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Temporal lines and dental development in chimpanzees J.Francis Thackeray, Transvaal Museum,
It is well known that in chimpanzees, as in some other mammalian species, sagittal crests can develop as a result of the need to increase surface area for the attachment of temporalis muscles. Such crests usually develop only in males, but it has been recognised that age is an important factor influencing the probability of the development of sagittal crests, associated with the meeting of temporal lines in the mid-sagittal plane (Prat and Thackeray, 2001). In order to quantify changes in the position of the temporal lines associated with the development of temporalis muscles in juvenile and adult chimpanzees, measurements were taken from crania in the Powell-Cotton Museum in England. Measurements of the distance between left and right temporal lines across bregma were obtained from juvenile and adult individuals, and dental stages of development for the same specimens (males and females treated separately) were also recorded. The results are summarised by calculating mean distances and where possible standard deviations, as follows : Distances between temporal lines across bregma (all distances in millimetres)
Conclusions As expected, the distance between temporal lines across bregma diminishes with individual age, in both male and female chimpanzees. For subadult individuals at the same stage of dental development, when first or second molars are developing, the distances are not significantly different. However, the differences between males and females become apparent after the third upper molars begin to erupt. Of particular interest is that in at least one subadult male chimpanzee (with third upper molar in slight wear), the temporal lines had already met. If that individual had survived into sexual maturity, it is likely to have developed a sagittal crest. There is a significant difference between males and females when one compares the mean distance between temporal lines in specimens with third upper molars in moderate to heavy wear. The mean value of only 9.1 +/- 4.3 mm, (n=5) for adult males is significantly lower than the mean value of 41.7 +/- 10.8 mm (n=19) for adult females. These results are potentially useful for purposes of assessing probabilities of the development of sagittal crests in hominoid primates with cranial capacities ranging between 400 and 500 cc. Individual age is an important factor that needs to be considered in relation to probabilities of the development of a sagittal crest (Prat and Thackeray, 2001). In the case of one specimen of Australopithecus africanus (Sts 5, "Mrs Ples"), we have a situation in which temporal lines were probably are almost touching (Prat and Thackeray, 2001), even though the individual may not have been fully adult, based on the fact that the roots of the upper third molars may not have been fully closed (Thackeray et al, in prep).
Acknowledgments We are grateful to the curators of the Powell-Cotton Museum for the opportunity to study modern chimpanzee skulls in their care. This study has been supported by a grant from the National Research Foundation (South Africa) and through the Embassy of France in South Africa (Cultural Services).
References Prat, S. and Thackeray, J.F. 2001. Position des lignes temporales sur le cranium de "Mrs" Ples (A. Africanus): une attribution sexuelle est-elle possible? C.R. Acad.Sci. Paris 322: 403-409. Thackeray, J.F., Braga, J and Treil, J. In preparation. CT analyses of the roots of the teeth of Sts 5 ("Mrs Ples"), an adult cranium of Australopithecus africanus.
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