March 26, 2010
AMHERST, Mass.—Explanations for the differences in appearance and behavior between males and females of many animal species may still elude scientists, but AmherstCollege biology professor Ethan Temeles and his colleagues are two steps closer to understanding how hummingbirds of both sexes eat and mate.
Temeles in the field in Dominica
Temeles and his coworkers published two separate papers recently that shed some light on the creatures’ bill size and shape and their mating preferences. Both studies offer new knowledge about how the tiny birds function but also provide strong support for Charles Darwin’s famous theories of natural and sexual selection.
The first paper, titled “Evolution of sexual dimorphism in bill size and shape of hermit hummingbirds,” was published in March issue of The Philosophical Transactions of the Royal Society B and was co-authored by fellow AmherstCollege biology professor Jill Miller and honors student Joanna Rifkin ’09. In it, Temeles, Miller and Rifkin report that the specific differences in the length and shape of the bills of male and female purple-throated caribs (also known as Eulampis jugularis, a species that Temeles has long studied in Dominica, West Indies) are quite common among several other birds in the hummingbird family. Their findings support the hypothesis that these “sexual differences,” as they are described, have apparently evolved to allow birds of each sex to feed from flowers that match the size and shape of their bills.
According to the study, the bodies of males are 25 percent larger than the bodies of females, although females have bills that are 20 percent longer and 40 percent more curved. As a result, males primarily feed at the flower Heliconia caribea—which has short, straight flowers—and females feed on the long, curved flowers of Heliconia bihai. Other species of birds exhibited similar sexual differences.
“This study adds further support to Charles Darwin’s theory that some sexual differences in animals evolved to allow males and females to differ in what and how they eat,” said Temeles.
A female purple-throated carib
“Ethan and his students have done, and continue to do, excellent studies documenting sex-specific differences in the purple-throated carib,” said Miller. “The real power of this research is that it extends previous work by identifying similar sex-specific patterns of bill dimorphism in the closely related hermit hummingbirds, thus broadening the significance of sexual dimorphism in feeding ecology. In addition, Joanna’s thesis and the resulting paper also provided an opportunity for collaboration between Ethan and myself, which was a real treat for me.”
The paper, Temeles added, is part of an invited volume honoring Darwin and his finches and appears alongside the work of other distinguished scientists. “For Jill, Joanna and myself to be included in this special volume of the Royal Society on Darwin’s finches is thrilling. The entire volume is very special.”
The second paper, “Mate choice and mate competition by a tropical hummingbird at a floral resource,” was published on the Web site of The Proceedings of the Royal Society B in February and focused on the purple-throated carib hummingbirds exclusively. The study details how Temeles and co-author W. John Kress at the Smithsonian Institution were able to determine for the first time the cue that these female hummingbirds use to choose males: food. “In this case, it’s quite simple: the females prefer to mate with males that have large amounts of nectar in their flowers,” said Temeles.
The pair also found that a male hummingbird’s ability to maintain high levels of nectar in his flowers depends on two things: (1) his ability to defend his territory against intruders that might steal his nectar and (2) his partitioning of his territory specifically into flowers that he uses and defends for his own nutritional needs and flowers that he uses and defends for attracting females as potential mates.
The work builds on previous research Temeles has done on the purple-throated carib that was also featured on the PBS program Nature recently.
“This study indicates that the larger size of male purple-throated carib hummingbirds is a result, in part, of their aggressive interactions with one another,” explained Temeles. “In other words, it is the larger male who will win a fight with a smaller male—and thus win the right to reproduce, which is exactly what Darwin hypothesized would happen in his theory of sexual selection.”
“To confirm what we thought by seeing such fights associated with the amount of nectar in males’ flowers, and then seeing females choosing and mating with males that have the most nectar, was amazing,” he added. “It was the best two and a half months I’ve ever spent in a rain forest.”
Temeles received a bachelor’s degree in biological sciences from CornellUniversity and went on to earn a master’s in zoology from LouisianaStateUniversity. He completed his doctoral work at the University of California, Davis, and joined the Amherst faculty in 1994.
Miller received a bachelor’s degree in biology from ColoradoCollege and earned a master’s degree and doctorate in ecology and evolutionary biology from the University of Arizona. She joined the Amherst community in 2003.
The full texts of the papers are available at http://rspb.royalsocietypublishing.org/content/early/2010/01/28/rspb.2009.2188.abstract?sid=3cb1436c-ab50-4e43-95fe-f1dbd42541ba and http://rstb.royalsocietypublishing.org/content/365/1543/1053.abstract?sid=de176c3e-787c-4ed7-8f46-908241529b80.
Founded in 1821, AmherstCollege is a highly selective, coeducational liberal arts college with approximately 1,600 students from most of the 50 states and more than 30 other countries. Considered one of the nation’s best educational institutions, Amherst awards the B.A. degree in 34 fields of study.