmann recalls, "they told me, 'You're crazy, it isn't going to work.' But it did."
Indeed, laboratory results showed that nearly 57 percent of the losing crickets regained their aggressiveness after being shaken and tossed, confirming the claims of ancient Chinese gamblers.
"All my colleagues in the lab ended up testing crickets," Hofmann says, "and we often gambled on the outcome of the fights. It was very entertaining."But is it the shaking or the tossing in mid-air that turns crickets from wimps to warriors?
To find out, Hofmann and Stevenson tried tumbling defeated crickets in a rotating tube, but all the spinning and shaking had no effect: The insects still refused to fight.To test the effect of being thrown in the air, losers were suspended on a wire and placed in a small wind tunnel, where they had no choice but to fly. The results were dramatic: Four out of five losers returned to the arena and immediately challenged their previous opponent to a wrestling match.
According to Hofmann, the experiment demonstrates that shaking a cricket has little effect on aggression, and tossing only works if the insect actually flies in mid-air if just for a few seconds.
"The effect of flight on aggression was independent of its duration," he writes. "A 10-second flight was as effective as a 15-minute flight, and the influence of a one-minute flight was still evident hours later. Furthermore, the flight treatment could be repeated many times, whenever an animal lost, without losing its effectiveness."
Insect aggression is controlled by the brain, but the command to fly comes from an electrical signal generated in the animal's thorax or chest area.
When researchers cut the nerves connecting the thorax to the brain of a defeated cricket, the animal was still able to fly, but flight did not restore its willingness to fightan indication that the act of flying releases a neurotransmitter or some other chemical signal from the thorax that directs the brain to "reset" the cricket's aggressiveness.
According to Hofmann, this may be the first example in nature in which a specific motor activity (flight) has an immediate effect on an unrelated subsequent behavior (aggression).
The discovery of a link between bodily movement and behavior in crickets might have applications for research on humans. Studies have shown that people who suffer depression often become euphoric after undergoing sleep deprivation.
"But no one ever investigated what these depressed patients did to stay awake for long periods of time," Hofmann points out. "Maybe they walked around and exercised a lot. Maybe we'll find a motor pattern that people can perform that will decrease their depression without having to stay awake all night."
By Mark Shwartz