This red eyed frog (Agalychnis callidryas) has large, bulging eyes near the top of its head. Don Farrall/ Getty Images

Frogs rely on their vision and hearing to catch prey and avoid predators. They have good hearing and vision, although their ears and eyes aren't situated quite like those of most other animals. Frogs don't have external ears. Instead, they have an eardrum called the tympanum that sits just behind each eye. Often, you can see the eardrum -- it's a flat area surrounded by a ring of cartilage. In some species, scientists can tell whether a frog is male or female by comparing the size of the eardrums to the size of the eyes.

Most frogs have wide, bulging eyes that sit on the top of their skulls. This gives the frog a wide field of view and helps compensate for its inability to turn its head. There's not a lot of overlap between what a frog can see with its left eye and what it can see with its right, though. This may mean that frogs don't have as much depth perception as other animals do, which makes some species' ability to catch flying prey with their tongues even more amazing. Most frogs also have a nictitating membrane, or a sturdy film that covers and protects the eye while the frog is underwater.



" " Frogs call by inflating their vocal sacs. Jeremy Woodhouse/ Getty Images

Frogs' eyes also play an important role in eating. Frogs don't have the skull structure or the necessary muscles to chew their food. Instead, they have to swallow their prey in a couple of gulps. This is tricky since, unlike people, their tongues aren't usually anchored in the back of their mouths. That means a frog can't use its tongue to push food down its throat and toward its stomach. For this reason, when a frog swallows, its eyes sink down into its skull to help push the food along.

Many frogs, particularly males, have vocal cords and a flap of skin called a vocal sac in the front of their throats. It's this sac that allows frogs to croak, trill and ribbit. The frog inhales, and fills the vocal sac with air, causing it to stretch out like a balloon. With its mouth closed, the frog forces air from this sac back and forth over its vocal cords, producing a loud, repetitive sound. Frogs that don't have vocal cords can also use a sharp intake of air to make a clicking sound.

Most of the time, this noise has something to do with mating. Next, we'll look at the frog mating process, which can last for days, and the metamorphosis that turns tadpoles into frogs.