Lately, I’ve been a little stressed. Long hours in the lab and moving into a new apartment have created the perfect storm for “treating myself” to restaurant food, served with a side of inactivity and sloth. As I sit here at my desk, chowing down on crackers and reading the latest issue of the PNAS, I can be thankful for one thing: At least I’m not an Orangutan.
Orangutans, as it turns out, burn fewer calories than almost every single animal alive (or at least studied). The one exception? The sloth, of course. Orangs even use relatively less energy than a mouse lemur in torpor. (For the past few minutes I’ve been walking around the building spouting that little factoid, and someone pointed out that it’s not exactly surprising that small animals burn relatively more calories. Like hummingbirds…)
Herman Pontzer and his colleagues reported this in the latest issue of the PNAS, and conducted the study using double-labeled water in which the hydrogen and oxygen had been replaced by rare isotopes of those elements. That way, the researchers could tell how much of each element was passing through their system, and how quickly. In other words, they could measure their metabolism.
When they did that, they found that their biggest, oldest male Orangutan, Azy, was only burning about 2,000 calories in his 255 pound frame. Their females- both around 120 pounds- burned about 1600, and a young male went through about 1250 calories.
Now, why might a great ape evolve to have a really slow metabolism? The explanation that the authors offer is an ecological one: Orangs have been living in an area in which their best foods- ripe fruit- are only available rarely and in patches. In order to cope, the orangs have evolved some way to reduce their daily caloric expenditure while still remaining fairly active.
This scenario fits in with a lot of other Orangutan weirdnesses that can be explained by ecological scarcity. Orangs are the only solitary ape. They have extremely low birth rates, and take a long time to reach adulthood. Males have two different mating strategies, where some males delay full reproductive maturity (which includes a whole slew of metabolically costly secondary sexual characteristics) until they are sure that there aren’t any competitors in the area. So an ecological explanation is an obvious choice.
But I’m interested in how exactly Orangutans achieve such a slow metabolism. Do they regulate their body temperature differently? Do they have more slow-twitch muscle fibers? Or have they rearranged their bodies so that their muscles don’t have to work as hard to move their bodies? It’s fine and good to suggest that the low metabolism is an ecological adaptation, but there might be some alternative explanations, too. Being a locomotion person, I automatically start thinking of ways that this could be a side effect of the adaptations for biomechanical efficiency.
Whatever the answer, it sure is fun to have a neat problem to think about for awhile!
Beautiful Orangutan drawing by Lois Cordelia at www.loiscordelia.com
Pontzer, H., Raichlen, D., Shumaker, R., Ocobock, C., & Wich, S. (2010). Metabolic adaptation for low energy throughput in orangutans Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1001031107
IIRC, the primary source of heat in endotherms isn’t muscle, but rather because they have loads of holes in their mitochondrial membranes, disrupting the the proton gradient and causing much of the energy from food to be wasted (and, therefore, requiring more sugar and more heat to make the same ATP). I suspect that lower or higher metabolism (both within primates and more broadly in mammals) is more due to differences in the level of expression of this “decoupling protein”.
Even if they altered their muscles, a 100 kg orang would still require 9810 Joules of physical energy to move 10 meters upwards, requiring 39,240 Joules of chemical energy if we use the typical 25% efficiency figure for muscles. Changes in anatomy can reduce the “wasted” work done by co-activated antagonists, for instance, but won’t change the minimum total mechanical and chemical work needed. Slow fibers may help, but not enough to account for this difference, and come with a penalty – slow fibers have lower power outputs, so a slow-fiber animal shouldn’t be able to rise 10m up a tree as quickly as a fast-fiber animal. Even brachiation is ~20% more costly that just walking/running (though it does have the benefit to keeping the animal at fruit-level, thus reducing costly vertical movement).
Beautiful comment!
I have no idea what to offer on this front, but I’d be curious to see comparisons between captive orangutans and wild orangutans. Does one burn more than the other? Does the provisioning from zoos offer any sort of advantage?
Oh, man. This is going to make me think for a few days.
The paper did have a “warning” of sorts to zoos- since they don’t require as many calories, we are probably overfeeding them and contributing to their obesity. I’m kind of interested in seeing what gorilla metabolism is like, since they are folivores, are big, and have lots of problems with obesity when in captivity.
The study used orangs from the Great Ape Trust in Iowa, which is supposed to be “similar” to the environment of wild apes.
Does this disagree with your june 10 post?
“In chimps, the border of the ilium extends further, so that the last two lumbar vertebrae are “entraped” by the ilium. Orangutans share this feature with chimps. This may seem trivial, but it demonstrates that animals with more active locomotor regimes can converge on the same degree of entrapment”.
Sloths and orangs have very low metabolisms, long thick fur coats, tropical rainforest canopy niches, and spend most of their waking hours hanging from branches while foraging, both have camouflage and generally lack big predators (which produce fast-escape-tactics in prey).
I think long daily periods of hanging while foraging produces this metabolic effect.
I suppose “dynamic” would have been be a better word to use than “active.” What I didn’t know then that I know now (thanks to this article) is that orangs don’t move around *as much.* But they still move around in a similar way and face similar evolutionary pressures and constraints, because of their great ape body plan.
I wonder, why would hanging in particular produce this kind of metabolism, as opposed to, say, standing up all day to forage like a cow does?
Well, hanging is passively stable – all you need to do is maintain enough muscular action to hold on (and isometric muscle contractions are cheaper than those when length changes, even for the same number of motor units). A standing animal must not only support its weight, but continuously activate additional muscles and motor units to maintain postural stability.
Plus, I wouldn’t be surprised if a cow’s BMR was lower compared to a similarly-sized active forager such as a large bear.
I think that slow, camouflaged branch hangers have much less predation.
Just a thought on how orangs might slow down their metabolism…by reducing circulating levels of “thyroid hormone” (triiodothyronine and thyroxine). As stated in Sherwood, Klandorf, and Yancey’s “Animal Physiology”, thyroid hormone “is the primary determinant of overall metabolic rates” or “idling speed” by, among other things, regulating the number of active sodium-potassium pumps in cell membranes. I’m not sure how circulating levels vary among the great apes but I wouldn’t be surprised if they were relatively low.
Interesting idea. I just found this article, which suggests that you may be on to something.