Many animals have areas of high acuity in their eyes. In humans and many other primates and in some birds, fish and reptiles, this takes the form of a fovea or “pit” that contains a high density area of photoreceptors. Other animals have evolved different strategies, but the idea remains the same: for many visual animals, there is an area of the receptive field of the eye from which the greatest amount of information can be gained.
So let me tell you about kangaroos. What I didn’t know, until recently, is that different types of kangaroos have very different habitats: there are plains dwelling kangaroos and arboreal kangaroos (which I’m guessing from context means ‘roos who live among trees, rather than ‘roos that live up trees). In both of these types of kangroo, instread of a fovea, there is an area of the retina that is more densely connected to ganglion cells (retinal nerve cells). But here’s where it gets interesting: arboreal kangaroos, which tend to have a limited horizon, have a roughly circular area of high acuity (in the form of densely packed ganglia) in the centre of their visual field. In contrast, plains dwelling kangaroos, whose environment tends to be characterised by a large horizon, have a horizontal area of high acuity that stretches across the visual field . What a brilliant example of form following function!