Senses
Sight
Hearing
Smell
Taste & Touch
ISOs
Crocodylians have very well developed senses. They don't really seem to discriminate too much between each sense like dogs and humans do.
Crocodylians have the typical vertebrate eye, being a large spherical eyeball enclosing the light sensitive retina. But there are a couple advances in the eyes of crocodylians. One such advance is the vertical pupil. This pupil can be found in cats, most geckos, vipers and other nocturnal (active at night) animals. A vertical pupil limits the amount of light coming into the eye far better than a spherical pupil. Since crocodylians are nocturnal animals that have adapted to seeing in the dark with its very limited amount of light. In the daytime the vertical pupil helps to limit the amount of daylight coming in, seeing as how too much could overload the senses and cause blindness. Now of course a vertical pupil isn't very helpful at night when you want an eye that can take in as much light as possible so the crocodylian eye (like other nocturnal vertebrate eyes using vertical pupils) dilates as far as it can, covering most of the iris and allowing maximum usage of available light.
Along with the pupil's ability to dilate to that extreme crocodylians along with most nocturnal animals have a special reflective layer of tissue that lies behind the retina. It's known as the tapetum lucidum and it works like this:
The cells in the tapetum contain guanin crystals, which form a mirror-like layer reflecting most of the incoming light back through the light receptor cells of the eye. So any incoming light that is missed by the retina is reflected back on it which allows crocodylians to see twice as good as an animal lacking such a layer (e.g. a human). Another secondary, yet nonetheless interesting effect of the tapetum lucidum is that when a person shines a light on an animal that has this layer, the eyes will reflect the light back and it will look like the animal's eyes are glowing. (Note: This effect no doubt helped contribute to the "crocodiles are evil monsters" scenario)
Another interesting thing about crocodylian sight is that even though crocodylians are mostly night animals, they can still see in colour. This is bigger than one might think. In most nocturnal creatures (especially mammals), having colour vision is pretty useless at night where colour doesn't show up. So colour vision is usually lost or in the case of most mammals never developed. Yet crocodylians have cones (which allows for colour) which means that they have colour vision. No one is real sure how well developed this is and it might be an evolutionary left over from a time when crocodylians were terrestrial hunters.
When going underwater the eye is covered by a nictitating membrane (which is a clear third eyelid) which allows the animal to see underwater. The membrane, although clear, is usually a bit cloudy underwater and crocodylian vision is impared. But, it doesn't seem to matter much as these animals are quite capable underwater.
Hearing in crocodylians is well developed also. Even though crocodylians like most reptiles (including birds), lack any external ear coverings (called auricles of which mammals seem to be the only ones) they still have a well developed sense of hearing. There is a movable flap near the external ear opening of crocodylians which is used to keep water from entering the inner ear. Crocodylians are also very vocal animals that have sounds ranging from sub-sonic adults to ultra-sonic hatchlings. The hearing in crocodylians is able to pick all this up, which is very amazing and more wide ranging than many animals.
Crocodylians have a great sense of smell. Their olfactory nerve endings lie in nasal cavities that open into paried nostrils placed on top near the front tip of the upper jaw. Crocodylians seem to have a whole set of chemical signals that we at the moment don't seem to know too much about. There was a test on baby alligators that had juveniles responding to the airborne odors from an adult male's cloaca. The meaning behind this is not yet understood.
Unfortunately there doesn't seem to be much information regarding these two senses, but field observations have shown that crocodylians are sensitive to touch and enjoy "fondling" each other with their mouths and bodies during the mating season.
ISOs or Integumentary Sense Organs, are special pits located around the mouths and bodies of crocodylians. There main function is uncertain at this point in time, but it is believed that these ISO pits perform the same functions that the lateral line performs in fish. The pits act as receptors that sense slight changes in water pressure, thus telling the animal which way a current is flowing or if a fish is nearby.
The distribution of the ISO receptor pits varies among species with alligators having them only around the jaw while most crocodylids have them all over the body. The pit distribution may depend a lot on the degree of underwater use, or perhaps the environments in which they are in. While ISOs around the jaws have been found to be mechanoreceptive in nature (i.e. they sense pressure), those around the body are less well understood and might have alternative roles (e.g. salinity testing for marine species).
A more thorough testing of these pits would be helpful in understanding their exact role in crocodylian life. In particular a cross species study. For instance, what is the distribution of these ISOs like in terrestriocrocs like Osteolaemis or Paleosuchus and are the highest ISO concentrations found in crocodylians such as C.porosus and Gavialis gangeticus which spend the most amount of time in water?
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