One of the more surprising features of nature is the ability of two unrelated animals to independently evolve extremely similar body parts. In a new study, scientists demonstrate that rorqual whales — whose members include the largest animal on Earth, the blue whale — have come to share very similar feeding mechanisms and jaw structures with pelicans, despite clearly independent pathways of evolution.
"You can have two organisms that are as completely different as a pelican and a rorqual whale are — you just have to look at them to immediately understand the differences — but, that said, they have these extraordinary similarities as well," said study leader Daniel Field of Yale University. The finding, he said, was the exciting result of a computer program he and his research team designed to examine bone strength.
Both the whales and pelicans use a strategy called engulfment feeding to capture lunch.
Basically they take advantage of stretchy tissue between their jaws to open up their mouths incredibly wide and let in huge rushes of water, hopefully filled with fish and other prey.
"This tissue is so stretchy that rorquals can increase their body volume by up to 50 percent during a feeding event, which is really remarkable because 50 percent of the largest animal on Earth is really huge," Field told LiveScience. And pelicans can increase their total body volume by up to 300 percent.
A strong lower jaw, one able to withstand the high drag forces encountered during engulfment feeding, independently evolved in both species.
When two animals arrive at the same endpoint — in this case, these similar feeding structures — via different processes, it's called convergent evolution.
To test just how similar the jaws of rorqual whales and pelicans are, the researchers designed a special computer program that can assess a bone's ability to resist bending, based on data from CT scans.
"We feel that the computer program is pretty important in its own right," Field said. "It enables scientists to study a bone's bending resistance without damaging rare specimens."
The researchers found that the jaws of both species exhibit the exact same pattern of bending resistance.
"The really interesting implication was the realization that what we had was a pretty textbook example of convergent evolution," Field said. "You have two unrelated animals contending with really similar selective pressures, which forces them to adopt similar adaptations."
Bats and birds
Convergent evolution, while sometimes startling, is not that rare. Whenever two organisms are exposed to similar environmental pressures, such as the challenges of finding prey in the ocean, or even the dryness of a desert, they sometimes end up developing similar ways of coping.
A famous example is wings. Both bats, which are mammals, and birds, which are thought to have evolved from dinosaurs, have wings that enable them to fly. Yet the animals otherwise have little in common. They did not evolve from a common ancestor with wings, but instead arrived at wings on their own. [Galleryof Colorful Wings]
"They evolved similar adaptations for flight, but they did it in totally different ways," Field said. "Convergent evolution is something really prevalent not just in animals but in biology in general."
Learning more about examples of convergent evolution in new organism pairs, such as the rorqual and the pelican, offers scientists a hope of better understanding of the intricacies of evolution.
"To get an example of convergent evolution that is so directly similar, with such close parallels in the feeding apparatus in really different organisms — it's a really compelling example that has the potential to show you how evolution proceeds," Field said.
The new study will be published in the July edition of the journal The Anatomical Record.