Not So Smart

Even simple-minded animals, such as fish, seem to size up situations. Male fish of nest-building species chase other males off their premises. They welcome nubile females. To the human observer it looks as if they judge approaching fish, determine their sex and promise, and act accordingly. But this, explains Professor N. Tinbergen in a recent Bulletin of the Wilson Ornithological Club, is giving them too much credit. Often all they see is a single characteristic of the approaching fish, which "releases" in them a train of automatic responses.

Such simplicity allows zoologists to do experiments with animals almost as if they were chemical compounds. Professor Tinbergen tells how he bothered a herring gull horribly by putting a red wooden egg in her nest. The shape and size of the egg were correct; they tended to release the gull's incubating reaction. But the red color touched off an attack reaction. The poor confused mother fought an exhausting battle between conflicting drives, until the "sitting drive" dominated long enough to allow her to sit down. She could feel the egg's tender shape beneath her, but could not see its color. So she finally settled down, in peace, to incubate it.

Bright Red Beacon. Most of Professor Tinbergen's work has been with sticklebacks, prickly fresh-water fish that make good subjects for "releaser" experiments. The male builds a nest of fine sticks and tries to persuade females to lay their eggs inside it. He wastes no time on nonpregnant females. But when he sees one with a swollen abdomen, her shape acts upon him as a powerful releaser. He rushes into the lists of love, displaying his own releaser: a bright red belly.

The female follows him to the nest and swims in. He pokes her with his nose. This is a "tactile releaser," which persuades her to deposit her eggs. Then a "chemical releaser," given off by the fresh eggs, goes into action. Attracted by it, the male swims into the nest triumphantly and fertilizes the eggs.

Dissected Love Affair. Professor Tinbergen observed this byplay; then he took it apart. First he proved that the male would attack as a rival a crude dummy fish with a red belly like his own. Professor Tinbergen then made a dummy with a swollen abdomen. This released a chain of mating reactions in the eager male, which tried to entice the dummy into his waiting nest.

The females, too, were susceptible. They followed a red dummy as if it were a male. When a female was in the nest, Professor Tinbergen gently poked her abdomen with the tip of a glass rod. She laid her eggs willingly.

Professor Tinbergen concludes that breeding sticklebacks do not see their mates as fish of the other sex. The male sees only an egg-distended shape; later he smells fresh eggs. The female sees a flash of red belly; later she feels a prodding. They sense little more of each other.

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