Social Cues in Animals

Image of human eye from: Source

 Seeing is a sense that we often take for granted.  Animals of all sorts have developed eyes that allow them to hunt and survive in their environment.   From the early beginnings of the eye which involved a pit that could only detect light and dark, animals have evolved and solved the problem of seeing what is out there in a variety of ways.  

Negative lens on the left and positive lens on the right. Source.

     One reason that eyes likely evolved is pressures from the need to find moving prey and to avoid being eaten.  In fact, we see some unique adaptations to the pressures of hunting in predator species.  Eagles and hawks as well as chameleons and the sandlance, a fish species, have all developed negative lenses.  Unlike our human eye lens which is fat in the middle and skinny on the edges, a positive lens, these animals have lenses that are skinny in the middle and fat on the edges.  This produces a telephoto zoom for these animals and allows them to see prey species accurately at a greater distance.  Note that the diversity of these species showing the negative lens suggests that this is an adaptation that has evolved repeatedly.  

Eagles have negative lenses that help them capture prey.  Source

 
     Other adaptions have evolved to meet different pressures.  Many birds have developed multiple points in the retina of the eye to focus light in order to solve the problem of flying straight ahead and watching for predators at their sides at the same time.  With their dual focus points, they likely can "multi-task" much better than our limited single point of focus.  Most nocturnal predators have a tapetum lucidem, a reflective layer in the eye, that allows light to collect even in very low light conditions.  This also produces the "eye-shine" commonly seen in photographs.

Raccoon demonstrating "eye-shine" from:
en.wikipedia.org

   Insects and crustaceans have solved the problem of seeing in a very different way than larger animals.  They have developed compound eyes that allows each of many small lenses to attach to a photoreceptor.  Each lens is similar to a mini version of our own simple eye.  The collection of little lenses allow the insect to form one large but very pixelated image.  To see what a bee might see, check out this website. Why did insect evolution solve the problem of seeing with the compound eye?  The most likely reason is that insects have such small eyes which would require a "simple" lens and pupil so small that it would be impossible to produce a clear image.

Compound eye of fly from: http://lis.epfl.ch/curvace#

The fascinating complexity of the eye has long led some to use eyes as an argument for the impossibility of evolution, for example you can read a typical argument here.  The obvious problem in studying the eye, is that it contains mostly soft materials that do not preserve well in the fossil record.  We can however look at the make-up of the eye and compare the diversity of eyes present in the animal kingdom today and see how the eyes could have progressed in evolutionary history.  For example, the cornea, which we now use to help focus light, was originally an adaptation for protecting eyes in underwater environments.  The proteins that make-up the eye are also used for  olfaction and were likely available within the organism to create the eye.
   The problem of being able to forage, navigate space, and avoid danger has led to the evolution of many different eyes in the animal kingdom.  These adaptations mean that the visual world of animals is very different from our own.  Clearly, what we see in the world around us really is in the eye of the beholder.

For further readings in the area check out these sources:
Scientific American- Evolution of the Eye

Fernald, R.D.  (2000).  Evolution of eyes.  Current Opinion in Neurobiology, 10, 444-

     450.

Land, M.F., & Fernald, R.D., (1992).  The evolution of eyes.  Annual Reviews of

     Neuroscience, 15, 1-29.    

Van der Horst, C.J.  (1933).  The optics of the insect eye.  Acta Zoologica, 14, 101-109.