How did Lucy and her ilk choose a mate? Did her Paranthropus robustus cousin, we’ll call her Elenor Rigby, turn to Lucy and say, “hey, look at the sagittal crest on that one!”? She might have.
Researchers from the Australian National University (ANU) have analyzed sagittal crest size in great apes and gibbons and now hypothesize that crest size may play a role in social signaling and by extension mate selection. They postulate that the social behavior they observed in apes can be useful in reconstructing behavioral aspects of our extinct sagittal crest bearing relatives.
A sagittal crest is a ridge of bone running front to back on top of the head, kind of like Yondu in Guardians of the Galaxy but under the scalp, not protruding. Powerful chewing muscles attach to the crest giving an animal tremendous bite force. However, ANU scientists Balolia, Soligo, and Wood, say their analyses shows muscle attachment alone does not fully account for observed sagittal crest sizes. Rather, the group found age related crest size differences in male apes that were not reflected in females and a significant relationship between male crest size and “tooth wear rank.” They also found a relationship between male crest development and wisdom tooth eruption, which, according to Dr. Balolia is important because:
"In terms of gorilla social structures, the males establish dominance shortly after their wisdom teeth emerge. We found the sagittal crest appears right after their wisdom teeth emerge, so that fits in with the timing of social dominance.”
"In contrast, in orangutans some males only become dominant quite late in their adult life, and the sagittal crest appears later.”
Their work was published in the Journal of Anatomy and can be found here
http://onlinelibrary.wiley.com/doi/10.1111/joa.12609/full
Balolia, K. L., Soligo, C. and Wood, B. (2017), Sagittal crest formation in great apes and gibbons. J. Anat., 230: 820–832. doi:10.1111/joa.12609
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