CRANIAL CRESTS IN THE ANTHROPOIDEA
Authors: ASHTON E. H. , ZUCKERMAN S. 
Published: August 2010

SUMMARY

In young monkeys and apes, and human skulls of all ages, the cranial attachments of the temporalis and nuchal muscles are completely separate.
As the permanent teeth erupt, the facial skeleton grows much faster than the cranium, and the correlated development of the temporalis and nuchal muscles results in the formation of bony crests in the occipital region (nuchal crests) and sometimes also in the mid‐sagittal line of the skull (sagittal crests).
The crests are most powerfully developed in the gorilla, especially in males, where a nuchal crest may eventually become a uniform shelf of bone as much as 4 cm. wide. A sagittal crest begins to form in the region immediately anterior to the inion. It is present in all males, and may be as much as 5 cm. high. It is found in only about two in every five females.
In the adult chimpanzee, the nuchal shelf is formed only in the middle part of the arc between the mastoid and inion, and even in males does not project more than 0–6 cm. A sagittal crest is never found in female chimpanzees. A sagittal ridge or small crest develops in about one male in six. It begins to form mid‐way between the bregma and lambda.
In the orang‐utan, the nuchal crest varies in its development. Occasionally it forms a continuous shelf from mastoid to mastoid, and sometimes it is present in only the middle part of the arc between the mastoid and the inion. A sagittal crest develops in up to seven out of every ten adult males, but in only about one adult female in twenty. It begins to form in the region mid‐way between the bregma and lambda.
In the gibbon the nuchal crest seldom projects more than 0–1 or 0–2 cm., and extends over only the middle part of the arc between the mastoid and inion. Even in adults, and in contrast to the condition found in most apes and monkeys, the temporal lines are often closest together in the region of the bregma. Very rarely they meet to form a sagittal ridge in this region.
In the adult white‐nosed monkey, the nuchal crest occupies at most only the middle third of the arc between the mastoid and the inion. A sagittal crest is present in about one adult male monkey in ten, but never in adult females. It may develop near the bregma, near the inion, or at both points simultaneously. It extends along the whole length of the sagittal suture in less than one male skull in twenty.
In the adult male baboon, the nuchal shelf may extend uninterruptedly from mastoid to mastoid, and may project as much as 0–5 cm. In some males and in all females it extends for only a short distance behind the mastoid. The sagittal crest forms in the region immediately anterior to the inion in practically all adult males. In the female the temporal lines may touch immediately anterior to the lambda, but a true crest does not develop.
In the woolly monkey, the nuchal crest is weakly developed behind the mastoid. Even in adult males it seldom extends for more than 2 cm. and projects at the most for 0–05 cm. In some adult females not even a nuchal crest is formed. No sagittal ridge or crest was found in any of the skulls of this type of monkey that were examined.
The growth processes underlying the formation of the cranial crests are similar throughout the Primates, variations between different species resulting from differences in the relative speed of growth and in the point at which growth stops. Thus:

In all primate species the posterior fibres of the temporalis muscles grow more rapidly than the middle and anterior groups, and consequently first approximate to the upper advancing edge of the nuchal muscles at a point between the mastoid and inion.
In young animals in which only the milk teeth are in place, the temporal lines are closest together immediately posterior to the glabella. Subsequently, however, as the middle fibres of the temporalis muscle grow more rapidly than the anterior ones, the point of least separation moves backwards to the region of the bregma. Further growth may result in their meeting in the mid‐sagittal line of the skull, at a variable point between the bregma and lambda to form a sagittal crest.

In both fossil and living man, the temporalis and nuchal muscles do not approximate, and neither a nuchal nor a sagittal crest is ever formed.
The evidence for the three available specimens of Paranthropus crassidens, in which a sagittal crest is present, suggests that the cranial crests of the Australopithecinae developed according to the general pattern which prevails in living monkeys and apes.