Pixiq

Whether taking a complete animal or just a head, it is essential to get the eyes in focus. Also if the eye is dark and set within black fur or feathers, it does need a catchlight, otherwise the animal simply does not look alive. This is why I constantly check the focus point and invariably move it away from the center of the frame. Even a tight crop on eyes alone needs careful focusing on the eye rather than the bridge of the nose or the snout.

Reflection from sky or water in the eye may be enough to gain a highlight; if not, use fill-flash by going with the ambient light exposure and dialing in -1.7 stops for the flash. However, beware of using a flash for a macro shot because the rectangular flash window is likely to be reflected in the eyeball and dominate it. This will not be a problem if standing back with a tele lens.

Animals with either a contrasting color around the pupil – such as an eagle owl – or eyes set within pale fur or feathers are the easiest to photograph because the eyes separate themselves and so it is not so critical to have a catchlight in the eye(s).

Nocturnal animals, such as bush babies and nocturnal lemurs, tend to have larger eyes than animals that are active by day and since they spend the day sleeping you need to search for them when they are active at night. This easiest way to do this is to use a flashlight to pick out the eye shine, while the photography will have to be using a flash. To avoid getting a false colored eye (red or green are most common) from the light being reflected back from inside the eye, the flash should preferably be mounted on a flash bracket off the camera.  

Chameleon eyes are particularly interesting because they move independently of each other, so they can sense prey moving anywhere around them.

Geckos are have an interesting behavior connected with their eyes; they keep their eye balls clean by periodic washing with the dexterous tongue. This is because the eye has neither an eyelid nor a nictitating membrane – a transparent or translucent third eyelid present in reptiles, birds, and sharks  that can be drawn across the eye to protect and moisten it whilst still being able to see.