PerspectiveECOLOGY AND EVOLUTION

Darwin's Hummingbirds

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Science  25 Apr 2003:
Vol. 300, Issue 5619, pp. 588-589
DOI: 10.1126/science.1084477

Islands [HN1] have had a long association with evolutionary biology ever since Darwin observed the myriad variations in species from the Galápagos and other archipelagos and used them to formulate his theory of evolution by natural selection. The simplicity of island flora and fauna and the replicate series of study sites that islands provide enable contemporary biologists to identify how ecological processes such as competition and predation have shaped evolution within communities (1, 2). On page 630 of this issue, Temeles and Kress (3) [HN2] present a comparative study of hummingbirds [HN3] and their Heliconia [HN4] food plants from islands of the Lesser Antilles [HN5]. Their study provides evidence for reciprocal evolution (coevolution) between species [HN6], in this case hummingbirds and Heliconia. In addition, they elucidate how ecological pressures shape the different morphologies of males and females (sexual dimorphism) within a species, evidence that has been difficult to obtain from studies on mainland fauna.

Hummingbirds consume flower nectar using sustained hovering [HN7] when feeding, a highly specialized behavior. Although all hummingbirds display this behavior, they show wide variations in their body mass, and in the size and shape of their bills and wings (4). Another curious feature of hummingbirds is the pronounced difference in color (5), bill shape (6), and body size between males and females. For example, females tend to be larger in small species and males tend to be larger in large species (7). A central challenge for comparative biologists is to understand the sources of this remarkable variation.

Sexual dimorphism [HN8] arises through one of three mechanisms: sexual selection, fecundity selection, and ecological causation. Darwin noted that ecological causes should be associated with the feeding apparatus and even suggested hummingbirds as an example (8) [HN9]. However, finding proof of ecological causation is very challenging because sexual dimorphism of feeding structures usually correlates positively with differences in body size. In previous work, Temeles et al. (9) [HN10] demonstrated ecological causation of bill size dimorphism in the hummingbird Eulampis jugularis [HN11] from the Caribbean island of St. Lucia [HN12]. In this species, males are larger than females but have shorter and less curved bills [HN13] (see the figure), thus exhibiting a reversal in the dimorphism between bill and body size.

A dance between hummingbird and Heliconia.

The feeding relationships between the Caribbean purple-throated hummingbird, Eulampis jugularis, and different morphs of two species of Heliconia (H. caribaea and H. bihai) on the islands of St. Lucia and Dominica. Solid lines indicate that hummingbirds regularly feed from that flower morph, whereas dashed lines indicate rare or occasional feeding forays.

CREDIT: KATHARINE SUTLIFF/SCIENCE

On St. Lucia, the hummingbirds feed primarily on two species of Heliconia: H. caribaea characterized by a red bract (the hardened structure that protects the flower's corolla) and H. bihai, which has a green bract (see the figure). Male hummingbirds defend patches of H. caribaea and feed on the flowers, which have relatively straight and short corollas [HN14], closely matching the shape of the male bill. In contrast, females imbibe nectar from the longer and more curved corolla of H. bihai flowers and forage over greater distances among undefended plants, a behavior known as “traplining.” In sites where H. caribaea is rare or absent, there appears an additional red-green-bracted morph of H. bihai with a corolla that is straighter and shorter than that of the green-bracted variety. The corolla of the H. bihai red-green morph has evolved a similar shape and size to that of the absent H. caribaea, and this altered morphology matches the shape of the E. jugularis male bill. The close match between bill morphology and flower shape demonstrates that feeding ecology [HN15] can drive bill dimorphism.

The morphs ofHeliconia on St. Lucia. (Left) H. bihai green morph, (middle) H. bihai red-green morph, (right) H. caribaea red morph. [From (3)]

These findings in St. Lucia hummingbirds strongly suggest that bill dimorphism stems from ecological causes. In their new study on the neighboring island of Dominica [HN16], Temeles and Kress (3) report that the male and female hummingbirds split floral resources in a new way, providing even stronger evidence that ecological pressures can induce bill dimorphism. Although the same Heliconia species are present on Dominica, there are two H. caribaea morphs (red- and yellow-bracted) and only one H. bihai morph (red with a yellow stripe). On St. Lucia, the corolla of the H. bihai red-green-bracted morph matches the shape of the male bill, but on Dominica it is the red-bracted H. caribaea corolla that has become longer and more curved to match the shape of the female bill (see the figure). Males prefer the yellow-bracted H. caribaea, which has a shorter and straighter corolla than the red-bracted morph. At some sites where only the two H. caribaea morphs are present, their corollas do not differ and both sexes of hummingbird feed equally from both flower morphs.

