EJERCICIO PRÁCTICO

Sexual dimorphism is defined as the anatomical difference between males and females, and can be seen on a number o f levels. The first and most fundamental differences are those relating to the organs o f reproduction. The next level, sometimes referred to as secondary sexual characters, comprises other differences between the sexes which are related to breeding or courtship, such as variations in the colour and nature o f the fur or plumage or the possession o f horns antlers or tusks. The third level is that o f size. It is usually reckoned as the mean male-female size difference, expressed as a percentage o f the mean female size. Overall and body size dimorphism is quite notable in some species for example gorillas, elephant seals, and some species may show 100% dimorphism - where the male is twice the size o f the female. These differences in size are also seen in the dentition, and are noticeable for example in the canines o f primates and carnivores. For most teeth, in most mammal orders studied, dental sexual dimorphism represents the tendency for the teeth o f the males (on average) to be larger in size than those o f the females (Hillson 1986, Kieser 1990). The simplest way to assess it is to use a t-test to ascertain whether there are statistically significant differences between the males and the females.

Much o f the work on sexual dimorphism o f the dentition has been done on primate species including humans. Some studies look specifically at the levels o f sexual dimorphism and attempt to suggest the causes for it genetically and developmentally but it is also one o f the possible sources o f variation in a metric dental study that is

measured in odontometric investigations o f populations.

The level o f sexual dimorphism in the teeth has been found to vary between populations. The mesio-distal tooth dimensions in a US population showed an average dimorphism o f 4% (Gam et a l 1964). A slightly lower level o f dimorphism was

observed in an Icelandic population, even though the teeth were larger than the those o f US population (Axelsson and Kirveskari 1983:182). The lowest levels o f dimorphism in the human dentition were found in a study o f Ticuna Indians where statistical tests failed to reveal any consistent male-female size differences (Harris and Nweeia 1980:81).

The dimorphism noted in human tooth size led to the development o f techniques for separating the sexes on the grounds o f tooth size. Such investigations generally focused on the differing results obtained when univariate or multivariate methods were employed in the analysis. Potter (1972) found that the degree o f separation possible between males and females varied according to the statistical method used, which method was most successful varied between populations and the dimensions included in the analysis (Potter 1972:720, Potter et al. 1981:40). It appears that in an odontometric investigation o f a population the wisest approach is to use both univariate and

multivariate methods in order to assess the presence and degree o f sexual dimorphism. Ditch and Rose (1972) successfully applied a multivariate sexing technique based on discriminant analysis to an archaeological sample, achieving percentages o f correct classifications o f 90% and above (Ditch and Rose 1972:63). More recently Cimrmancic

et a l used discriminant analysis to determine the sex o f an archaeological sample (for

which written records were available to check the sex o f individuals). They employed an ethnic specific (in this case Caucasian) discriminant function which produced accuracies between 85% and 95% (Cimrmancic et a l 1996:362).

The level o f sexual dimorphism in the deciduous teeth is o f a smaller magnitude that that seen in the adult dentition. However, a study by De Vito and Saunders

(1990:856), found that the dimensions o f the deciduous teeth could be used to correctly sex individuals in over 75% o f cases, noticeably less accurate than studies on the

permanent dentition. All o f these techniques for discriminating the sexes using tooth size rely upon having the whole dentition available, if one tooth type is removed the results can be quite different. The canine, for example, supplies most o f the ability to

discriminate the sexes in human dental population studies, if the canine is removed, the discrimination would drop significantly.

The variation in size and shape between male and female teeth has been used as an aid to identification in other mammal species. Most work has concentrated on the canines as these teeth are the most dimorphic, in some species extremely so. Schmid (1972) includes figures o f pig canines as a means for sexing archaeological material. This idea was taken further by Mayer and Brisbin (1988). In a study based on the measurements fi*om almost 300 individuals all dimensions were found to be larger in the males (Mayer and Brisbin 1988:408). The problem with the metric method was the

degree o f overlap in the male and female ranges - a problem also encountered in studies o f human dental dimorphism. Kurten (1969,1976) in a study o f cave bears found that the male and female canines could be distinguished metrically. The level o f sexual

