Symmetry in nature?

August 8, 2005 6:04 PM

Are all animals basically symmetrical? Even a lobster, which has specialized claws, seems to be basically symmetrical. More specifically, are there animals that are clearly not symmetrical?
posted by R. Mutt to science & nature (15 comments total)

Sponges are asymmetrical. Sperm whales and narwhales are skeletally asymmetrical. The BBC dealt with this topic more in-depth arguing that many animals are at least mildly asymmetrical. This article over at the Smithsonian discusses the role that symmetry plays in evolution.
posted by jessamyn at 6:09 PM on August 8, 2005

Flounder are generally asymmetrical.
posted by tellurian at 6:21 PM on August 8, 2005

It is better to say bilateral symmetry. It has to do with the evolution of species, or God's divine hand.

The sponges were asymmetric and from them presumably we develop the radial symmetry of cinidarians followed by our earlier ancestor of the flatworm. It fed along the bottom and needed bilateral symmetry but not the other way around. We aren't totally symmetrical (our heart and other organs) but unless there was a compelling reason not to be bilateral it just kind of stuck.
posted by geoff. at 6:21 PM on August 8, 2005

Most animals are basically symmetrical in at least one way. All vertebrates (and many more) share a basically bilateral body plan. There are also radially symmetrical animals, such as starfish and jellyfish.

Not all, though - I'm thinking of the sole, which has both eyes on one side of its head. Note, though, that by definition vertebrates aren't symmetrical front-to-back, only left-to right.

Actually, hold on a second. The plain old annelid worm isn't strictly symmetrical in any way at all. Sure, it looks like a donut in cross-section, so it's got some radial symmetry. But its aorta (basically a very long and weak heart) is only on the dorsal side. That means it's not strictly symmetrical at all, in any direction. But by that argument, neither are we; our stomach is only on the left side, our liver on the right, and so on.

That kind of qualification rules out all sorts of things, including most vertebrate animals. So, is any animal STRICTLY symmetrical? Echinoderms (starfish and whatnot) aren't, as their reproductive organs are only in one segment. And, as J says, whales too.

Jellyfish, I think, may actually be symmetrical. I'll have to think about that.

Now, why most animals are symmetrical is an interesting question that's had a lot of attention in the last few years. Most of the morphological asymmetries we see in animals are very minor - and are internalized, rather than explicit in the..

Oh hell. That smithsonian article is much more intelligent than anything I have to write. I'll just add that one of the biggest proponents of the "directed asymmetry" theory (that asymmetry may actually be beneficial in animals, whichever side it's on - a completely fascinating theory) has recently been accused of fabricating a _lot_ of data. And the accusations seem to stick. So while directed asymmetry was a fad in biology until about 2002, its lost a lot of credibility now.
posted by metaculpa at 6:26 PM on August 8, 2005

Slugs and snails have a breathing pore only on one side of their body...though this is probably a vestige of the helical symmetry of their shells.
posted by randomstriker at 6:28 PM on August 8, 2005

Don't forget the mighty Fiddler Crab!
posted by Who_Am_I at 6:29 PM on August 8, 2005

Symmetry requires less information to produce than asymmetry.
posted by nebulawindphone at 6:30 PM on August 8, 2005

Patterns are repeated: two bones in the forearm and lower leg; single bones in the upper arm and upper leg. Four bones for each finger. Etc.

Symmetry is just another pattern.

Why patterns? Efficiency in encoding, perhaps; or redundancy provides better error-checking.

Not all, though - I'm thinking of the sole, which has both eyes on one side of its head.

Not to start off with, though.
posted by five fresh fish at 8:39 PM on August 8, 2005

Why patterns? Efficiency in encoding, perhaps; or redundancy provides better error-checking.

Or, more likely, don't mess with what works. I have trouble imagining how effective our hands would be if we had a different number of joints in each finger.
posted by randomstriker at 9:57 PM on August 8, 2005

Most crabs are not symetrical, not just fiddlers crabs.
posted by bat at 11:41 PM on August 8, 2005

I found the Smithsonian argument for symmetry somewhat backwards. Selecting the beauty of symmetry doesn't ensure survival--survival has produced our attraction to symmetry. Those who see it as a choice are missing the point.

I figure most animals have a lot of asymmetry on the inside because it's the most efficient way to store organs that have different shapes and functions--heart, kidney, etc. The exceptions are protective and supportive parts, like ribs and spine, which benefit from bisymmetry, since danger in the natural world tends to be symmetrical.

