Saturday, March 29, 2008

Picky, picky

A few days ago, Geoff gave us a beautiful example of "imprinting" from the work of Peter and Rosemary Grant with the finches of the Galapagos. Here's another example that struck me as rather amazing.

Back in 1967, Neal Griffith Smith of the Smithsonian Institution reported in Scientific American on a series of experiments with gulls. The problem that attracted his attention was this: Some species of gulls that live together look very nearly alike, yet do not interbreed. How is it that gulls of one species recognize others of their own kind?

For example, at one site on the eastern coast of Baffin Island in the Canadian Arctic, four species of gulls inhabit the same breeding ground without interbreeding. The only visual differences between the species are the color of the eyes and the fleshy rings around the eyes, and slight variations in the shading of the back and wings. These subtle differences are easily recognized by ornithologists. Are they equally important to the gulls?

To test the significance of visual cues for species isolation, Smith captured hundreds of gulls by lacing bait with drugs. Once the birds had been rendered immobile, Smith painted their eye rings. Gulls with light-colored eye rings were painted dark, and vice versa. As a control, other groups of birds were drugged but not painted, or drugged and painted with their own color. Still others, of course, were neither drugged or painted.

The result: Eye-ring color (or contrast) did make a difference in several interesting ways. For example, females chose mates that looked like themselves. Light-eyed females chose light-eyed males, and ignored birds of their own species that had their eye rings painted dark. In a word, gulls are gullible.

The next question, then, was how do female gulls know the color of their own eyes? Smith's conclusion: Since mirrors are generally absent on the cliffs of Baffin Island, female gulls must "imprint" on their parents soon after birth, and thereafter seek a mate with eyes like mom and dad.

This business of imprinting is rather wonderful. Even more wonderful, perhaps, is the idea of the intrepid zoologist traveling the Arctic by dog sled and kayak, with paint box and drug capsules, bringing the light of science to bear upon the sex lives of gulls.