Lemurs and Gummy Bears

For lemurs taking part in a new research project at Duke's Primate Center, the experience will be a feast of gummy bears, apples, popcorn kernels, hard nuts, dried fruits, and local foliage. But to the scientists feeding them, the aim will be to gain new insights into the evolution of a critically important process to all mammals--chewing.

The scientists say the study will also add a piece to the puzzle of "convergent or parallel evolution," the independent evolution of the same structure in disparate species. Convergent evolution is seen in bats and birds as they have independently evolved similar flight structures. Similarly, dolphins resemble fish, even though, as with birds and bats, they are far removed from one another on the evolutionary tree.

Under a new $186,000 National Science Foundation grant, center director William Hylander and his colleagues will conduct detailed studies of the complex activity or firing patterns of jaw muscles of four lemur species as they chew various foods. While the lemurs chomp away at their treats, scientists will measure with hair-thin electrodes the electrical activity of their jaw muscles.

One puzzle the scientists seek to solve is the evolutionary significance of symphyseal fusion, the fusing of the two sides of the lower jaw at the chin. For reasons still not well understood, such fusion occurs independently in many but not all mammalian species. "Scientists have known for a long time that symphyseal fusion has occurred in many different orders of mammals, including humans and some but not all primates," says Hylander. "And that makes it interesting to us because it suggests that by studying the underlying biomechanical correlates associated with fused and unfused symphyses, we have an opportunity to understand why fusion occurs."

In their studies, Hylander and his colleagues will study the chewing patterns of four lemur species with very different degrees of such jaw connections. The eerie, batlike aye-aye has a very weak connection, even though it is known for its ability to gnaw through concrete blocks and metal locks with its rodent-like front teeth. Ring-tailed lemurs and bamboo lemurs have intermediate levels of such connections, whereas the agile, acrobatic sifakas develop a tight jaw fusion as they grow to adulthood.

Through their experiments, the scientists will seek evidence for their hypothesis that such jaw fusion enables an animal to bring to bear the force of muscles on the opposite side of the jaw from where the food is being chewed. As anyone knows who has tried to chew a tough steak, such muscle-force transfer could be critical to the ability to eat difficult-to-chew foods, says Hylander. And thus, it could have been important for the ability of animals such as the ancestors of humans and certain other primates to expand their dietary sources, giving them a better shot at evolutionary success.