Sexual dimorphism in A. afarensis, again

5 minute read

Earlier this year, Michael Plavcan et al. (2005) had a critique in Journal of Human Evolution of the 2004 paper by Philip Reno et al. in PNAS concerning sexual dimorphism in A. afarensis. Now, in the August issue of JHE, Reno and colleagues have a reply. I have previously written about the Plavcan critique and a news report on the issue.

The reply by Reno et al. (2005) covers the four areas raised by Plavcan et al. (2005) thusly:

  1. Is the AL 333 sample biased? And how many individuals are there? Reno et al. (2005) present new simulations to show that a very small number of individuals at the site would still result in an interpretation of low dimorphism. So a smaller sample size than they originally assumed does not explain their interpretation of low dimorphism (although it does limit the information provided by the data). They further argue that it is likely the non-AL 333 sample that is biased, because it does not represent the number of intermediate-sized (presumptive female) individuals found at AL 333.
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  3. How does temporal variation affect the estimate of dimorphism? Plavcan et al. (2005) used a clever illustration to show that temporal variation may have no effect on dimorphism estimates. Reno et al. (2005) respond with another clever illustration showing that temporal variation may affect dimorphism estimates substantially, usually by causing overestimation. This point is essential to their argument that the AL 333 site is more representative of dimorphism than the other, temporally dispersed, localities. </p>
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  5. Is skeletal dimorphism well-related to body mass dimorphism? This problem is third on the list generated by Plavcan et al. (2005), but Reno et al. (2005) tackle it first. That may be because they caught the prior authors in an error: </p>
    Plavcan et al. make a fundmental error in their Figure 3. This figure shows body mass dimorphism (BMD) plotted against femoral head dimorphism in apes and humans. They also plotted values for A. afarensis calculated from our template estimates of FHD on this figure. They claim that these values imply marked femoral head dimorphism for A. afarensis. They do not. This figure mistakenly commingles FHD estimates for A. afarensis, i.e., template sexual dimorphism (our TSD) and actual values obtained by direct measurement of specimens with known sex, i.e., direct sexual dimorphism (our DSD) (Reno et al. 2005:280).

    The two are not comparable, so this is an apples-and-oranges comparison. For the rest, Reno et al. (2005) correctly point out that body size dimorphism is not what they are trying to estimate, since skeletal dimorphism is all that fossils present:

    Skeletal dimorphism is only one aspect of size dimorphism. It is not, and can never be, a simple surrogate for dimorphism in body mass. First, these characters have only partial association in hominoids; humans display moderate skeletal dimorphism and low levels of mass dimorphism, but chimpanzees show the opposite relationship (as we discussed, see: Reno et al., 2003). Second, body mass (and therefore body mass dimorphism) is unknowable for fossils. It is therefore impossible to derive a regression with which to estimate it without substantial error, because such regressions must be based on extant species that are likely to be biologically dissimilar to extinct ones. Skeletal dimorphism, however, is dependent only upon skeletal dimensions, which are directly measurable in fossils. Plavcan et al.s discussion of scaling and allometry robustly demonstrates the regression problem. That is why we do not use regression. However, we certainly understand why they were able to "corroborate previously reported findings that A. afarensis [body] size dimorphism falls between that of chimpanzees and gorillas." (p. 318). For skeletal dimorphism this range includes virtually all primates (Reno et al. 2005:281).

    This is certainly the safe route, although the claim does raise a critical question, discussed below.

  6. Does body mass dimorphism predict social or behavioral features of primates? Of course, what primatologists usually study is body size dimorphism, and not skeletal dimorphism. So if skeletal dimorphism is what we are limited to studying in fossils, then where is our comparative data? Here, Reno et al. (2005:285) present some theory, focusing on bimaturational patterns: </p>
    There has long been a sub rosa assumption that body mass dimorphism is the primary target of sexual selection. However, body mass is a complex character and incorporates several morphological components (Leigh, 1992), particularly (but not exclusively) both skeletal and muscle mass. Muscle mass and skeletal dimorphism can be differentially regulated during mammalian development (McMahon et al., 2003), and our results suggest that this is likely to be the case in chimpanzees and humans. Thus, sexual selection can independently affect skeletal and muscle growth.

    That's certainly something to chew on, as are the details of chimpanzee and gorilla developmental hypotheses presented later. They conclude that no extant ape or primate provides a valid model for A. afarensis, and therefore it is necessary to construct one.

At the end, the paper asks an interesting question of A. afarensis:

If its skeleton was largely modern in structure, is it not also likely that much of this derived physiology and anatomy had evolved by the dawn of the Pliocene in ecogeographically unique and cosmopolitan A. afarensis? (Reno et al. 2005: 286-287, emphasis in original).

"This derived physiology and anatomy" includes

...concealed ovulation and permanently enlarged mammary glands implying female reproductive crypsis, elaborated epigamics in both sexes (implying bi-directional mate choice), minimal semen coagulation, moderate muscularity of the vas deferens, relatively small testes and sperm midpiece, retention of scrotal rather than peritoneal testes, a remarkably rapid loss of olfactory receptors, a loss or failure to develop significant vocal sacs, and hormone profiles potentially paralleling those of some extant monogamous mammals (ibid., 286, citations elided).

I have only one thing to say. If the "largely modern" skeleton of A. afarensis is enough to infer all this stuff, then why are we talking about Neandertals? Of course, it's because a "largely modern" skeleton isn't enough to infer anything.


Plavcan JM, Lockwood CA, Kimbel WH, Lague MR, Harmon EH. 2005. Sexual dimorphism in Australopithecus afarensis revisited: How strong is the case for a human-like pattern of dimorphism? J Hum Evol 48:313-320.

Reno PL, Meindl RS, McCollom MA, Lovejoy CO. 2005. The case is unchanged and remains robust: Australopithecus afarensis exhibits only moderate skeletal dimorphism. A reply to Plavcan et al. (2005). J Hum Evol 49:279-288.