Crocodile eyes are optimized for lurking

Years ago, when I visited the Daintree Rainforest in northeastern Australia, I couldn’t help but notice the signs — several of them — warning of crocodiles. Australia is home to two species of the ferocious reptiles, freshwater and saltwater — the latter of which can be found in the Daintree River. And the signs are no joke. Croc attacks aren’t common, but a few do happen every year, and some result in deaths.

Crocodiles aren’t terrifying just because they have huge teeth and a deadly bite, though. It’s that an attack appears to come from nowhere. The animals lurk just beneath the water, with only their eyes keeping a lookout for something tasty — like one of us. Now, new research shows that, while a croc may not see as well as you or I, its eyesight is quite good and well adapted for lying in wait at the water’s surface.

Nicolas Nagloo and colleagues from the University of Western Australia in Crawley took a detailed look at eyes from three young saltwater and two young freshwater crocodiles. “Both Australian species possess a bright yellow iris, a slit pupil and a relatively large lens,” the team notes May 4 in the Journal of Experimental Biology. Such features, which were known before this study, are helpful for seeing in dim light. (The animals, though, don’t have great vision underwater.) Crocs are also equipped with a “mobile slit retina” that helps the animals control how much light reaches the eye during daylight.
Dissections and examinations of the cells of the eyes revealed that both species have three types of single cones, a type of double cone and one type of rod. This means that the animals can see colors well. But the freshwater crocs appear to be a bit more sensitive to red than their saltwater counterparts (known as “salties” in Australia); that may help the freshies see in streams and rivers where there is more red light. Both species also have a horizontal streak of high spatial acuity, which allows the reptiles to scan back and forth for prey without ever moving their heads.
That the two species have eyes that are so similar is somewhat surprising given that they are separated by some 12 million years of evolution, live in different habitats and prefer different prey, with the freshwater crocs preferring smaller animals and more fish. But both species have adopted a similar hunting style in which the animals hide just beneath the water and scan the flat environment for a suitable meal. Their eyes, this study shows, are specialized to aid in such attacks.