We are in the midst of the seemingly unrelenting winter nights. The only good thing for most of us is that when the sun sets, we can lock ourselves away in centrally heated houses, with the lights on. The very thought of being outside in the cold and dark makes us shudder. We humans are, after all, diurnal animals – we have always preferred daylight.
The winter night might seem uninviting, but what might it be like if we dragged ourselves away from our comfort zone and ventured out into the night – say, in a park, or wood?
At first, the murk would seem all-encompassing, as if a blanket had been thrown over our world. Our environment would be bereft of the vast amount of information that daylight gives – a dark night provides a hundred million times less light than a bright day.
In darkness we lose much of our spatial resolution, so we don’t know where we are. Our sensitivity to contrast drops so that things seem blurred, and our colour vision fails. True, if we stayed out for more than 30 minutes, our eyes would adapt to the dark and we would begin to make out far more. But we would still be stumbling.
As visitors to this alien world, however, we would soon be aware of something else: company, and lots of it, apparently going about business as usual. A fox might make its melancholy triple-bark in the distance, a deer might snort, and a tawny owl is sure to add atmosphere with its quavering hoot or its whip-crack “ke-wick”. And then there are the rustles at our feet from a whole suite of small creatures, such as mice, voles and shrews.
Almost every British land mammal is active by night. In winter, bats, hedgehogs and dormice hibernate, but almost everything else is active during the coldest months, from badgers to hares and from wild boars to weasels. They are all about, hunting, grazing, running and, in the case of foxes, breeding. And so are owls, some reedbed birds such as moorhens, while robins keep singing. They all seem to thrive in the dark, seemingly oblivious to the difficulties, getting on with their lives in the shadows.
So how do they do it? In many cases, the answer lies in their vision. They have super-sensitive eyes that are far superior to ours.
How the eye works
We can only appreciate how well nocturnal animals can see by understanding a little about how eyes work. The animal eye detects light photons and sends them to the brain as electrical signals. Rays of light pass through the transparent outside border of the eye, the cornea, through a gap called the pupil, and finally through a lens which focuses them on the retina at the back of the eye. In the retina are fields of many millions of light-sensitive cells, which send the signals for the brain to decode.
There are two sorts of these special cells – rod-shaped and cone-shaped – and they play very different roles in our vision. Cones are sensitive to high light levels and colour. Rods, in contrast, are particularly sensitive to low levels of light. Pack the eye with rod cells, and the eye will deliver vastly improved vision in low light levels. The retina of an owl, for example, is almost entirely composed of rods, and the same applies to deer, foxes and cats, among others. On the downside, these animals can see no colour, or barely any, but they are more likely to pick up and encode what light there is in the gloom of the night.
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A mirror in the dark
But the eyes of nocturnal animals have another feature that human eyes don’t. That’s a layer of reflective tissue behind the retina, close to the edge of the eye. Called the tapetum lucidum, it is ‘retro-reflective’ – sending light back out again. This means that any ray of light that fires a rod or cone cell can pass through it, bounce off the tapetum and be detected by the retina a second time. This increases the quantity of light detected by the retina; in a cat, the tapetum increases vision sensitivity by almost half. But interestingly, owls do not have a tapetum.
A by-product of the tapetum is known as eyeshine, which shows up when light from a torch or headlight meets the eyes of nocturnal animals. The tapetum is like a mirror, reflecting some of the light that the eye receives back out of the eye again and making the eyes glow. The colour of the eyeshine varies according to species, depending on the minerals that make up the tapetum.
Size and shape matters
In addition to the retinal cells and the tapetum, nocturnal animals also have some special adaptations at the front of the eye to boost their night vision. For example, the pupil – the gap through which light passes on its way to the lens – can be increased in size, allowing more light in for the retinal cells to detect.
The size of the pupil is controlled by the iris surrounding it. The human iris contracts or expands, making a larger or smaller circle. But in the slit eyes of a fox, the effect is more like curtains closing or opening.
The span of light entering an animal’s eye can also be increased by bulging the cornea and by flexing the lens. An easily overlooked aspect is the overall eye size. The bigger the eye, the greater amount of light it can process; for example, while an average man weighs around as much as 140 tawny owls, the eyes of a tawny are relatively huge – the same size as ours. Robins, which feed in deep shade, have bigger eyes than many other songbirds.
Scratch and sniff
Of course, for nocturnal animals, vision isn’t everything – after all, if no light is present, no amount of sensitivity will help. Instead, they all have a battery of senses that enhance and complement vision, including super-sensitive ears (in owls, especially), noses and touch. The small world of voles and mice means much of what they need to find is literally a whisker away.
There is one more trick that many a nocturnal animal possesses, and that is local knowledge. Rodents, owls and badgers, for example, have home ranges in which they remain all their lives. Such animals soon learn all the nooks and crannies in their patches – gaining information by day, dusk or on moonlit nights – and know them like the back of their hands, even in complete darkness.
In fact, the apparent ease nocturnal animals display at night-time is often a result of pure familiarity. If humans spent more time outside at night, we might find it less of an alien world, too. Our night vision isn’t as keen as that of many animals, but it certainly isn’t bad. Perhaps we should be adventurous and use it more?
Animal eyes: species by species
Deer
The deer’s eyes are remarkable, but they only see 20% of the detail that we do. The eyes are slit horizontally, perhaps allowing them to see much of the horizon at once; an effect boosted by the position of the eyes on the side of the head, allowing them a roughly 310˚field of view. On the other hand, this means they are not very good at perceiving depth. Their retinal cells mostly have rods, for low light, but their cones are especially sensitive in the blue-green part of the spectrum, and they can also see ultraviolet.
Tawny owl
The eyes are relatively large and forward-facing, which helps the owl to judge distance. The retina is packed with rod cells. Technical measurements have shown that a tawny owl sees 2.7 times better in the dark than a dark-adapted human. Owls can’t turn their eyes, which are fixed in their sockets, so their heads have adapted to turn up to 270˚.
Barn owl
It has surprisingly small eyes, albeit packed with rod cells. Its great adaptation, though, is hearing. The heart-shaped facial discs reflect sound toward the ears and the ears themselves are asymmetrical in the vertical plane – one slightly higher than the other – allowing for three-dimensional hearing.
Fox
The fox has vertically slit pupils. This means in daylight the ‘curtains’ are half-shut to let in limited light, and are completely open at night to let in as much light as possible. Foxes are active day and night, so have plenty of rods and cones.
Badger
For a nocturnal animal, the badger has surprisingly small eyes, but then it spends much of its life underground, and large eyes would be prone to damage in a tunnel. Instead, its sensory world is close at hand, with a phenomenal sense of smell in particular. Nevertheless, a badger does have a tapetum – a reflective membrane to recycle light, enhancing its vision.
Hedgehog
Another nocturnal mammal with relatively poor eyesight, which relies more on smell and other senses. It only has 4% cone cells, as opposed to 25% in humans, so probably sees virtually no colour. It can discern shapes and movement.
Rabbit
It has large eyes with high concentrations of rod cells and can apparently see six times better in low light than a human. Its eyes are on the side of the head, allowing a wide field of view and long sight, but very poor detail.
