Monday, July 26, 2010

Rotating Eyeballs

Goats, like most hoofed mammals, have horizontal pupils. The purpose of those elongated pupils is to allow them to scan the horizon for possible predators.


When a goat’s head tilts up (to look around) and down (to munch on grass), an amazing thing happens. The eyeballs actually rotate clockwise or counterclockwise within the eye socket. This keeps the pupils oriented to the horizontal.

That seems impossible to believe. Eyeballs might be able to scan from side to side, or swivel up and down, but how could they actually rotate clockwise or counterclockwise?

To test out this theory, I took photos of Lucky the goat’s head in two different positions, down and up. I then tilted one of the photos so that the slope of his forehead (marked by arrows) was constant. The photos prove that the pupils actually swivel about 25 degrees within the head.

27 comments:

Julia Kelly said...

that is disturbing- got to go find a goat and test it out myself!

Super Villain said...

i noticed this awhile ago with my own eyes in the mirror, check for yourself human eyes do the exact same thing! crazy!

Bob Mrotek said...

Thanks, James. This will make a wonderful conversation topic at dinner tonight :)

Johan said...

Wasn't aware of this. The more you know!

The Surfin' Squid said...

Wow, that's amazing. I never even noticed this (then again, I'm not fortunate enough to get to spend as much time around livestock as you). This will actually come in handy for a few things of mine.

Hubert de Lartigue said...

You are amazing! You seems like Leonardo da Vinci!

Kendra Melton said...

WHOA! what a cool fact. I'm going to have to take a closer look at the hoofed animals at the zoo next time. Very neat!

Torbjörn Källström said...

No need to go further than your own mirror.. Humans do it too. :)

Don Cox said...

Wikipedia has diagrams of the eye muscles here.

Two of the diagrams show the two oblique muscles rotating the eye. I was always vaguely puzzled by these muscles, but I never realised that human eyes actually rotate. So thanks to Super Villain for noticing it in human eyes and to James for noticing it goat eyes.

If it happens in human eyes as well as in goats, it may not have anything to do with pupil shape.

"The purpose of those elongated pupils is to allow them to scan the horizon for possible predators."

I doubt this. The shape of the pupil would give greater depth of field for vertical than for horizontal lines. It cannot determine the angle of view. That depends on the position of the eyes in the head.

I think the slit is more likely to be a camouflage effect.

Don Cox said...

"greater depth of field for vertical than for horizontal lines."

Woops - wrong way round.

Nick Woolridge said...

Hi James,

This occurs in people, too, when you tilt your head. The Superior and inferior oblique extraocular muscles can rotate the eye (to a limited degree) around the visual axis. These movements are called intorsion (rotation of the top of the eye towards the nose) and extorsion (rotation of the top of the eye towards the side of the head).

This link animates it nicely:

http://edissertations.library.swmed.edu/pdf/HowdyR122005/media/diplopia/d_int/int_extorsion.htm

Best,

Nick

James Gurney said...

Wow! My turn to be surprised. I didn't know it happened in humans. Thanks, Super V. and Torbjörn.

Nick W, thanks for the animated link showing human "intorsion" and "extorsion."

For those who don't know who Nick Woolridge is, he runs the biomedical communications (aka medical illustration) department at the University of Toronto, a great place to go to study such things. Search in the GurneyJourney search box under "biomedical" and you'll get my report of the visit there.

Don, it's good to be skeptical of asserted facts. I don't know for sure whether horizon-scanning for predators is the function of elongated pupils. I'm taking the word of David Attenborough, from one of the BBC "Life of Mammals" programs.

Don Cox said...

I have great respect for David Attenbrough, but in this case he is wrong. The shape of a pupil, or the aperture in a camera lens, affects the shape of the blurs in out-of-focus parts of the image. Photographers use the word "bokeh" to refer to the quality of the blur - some lenses have "better" bokeh than others.

When printers used cameras to photograph artwork, they had all kinds of odd-shaped apertures to control the shapes of the half-tone dots (which are basically blur shapes). There are also "portrait" lenses with diaphragms designed for taking romantic soft-focus portraits, particularly of ladies who are not quite as young as they would like to be.

