Sagittal crest formation in great apes and gibbons

Abstract

The frequency of sagittal crest expression and patterns of sagittal crest growth and development have been documented in hominoids, including some extinct hominin taxa, and the more frequent expression of the sagittal crest in males has been traditionally linked with the need for larger-bodied individuals to have enough attachment area for the temporalis muscle. In the present study, we investigate sagittal cresting in a dentally mature sample of four hominoid taxa (Pan troglodytes schweinfurthii, Gorilla gorilla gorilla, Pongo pygmaeus pygmaeus and Hylobates lar). We investigate whether sagittal crest size increases with age beyond dental maturity in males and females of G. g. gorilla and Po. pyg. pygmaeus, and whether these taxa show sex differences in the timing of sagittal crest development. We evaluate the hypothesis that the larger sagittal crest of males may not be solely due to the requirement for a larger surface area than the un-crested cranial vault can provide for the attachment of the temporalis muscle, and present data on sex differences in temporalis muscle attachment area and sagittal crest size relative to cranial size. Gorilla g. gorilla and Po. pyg. pygmaeus males show significant relationships between tooth wear rank and sagittal crest size, and they show sagittal crest size differences between age groups that are not found in females. The sagittal crest emerges in early adulthood in the majority of G. g. gorilla males, whereas the percentage of G. g. gorilla females possessing a sagittal crest increases more gradually. Pongo pyg. pygmaeus males experience a three-fold increase in the number of specimens exhibiting a sagittal crest in mid-adulthood, consistent with a secondary growth spurt. Gorilla g. gorilla and Po. pyg. pygmaeus show significant sex differences in the size of the temporalis muscle attachment area, relative to cranial size, with males of both taxa showing positive allometry not shown in females. Gorilla g. gorilla males also show positive allometry for sagittal crest size relative to cranial size. Our results suggest that although patterns of sagittal crest expression have limited utility for taxonomy and phylogeny reconstruction, they could be useful for reconstructing aspects of social behaviour in some extinct hominin taxa. In particular, our results in G. g. gorilla and Po. pyg. pygmaeus, which suggest that the size of sagittal crests in males cannot be solely explained by the surface area required for attachment of the temporalis muscle, offer partial support for the hypothesis that large sagittal crests form in response to sexual selection and may play a role in social signalling.

Masculinity and the Mechanisms of Human Self-Domestication

“…Relevant structures include: brow ridges, teeth (especially canines), cheek flanges, and sagittal crests, each of which are derived from embryonic NCC progenitors. Interestingly, male cheek flanges and exaggerated sagittal crests emerge at sexual maturity, or even later in adulthood (Balolia, Soligo, & Lockwood, 2013;Balolia, Soligo, & Wood, 2017). This delayed developmental timing demonstrates both stem-cell-like extended potency of the NCC-derived cells involved, as well as interactions with androgen hormones and related social signalling.…”

Evaluating the muscle attachment hypothesis for sagittal cresting in Gorilla and Pongo

“…These findings have implications surrounding the associations between social behaviour and the morphology of the craniofacial complex. Male-male competition is associated with the development of sexually dimorphic traits, including the sagittal crest in gorillas and orangutans (Atmoko & van Hooff, 2004;Balolia et al, 2017;Breuer et al, 2012;Caillaud et al, 2008;Cassini, 2020). Caillaud et al (2008), based on photogrammetric data, found male sagittal hump size to be related to the number of females in a breeding group, which suggests that the size of the sagittal hump is a signal of male health, as the male is healthy enough to produce excess adipose tissue surrounding the crest.…”

“…They determined that sagittal hump size is an indicator of reproductive success and offspring survivorship. Balolia et al (2017) showed that sagittal crests emerge in early adulthood in the majority of G. g. gorilla males, and in P. p. pygmaeus there is a three-fold increase in the number of males exhibiting a sagittal crest in mid-adulthood.…”

Biocycle between Design and Additive Manufacturing

“…The strong bite of this fish is related to the characteristic shape of its skull, since the sagittal crest they present allows a greater insertion of muscle fibers, that results in the generation of a greater bite force which some species require to feed. [ 8 ] This crest is also present in males of the California sea lion ( Zalophus californianus ), a species that also requires great strength to catch its prey. [ 9 ] Because of this increased strength, the skull requires a structure that resists both forces: tension as the large muscles from the crest shorten, and compression as the strong jaw closes.…”