Figure 13.27: Two Nile crocodiles, showing the basic crocodilian body plan. ( 11 )are known to swallow stones, known as gastroliths, which act as a ballast in addition to
aiding post-digestion processing of their prey. The crocodilian stomach is divided into two
chambers, the first is powerful and muscular, like a bird gizzard, where the gastroliths are
found. The other stomach has the most acidic digestive system of any animal and can digest
mostly everything from their prey: bones, feathers and horns.
The sex of developing crocodilians is determined by the incubation temperature of the eggs.
This means crocodilians do not have genetic sex determination, but instead have a form
of environmental sex determination, which is based on the temperature that embryos are
subjected to early in their development.
Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more
advanced than those of other reptiles. As in many other aquatic or amphibian tetrapods, the
eyes, ears, and nostrils are all located on the same plane. They see well during the day and
may even have color vision, plus the eyes have a vertical, cat-like pupil, which gives them
excellent night vision. A third transparent eyelid, thenictitating membrane,protects
their eyes underwater.
While birds and most reptiles have a ring of bones around each eye which supports the
eyeball (the sclerotic ring), the crocodiles lack these bones, just like mammals and snakes.
The eardrums are located behind the eyes and are covered by a movable flap of skin. This
flap closes, along with the nostrils and eyes, when they dive, preventing water from entering
their external head openings. The middle ear cavity has a complex of bony air-filled passages
and a branching Eustachian tube. Eustachian tubes will be discussed in the chapter titled
Controlling the Body.