You probably already know that birds are just dinosaurs that survived the last apocalypse. They have a lot of big differences when compared to mammals (feathers! eggs! a cloaca!), but there are also differences that are less obvious, such as the ones in the eyes.
(Photo credit: Hans Marth [Pexels])
But First, A Refresher
High school science classes are big on memorizing things, so you’ve probably heard of the three types of muscle:
- Cardiac: In the heart, not under voluntary control. (If cardiac muscle were under voluntary control, most people wouldn’t survive their first breakup.)
- Smooth: Around blood vessels and organs, also not under voluntary control. (If it were under voluntary control, pharmaceutical companies wouldn’t be making so much money on little blue pills.)
- Skeletal: These are the muscles you can control directly to move your skeleton.
If you are reading this and you are not a bird, you have smooth muscle controlling your iris. A mammal’s pupils dilate based on how much light there is in the room along with other factors that they can’t directly control, such as state of arousal. Pro-tip: if you’re playing with a cat whose eyes suddenly fully dilate, watch out — it’s no longer a game.
Pretty Little Dinosaurs
Birds, though, have skeletal muscle controlling their iris, which means they can dilate and constrict their pupil whenever they want to.
Pupillary dilation is a form of communication with birds, and you’ll often get a quick flash of the iris when a bird greets you or is excited about something. Watch this Amazon’s eyes as the owner is whistling to him.
Why Does It Matter?
That’s great, you say, but why do we care? As with so many other questions, the answer is “drugs”.
Drugs work very differently on smooth and skeletal muscle. That’s a good thing — if they didn’t, drugs to control asthma by relaxing smooth muscles in the airways might leave you paralyzed on the floor.
A bird under anesthesia doesn’t need to be able to fly, but it does need to breathe, and most of the muscles involved with breathing are skeletal muscles. If you use a drug to paralyze a bird’s skeletal muscles so you can work on the eye (during cataract surgery, for example), you’d better have a ventilator ready to go, because that bird isn’t going to be able to breathe on its own.
It’s fun to think about how cool it would be if humans could dilate or constrict their pupils at will, but it would certainly make eye exams a lot more difficult!
Links With More Information
Different types of muscle: https://medlineplus.gov/ency/imagepages/19841.htm
Everything you ever wanted to know about avian ophthalmology: https://www.vin.com/apputil/content/defaultadv1.aspx?pId=11147&catId=29502&id=3846255
During Fun Facts Friday, I talk about something I think is interesting. Do you have questions? Suggestions for a future post? Add a comment below!
