'The bigger the eyes the faster the animal'

Washington, May 3 - Mammals which have bigger eyes compared to their body size are more likely to be the fastest runners in the animal kingdom, researchers have found Animals with bigger eyes usually have better visual acuity which is beneficial when moving quickly, said study author Chris Kirk, an associate professor of anthropology at University of Texas, Austin.

ÒEye size is one of the most important factors for understanding visual abilities and the evolution of visual adaptations,Ó Kirk told LiveScience.

ÒIn vertebrates, increases in eye size can benefit both acuity (detail vision) and sensitivity (vision in dim light).Ó Some animals rely more on senses other than eyesight, but for many it plays an important role. This seems to be especially true in fast runners, the researchers said.

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

To figure out what factors influence the eye size, the researchers collected measurements of eye size, body size and running speed in 50 mammals.

The findings, published in the journal Anatomical Record, showed that 89 per cent of the variation in eye size among these mammals was due to body mass and maximum running speed.

ÒYou start looking at comparative data and one thing that will always influence eye size is body size. An elephant is always going to have bigger eyes than a mouse,Ó Kirk said.
Even after adjusting for body size, the researchers found a significant effect of running speed on eye size.

ÒThere is going to be the effect of body mass, but when you look at maximum running speed in isolation or when you hold body mass constant, it's still significantly related to eye size,Ó Kirk said.


And when you combine maximum running speed and body mass as two variables influencing how big an eye is, they can tell almost all of the differences observed between species, the researchers said.
Interestingly, humans seem to buck this trend, Kirk said.

ÒWe have large eyes, comparable to those of fast runners, like wolves, but our maximum running speeds are pitifully slow compared with other mammals.

ÒI'm often reminded of the Pekingese dog that I grew up with, who could run faster on four tiny legs than I could run on two long legs.Ó ÒAt least in the case of humans, it's safe to conclude that fast running was not the selective factor that led to the evolution of our large eyes,Ó Kirk added.