Is this zoo statement about tree branches (and apes) true?

May 28, 2010 12:23 PM

What's the real reason apes evolved to move through trees using arms rather than legs and a tail; why do they swing rather than jump? Surely not because "a tree branch can hold more weight above than it than below it" (as purported by a suspect sheet of "fun facts" at the zoo).

My brother's friend just got a job at the zoo. The quote above is from the page given to them of "fun facts" to share with visitors.

Strictly from a mechanics point of view, the statement doesn't make any sense to us (but perhaps there's something we're missing?). Is the statement incomplete, imprecise, or just plain wrong, and how so?

posted by theDTs to science & nature (21 answers total)

"above IT THAN below it"

*sigh* Preview button, you let me down this time.
posted by theDTs at 12:26 PM on May 28, 2010

Using your arms allows you to brachiate. A gibbon can brachiate at speeds as high as 35 mph (55 km/h) and can travel as far as 20 feet (6 m) with each swing.

I may be mistaken but I doubt that any animal can jump from branch to branch that quickly.
posted by alms at 12:32 PM on May 28, 2010

I had to re-read your question twice to figure out what mistake you were talking about - I just naturally corrected it.

I would suggest that you use less stregth in a swing than a hop, because you're taking advantage of your momentum. With a jump, you have to bring yourself to a stop (difficult) and then put all your power into reaching the next branch. It takes more energy and is more difficult to control - think of the control you have landing on a branch with your toes versus wrapping your fingers around it.
posted by Dasein at 12:34 PM on May 28, 2010

Less points for failure for brachiating compared to jumping because if one doesn't quite reach the next branch, one is still has a hold onto the previous branch with the trailing arm. IANAG [I'm not a gibbon], but I learned this while playing on the monkey bars as a child.

Less energy expended too: jumping from limb to limbs would require four limbs (two for strength, two for balance) while brachiating requires only two plus takes advantage of momentum and arcs. If you've ever watches lesser apes at a zoo, they can swing around for hours without seemingly expending much energy at all.

Speed: apparently gibbons can brachiate as quickly as 35 mph. Reseting (target acquisition, tensing muscles) in between each jump in an environment as complex as their arboreal world would take more time than just swinging in the general direction of the next limb.
posted by jamaro at 12:38 PM on May 28, 2010 [1 favorite]

What's the real reason apes evolved to move through trees using arms rather than legs and a tail

Apes aren't really bipeds. They're not hairy humans with long arms. If anything, we're the weird ones in the animal kingdom.

Just as squirrels use all four limbs to climb trees, apes and monkeys will use their arms to motor about, too. Brachiation is just faster and more efficient.

a tree branch can hold more weight above than it than below it

This is probably just a clumsy way to express that a bent tree branch can act like a spring, which the apes can use to their advantage.
posted by Cool Papa Bell at 12:38 PM on May 28, 2010

Interesting: Brachiation may be a pre-adaptation to bipedalism.
posted by Cool Papa Bell at 12:41 PM on May 28, 2010

If you swing from flexible branches then the branch moves with you and carries you further. If you try to jump from a flexible branch then you just end up pushing the branch away from yourself while you stay in pretty much the same place. Then you fall, and the branch snaps back and hits you in the head.
posted by jon1270 at 12:42 PM on May 28, 2010 [3 favorites]

I think you might be giving the author too much credit, Cool Papa Bell. :)
posted by theDTs at 12:48 PM on May 28, 2010

Brachiation gets the most done for the least expenditure of energy.

This is why every animal & plant does everything, by the way.
posted by Aquaman at 12:50 PM on May 28, 2010

Cool link about pre-adaptation, btw. Until I actually read it, I was wondering if it was some sort of Lamarckian thing.
posted by theDTs at 12:50 PM on May 28, 2010

Thanks for the responses, guys! Aquaman's answer seemed to sum it up most succinctly. After reading everything and discussing, my brother had the point that brachiating is probably also less jarring on branch--because the momentum is largely redirected, rather than fully absorbed by branch.

