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. 2003 Aug 5;100(16):9404-9.
doi: 10.1073/pnas.1133180100. Epub 2003 Jul 23.

Sexual dimorphism in Australopithecus afarensis was similar to that of modern humans

Philip L Reno  1 , Richard S Meindl, Melanie A McCollum, C Owen Lovejoy
Affiliations

Sexual dimorphism in Australopithecus afarensis was similar to that of modern humans

Philip L Reno et al. Proc Natl Acad Sci U S A. .

Abstract

The substantial fossil record for Australopithecus afarensis includes both an adult partial skeleton [Afar Locality (A.L.) 288-1, "Lucy"] and a large simultaneous death assemblage (A.L. 333). Here we optimize data derived from both to more accurately estimate skeletal size dimorphism. Postcranial ratios derived from A.L. 288-1 enable a significant increase in sample size compared with previous studies. Extensive simulations using modern humans, chimpanzees, and gorillas confirm that this technique is accurate and that skeletal size dimorphism in A. afarensis was most similar to that of contemporary Homo sapiens. These data eliminate some apparent discrepancies between the canine and skeletal size dimorphism in hominoids, imply that the species was not characterized by substantial sexual bimaturation, and greatly increase the probability that the reproductive strategy of A. afarensis was principally monogamy.

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Figures

Fig. 1.
Fig. 1.
Sample of some original specimens of the distal humerus from A. afarensis. (A) A.L. 288-1; (B) A.L. 322-1; (C) A.L. 137-48A; (D) A.L. 333-29; and (E) MAK VP 1/3. A simple nonsystematic comparison of A and E yields very high dimorphism; however, the presence of other specimens of intermediate size verifies that such estimates cannot be correct (see Table 1).
Fig. 2.
Fig. 2.
(A) Histograms generated by simulations of A.L. 333 sample for 22 metrics treated separately and for 22 metrics randomly obtained from only nine individuals (1,000 iterations each). The arrow and vertical line in each plot indicate dimorphism for the A. afarensis sample from A.L. 333. Note that restricting the source of these metrics to only nine individuals (the minimum number of individuals at A.L. 333 is nine) has no substantial effect on the dimorphism estimate. (B) Histograms generated for samples of 22 (A.L. 333) and 29 (C.A.) metrics. Note that the behavior of BDI and CV are almost identical, but that MMRs greatly overestimate dimorphism (see Table 3). Nonetheless, MMRs predict the same degree of dimorphism in A. afarensis, which is substantially greater for the C.A. sample than for A.L. 333, but still most similar to humans.
Fig. 3.
Fig. 3.
Mean absolute differences (and standard deviations) between simulations of actual dimorphism (DM) and BDI and MMR using the template method (TSD) for the C.A. simulation (Table 3). BDI and MMR are the two methods in our study that produce direct estimates of dimorphism. BDI overestimates chimpanzee dimorphism, because the species is actually nondimorphic. For species with dimorphism as great as Homo and Gorilla, however, the actual DM is well within the 2-SD range of the dimorphism indices.
Fig. 4.
Fig. 4.
Simple ratio of male/female FHDs (using A.L. 288-1 as a template) for A.L. 333 (for Combined Hadar, see the supporting information) as a function of the number of females preserved [i.e., the full range of all ratios used to calculate BDI (TSD in Table 3)]. A gorilla-level dimorphism was present at this site only if it contained a single female specimen. In all other cases (female n = 2–21), dimorphism was near or well below the average modern human BDI (estimated by TSD; Table 3). This calculation presumes that males are always larger than females, which is almost certainly incorrect at this level of probable dimorphism, and thereby is also an overestimate of actual dimorphism. Indeed, if more than one-half the sample is female, its BDI (calculated by TSD and thereby overestimated) is still very close to human values of actual (known sex) dimorphism (see Table 3).
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