A northern gannet scans the sea for fish from 100 feet in the air. Upon seeing a school, it pitches its body into a nosedive, somehow breaking the surface at 70 miles per hour without breaking its neck. This daring maneuver seems to defy the laws of nature, but it's standard hunting behavior for plunge-diving seabirds.
A University of Washington study published Oct. 12 in the Anatomical Record shows that gannets and boobies have unique morphological adaptations that allow them to survive impact. Several of their neck vertebrae are joined by an additional contact surface that helps stabilize the cervical column. The overall shape of the vertebrae is also stockier than other birds.
"We wanted to figure out what makes gannets so special, so good at diving and looking at their biology helped us understand how they can survive these impacts," said senior author Bart Boom, a postdoctoral scholar at the UW's Friday Harbor Labs.
The ecological record shows repeated instances in which birds develop physical traits in response to their environment. Charles Darwin was among the first to record this in in the 1830s - in finches on the Galápagos Islands - and researchers are still discovering new examples today. The connection between beak shape and foraging behavior is well documented, but the link between neck structure and feeding has received less attention.
The UW researchers began studying the mechanics of plunge-diving years ago. Gannets are built to withstand immense pressure on the head and neck that compresses the cervical column. Engineers typically favor short and fat structures for things built to tolerate compressive force, but these birds have long, slender necks with segmented vertebrae.
Gannet necks have 15 stacked vertebrae that bend slightly in an s-curve. Previous studies have shown that the muscles surrounding their vertebrae help lock them in place to prepare for impact, but few studies have looked at the actual morphology of diving birds' necks.
To do so, the researchers collected specimens of diving birds and non-diving birds from the Burke Museum. The team performed CT scans to create detailed three-dimensional models of the cervical vertebrae and then analyzed the images with geometric morphometrics, a statistical method for studying biological samples that establishes landmarks for comparison. They considered the forces associated with various feeding styles and chose birds that represented a spectrum, pulling in additional morphological data from previous studies.
"Birds that rotate their heads side to side, or move them front to back, separate out from birds that use their heads in compression," said lead author Megan Vanderberg, a graduate student of biology at UW's Friday Harbor Labs. "We see that the birds that plunge-dive have more robust vertebrae. They're thicker, wider, and also shorter, shaped more like a square."
The stocky shape of the vertebrae provides added support, as does the additional contact surface, called an articular facet, in second through the sixth vertebrae. In this case, researchers think the extra facet keeps the birds' necks from bending backward when they hit the water at an angle. Instead, the load gets transferred down the cervical column.
The researchers also observed more blocklike vertebrae in other diving birds, such as the kingfisher, but did not detect the extra facet. Kingfishers belong to a different avian family and are much smaller than gannets and boobies, but their necks seem to have evolved in a similar way. The structural relationship is also proportional: the larger the neck, the larger the vertebrae.
Kingfishers and the bigger plunge-divers also experience the same amount of force per neck area during a dive. Although total force scales with neck size, the amount of strain on the tissue is the same. Both groups experienced peak force nearly five milliseconds after impact despite the size difference.
Moving forward, the researchers hope to explore how force transfers down the neck in more detail. They also think this link between structure and function could help paleontologists identify plunge-diving birds from fossils.
"It feels like something I didn't previously think about and probably overlooked before," Vanderberg said. "I'm curious to start looking more closely at vertebrae in general, not just in birds."
Co-authors include Adam P. Summers, professor of biology and of aquatic and fishery sciences at UW; Ed Habtour, assistant professor of aeronautics and astronautics at UW; John Michael Racy, graduate student of aeronautics and astronautics at the UW; and Alissa Perez of the University of California, Riverside.
This research was funded by the U.S. National Science Foundation, Beatrice and Samuel A. Seaver Foundation, Friday Harbor Laboratories Research Endowment Fellowship, University of Washington Friday Harbor Laboratories Award, and Jim and Camille Uhlir.