This would probably be more important the bigger the animal got. A monkey (or squirrel) has plenty of handy branches it can jump to and from it's so light they would be a more solid base for it. On the other hand, a gibbon-sized jumping animal would break a lot of those same branches. It seems that brachiating allows it to safely use a wider range of branch sizes, while also allowing it to reach more of them.
posted by theDTs at 1:23 PM on May 28, 2010

A jump from above temporarily puts far more force on the branch than a swing from below, and thus is more likely to break the branch. The force curve on the branch for a swing is longer and not as violent. (That's what the zoo meant.)
posted by Chocolate Pickle at 1:50 PM on May 28, 2010 [2 favorites]

How about looking at it from a balance perspective? If you're hanging, and you get jostled, you automatically go back to center when the jostling is over. If you're on top of the branch and you get jostled, you fall off.
posted by notsnot at 2:11 PM on May 28, 2010 [1 favorite]

In the spirit of Aquaman's expenditure of energy point -- swinging by your arms allows you to rest the bulk of your body weight on your skeletal structure. The only muscles that are really being engaged intensively are the forearms, to hold onto the branch. Same reason gymnasts and climbers are doing their fastest, most dynamic movements on unbent arms.
posted by Pantengliopoli at 2:30 PM on May 28, 2010

This Wikipedia article points out several problems posed by biomechanics of arboreal travel, including this:

"On horizontal and gently sloped branches, the primary problem is tipping to the side due to the narrow base of support. The narrower the branch, the greater the difficulty in balancing a given animal faces."

Later on, it mentions that "Some species of primate and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip."

I find this argument far more convincing than the notion that tree branches can hold more weight if most of the mass is suspended beneath the branch itself.
posted by centerweight at 3:31 PM on May 28, 2010

Yeah, I would go with balance as a primary factor. Above a branch, you have to balance. Balancing on a branch is hard. Below the branch, you're hanging. Easy.

Also, landing on a branch from above requires your center of mass to be heading directly for the branch, else you'll miss or at least swing around it when you land. Might as well swing around it on purpose, making the required aim much less precise and using the swing to your advantage to redirect momentum for the next branch.

This all depends on your body being larger than the branch. If the branch is larger than you, swinging makes no sense and balancing on top is easy.
posted by whatnotever at 4:22 PM on May 28, 2010

Here's my theory, developed in an undergrad advanced physical anthropology paper. The fossil record shows that the first monkeys (i.e. not prosimians) were so-called "dental apes", fruit-eating monkeys with teeth like ours, which are usually associated with fruit eating. All monkeys came out of this group. At some point, a second group arose out of the first, eating leaves. These monkeys, the cercopithicoids, developed complex sacculated stomachs for breaking down leaves. But what happened was they started eating fruit again too. With their powerful stomachs, they started to be able to eat the fruit earlier in the year when it was less ripe. This left the "dental apes" in trouble, and they died out rapidly. It is known that only a few species survived this cercopithecoid radiation into their food niche.

Our ancestors were amongst the survivors of this group. They all have "ape-style" teeth and they all brachiate and they are all smarter than other monkeys.

In my paper, I theorized that the brachation allowed them to cover more ground to get sparse food in a shorter time, and that the intelligence allowed them to remember the location of scarce food and navigate to it.

Also, it turns out the appendix was created to store the toxins from less-ripe fruit, allowing us to compete again.
posted by Ironmouth at 11:10 PM on May 28, 2010

This is a great ask.me: You should turn the combined question & answers into an FPP!
posted by pharm at 1:30 AM on May 29, 2010

Brachiation gets the most done for the least expenditure of energy.

This is why every animal & plant does everything, by the way.
posted by Aquaman at 3:50 PM on May 28 [+] [!]

This is not true at the molecular level in eukaryotes (including all animals and plants). Huge biomolecules are assembled and then degraded at both the mRNA (it's not rare that >90% of the message is spliced out and degraded) and protein levels. This turnover is a huge waste of chemical energy with no clear function.

It may be that at the molecular level, exploring the adaptational or genetic information "space" is selected for in eukaryotes as opposed to energy efficiency.
posted by Jorus at 3:30 AM on May 29, 2010

Jorus, I think Aquaman's statement of "Whatever does the job for least" (paraphrasing) could apply to the process you're describing in eukaryotes, if that process were defined as a "job" being "done" (regardless of whether it is vitally important, but with a function simply not clear to us yet--or wasteful molecular busywork that evolution has just never managed to eliminate from the organism's design).

I might add that Aquaman's statement is not entirely precise or comprehensive--for instance, I myself am an animal after all, and expending an inordinate amount of energy to quibble with some poor zoo's "Fun Facts"--but that would, of course, also be a quibble.

:)
posted by theDTs at 4:08 PM on May 29, 2010

If by process and job, you mean improving this genotype's chances of being selected, I can see what you're saying.

I don't specialize in evolution, but I do caution against taking a "best of all possible worlds" view. Gould writes about it better than I can: Male Nipples and Clitoral Ripples. He agrees with your point on human mentality as well.
posted by Jorus at 9:01 AM on May 30, 2010