Aquatic Ape – the body fat observation

Recently I wrote about the Aquatic Ape Hypotheses, inspired by a conference being held on the subject.

Unsurprisingly, other people have also been busy discussing the topic and there has been a fun pseudoscientific parody called the “Space Ape Theory” which took off rather nicely on Twitter and is doing a good job of highlighting problems with the AAH.

Space Ape

This post will only be short, because other people have been dealing with this issue perfectly well without the need for my input, but I thought it might be useful to make visible the outcome of an extended conversation I had with AAH proponent Marcel F. Williams, in the comments section of my earlier post.

I decided to check some independent evidence about an inference made by Alister Hardy that led to the development of the AAH in the first place – the idea that having layers of subcutaneous fat was a trait unique to humans and aquatic mammals. This idea is still regularly cited by AAH proponents (especially Elaine Morgan) as a line of evidence for evolutionary convergence between humans and marine mammals due to the sharing of an aquatic habitat.

However, on checking the literature on primate husbandry it turns out that other primates have levels of subcutaneous fat that are directly comparable to humans if the animals have a ready supply of food, suggesting that humans are no different to other primates except in having a more stable food supply and leading a more sedentary life. This is supported by data from modern hunter-gatherer groups, who exhibit far lower fat levels than either farming or Western populations. Here’s a summary of the data with a link to the research:

Body fat – man vs monkey
Rhesus macaques in labs that are identified as being in the ‘optimal’ weight range have an average body fat content of 25% with individuals in the obese range averaging 42.7% [link opens pdf]

Autopsies of male orangutans show body fat levels range from 15% to 45% [link opens pdf]

Average body fat in Hadza hunter-gatherers averages 13.5% for men and 20% for women. In Western populations men average 22.5% and women average 37.9% [link opens pdf]

In short, humans are by no means unique in the primates with regard to their proportion of subcutaneous fat, so if any AAH proponent pulls out that old chestnut in conversation, be sure to put them right.

Oshine the morbidly obese Orangutan

Oshine the morbidly obese orangutan

12 thoughts on “Aquatic Ape – the body fat observation

  1. Another couple of references re human fat levels, from an old forum post of mine:

    about 13% for males and 24% for females in a study of BAKA in the Cameroon; 10.6% for males and 19% for females in a study of the Hadza). Naturally we have lots more info on peoples living in cities and the like and they tend to be on the fattier end of the scale. In general humans can accurately be said to be fattier than many mammals, apes certainly; it just never is good to forget the range exhibited in a feature like this. And not only an overall range, but the range throughout the year, which in the Hadza study just mentioned “fluctuated within individuals by as much as 20% of initial values for both sexes” (I’ve been at a LSE conference reception with a Hadza man and can testify that a fit looking Hadza hunter can put away one hell of a lot of food when it’s available).

    “Anthropometric data indicate nutritional homogeneity in Hadza foragers of Tanzania”
    Diana S. Sherry, Frank W. Marlowe
    American Journal of Human Biology Volume 19, Issue 1, pages 107–118, January/February 2007

    “Nutritional Status, Activity Pattern, And Dietary Intake Among The Baka Hunter-Gatherers In The Village Camps In Cameroon”
    T. Yamauchi, H. Sato, and K. Kawamura
    African Study Monographs, 21(2): 67-82, April 2000

    Also, I don’t have the ref handy but I remember reading a paper about fat levels in macaques — wild macaques — and they exhibited fat levels approx. the same as those humans in the studies I mentioned above.

    • Fantastic – many thanks!

      There was a paper on Japanese Macaques kept outdoors which also showed a seasonal variation in fat levels. I think it’s fair to conclude that primates (including humans) use subcutaneous fat as a way of making a seasonal or occasional abundance of food resources physiologically available as energy at other times. It’s such a self evident observation that I feel stupid stating it, but it has to be done.

      • There’s another paper that I haven’t read but have seen reported, measuring fat level in chimps. It’s “Water-Contact Behavior of Chimpanzees”, Shannon Angus, Folia Primatol 1971;14:51–58. That one used a ref from an unpublished manuscript (this would therefore probably be hard to dig up and actually read) which showed an average of 9% body fat in chimps, which I’d bet were captive. That’s not unlike the numbers in the refs regarding humans we found.

