Sex drives birds apart

CORIE LOK

The male peacock may owe his tail to promiscuity

New evidence from more than 1,000 of the world's bird species backs up Charles Darwin's theory that male birds tend to be bigger, brighter and more colourful than females because of their differing approaches to sex¹.

Peter Dunn of the University of Wisconsin-Milwaukee and his colleagues collected bird size and plumage data from the scientific literature and from 12 museums around the world. They found that more promiscuous species display greater sexual differences in body mass, feather colour and patterns, and wing and tail length than monogamous species.

Nest height also plays a small role. Birds living in shrubs show greater sexual differences in plumage. Presumably, brighter females in these nests are more susceptible to predators, leaving the duller ones to survive and mate.

"This is the most comprehensive look at such a large number of bird species from all over the world," says Alex Badyaev of Auburn University in Alabama, who studies sexual differences in birds.

In 1871, Darwin proposed that bigger, flashier males attract more female attention, mate more often, and hence pass on their longer tails and brighter feathers more frequently.

Brighter females nursing their young are attractive targets for predators, so duller females tend to survive and mate.

This, he reasoned, generates large differences in plumage and body size between males and females. At the same time, females inherit the preference for the attractive traits which speeds up their evolution.

Evolutionary biologists have been seeking evidence to support this hypothesis ever since. Matters are complicated by the fact that most birds are apparently faithful to one mate.

But even 'monogamous' birds sometimes fool around. DNA fingerprinting paternity tests reveal that around 20% of the brood of many bird species are fathered by a male other than the supposed father.

This sneaky mating can also contributes to sexual differences. Dunn's team looked at testis size, a known indicator of adulterous mating, and found that species with larger testis have larger differences in tail and wing length than more faithful species. Polygamous species do differ more widely, however.

Dunn's team statistically controlled for other factors potentially affecting sexual differences, such as migratory behaviour, to show a clearer connection between mating behaviour and sexual differences.

There are several other possible reasons for sexual differences which this study did not address, says Trevor Price of the University of California at San Diego, who also works on differences between bird species. For example, he says certain territorial species that fight a lot show large sexual differences, perhaps because bigger, brighter males intimidate invaders and win more fights -- and more mates.

Nonetheless, these male populations maintain diversity, Price says, as duller males can sneak in some copulations while flashy males are busy fighting.

1. Dunn, P. O., Whittingham, L. A. & Pitcher, T. E. Mating systems, sperm competition and the evolution of sexual dimorphism in birds. Evolution 55, 161 - 175 (2001).

© Nature News Service / Macmillan Magazines Ltd 2001