Animal eyes at Auckland Zoo: simple vs compound

The Eyevolution crew headed to the Auckland Zoo over the Easter weekend, for field trip with a focus on eyes. Our aim was to see for ourselves the diversity of eyes throughout the animal kingdom. This was not an open and shut case however, as many animals hid themselves away from the prying eyes of their human observers.

We looked from big to small, and simple to compound. Most similar to our own eyes were the 'simple' eyes of large mammals and other vertebrates: orangutans and monkeys, to giraffes and zebras.

The structure of these simple eyes consists of a single concave surface (the retina) that focuses light. Variations in eye structure between these simple eyes have evolved due to the different visual requirements of these animals. Primate eyes focus light in a central area of the retina known as the fovea, giving us a small region of high acuity vision that diminishes in the periphery. However, giraffes and zebras possess a horizontal streak of light-sensitive cells across their retina, optimising their vision for the open plains of the African savanna.

The fearsome eyes of spiders and insects are structurally very different from our own. Instead of a single light-focusing chamber, 'compound' eyes are made up of many hundreds of individual 'eye units' or ommatidia, formed on a concave surface. Each points in a slightly different direction, so that the combination of inputs produces a final image with a very large field of view.

We think of eyes as two light-gathering spheres located on an animal's head. However, evolution has the tendency to think outside the sphere. Here comes the Tuatara - a reptile-like creature native to New Zealand.


 The Tuatara is the only living species of the order Rhynchocephalia, that stretches back to the Mesozoic Era. For this reason, Tuatara can be considered a living fossil from the age of the dinosaurs. The Tuatara, as well as some species of lizards, frogs, and fish possess a parietal eye, a 'third eye' made up of light-sensitivite cells located on the top of the head that is responsible for regulating the circadian rhythm.

As we left the zoo, we reflected on the diversity of eyes that we had witnessed that day. We considered how common the eye is in the animal kingdom, having evolved many times along different branches of the evolutionary tree.


Further reading:

How do insects see the world? Nature's Web
Tuatara The Kiwi Conservation Club, Animal Corner
Evolution of Retinal Structures Avian Visual Cognition
The animal kingdom's most amazing eyes Environmental Graffiti

Photo credit: Emma Goodman