On both islands, the energetic rewards of the flower morphs correspond to the body sizes of male and female hummingbirds. Because males are larger than females, it follows that they require more energy (10). If the flower morphs offer the same amount of energy, then the same hummingbird body size would be optimal for nectar extraction, and we would reject the hypothesis that foraging ecology drives size dimorphism. On both islands, H. bihai offers slightly less nectar than H. caribaea, corresponding to the difference in body size of the male and female hummingbirds that feed on the two different morphs. Similarly, on St. Lucia, the red-green replacement morph of H. bihai offers more nectar than the green morph, whereas on Dominica the red replacement morph of H. caribaea offers less nectar than the yellow morph.

The Antillean islands of St. Lucia and Dominica provide two striking examples of ecological environments that produce sexual dimorphism in hummingbirds. The rearrangements of flower morphology and nectar productivity are surprisingly plastic. Hummingbird and Heliconia engage in a coevolutionary dance, with flower shape evolving in response to hummingbird bills, and bill shape evolving in response to flower shape. By offering nectars containing different amounts of energy, Heliconia species select for different body sizes. Although this is not evidence that size dimorphism in E. jugularis evolved solely in response to foraging ecology, it casts doubt on assumptions that sexual dimorphism is a measure of sexual selection (11).

On the islands of St. Lucia and Dominica, this coevolutionary dance is finely tuned. These island ecosystems provide a natural laboratory for further studies of reciprocal evolution. The hummingbirds are clearly benefitting from the plants: Males control an energy-rich accessible food source and females have nearly exclusive use of an alternative food source. On the other side of the equation, how do differences in animal feeding behavior and efficiencies affect plant fitness (12)? Gene flow is clearly available to plants with traplining pollinators but how do plants with territorial pollinators accomplish genetic outcrossing? Also, does the consumption of insects by hummingbirds directly influence hummingbird morphology (13) and indirectly influence flower morphology? Once again, islands may provide the ideal setting for answering these questions.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

The xrefer Web site provides scientific dictionaries and other reference works.

The University of California Museum of Paleontology offers a glossary of natural history and biological terms.

A glossary of ecology terms is provided by the companion Web site for Essentials of Ecology by C. Townsend, M. Begon, and J. Harper.

A glossary is provided by the Animal Diversity Web of the University of Michigan Museum of Zoology.

A glossary of biogeography is provided by S. Woodward, Department of Geography, Radford University, VA.

Web Collections, References, and Resource Lists

The Google Web Directory provides links to Internet resources in ecology and evolution.

Biology Links: Evolution are provided by the Department of Molecular and Cellular Biology, Harvard University.

The Ecology WWW page is maintained by A. Brach, Harvard University Herbaria.

K. Holsinger, Department of Ecology and Evolutionary Biology, University of Connecticut, provides links to Web sites of interest to botanists, ecologists, and evolutionary biologists.

Evolution Links are provided by J. Banta, Department of Ecology and Evolutionary Biology, University of Tennessee.

Evolution Update is maintained by S. Spencer.

The International Center for Tropical Ecology at the University of Missouri, St. Louis, provides a collection on ecology links.

The Ornithological Council's BIRDNET provides links to ornithology resources.

OWL (Ornithological Web Library) provides links to Internet resources.

Online Texts and Lecture Notes

Evolution is a presentation of the Public Broadcasting Service. A collection of educational resources on adaptation and natural selection is included.

Kimball's Biology Pages offer presentations on evolution and adaptation and speciation, which has a section on Darwin's finches.

The Bird Division, University of Michigan Museum of Zoology, offers a collection of resources on bird biology, including a presentation on bird beaks.

C. Lindell, Department of Zoology, Michigan State University, offers lecture notes for a course on the biology of birds.

R. Koford, Department of Natural Resource Ecology and Management, Iowa State University, offers a study guide for an avian ecology course.

Ornithology.com offers lecture notes on ornithology by R. Lederer.

G. Ritchison, Department of Biological Sciences, Eastern Kentucky University, provides lecture notes for courses on ornithology and behavioral ecology.

D. Straney, Department of Cell Biology and Molecular Genetics, University of Maryland, makes available lecture notes for a course in plant biology.

P. Ganter, Biology Department, Tennessee State University, Nashville, provides lecture notes for an ecology course.

D. Walters, Biological Sciences Department, California Polytechnic State University, San Luis Obispo, provides lecture notes for a course on evolution. Lecture notes on coevolution are included.

K. Holsinger, Department of Ecology and Evolutionary Biology, University of Connecticut, offers lecture notes for a course on evolutionary biology.

D. Rand, Program in Biology, Brown University, provides lecture notes for a course on evolutionary biology.

D. Fitch, Department of Biology, New York University, offers lecture notes and other resources for a course on evolution.

