Amherst Biology Professor Reports on Sexual Dimorphism among Hummingbirds in Science

May 8, 2003
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AMHERST, Mass.-The difference between boys and girls may result from small changes in their environment-if you're a hummingbird. Ethan J. Temeles, associate professor of biology at Amherst College, and his colleague W. John Kress of the Smithsonian Institution report in the April 25 issue of Science, that the shape and length of the bill of the male purple-throated carib hummingbird has evolved differently from the bill of the female, to better fit the flowers that provide the birds with nectar. At the same time, the flowers of the Heliconia plant have adapted to the bills of the birds that are their means of reproduction.

Temeles and Kress found evidence of ecological causes of sexual dimorphism in bill morphology of Eulampis jugularis on the islands of St. Lucia and Dominica.

The flower and the bill have co-evolved in an unusual manner. The male birds, which feed predominantly on a red-bracted variety of heliconia, have relatively short and straight beaks. The females feed on a green-bracted heliconia, and their beaks are longer and more curved.

Sexual dimorphism in bill morphology and body size of the Caribbean purple-throated carib hummingbird is associated with a reversal in floral dimorphism of its Heliconia food plants. This hummingbird is the sole pollinator of H. caribaea and H. bihai, with flowers of the former corresponding to the short, straight bills of males, the larger sex, and flowers of the latter corresponding to the long, curved bills of females.

On St. Lucia, H. bihai compensates for the rarity of H. caribaea by evolving a second color morph with flowers that match the bills of males, whereas on Dominica, H. caribaea evolves a second color morph with flowers that match the bills of females. The nectar rewards of all Heliconia morphs are consistent with each sex's choice of the morph that corresponds to its bill morphology and energy requirements, supporting the hypothesis that feeding preferences have driven their coadaptation.

Temeles has a Website at www.amherst.edu/~ejtemele/; Science magazine at www.sciencemag.org/.

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