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The ontogenetic basis of the observed sexual shape dimorphism was investigated. Unlike G. gorilla and P. troglodytes, the preceding adult analyses determined that the overall form of the facial skeleton of P.

paniscus is not sexually dimorphic overall, however, previous studies (Cramer, 1977; Fenart & Deblock, 1972) have reported sexual dimorphism in P. paniscus in bizygomatic breadth and the mesiodistal dimension of the upper canine (which may be reflected in the bony morphology). Therefore the ontogeny of form variation was also investigated in P. paniscus in order to identify any such dimorphism and, as with the other two species, determine whether or not its ontogeny is size related.

There is no significant angle between the scaling trajectory of juvenile males and the scaling trajectory of juvenile females within each

species. This indicates that, within each of the three species, any shape differences between the juvenile males and females are due to ontogenetic scaling. It should be noted that this finding is not stated with absolute confidence because positional differences between the scaling trajectories of the juvenile males and juvenile females have not been statistically tested. The limited sample sizes within each species of males and females from dental stages 1 and 2 (see Table 2.1) precludes the statistical analysis of differences between the start shapes of these trajectories. By definition, if the male and female juveniles are ontogenetically scaled, individuals of the same size will share the same shape. Therefore, in a future study such trajectory transpositions could be statistically tested using older juvenile individuals which were sampled in greater numbers (e.g. dental stage 3). This provisional finding of intraspecific ontogenetic scaling prior to adulthood in turn indicates that the single trajectory constructed from a juvenile combined sex sample, for each species, is an adequate estimate of scaling for the sample.

The intraspecific form variation associated with both ontogeny (i.e. size increase) and sex was investigated, and is discussed for each species in turn below. Ontogenetic shape variation was examined using PCs correlated with centroid size, sexually dimorphic shape variation was assessed by examining PCs on which adult males and females are significantly separated. Thus, based on the above finding, that within each species the juvenile males and females are ontogenetically scaled, aspects of sexually dimorphic shape variation due to ontogenetic scaling were assessed by examining PCs on which both the juvenile sample correlates with size and the adult males and females are significantly separated.

3.4.2.1 Gorilla gorilla

The ontogenetic shape changes of the male and female juveniles are well represented by PCI. This is the only PC with which the

combined juvenile male and female sample significantly and strongly correlates with increasing size.

PC1 is also representative of sexually dimorphic adult shape variation. In addition, the graph in Figure 3.4a and strong and significant size correlation of the full sample (adults and juveniles combined) demonstrate that PC1 represents shape change associated with increasing size amongst the adults. More specifically, the adult males appear to have a more curvilinear relationship between PC1 shape variation and increasing centroid size. However, the larger adult males still have the lowest PC scores (as would be predicted bearing in mind that the smallest individuals have the highest PC scores) except the scores are not as low as would be predicted by linear extrapolation for their size. The adult females, on visual inspection correspond more closely to the linear relationship amongst the juveniles than do the adult males. PC1 represents approximately 34% of the total shape difference between the mean adult male and female (Table 3.6), however, the adult male curvilinear relationship between size and this PC indicates that the shape dimorphism on PC1 is not a simply due to ontogenetic scaling. Rather it seems that the variation of the adult females on PC1 is due to ontogenetic scaling of the juvenile trajectory whereas that of adult males does not share the same relationship with centroid size.

What are the shape changes associated with increasing size common to both male and female juveniles and adults in the G. gorilla

sample? As Figure 3.5 demonstrates, overall there are two major types of shape change which relate to the orbit and the masticatory apparatus.

With increasing size there is a relative reduction in the overall contribution of the orbit to the whole of the facial skeleton. This is quite expected because neural structures, such as the eyes themselves, are known to cease growth long before the rest of the face and body as a whole (Gould, 1975; Shea, 1983brain).

The second shape change is the relative increase in the proportions of the structures associated with the masticatory apparatus.

In particular are the anteroposterior elongation of the palate, and the relative increase in both width and height of the zygomatic and infraorbital region. The relative increase in the length of the palate is clearly related to the need to increase the alveolar maxillary region in order to accommodate the progressive eruption of the deciduous dentition and then the yet larger permanent dentition. This elongation of the palate is not uniform along its length, there is a relative decrease in the region successively occupied by the deciduous molars and permanent premolars. In fact according to the data of Ashton & Zuckerman (1950) there is very little absolute difference in the total mesiodistal length of the deciduous molars and the permanent premolars with which they are replaced. On the other hand, there is a large relative increase in the length of the alveolus posterior to this region which comes to house the permanent molar dentition. This portion of the dentition does not replace any of the deciduous dentition and so “de novo" alveolus, as it were, is required to house the successive eruption of each of the three permanent molars. It is the addition of this bone for the alveolus of the permanent molars which contributes most to the relative increase in the length of the palate. In relation to this great increase in length the relative width of the palate decreases, presumably because, even with the more anteriorly orientated incisor alveolus, the total width of the permanent incisors (mesiodistal) and canine (buccolingual), which contribute to the maximum width of the palate, is proportionally dwarfed by the maxillary elongation associated with the eruption of the above mentioned permanent molars.

