It is a machiavellian tale of strategy and counter-strategy. The whirligig beetle produces a blend of noxious substances, the main component of which is an organic compound 'gyrinidal'. This is toxic to freshwater fish -- not enough to kill them, but sufficient to make them write beetles off as too bitter a pill to swallow -- sometimes. Eisner and Aneshansley find that victory does not always come so easily to beetles.
With a beetle in its mouth, a fish repeatedly opens and closes its jaws. Occasionally it spits the insect out, only to take it again into its mouth. At first the researchers thought that the fish must be gagging as the beetle let forth its chemical weapon; but the behaviour never culminated in vomiting.
Eisner and Aneshansley conclude that instead the fish is 'orally flushing' its prey: milking it of poison before swallowing it. It is an evolutionary adaptation uniquely available to predators living in water. Although often the fish seemed to give up hope that the beetle would become palatable enough, and spit it out for good.
To investigate how much gyridinal a fish can stomach, the researchers fed fish mealworms dipped in varying amounts of the poison. The hungrier a fish was, the more toxin it could stomach. Well-fed fish spat out lightly treated worms that hungrier fish swallowed. The fish subjected all the worms to the same oral flushing to try to rid them of their bitterness first.
But the beetles have got wise to the fishes' washing' trick. Rather than squirt out their poison in a quick burst, as many other toxin-releasing creatures do, they let it ooze out of their glands slowly, playing for time in the hope that the fish will give up before their gyrinidal reserves are exhausted.
"What better way to counter oral flushing than to deliver one's deterrent as a trickle?" say the researchers. "Beetles pinched briefly to mimic the closing of jaws around it emit gyrinidal for about 42 seconds; if the pinch is sustained they discharge it for twice as long.
Even a hungry fish can rarely afford to spend a minute and a half washing each tiny prey before swallowing it. The researchers found that, of 96 beetles offered to seven bass, only three were swallowed. Another 17 were tried and spat out, and the rest were ignored.
In this cunning battle for survival, then, the beetles seem to have gained an upper hand. These findings are reported in