When heading back to the lodge after dark, either the ranger or tracker will most likely be swinging a spotlight back and forth, looking for the denizens of the night. Techniques vary, with some zipping that beam around so quickly you are convinced it’s just for show as there’s literally no physical way their eyes could be following it, while some are far more methodical, scrutinising trees, gaps in the bushes, and every possible hiding spot that might have a nocturnal creature concealed in it.
As impressive as many of the trackers are at picking things out of the darkness, their secret is that they’re not actually looking for animals. What they’re really looking for (and this is where nature has played right into the safari industry’s hands), is the eye-shine of the nocturnal creatures, that acts like a beacon, pinpointing an animal’s position even from hundreds of metres away.
Without eye-shine, it would be infinitely harder to find things after dark, as colours, shapes and patterns are far more difficult to discern in the limited beam of the spotlight. So skilled are many of the trackers that from a simple combination of the colour of the eye-shine, the height of the eyes and their width apart, they can immediately tell what species it is, without actually seeing the animal itself.
But what is it that causes the actual eye-shine?
If one looks at the above diagram of an eye, you will notice that just behind the retina (the part of the eye that converts light into neural signals to send to the brain), there is a layer called the tapetum lucidum. This is a reflective layer which sends light back through the retina, greatly increasing the amount of light available to be interpreted into an image. Animals that are in possession of such layers therefore have far better nocturnal vision than those that don’t, and it is this reflected light that we call eye-shine.
Lions have a tapetum lucidum, humans do not, which is why we cannot see as well at night as the great felines.
That’s the simple part.
The complex reality is there is not one single kind of tapetum lucidum, but rather four different classifications, all with long Latin names that we’re not going to go into here. Suffice it to say that the different types of tapeta lucida vary significantly in their makeups, with some being composed of reflective crystals and some of extracellular fibers, with their positions in eyes also varying between species. Bats, sheep, crocodiles, cows, leopards and even spiders… all will reflect light back through their eyes if you shine a spotlight at them. Eye-shine is really a type of iridescence, which is why some animals’ eyes shine in different colours, depending on what the crystal makeup is of the tapetum lucidum, which is how most trackers and some rangers can become so good at distinguishing between species without seeing the actual animal.
The red-eye you often see in photos of people taken with a flash is not the reflection of a tapetum lucidum. We don’t have one. That is merely light reflecting off the capillaries in the back of our eyes. Sadly we are doomed to have significantly poorer night-vision than many creatures, which might only be rectified by a few hundred thousand years of evolution.
Can’t wait that long? The good news is there’s a solution. It’s called a torch.