James Gurney said...

Don, thanks, that's interesting. Does that mean that to an antelope things would be blurry in one direction, or would things appear sharp? Would elongated pupils give a particular adaptive advantage for prey animals?

Kat said...

Well, it looks like everyone is right about the pupil shape matter -- for the time at which it was posited. Sir David (who I adore) gave the current hypothesis for slit pupils in diurnal hoofed mammals in Life of Mammals in 2003; the work demonstrating slit pupils as an adaptation to correct for chromatic aberration in multifocal lenses came about in 2005. So everyone comes out of this intact. Science is wonderfully self-correcting.

Kat said...

By the way, the real reason for slit pupils is here:

http://jeb.biologists.org/cgi/content/full/209/1/18

...and it's fantastic. Multifocal lenses focus different wavelengths onto the retina across the diameter of the lens, thus to get a complete chromatic picture -- in order not to omit, say blue, in bright light when a circular pupil would constrict, a slit pupil solves the problem nicely.

Don Cox said...

Kat, thanks for that link.
I also found a paper on Gecko pupils here.

Fascinating topic. I learned something today. Thanks to James for starting it off.

Natalia M. said...

I'm glad this generated so many links. Time to read up!

I had noticed this in horses some time ago, but i didn't know they rotated like that! Very interesting, but is this only for mammals (or more specifically, ungulates)? i find myself if something like a Triceratops would have a similar adaptation. If humans and other mammals have some degree of rotation as well, maybe this trait goes even deeper into the animal kingdom...?

Mario said...

This blog is a real treasure, for both the original posts and the comments.

As far as I understand (but I haven't read the paper yet), the elongated pupil and the intorsion movement are not related: humans have round pupils and still have intorsion; animals have both vertically and horizontally elongated pupils, there seems to be no reason to keep the slit pupil horizontal.
I guess keeping the eye at a fixed angle makes things easier for the brain.

P.J. Magalhães said...

Man this blog is like the old comics and the letters pages. LOVE IT!! :) This is an awesome topic too.

Tidah said...

That explains why when I tilt my head the only thing that rotates in my vision is my glasses! I noticed it before but I hadn't realized it was because my eyes were rotating.

Anderhowl said...

This isn't quite as scientific, but it reminds of the hilarious feeling you can get when looking in a mirror and trying to see your eyes from any angle other than straight on.

Erik Bongers said...

What an interesting scientific discussing.
Funny thing that I got contacted by someone from the U of T just 2 days ago. An assistant professor psychiatry who took a keen interest in my work. No need to get scared Erik...

ampsanne said...

We had four goats and one of them lived to be 16 yrs. old, but we had never seen their eyes change as you put it. Maybe not all goats do it. We had nubians and their eyes never changed. Perhaps you have a rare goat there or something.

Lazenby Family News said...

I've known about goats & related species eyeballs for over 30 years. The repeated rotation of the head, in especially the Alpine goat breed, is considered a disqualification in judging & called "star-gazing"

I am still amazed at the incredibly accurate hoof kicking ability of Amertifax heifers (Jerseys, too) I was a spotter for my daughter's heifer at our local fair..watching for children balloons and strollers. Everyone wants to pet them & all are usually "in-heat" because of the environment. Our dear red heifer kicked staight-out & knocked full, red can of Dr. Pepper, that I wad holding 20 ft. w/o touching my hand. Their proliferial vision is accurate beyond description : ) Glad it was my soda-pop and not me.

Mark Tedin said...

All hail Hypnotoad!

Louis said...

I saw a goat who got her head stuck in a fence for several days before the rancher could free her. Now she has a permanent deformed neck, with her head turned fully to one side and chin down. Her jaw is almost vertical. Her left eye faces straight ahead, in line with her spine. Her right eye constantly faces her right shoulder. Over time, her eyeballs have rotated almost 90 degrees, so that the slit is aligned with the earth's horizon, not with the goats head. I don't know what this means, but it sure is interesting.