Frogs and toads have extraordinary eyes. The eyes of frog and toad species come in a collage of colors and pupil shapes. The color of the iris will either stand out from the rest of the body or it may blend in, taking the same coloration as the rest of the body. Yet, even more interesting are the pupils which it is believed that pupil shape has great influence on the way a frog species brain will see and interpret an image. Common shapes associated with frog and toad pupils are…can you guess? Horizontal and vertical. In fact, many species from the North and South American continents have one of the two of these shapes. Other known pupil shapes in frogs are diamond, cross, round, hourglass, and triangular to heart.
Besides the pupil there is the eye lens itself. The lens of frog eyes focus by moving back and forth in the socket. It’s how they zoom in and out to focus. The eyes themselves can focus on close, moving visual stimuli, but the background is blurry. This is especially helpful for detecting nearby predators or even a snack. When food is nearby frogs and toads react so quickly to what they see that the insect or whatever the meal is, is in their mouth in an instant. Then, to swallow the food they pull back the eyes into the sockets and use those muscles behind the eye balls to push against the roof of the mouth.
Their brain can handle only so much visual stimuli at a time. If there is more than one stimuli occurring simultaneously, (at the same time), the frog might react like a deer in headlights. When in motion a frog continually closes its eyes. Scientists think this habit is to keep the visual stimuli from overwhelming the brain and giving the frog “motion sickness”. But, in the air during a jump or leap the eyes are pulled back to help with aerodynamics.
Frogs are nocturnal creatures. But some species are diurnal, meaning they are active during the day. For these species color vision is helpful, that’s why they have developed such good color vision.
All this talk about eyes and we haven’t gotten to their location yet. The eyes are located smack on the top of the head. They are more to the side, but slightly forward facing, toads even more forward facing. The benefit to having eyes to the side is being able to see partly front, side, and partly behind. This almost 180˚ range of vision is great for their lifestyle. The downside is, front and center on the head, around the snout, is a blind spot.
So, if frogs rely on their eyes so much then how do they protect them? Well, frogs have three—yes 3!—eyelids. The first to two are just eyelids like we mere humans have. The third is called the nictitating membrane. This eyelid is used when the frog is partly or mostly submerged in water. It can be pulled up to cover as much of the eye that is underwater at the time. But the frog still has to see. So, this eyelid has a cloudy transparency still allowing some vision. And finally, outside of water the eyes need to be kept moist. This is taken care of by the frog’s moisture-producing glands.
Works Cited:
Behler, J. L., & Behler, D. A. (2005). Frogs: A chorus of colors. New York, NY: Sterling Publishing Co., Inc.
Beltz, E. (2005). Frogs: inside their remarkable world. Buffalo, NY: Firefly Books Ltd.
Credit: U.S. Fish and Wildlife Service
Photo by Gary M. Stolz/U.S. Fish and Wildlife Service
(above)
Mongabay.com
Rhett A. Butler - San Francisco, CA.
1999-2008.
(left)
Mongabay.com
Rhett A. Butler - San Francisco, CA.
1999-2008.