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. 2015 Apr 28:3:e925.
doi: 10.7717/peerj.925. eCollection 2015.

From Lucy to Kadanuumuu: balanced analyses of Australopithecus afarensis assemblages confirm only moderate skeletal dimorphism

Philip L Reno  1 , C Owen Lovejoy  2
Affiliations

From Lucy to Kadanuumuu: balanced analyses of Australopithecus afarensis assemblages confirm only moderate skeletal dimorphism

Philip L Reno et al. PeerJ. .

Abstract

Sexual dimorphism in body size is often used as a correlate of social and reproductive behavior in Australopithecus afarensis. In addition to a number of isolated specimens, the sample for this species includes two small associated skeletons (A.L. 288-1 or "Lucy" and A.L. 128/129) and a geologically contemporaneous death assemblage of several larger individuals (A.L. 333). These have driven both perceptions and quantitative analyses concluding that Au. afarensis was markedly dimorphic. The Template Method enables simultaneous evaluation of multiple skeletal sites, thereby greatly expanding sample size, and reveals that A. afarensis dimorphism was similar to that of modern humans. A new very large partial skeleton (KSD-VP-1/1 or "Kadanuumuu") can now also be used, like Lucy, as a template specimen. In addition, the recently developed Geometric Mean Method has been used to argue that Au. afarensis was equally or even more dimorphic than gorillas. However, in its previous application Lucy and A.L. 128/129 accounted for 10 of 11 estimates of female size. Here we directly compare the two methods and demonstrate that including multiple measurements from the same partial skeleton that falls at the margin of the species size range dramatically inflates dimorphism estimates. Prevention of the dominance of a single specimen's contribution to calculations of multiple dimorphism estimates confirms that Au. afarensis was only moderately dimorphic.

Keywords: Chimpanzee; Gorilla; Homin; Hominid; Human evolution; Reproductive behavior; Sexual dimorphism; Sexual selection.

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Conflict of interest statement

The authors declare there are no competing interests.

Figures

Figure 1
Figure 1. Demonstration of the calculation for the template method.
(A) Many Au. afarensis specimens can be used to judge the species’ skeletal size distribution, but only a few represent the same skeletal site (horizontal axis) and therefore cannot be simply compared with one another. However, many sites are also found in the Lucy skeleton (diamonds). (B) The relative size of each non-Lucy specimen can therefore be calculated as a simple ratio (vertical axis) of each specimen to the same site as preserved in Lucy. These (dimensionless) ratios can then be used to compute a CV or BDI for the species. Note that the template ratios all equal 1 for the template specimen, so the choice of metric used for this individual has no effect on the dimorphism calculation. (C) To convert ratios to “real dimensions” each can be “normalized” using Lucy’s FHD. This has no effect on the sample’s dispersion (i.e., compare B to C), and therefore has no impact on the value of the CV and BDI when Lucy (or other specimen) is used as a “template.” See Table 1 for variable definitions.
Figure 2
Figure 2. Sampling procedure used to simulate the Template and Geometric Mean Methods in extant humans, chimpanzees and gorillas.
Figure 3
Figure 3. Size distribution of fossils using Lucy as a template.
Each specimen’s assigned sex and the number of times it is included in the respective GMM analyses are indicated. (A) The addition of Kadanuumuu elevates the non-333 range to greater than that of A.L. 333. Note that Lucy and A.L. 128/129 account for 10 of 11 female assignments in the Gordon, Green & Richmond (2008) analysis. (B) In the full, unmodified CA sample these two specimens account for 18 of 30 female assignments resulting in minimal overlap between sexes. (C) When Lucy values are increased to 130% her original size or (D) when Lucy and A.L. 128/129 are allowed to contribute only once to the sample, the sex assignments overlap substantially.
Figure 4
Figure 4. Frequency histograms of the simulations modeling the Template Method using extant chimpanzee, human and gorilla reference samples (1,000 iterations each).
The vertical line and number indicate the dimorphism value (CV) for the respective Au. afarensis sample using either Lucy (A, B, and D) or Kadanuumuu (C) as a template. In all cases the Au. afarensis value is compatible with human levels of dimorphism. However, as sample sizes decrease (B–D), the loss of intermediate specimens in the Au. afarensis sample increases the value of hominid dimorphism and/or the range of generated dimorphism values for each of the reference taxa. In these cases, the Au. afarensis values fall in the overlap between humans and gorillas demonstrating the importance of maximizing sample size when estimating dimorphism.
Figure 5
Figure 5. Estimated distal tibial articular breadth using Kadanuumuu as a template and sex assignment from the GMM.
This smaller sample includes the full Au. afarensis size range from Lucy to Kadanuumuu but excludes numerous intermediate sized specimens available when Lucy is used as template. Note that Kadanuumuu, Lucy and A.L. 128/129 account for 10 of 21 measurements included in the calculation of the GMR.
Figure 6
Figure 6. Frequency histograms of the simulations modeling the Geometric Mean Method using extant chimpanzee, human and gorilla reference samples (1,000 iterations each).
The vertical line and number indicate the dimorphism value (GMR) for the respective Au. afarensis sample and the gray line in (A) represents GMR if Lucy is increased to 130% original size. In (B) Lucy and A.L. 128/129 contribute only femoral head diameter or proximal tibial breadth respectively. When small and more complete specimens such as Lucy and A.L. 128/129 contribute multiple metrics, predicted dimorphism values fall in the upper range of the gorilla distribution (A and C). However, when Lucy is scaled to an intermediate size or Lucy and A.L. 128/129 are restricted to contributing a single metric, the Au. afarensis falls in the human distribution and significantly outside those of the apes (A and B).
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