ENHS Nature Trails October 2000 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

The hummingbird's beak: A tale of ecology driving evolution

Sexual dimorphism, differences in size and shape between males and females of the same species, is widespread among the animal kingdom. Nearly 150 years ago Charles Darwin observed that sexual dimorphic characteristics are likely to be the result of sexual selection, fecundity selection, or ecological causes. The first two explanations, sexual and fecundity selection, have been unambiguously and repeatedly demonstrated in numerous species. For example, the antlers of male deer or the brightly colored body patches of certain male lizards play an important role in sexual selection, the process of obtaining a mate. Fecundity selection explains other sexually dimorphic species, as in the case of the increased size of the female pray mantis, who is half again as long as her mate due to the large mass of eggs she carries. Unfortunately Darwin's third explanation for sexual dimorphism, ecological causes, has been much more difficult to demonstrate experimentally.

Ecological explanations for sexual dimorphic characteristics have been notoriously problematic because most species use the same environmental resources for feeding, sleeping and other day-to-day activities. Darwin himself was well aware of this difficulty and suggested that sexual differences may be confined to cases where each sex fed on different foods and thus evolved different feeding apparatuses. To illustrate this type of sexual dimorphism, Darwin used the New Zealand huia, a now extinct bird. Male and female huias had similar body sizes and plumage, but the bill of the male was short, thick and straight whereas the female's bill was longer, slender, and slightly curved. Darwin proposed hummingbird beaks as second example of sexual dimorphism due to differences in food use but did not pursue this further. It took about 150 years to follow up on Darwin's keen observation about hummingbirds, but was worth the wait since the study is the first clear cut example of an ecological cause for sexual dimorphism.

The study, by Temeles and colleagues and published in Science (Vol. 289, pp441-443, 21 July 2000) was performed in on the island of St. Lucia in the West Indies. This island is the home of purple-throated Carib hummingbird, Eulampis jugularis, a moderately large, upland hummingbird (see Figure 1). Temeles and his co-workers showed that two sexes of this species feed on different plants. The male hordes nectar from Heliconia caribaea, whose flower is short and slightly curved. In contrast the female shuns the plant favored by the male and primarily eats nectar from a related species Heliconia bihai, which contains longer and very curved flowers.

What makes this study so striking is that the differences in food sources among male and female purple-throated Carib hummingbirds is precisely mirrored by differences in beak size. Male beaks are short and curved at a moderate 15o angle whereas female beaks are 30% longer and have a 30o curvature. In each case the bill and flower match precisely, which allows each sex to feed more quickly and presumably more efficiently on their flower of choice.

How did this sexual dimorphism evolve? Temeles speculates that when hummingbirds first invaded St. Lucia about ten thousand years ago, the larger, dominant males likely preferred feeding on Heliconia caribaea, a plant bearing more flowers. The smaller, more submissive females were likely excluded from feeding on this plant and resorted to obtaining food from the less showy Heliconia bihai. Over time, the bills of each gender adapted to fit their flower of choice in order to maximize feeding. This study demonstrates, for the first time, that ecology drives evolutionary change and should provide the impetus for additional work in the future on the relationship between ecology and evolution.

Nathan Tublitz
Professor of Biology
Institute of Neuroscience
University of Oregon
Eugene OR 97403
Phone: 1-541-346-4510 FAX: 1-541-346-4548

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