The bones of long-dead species usually reveal little about their social behavior, but a study of four ape species suggests there might be an exception in the sagittal crest, the bony ridge some have at the top of the skull. These ridges provide space for muscles to attach to, allowing stronger chewing, but new research indicates they also act as a form of male social signaling.
Gorillas' diets involve a lot of chewing, so it helps to have very strong temporalis muscles, and these need somewhere to grip. Sagittal crests provide this, and for a long time this was thought to be the end of the story. However, Dr Katharine Balolia of the Australian National University pointed out that the crests are often larger than they need to be for muscle attachment alone. Previous research has shown that female gorillas find a big crest a bit of a turn on, and offspring born to large-crested males are more successful.
Balolia also noted that some extinct human relatives, particularly some members of the Australopithecus genus, had prominent crests, although by the time our own genus had evolved crests were almost entirely gone. She wondered if the crests could inform us, not just about the foods our ancestors ate, but how they interacted.
In the Journal of Anatomy Balolia reports on a comparison of the crests in four ape species. She shows that in western lowland gorillas and orangutans the timing of crest formation coincides with males becoming dominant and competing for mates. In gorillas, this happens shortly after the wisdom teeth emerge, but male orangutans have a long period of sub-dominant adulthood. The crest only emerges when they are ready to claim territory, later in life.
Female gorillas also develop a crest, but it is smaller and emerges more slowly, suggesting its purpose is for chewing, not show.
On the other hand, lar gibbons seldom develop a crest at all. Balolia thinks this reflects lower levels of male-male conflict, and therefore less need for showy signs of dominance.
Species have plenty of other forms of sexual signaling, but most don't fossilize well. If, as Balolia suspects, the absence of a crest among apes indicates a capacity for multiple adult males to live together, its disappearance could help us understand the evolution of social behavior in long-gone ancestors.
“If sagittal crest size and social behavior are linked in this way, then we could potentially establish that some of our extinct human relatives had a gorilla-like social system,” Balolia said in a statement. "This would be a first because otherwise, the human fossil record provides precious little about how our extinct relatives chose their mates."
That's a very impressive sagittal crest you've got there. Bet the ladies love it. Kari K/Shutterstock