Mammals running speed determines eye size

Washington, May 3(IANS) Maximum running speed practically determines mammalian eye size other than body size, says a new study.

Species with larger eyes usually have higher visual acuity, says Chris Kirk, associate professor of anthropology at University of Texas, Austin. But what are the ecological factors that cause some mammals to develop larger eyes than others?

"If you can think of mammals that are fast like a cheetah or horse, you can almost guarantee they have got really big eyes," says Kirk. "This gives them better vision to avoid colliding with obstacles in their environment when they are moving very quickly."

Kirk and physical anthropology doctoral student Amber Heard-Booth are the first to apply Leuckart's Law - a hypothesis that was developed specifically for birds and speed of flight - to 50 species of mammals, the journal Anatomical Record reports.

Previously it was thought that the time of day that an animal is active (nocturnal or diurnal) would be the main factor driving the evolution of mammalian eye size, according to a Texas statement.

However, comparative research on the anatomy of the eye has shown that although nocturnal and diurnal species differ in eye shape, they often have similar eye sizes. Although nocturnal species may appear to have bigger eyes because more of the cornea is exposed to let in more light, activity pattern only has a modest effect on eye size.

By comparison, body mass plus maximum running speed together can explain 89 percent of the variation in eye size among mammals.

The researchers found that eye diametre and maximum running speed is stronger than the link between body mass and running speed. Heard-Booth presented the findings at the 2011 American Association of Physical Anthropology Meeting.