What do other animals see?

Why do we see such huge variation in vision in the animal kingdom? Evolution by natural selection is the key. In such a world rich in light, animals that are able to detect visual signals in their specific environments inevitably enhance their survival and reproduction. Predators have evolved high visual acuity and superior colour vision. And in response, prey have evolved colour patterns that decrease the likelihood of being detected by predators, in the form of camouflage. 

But yet, many animals don't just find food using visual signals, but also mates. So, colour patterns have also evolved as sexual signals, communicating genetic quality to potential mates or fighting ability to sexual rivals.  We only have to look at the beautiful peacock above to remember just how strikingly conspicuous animals can be. 

Visual signals obviously play a very important role in the lives of animals. Even humans seem to use visual signals to enhance their reproductive success: for instance, by using 'ornamentation' like lipstick in women and fast cars in men, they signal sexual receptiveness and future paternal resourcefulness respectively (Low, 1979).  

Until recently, studies of animal visual signals and colour patterns have used human assessments of colour. If you've read my previous post ("ChuckleVision") you would know by now that there is a wide variation in visual systems throughout the animal kingdom, and humans tend to be pretty inferior in their colour vision capabilities. 

So, knowing this, if you were to assess the colour of say, a lizard, which would be more appropriate: seeing it through the eyes of a) a human, b) another lizard or c) a lizard predator? 

The correct answers are b) and c). Using human assessments is extremely subjective, and a human has had no impact on the evolution of the lizard coloration, whereas the visual systems of its potential mates and predators have. You would agree with me then, that the correct way to assess animal coloration is objectively and through the eyes of other, ecologically relevant, animals? I hope so.

This is one thing I aim to do in my PhD. My supervisor was one of the pioneers of objective assessments of animal colour through the eyes of other animals (specifically birds). Using spectrophotometry, digital photography and visual computer modelling, this is fast becoming a super-cool way of understanding why animals look like they do.

My study species Podarcis erhardii have colour patterns that may be for camouflage, sexual communication, or even a combination of both. But I'm not going to assess this. Instead, I'm going to look through the eyes of their mates and predators, and tell you what they see. 

ChuckleVision

Photo credit: comedy_nose

Let's face it: we humans have pretty ordinary vision. Unlike birds of prey, we can't see tiny mice running around several metres below us, and we can't detect the ultraviolet 'signature' they leave behind by their urine trails (as in kestrels; Viitala et al., 1995). Unlike snakes, we don't find our dinner by detecting combined movement and heat (infrared) signals. I could go on, but you get my point.

In particular, our colour vision is rather inferior. I'll come onto that in a moment. But first, how do we and other animals see colour? All around us, we see light bouncing off objects, which we perceive as different colours: blues, greens, reds, yellows, purples, pinks... But how is this possible? How does the light give us such rich information about the world?

In a nutshell: The light is made up of different properties ('wavelengths' measured in nanometers [nm]) across a spectrum of different wavelengths (shown below). The eye has a specialised internal tissue called the 'retina' which contains cells that are sensitive to different wavelengths of light ('photoreceptors' or 'cone cells') (see cross-section of human eye below). When light hits the retina, certain photoreceptors are stimulated (depending on the wavelength of the light) and if enough are triggered, then they fire signals or nerve impulses to the brain's visual cortex where it is recognised as a specific colour. 

For instance, in humans, we have cone cells that are sensitive to three different types of wavelength: short wavelengths (≈400-500nm; 'blue light'), medium wavelengths (≈500-600nm; 'green light') and long wavelengths (≈600-750nm; 'red light'). In other words, we have 'trichromatic' vision, as we have three types of cone cell. 

Look at the blue chair below. It is reflecting light of short wavelengths (≈400-500nm), which is then stimulating enough of our short-wavelength-sensitive photoreceptors for them to fire a signal to our brain which recognises the light as a 'blue' colour. All in milliseconds. Pretty amazing eh.

Now, yes, this is pretty amazing, but not as amazing as some other animals' colour vision. Not only can other animals see that elusive 'ultraviolet' part of the spectrum on the left, but they can also see the very-long-wavelength infrared part of the spectrum on the right. And some animals possess not just three cones, but four, five, six, and up .... to even 16. And the last one is just a mantis shrimp. Why do we see such variation in vision, and what are the consequences? And why do I care? I'll have to save these questions for next time...