dimorphism in this species, explaining a bimodal distribution o f measurements. The possibility o f confusing sexual dimorphism with the variation seen between closely related taxa is a significant problem in palaeontology (and zooarchaeology), Kurten suggests that only by comparison with related extant species can the correct pattern o f dimorphism be identified for a fossil taxon (Kurten 1969:233). Sexual dimorphism has also been identified in the canines and camassials o f various carnivores. Gittleman and Van Valkenburg found the level o f canine dimorphism related to the breeding system o f the species (Gittleman and Van Valkenburg 1997:108). The camassials were dimorphic to a lesser extent and appeared to be linked to diet {ib id .\\0%).

Influence o f sex chromosomes

Sexual dimorphism is then due ultimately to differences inherited on the X and Y chromosomes, so it has seemed appropriate to look for evidence o f this. A number o f twin or family studies have been carried out, investigating either normal chromosomes, or individuals with abnormalities in the number o f sex chromosomes.

In an investigation o f a number o f sibling pairs the highest correlation was found to exist between sister-sister pairs which let to the conclusion that the X chromosome was linked to the inheritance o f tooth size (Gam et a l 1965:440). However the possibility o f some effect by the Y-chromosome was not mled out particularly in terms o f canine size (Gam et al. 1965:440). In a study o f an Australian Aboriginal population where the sample provided half as well as full sibs Townsend and Brown carried out an odontometric analysis. The results however, failed to show a stronger correlation for sister-sister pairs or for brother-brother pairs, thus it was concluded that both X and Y linkage were not proven in this case (Townsend and Brown 1978a: 313).

The use o f data from individuals with chromosomal abnormalities to investigate the possibility o f X or Y-linked inheritance has given mixed results, both the

chromosomes influence dimorphism in the dentition but in different ways. In a study o f females with only one X chromosome Mayhall and Alvesalo measured not only the

volume using moiré contour photography (Mayhall and Alvesalo 1992). It had already been noted that individuals with this abnormality tended to have a simplified molar morphology (Mayhall and Alvesalo 1992; 1039). The XO females had noticeably

reduced basal cusp area in relation to the cusp height more similar to the normal females; resulting in cusps that appeared very steep contrasting with normal male cusp which appeared more shallow due to the larger basal area. The XO females were found to have thinner enamel. It was concluded that the X chromosome had control over the ultimate enamel thickness as the XO had thinner enamel than XX. Whilst the Y chromosome influenced ameliogenesis and dentinogenesis thus increasing the basal area. The suggestion o f an additive effect fi'om one X to XX to XY was evident in the variables measured. In a radiographic study Stroud et a l (1994) found that there were significant differences between males and females for dentine thickness but not for enamel thickness (Stroud et a l 1994:170-1). They conclude that “interproximal enamel thickness is more closely related to the timing and duration o f amelogenesis than with anticipated

functional demands” {ib id .A ll), otherwise males would have thicker enamel related to the exertion o f greater mechanical force. A study o f deciduous teeth by Hicks and Harris produced similar results:

“The noteworthy finding was the absence o f any significant sex difference in enamel thickness Sexual dimorphism in the molars apparently is wholly attributable to the size o f the dentin” (Hicks and Harris 1998:149).

Supernumerary teeth are more common in males whilst missing teeth are more often seen in female dentitions this may suggest that the Y chromosome has an influence on mitosis in the lamina where the tooth germs form (Alvesalo 1997:3).

Although the mechanism o f sexual dimorphism is not yet fully understood its existence in the dentition o f mammals has been observed. The degree o f dimorphism varies between species and between populations o f the same species. In any metric study o f the dentition o f a population, sexual dimorphism should be tested for using

appropriate statistical methods. In this project the level o f sexual dimorphism in the tooth measurement data was assessed by means o f a t- test. The data are only useful for comparing the individual variation between populations if the level o f sexual dimorphism could be shown to be negligible. The selection o f cheek teeth for this study was to minimise the hkely influence o f sexual dimorphism, but it was still necessary to measure the statistical significance o f the sexual dimorphism in the data set collected.