An international research team, including UC Riverside's Theodore Garland, has documented the link between the way an animal moves and the dimensions of an important part of its organ of balance, the three semicircular canals of the inner ear on each side of the skull. The research shows that a fundamental adaptive mechanism links a species' locomotion with the sensory systems that process information about the species' environment.

The vertebrate ear is a complex structure with multiple functions. The semicircular canals of the vertebrate inner ear are bony tubes, filled with a fluid, called endolymph, which moves within the canals when the animal moves. The fluid's movement is sensed by special cells that send signals to the brain, which, in turn, uses this information to help coordinate posture and body movements during locomotion. It also triggers the neck and eye muscles to reflexively keep the visual image stable.

"We show quantitatively that primate and other mammalian species that are agile and have fast, jerky locomotion have significantly larger canals relative to body mass than those that move more cautiously," said Garland, a professor of biology, who joined UCR in 2001. "Therefore, future studies can use the relative size of the semicircular canals as a guide to the behavior of fossil (extinct) species, including ancestors of modern human beings."

The researchers studied 91 separate primate species, including all taxonomic families. The study also included 119 additional species, most of which are mammals ranging in size from mouse to elephant, that habitually move in diverse ways in varied environments.

The project is the first large-scale study to document the relationship of the dimensions of the semicircular canals to locomotion.

Study results will be published on 26 June in the print edition of the Proceedings of the National Academy of Sciences and in the journal's online early edition during the week of 18 to 22 June.

The basic hypothesis of the project was that the organ of balance - which helps stabilize an animal's gaze and coordinate its movements as it travels through the environment - should be irrevocably linked to the type of locomotion produced by its limbs.

"If an animal evolves a new way of moving about the world, its organ of balance must evolve accordingly," said Alan Walker, Evan Pugh Professor of Anthropology and Biology at Penn State University, one of the team's leaders. From the visual information, the animal tracks its position relative to stationary objects such as tree trunks, branches, rocks or cliffs, or the ground. Having a stable image of the environment is especially crucial for acrobatic animals that leap, glide, or fly.

To make the discovery, the scientists scanned skull samples of each species, measuring the size of each semicircular canal and calculating the radius of curvature. Most of the specimens were scanned at the Center for Quantitative Imaging at Penn State using a high-resolution x-ray computer tomography (CT) scanner, which can resolve features approximately 1/100 the size of those detected by medical CT scanners.

In addition, experienced field workers used personal knowledge or film of animals in the wild to classify species into one of six locomotor categories ranging from very slow and deliberate to fast and agile. The scientists then compared the canal size of each species to its category of movement.

The results revealed a highly significant statistical relationship between the radius of curvature of the semicircular canals and the species' habitual way of moving. More acrobatic species consistently have semicircular canals with a larger radius of curvature than do slower-moving ones. For example, a small, fast-moving leaper like a bushbaby has semicircular canals that are relatively and absolutely much bigger than those of the similar-sized, slow-moving loris.

However, because larger animals have absolutely larger canals, the analysis had to take body size into account. The research revealed that this functional tie between the semicircular canals and locomotor pattern is evident both within the primates alone and within the entire mammalian sample.

"How an animal moves is a basic adaptation," said Walker, an expert in primate locomotion. "Now we have a way to reconstruct how extinct species moved that is completely independent of analysis of the limb structure. For the first time, we can test our previous conclusions using a new source of information."

Co-leader of the team was Fred Spoor, a professor of anatomy at University College, London. Spoor originally studied a small number of species for his doctoral research and suggested conducting a detailed investigation using modern techniques. Other researchers on the project were Senior Instructional Design Consultant Gail Krovitz, of the eCollege company; Research Associate in Anthropology Timothy M. Ryan of Penn State University; and Associate Professor of Anthropology Mary T. Silcox of the University of Winnipeg in Canada.

The research received financial support from the U.S. National Science Foundation.

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