'The bigger the eyes the faster the animal'

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.

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

"In vertebrates, increases in eye size can benefit both acuity (detail vision) and sensitivity (vision in dim light)."

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.

"If 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," 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.

"You 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," Kirk said.

Even after adjusting for body size, the researchers found a significant effect of running speed on eye size.

"There 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," Kirk said. (More) PTI SKP AKJ SKP