General Reports and Articles

The January-February 2002 issue of the Zoogoer had an article by T. Dunn titled “Hummingbirds: Frantic and fascinating.”

The Fall 2001 issue of California Wild had an article by L. F. Baptista titled “Feathered gems: The song and dance of hummingbirds.”

The 26 April 2002 issue of Science had a Research Article by P. R. Grant and B. R. Grant titled “Unpredictable evolution in a 30-year study of Darwin's finches” and a News of the Week article by C. Zimmer titled “Darwin's avian muses continue to evolve.”

The 24 January 2003 issue of Science had a report by R. A. Schneider and J. A. Helms titled “The cellular and molecular origins of beak morphology” and a Perspective by P. Trainor titled “The bills of qucks and duails.”

The May 1999 issue of Natural History had an article (available on the Find Article Web site) by E. J. Temeles and P. W. Ewald titled “Fitting the bill?”

Numbered Hypernotes

1. Island ecology. Centres for Plant Diversity: The Americas, a presentation of the Botany Program, Smithsonian National Museum of Natural History, includes a section on Middle America and Caribbean Islands. C. Lindell provides lecture notes on islands for a course on zoogeography. A chapter on islands is included in Biodiversity and Conservation, a hypertext book by P. Bryant, School of Biological Sciences, University of California, Irvine. F. Davis, Donald Bren School of Environmental Science and Management, University of California, Santa Barbara, offers lecture notes on island biogeography for a biogeography course. H. Grissino-Mayer, Department of Geography, University of Tennessee, makes available lecture notes on island biogeography (parts one, two, three, and four) for a course on biogeography at Valdosta State University.

2. E. J. Temeles is in the Biology Department, Amherst College, Amherst, MA. W. J. Kress is in the Department of Botany, National Museum of Natural History, Smithsonian Institution.

3. Hummingbirds. The Columbia Encyclopedia provides an introduction to hummingbirds. D. Roberson's Bird Families of the World includes a section on hummingbirds. The eNature.com Web site offers an illustrated guide to North American hummingbirds. The chapter on neotropical birds of J. Kricher's A Neotropical Companion has a section on hummingbirds. The Hummingbird Web Site is provided by Portal Productions. The Mildred E. Mathias Botanical Garden at the University of California, Los Angeles, makes available a newsletter article about hummingbirds. Chloe's Hummingbirds Web site includes illustrated descriptions of Caribbean hummingbirds. J. Feldhusen's Hummingbird Web site includes a collection of web links.

4. Heliconiaceae. Heliconia is defined in xrefer's Macmillan Encyclopedia. J. H. Frank, Entomology and Nematology Department, University of Florida, provides an introduction to Heliconia. The Plant Systematics Collection Web site, an image collection provided by the Department of Botany, University of Wisconsin, includes information on the Heliconiaceae. L. Watson and M. J. Dallwitz's Families of Flowering Plants, made available by the Biodiversity and Biological Collections Web site, includes a section on the Heliconiaceae. Images of Heliconiaceae are provided by the Vascular Plant Image Library of the Digital Flora of Texas. The Heliconia Society International provides introductions to Heliconia species.

5. The Lesser Antilles in the Caribbean. Lesser Antilles is defined in xrefer's Macmillan Encyclopedia. Maps.com offers an overview map of the Caribbean. The University of Texas's Perry-Castañeda Library Map Collection makes available a map of the Greater and Lesser Antilles, as well as a CIA reference map of the Caribbean. The World Sites Atlas provides a map of the Lesser Antilles. The National Geographic Society provides a zoomable West Indies map.

6. Coevolution. Coevolution is defined in xrefer's Dictionary of Biology. The Birds of Stanford Web site offers an article on coevolution. D. Rand provides lecture notes on coevolution for a course on evolutionary biology. J. Stein Carter, Department of Biology, University of Cincinnati Clermont College, offers a presentation on coevolution for an ecology course. D. Straney makes available lecture notes on adaptations for a course in plant biology. R. Koning, Biology Department, Eastern Connecticut State University, provides lecture notes on pollination adaptations for a course on plants and human affairs. W. Gaud, Department of Biological Sciences, Northern Arizona University, offers lecture notes on coevolution for an ecology course. G. Folkerts, Department of Biological Sciences, Auburn University, provides lecture notes on coevolution for a course on evolution and systematics.

7. Hummingbird hovering. The About.com Guide to Birding/Wild Birds provides an introduction to bird flight. The Hummingbird Society provides an introduction to hovering. G. Ritchison offers lecture notes on bird flight (part one and part two, which discusses hummingbird hovering) for an ornithology course. The Hummingbird/MAV Project offers a presentation on the background theory of hummingbird flight. The August 2002 issue of the Journal of Experimental Biology had an article by D. Altshuler and R. Dudley titled “The ecological and evolutionary interface of hummingbird flight physiology.”

