Wednesday, June 11, 2014

Night stalkers -- how cats see in the dark

In the case of eyes, size matters, but so does shape.

the perils of blogging
I'm speaking in particular about the eyes of nocturnal animals.

Cats (tigers, leopards, lions, ocelots...), dogs (foxes, wolves, coyotes...), and raccoons and their cousins are mainly nocturnal, i.e. active at night, or crepuscular, active at dawn and dusk.

Either way, they need to be able to find and catch prey and generally get around, without tripping over logs or branches, even when there is little to no light.

And they need to be able to see during the day without being blinded. How do they do it?



Adapted to an active nightlife


There are 3-4 main adaptations in the eyes of nocturnal predators, such as cats, that allow them to move and hunt successfully at night: size, pupil shape, light receptors, and mirrors.


1. Size: Big

the tarsier (aka Yoda),
famous for its enormous eyes
Check out the face of this tarsier. Its eyes are half of its head.

The eyes of nocturnal animals like Yoda here tend toward large- proportionally larger than ours and of other daytime specialists.

Remember, even if you feel like more of a night owl, you evolved before the days of electricity!

Cats and many other nighttime specialists also have a relatively wide cornea, which aids in letting more light enter the eye, and relatively large, often spherical lenses, allowing them to form a clear image using light sources from any angle.

So where you and I may see pitch black, a cat or tarsier can see some dim light, perhaps enough to jump to the next branch or find a moving rodent to hunt.


2. Pupil Shape: Linear

Think of a cat's eye - that slit-shaped pupil gives them a major advantage for nighttime hunting.

sophisticated eyewear - linear pupils help cats thrive at night
photo: KatyCat

It's highly adaptable. According to Dr. Richard E. Goldstein, chief medical officer of the Animal Medical Center in New York City, the long, thin shape of a cat's pupil adjusts to changes in lighting conditions far more quickly than ours can. The muscles on the sides of a linear pupil can open and shut it much more quickly than those around a circular pupil (like ours).

A rounded pupil lets in light more effectively, though, so under dark conditions those narrow pupils of nocturnal animals widen and enlarge to become rounded like ours, to let in maximum light. We see them during the day, when they are trying to restrict the amount of light entering their eyes.

Says Dr. Goldstein: “A cat has the capacity to alter the intensity of light falling on its retina 135-fold, compared to tenfold in a human, with a circular pupil.”

The slit-shaped pupil can be quickly covered by partially closing the eyes and can be contracted so that cats can actually close their pupils when light is too bright. Like when they want to rest during the midday heat.

Bengal tiger: sleeping off a big night...
...but will check out the scene if things look interesting


3. Light receptors: Rods, not cones

The pupil, whether round or linear, allows light to enter the eye, where it is focused by the lens onto the retina, which is connected to the brain by the optic nerve.

does the Tasmanian Devil eat red meat? Or just meat?
The retina structure houses two different kinds of light receptor cells—called rods and cones due to their respective shapes.

Cones work in bright light and work together to register detail and enable color visionRods work in low light levels, detecting motion and basic visual information, but not color. You can picture this next time you are looking for something in the dark.

Needless to say, the eyes of cats and other nocturnal animals have more rods and fewer cones than their diurnal counterparts. In fact, cats can distinguish objects with less than 20% of the light intensity that we would need.

Perhaps not having great color vision is a small price to pay to dominate the night?


4. Mirrors: Tapetum lucidum

spotlight on a dwarf lemur shows his tapetum
Tapetum lucidum is: (a) a Hogwarts spell, or (b) a mirror-like membrane in the eyes of many nocturnal animals. Good guess.

The tapetum lucidum reflects light that has already passed through its retina once back through the retina a second time.

As this reflected light hits the rods (those light receptors, see above) again, the animal gets an enhanced version of each image.

Shine a flashlight on your dog or cat's face and watch its eyes glow yellow -- this is its tapetum reflecting back some of that light. Now you need to give it a dog biscuit for its trouble.






bonus points to whoever can guess this nocturnal monster predator!
photo: AREAS project


2 comments:

  1. Thanks David --- are you studying a nocturnal mammal? Or a cat fan?

    ReplyDelete