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Academic Minute

Dr. Chris Kirk, The University of Texas at Austin – The Nocturnal Mammalian Eye

In today’s Academic Minute, Dr. Chris Kirk of the University of Texas at Austin reveal s what the eyes of early mammals have to say about their nocturnal lifestyle.

Chris Kirk is an associate professor of anthropology at the University of Texas at Austin. As a physical anthropologist, his research interests include sensory ecology, functional morphology, and paleontology. He is currently conducting research on the relationship between visual anatomy and visual ecology, the functional morphology of the inner ear, and Paleogene primate evolution. He holds a Ph.D. from Duke University.

About Dr. Kirk

Dr. Chris Kirk – The Nocturnal Mammalian Eye

One of the central questions for scientists who study vision is how eyes adapt to function across a broad range of light levels. Light intensities on a moonless night are 100 million times less intense than those on a bright sunny day. My colleagues Margaret Hall, Jason Kamilar, and I wanted to study how the time of day that an animal is active influences the anatomy of its eyes.

To explore this issue, we gathered measurements of eye and cornea size for 840 species of living mammals, birds and lizards. Within birds and lizards, we found that diurnal species have the smallest corneas for their eye size and nocturnal species have the largest corneas for their eye size. This is the expected pattern because nocturnal species need larger corneas to increase the light gathering capacity of their eyes at night. However, when we compared mammalian species with other vertebrates, we found that nearly all mammals have eye shapes that are most similar to nocturnal birds and lizards. These results show that even mammals that are active by day would be judged to have eye shapes that appear “nocturnally adapted” when compared with non-mammalian vertebrates.

One explanation for this finding is the fact that all mammals probably share a long period of adaptation for nocturnal life deep in their evolutionary history. During the Mesozoic Era, when dinosaurs were the dominant vertebrates on land, mammals were mostly small insectivores that were nocturnally or cathemerally active. It’s during this time period that mammals evolved their keen senses of smell, hearing, and touch – all of these senses function well regardless of whether it’s day or night. But it’s also during the Mesozoic that mammals probably evolved their characteristically nocturnal eye shapes. Our study of eyes in living species suggests that evidence of this past still persists 65 million years after most dinosaurs went extinct and mammals took their place as the dominant vertebrates on land.

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