8. Sexual dimorphism. Dimorphism is defined in the Animal Diversity Glossary and in xrefer's Dictionary of Biology. Sexual dimorphism is defined in xrefer's Dictionary of Earth Sciences. E. J. Temeles offers a presentation on sexual dimorphism in a hummingbird. I. Owens, Department of Biological Sciences, Imperial College of Science, Technology and Medicine, makes available (in PDF format) a 1998 article by I. Owens and I. Hartley titled “Sexual dimorphism in birds: Why are there so many different forms of dimorphism?” The 22 December 1999 issue of the Proceedings of the Royal Society of London B had an article (full text available in PDF format) by R. Bleiweiss titled “Joint effects of feeding and breeding behaviour on trophic dimorphism in hummingbirds.”

9. Darwin on hummingbirds. Sexual dimorphism in hummingbird beaks is discussed (p. 359–360) in chapter 13 (“Secondary sexual characters of birds”) of Darwin's Descent of Man; subsequent chapters on birds (14 through 16) include other hummingbird observations.

10. Previous hummingbird study by Temeles. The 21 July 2000 issue of Science had a report by E. J. Temeles, I. L. Pan, J. L. Brennan, and J. N. Horwitt titled “Evidence for ecological causation of sexual dimorphism in a hummingbird” and a News of the Week article by K. Brown titled “Food fight drives evolution.” The 22 July 2000 issue of Science News had an article (available from the Find Article Web site) by S. Milius titled “Flowers, not flirting, make sexes differ.”

11. Eulampis jugularis. InfoNatura includes an entry for Eulampis jugularis; a distribution map is included. A sample plate from volume 5 of Handbook of the Birds of the World includes an illustration of E. jugularis. The Greg Lasley Nature Photography Web site offers photos of E. jugularis. A. Fossé's Digimages Naturalistes Web site includes a collection of photos of E. jugularis. E. J. Temeles provides photos of male and female E. jugularis showing bill differences.

12. St. Lucia. The Columbia Encyclopedia has entries for Saint Lucia and the Windward Islands, the southern group of the Lesser Antilles in the West Indies. The World Factbook 2002 has an entry for St. Lucia. The Holt, Rinehart and Winston World Atlas provides a map of St. Lucia as well as a map of the Caribbean region. Information about St. Lucia is provided by the St. Lucia Tourist Board, the St. Lucia Hotel and Tourism Association, and the government of St. Lucia. The University of Texas's Latin American Network Information Center (LANIC) provides links to Internet resources about St. Lucia.

13. H. caribaea and H. bihai. An image of H. caribaea is included in W. J. Kress's Key to the Suborders and Families of Zingiberales. E. J. Temeles makes available photos of H. caribaea and H. bihai (one and two). The Heliconia Gardens, R_o Grande, Puerto Rico, offers an introduction to H. caribaea, as well as a presentation on the evolution of heliconias in the Americas. Aloha Tropicals provides images of commercial H. caribaea and H. bihai varieties. The Environmental Horticulture Program at the University of Florida makes available a fact sheet (in PDF format) on H. caribaea.

14. Flower structure. Bract and corolla are defined in xrefer's Dictionary of Biology. Garden Botany from the Brooklyn Botanic Garden offers definitions of the parts of a flower. S. Wolf, Biology Department, California State University, Stanislaus, offers lecture notes on floral terminology (with a diagram) for a course on flowering plants. D. Nickrent, Department of Plant Biology, Southern Illinois University, offers lecture notes on flower structure and pollination for a course on plants and society. H. Wilson, Department of Biology, Texas A&M University, provides lecture notes on flower structure (parts one and two) for a course on the taxonomy of flowering plants.

15. Foraging ecology. Ornithology.com makes available R. Lederer's lecture notes on foraging and nutrition. G. Ritchison provides lecture notes on foraging for a course on behavioral ecology. M. Johnson, Department of Wildlife, Humboldt State University, Arcata, CA, offers lecture notes (in Word format) on foraging ecology for an ornithology course. R. Curry, Department of Biology, Villanova University, makes available a laboratory exercise handout on foraging behavior of pollinators for an ecology course.

16. Dominica. The Columbia Encyclopedia has an entry for Dominica. The World Factbook 2002 has an entry for Dominica. The Holt, Rinehart and Winston World Atlas provides a map of Dominica. Information about Dominica is provided by the Commonwealth of Dominica, VisitDominica.com, and Nature Island Destinations. AVirtualDominca.com offers an introduction to the flora and fauna of Dominica. LANIC provides links to Internet resources about Dominica.

17. D. L. Altshuler is in the Department of Bioengineering, California Institute of Technology.

18. C. J. Clark is in the Department of Integrative Biology, University of California, Berkeley.

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