Animals are bisymmetrical on the outside because disease, injury, and weakness are easier to spot, so the odds of finding a healthy mate are increased, and the tendency to be attracted to symmetry is passed on to future generations whether or not that was the intention. The cave woman who was attracted to the guy with the withered leg (because she liked his personality) placed her kids at a disadvantage. "Nice personality" remains a mating joke to this day. Symmetry rules. My guess is that asymmetrical animals, like the flounder, use senses other than vision as a primary means of choosing a healthy mate. Or perhaps their asymmetry has conferred a niche advantage, akin to the left-handed boxer or tennis player.

But why are we slaves to symmetry when we create our own machines? Perhaps asymmetrical cars would be safer and more efficient, since the threat from collisions is not symmetrical. But would humans, with our hard-wired need for symmetry, buy them?
posted by weapons-grade pandemonium at 3:10 AM on August 9, 2005

Strangely enough, there's just been an announcement that the oldest fossil of bilaterian lifeform (this specimen is called: Vernanimalcula) has been discovered.
posted by peacay at 6:19 AM on August 9, 2005

If you look at Drosophila development, there is an ordered progression of signaling protein expression very early on in the development of the zygote. These proteins turn each other on and off by virtue of how high their concentration is in specific cells. First an anterior-posterior gradient is established; then, a series of stripes; then, yet more specialized signaling in different stripes makes cells positioned in them form into different tissues and organs.

If you stare at it long enough it almost looks like symmetry, in the fly at least, is a by-product of this process which allows tissue specialization ("organogenesis").
posted by ikkyu2 at 7:42 AM on August 9, 2005

But cars aren't symmetrical! Just try importing a Holden to the U.S...

I've long advocated a center-mounted driver's seat, since I had such trouble keeping the car centered in the lane while learning to drive. It still strikes me as thoroughly unnatural to have one A-pillar right next to my face, and the other one way over on the passenger side. Why can't I look out the center of the windshield?

Tractors, formula 1 racecars, fighter jets, and bicycles are symmetrical. It's only regular cars, which still owe their design to the horse-drawn buggy (which, by the way, had a bench so the driver could be centered if nobody was riding shotgun), that inexplicably wedge a useless seat in next to the driver. The passenger has no claim to my prime windshield-space unless they're going to have a set of co-pilot's controls like in an airplane.

As far as biology goes, the initial spherical shape of the zygote lends itself well to radially symmetrical development, as ikkyu2 pointed out. Breaking that pattern would take special effort, so it only happens once in a while. Putting a spinal cord on the dorsal but not the ventral side is a neat trick. Everything since builds on that. Introducing an extra stage of assymetrical differentiation is a pretty big evolutionary hump to get over, so we don't see it often.
posted by Myself at 5:05 PM on August 9, 2005

But why are we slaves to symmetry when we create our own machines? Perhaps asymmetrical cars would be safer and more efficient, since the threat from collisions is not symmetrical. But would humans, with our hard-wired need for symmetry, buy them?

Admittedly, this is a bit off the original topic, but:

Yes, humans tend to find symmetry appealing. (Some studies have attempted to find a correlation between human beauty and symmetry; as far as I can tell the connection probably weak, with symmetry being more closely linked to health than beauty. And, as artists and photographers know, a little asymmetry in the composition of a picture can be much more attractive than a perfectly centered subject, though in the absense of symmetry people still look for balance, which you could argue is a type of high-level symmetry.

Although we find bilateral symmetry natural, for obvious reasons, I'd say symmetry in machines is mostly a matter of simplicity. Sure, some people might not buy an asymmetrical car because of how it looked, but would you really want a vehicle that handled differently in left and right turns?

There have been some interesting asymmetrical vehicles over the years. Outrigger canoes come to mind. There have also been some really intriguing asymmetrical aircraft. I think this is largely because it is possible to build an asymmetrical aircraft with very symmetrical handling characteristics. Two of the most dramatic have been the German WWII prototype Blohm & Voss BV 141 and the asymmetrical light twin demostrator Boomerang, designed by Burt Rutan (now of SpaceShipOne fame). Interestingly, this design was intended to mitigate the dangerously asymmetrical condition that occurs when one engine in a light twin fails, which represents the primary reason that light aircraft with two engines actually have a higher accident rate than those with one.

I can't think of any asymmetrical animals not already mentioned, though this BBC article mentions that "owls have one ear slightly higher on the head than the other, which helps them pinpoint sounds accurately." Including fictional animals, though, brings up a couple of interesting examples from the human imagination: Dr. Doolittle's more-symmetrical-than-normal pushme-pullyu (sorry, couldn't find a screencap) and the lopsided mythical dahu.

It's also interesting to note that many animals display handedness just as humans do. The BBC article above mentions handedness in chimpanzees; personal experience shows that horses also have analogous lateralization, and this article discusses the matter in great detail.
posted by musicinmybrain at 6:35 PM on August 9, 2005