        Also, Adrienne Zihlman did dissections of 4 captive gorillas and found “Maximum estimates of body fat range between 19.4-44%.”
        “Body mass in lowland gorillas: a quantitative analysis.” Zihlman AL, McFarland RK., Am J Phys Anthropol. 2000 Sep;113(1):61-78.

        • That’s really useful – thanks! I thought gorillas would be on the leaner side given their low quality but fairly seasonally stable diet in the wild, but it seems that fat is used as a ubiquitous energy store throughout the apes.

          • They were captive gorillas. From repeated studies captive animals tend toward being fatter than wild, just as “Western” humans tend toward being fatter than others. For instance Caroline Pond has found that Canadians who live in cites are fatter on average than Inuit.

            One thing that seems clear in studies of fat (and Caroline Pond has been the go-to researcher for info on the evolutionary significance of fat) is that every animal tends to get as fat as it can. We think of fat as a minus but it’s a plus when it comes to a world where times are lean, which is most of our existence. There are brakes on getting too fat and they are getting food (lots of seasonal differences in fat among most wild animals and among gather-hunters) and predation. Fat slows you down. Animals which live in relatively predator-free areas are fatter than their relatives elsewhere. And humans have created their own relatively (stress relatively) predator-free environment for a long long time, probably since we came up with controlled fire and “advanced” weapons like spears.

            • Not to beat this to death, but one thing that Hardy’s observation ignored, and this has been carried on since, is that humans’ body fat over our lifespan is nothing like that seen in seals or whales. They basically get fat, both sexes, when they’re ready to start swimming (Caroline Pond has shown this is mostly due to body shaping requirements) while humans are born pretty fat (unique among mammals) due to our need for nutrition during stress because of our large brains (not brain growth per se, just that it’s a very large organ with inflexible energy requirement), then we got through a very lean period during later childhood (excepting modern peoples who have plenty of food available and can and do stay fat), then pile on fat right at puberty, with females fatter than males. There’s another change in later life, after reproductive hormone levels go down.

              It just isn’t like the fat of seals and whales at all.

            • It should be beaten to death – that’s what science is meant to do with hypotheses that can’t handle the burden of evidence.

  2. We did not descend from aquatic apes, of course, although our ancestors were anatomically & physiologically not adapted to running over open plains as some anthropologists still believe. Instead, Pleistocene Homo populations simply followed the coasts & rivers in Africa & Eurasia (800 ka they even reached Flores >18 km overseas), google, eg, “econiche Homo”.
    –eBook “Was Man more aquatic in the past?” (introd.Phillip Tobias)
    –guest post at Greg Laden’s blog

  3. Having just watched a video showing the semi-aquatic life of 40,000 bce man in South Africa, and the supposition that this lifestyle enabled survival through the last Ice Age, I have questions.
    Was the subcutaneous fat layer inherent in all of us the saving grace? After all, the access to shellfish, fish, and the aquatic biome and it’s attendant take-up of the omega-3 fatty acids as crucial building blocks for neuron creation and function all seem to point to requiring that layer to navigate and thrive in that watery ‘supermarket’.
    AAH, while not proving the exclusivity of Homo development of the fat layer, none-the-less also highlights other physical adaptations we have that can more easily (occam’s) be explained by the lifestyle we enjoyed so long ago.

    • TBH there is no doubt that human ancestors utilised lacustrine and littoral habitats – all opportunistic species do, like foxes, raccoons, crows, bears, etc. However, there is no good support for those particular habitats to have had a significant impact on adaptations in human ancestors. At best the evidence presented by AAH is circumstantial and usually it requires cherry-picking of observations to fit the hypothesis, while discounting the problems.

      Body fat is the one that started the AAH speculation and we now know that one is flawed. Each other observation can be similarly explained in other ways or discounted by a deeper investigation of the observation.

      The trouble with Occam’s razor is that while a useful tool for limiting possibilities, it isn’t good for dealing with complex systems with multiple interactions and influences, so it can lead to oversimplification, which is what I think has happened with the AAH.

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