The Nature of Personalities

Staff scientist Wesley Hochachka shares perspectives on the debate about nature versus nurture, from the world of birds

Fundamentally, a peacock's tail seems to be an incredibly stupid thing...gorgeous but stupid. Why should any animal have to haul around something that cumbersome in the name of being attractive? Wouldn't a tail half the length be sufficient? One possible explanation is that a "more-is-better" preference is hard-wired into potential mates in the same way that tail length is inherited. The result would be a runaway increase in tail length over the course of generations that would only cease when death by ornamentation sufficiently diminished chances of living long enough to attract mates.

Are mating preferences really genetically hard-wired, or do other influences determine a bird's mating behavior? These questions were addressed by Wolfgang Forstmeier of Germany's Max Planck Institute for Ornithology and colleagues in the November 2004 issue of Evolution. Studying a fully pedigreed captive population of Zebra Finches, these researchers shuffled eggs among nests to separate the roles of parents and rearing environments on the offsprings' mating behavior. They also examined differences among siblings by seeing whether mating behavior differed between birds hatched from early-laid and late-laid eggs.

The results were surprising. There was no clear sign of any genetic basis for variation in male mating behavior (aggressiveness and song rate) or for female preference. However, there were consistent differences. Some mothers produced consistently more aggressive sons and sons that sang more frequently, and it was the mothers who laid the eggs, not the foster mothers who reared the offspring, that were responsible for these differences. Likewise, some mothers produced eggs that yielded daughters who preferred more aggressive males and males who sang more frequently. Not only that, but daughters from first-laid eggs in a clutch were choosier than daughters from last-laid eggs. However, daughters who preferred males who sang frequently did not have brothers who sang at high rates.

I still have not sorted out in my own mind what all of these results might mean, but two conclusions are obvious. First, aspects of adult behavior can be fixed at an extremely early age, before an egg is even laid. Second, a mother influences her grown offsprings' behavior, not just through the genes she provides, but in other ways, probably through variation in the levels of hormones inside the eggs. The whole nature versus nurture argument has just become a lot more interesting.

For permission to reprint all or part of this article, please contact Miyoko Chu, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-2451. email: mcc37@cornell.edu