The purple-throated carib hummingbird's bill has evolved different lengths and curvatures for males and females to best exploit the shape of the flowers that provide them nectar.
The flowers of the Heliconia plant have evolved shapes adapted to the bills of the birds that are their means of reproduction.
But the adaptations that keep the birds and flowers thriving on the Caribbean island of St. Lucia differ from those that occur among the same birds and plants about 100 miles north on Dominica, researchers report in Friday's issue of the journal Science.
"We can't really say which came first," said Ethan J. Temeles of Amherst College. "Over time, both the bills of the birds and the (structure) of the flowers adapted to each other."
"It is probably best to view the interaction between these hummingbirds and their flowers as a dynamic one, in which each party can drive the evolution of the other," Temeles said.
By studying museum collections, he said he had found that bird bill length and the shape of flowers vary by date and location.
"Birds in the north end of the Lesser Antilles are larger and have longer bills than birds in the south. ... Birds from the same island may have different bill lengths depending upon whether they were collected in 1920, 1970, or the present," Temeles said. "We see the same changes in the flowers, too. So, the answer is that both parties likely change with each other over time."
But co-author W. John Kress said he believes that the plants colonized the Lesser Antilles islands from the south when the island chain formed thousands of years ago and the differences between the male and female birds led to the differences among the flowers.
"We know it led to the difference in the plants" he said. The question is whether the birds caused separate species of the plants to develop, said Kress, head of botany at the Smithsonian's National Museum of Natural History.
In the type of hummingbirds studied by Temeles and Kress, the males are larger than females, while the females' bills are longer and more curved.
They observed that on both islands the males eat from and defend the caribaea, which has short flowers but produces more nectar. The smaller females fed on the bihai, which has longer flowers and not as much nectar.
As a result, males control a rich source of nectar while females have exclusive use of a separate food source that their bills can reach.
On St. Lucia, the researchers found some forest reserves where the caribaea preferred by the males is scarce or missing altogether. In those areas, a type of bihai has developed with shorter flowers that the males can feed on.
On Dominica, instead of two types of bihai found on St. Lucia, the researchers saw two forms of caribaea, with one of them having longer flowers that the females could use.
On St. Lucia both plants live in close proximity to one another. On Dominica, the caribaea preferred by the males lives in the lower slopes of hills and bihai occupies higher elevations. It is in the areas where the two came into contact with one another that the caribaea had a long-flowered version that female hummingbirds could exploit.
Heliconias are also known as "lobster claws" and they belong to a group of plants that include bananas, gingers, and bird-of-paradise flowers.
"The study provides evidence for reciprocal evolution between species, in this case hummingbirds and Heliconia," Douglas L. Altschuler of the California Institute of Technology and Christopher James Clark of the University of California, Berkeley, said in a commentary on the paper.
"Hummingbird and Heliconia engage in a co-evolutionary dance, with flower shape evolving in response to hummingbird bills and bill shape evolving in response to flower shape," Clark and Altschuler added. "By offering nectars containing different amounts of energy, Heliconia species select for different body sizes" in the birds.
By Randolph E. Schmid