Sexual Dimorphism & Social Behavior of Paranthropus robustus

This week, we saw a short paper in Science on Paranthropus robustus sexual dimorphism and the implications the differences between sexes had on this early hominid social behavior. Here’s the title and a link to the original publication, “Extended Male Growth in a Fossil Hominin Species.”

Sexual dimporphism is what scientists use to define the differences between male and female body sizes and anatomical variation. In great apes like gorillas and orangutans, there is a large degree of sexual dimporphism. Males are physically much larger, for example, some male silverback gorillas are twice the size of female gorillas. Male gorillas also have pronounced sagittal crests and other skeletal features that support larger bodies. Chimpanzees and bonobos do not exhibit sexual dimporhism to the degree gorillas and orangutans do. They fall more in line with humans. Most modern day human skeletons have finite differences that aren’t really cut and dry, but almost all male ape primates have two periods of growth and development during adolescence, one prior to females and one after female adolescence development wraps up. This is understood as an adaptation to complete growth after all the competition has passed its prime.

As the binomial name implies, the specimen analyzed in this last week’s issue of Science, Paranthropus robustus is a robust australopithecine. Robust means it has a lot of pronounced features, and this species makes for a great example to study robusticity because there’s a lot of Paranthropus fossils.

There is some really ignorant press that confused Paranthropus robustus as a human ancestor, such as this National Geographic News headline, “Early Human Ancestors May Have Had “Harem” Societies.” This headline is not particularly true. True, these australopithecines were bipedal and succeed after Australopithecus afarensis and africanus. A branch off of the Australopithecene lineage most likely gave rise to early Homo, but the current consensus among plaoeanthropology is that the Paranthropus lineage did not. Another separate australopithecine lineage gave rise to the Paranthropus genus.

Furthermore, all known P. robustus fossils are not older than 2 to 2.5 million years ago, which means they succeeded A. africanus and appeared after the Homo lineage already diverged. I’d like to clarify that not everything hominid is directly related to humans. For all we know now, we can only say that
the Paranthropus genusshared a commn ancestor with humans. So keep that in mind as I review this publication.

This study used 35 P. robustus specimens from Swartkranns, Kromdraai, and Drimolen sites in South Africa. Like I mentioned in the paragraph above, the deposits these fossils came out from are no older than 2 million years ago and no younger than 1.5 million years ago. The authors limited their pool of specimens to analyze into three criteria. The fossils had to show evidence that the last molar tooth had erupted. M3, the last molar, usually coincides with adolescence, a time during life history where sexual differences manifest while the primate undergoes puberty and sexual maturity. Other logistical criteria included having sufficient parts of the fossil fragments to make a complete analysis. Ultimately 19 face fragments, most of which came from Swartkrans and most of the 16 jaw fragments also from Swartkrans were studied. In the sample there were about 29 males and 6 females P. robustus represented, which they account for because males were less shielded from predation than females.

The amount of wear on each specimen’s teeth was studied to estimate the ages of the individuals at the time of death. The authors state that they had a sample that represented ‘nearly every stage of dental wear from young adulthood to old age.’ The authors make note of an observation that the P. robustus males exhibited a bimaturation pattern, and older males were much larger than the younger males.

Size & Morphological Comparisons of three Paranthropus robustus specimens

They authors go onto infer how the differences in the pattern of development in P. robustus males and females represent gorillas and not really other australopithecines. The pressed loved this, eating it up and spitting it out to the public that early hominids had harems much like a male silverback does. I’m not too certain about that, it is a strong possibility that’s the case. The adaptionists understand this is an excellent reproductive and evolutionary strategy, larger the male the more he can dominate females as well as thwart out competition from other males. That’s how silverbacks do it, and other animals such as lions, elephant seals, etc.

But I’m not completely sold. I’m uncertain about this all because one of my undergraduate professors, Dr. Adrienne Zihlman, instilled a lot of uncertainty in me about sexual versus species differences, especially in australopithecines. She argued that lots of the robust australopithecines, such as P. robustus can represent only the males of the species while the gracile ones represent the females. Ultimately, she was saying what we maybe calling different species could be just the differences between males and females. If you ever seen male and female orangutan or gorilla skeletons, and didn’t know who was who, you could divide them up into 4 different species. For this reason, I’m not sold that P. robustus was harem like, I do agree that the authors made an astute observation that there is a bimaturation processes in the 29 of the specimens analyzed. That’s about all I feel comfortable with settling on.

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