Based on centroid size correlations, no other PC can be considered here to represent shape changes associated with size increase amongst the juvenile G. gorilla sample.

The remaining portion of sexually dimorphic shape variation amongst the adult G. gorilla sample is represented on PCs that are uncorrelated with size amongst the juvenile only sample and each of the adult sex samples (Table 3.6) and as such cannot be regarded as the product of either ontogenetic scaling or allometry in general. These are

PCs 4, 5, 6, 7 and 8, which contribute approximately 18%, 11%, 4%, 12% and 9%, respectively, to the total shape difference between the mean adult male and female (Table 3.6). As such these PCs represent the adult sex specific adult shape variances that are not the product of an extension of the juvenile scaling trajectory to adult sizes. Rather these PCs indicate that the shared juvenile scaling trajectory, as represented by PCI, diverges around the point of adulthood (determined in this study as the complete eruption of the permanent maxillary canines and third molars) into a separate male and female trajectory. The divergence of the adult sexes is thus represented across these five PCs (PCs 4, 5, 6, 7 and

8).

What are the non-allometric shape changes associated with sexual dimorphism in the adult G. gorilla sample? The interpretation of these shape changes proves quite complex given the number of PCs which represent this shape variation. The major shape changes represented by each of these PCs, which represent shape change that is both non- allometric and sexually dimorphic (individually described in detail in section 3.3.2.1) are below combined to give an overview of the shape change represented by this trajectory divergence. The shape differences are described in terms of the mean male shape relative to the mean female (the PCs that contribute to each shape change are given in brackets):

• relative increase in midfacial prognathism (PC4; Figure 3.6)

• relative increase in the mesiodistal dimension of the canine alveolus (PC4 and PC7; Figure 3.6 and Figure 3.9, respectively)

• relative decrease in palatal width (PC4 and PC6; Figure 3.6 and Figure 3.8, respectively)

• relatively more anteriorly projecting superior nasal margin, resulting in "beaking" appearance (PC5 and PC7; Figure 3.7 and Figure 3.9, respectively)

• relative lateral expansion of infraorbital and zygomatic regions, with contributions from both maxilla and zygomatic bone (PC5 and PC8; Figure 3.7 and Figure 3.10, respectively)

Thus the facial form in G. gorilla, as determined in this study, is the product of a shared male and female trajectory up until the eruption of the canines and third molars, at which point the shape of the adults come to differ by divergence of trajectories which are a combination of ontogenetic scaling along this common juvenile trajectory and sex specific non- allometric shape variation.

3.4.2.2 Pan paniscus

The ontogenetic shape changes associated with increasing size amongst the juvenile P. paniscus sample is represented by PCI. This is the only PC on which the juvenile sample is strongly and significantly correlated with centroid size. When the full sample is included, that is male and female adults as well, there is still a strong size correlation on P C I, however, neither the male nor the female adult sample alone on PCI is correlated with size. In addition, the adult males show a correlation between their PC3 scores and size (there is a weak to moderate correlation for the adult females on PC3). Although the size correlation of the female adults on PC3 is not very convincing, a t-test for this PC shows there is no shape difference between the sexes (Table 3.9) suggesting that shared aspects of scaling of both sexes are described to some extent on this PC. The fact that the shape variation on this PC is size correlated means that this shape variation cannot just be regarded as representing the increased individual shape variation that is expected amongst adults as compared to younger and smaller individuals. This indicates that, although the scaling of the male and female juveniles can be adequately represented by a single linear trajectory (PCI), around the point of adulthood this trajectory no longer completely describes the scaling of the male and female adults, size

related shape change is also represented on PC3. It would appear therefore that the adults diverge from the linear course of the juvenile. It should be borne in mind at this point that the shape variance described by PC3 is only 3.5% (Table 3.8) of the shape variance of the whole sample, and as such this adult divergence from the juvenile scaling trajectory is subtle but nonetheless detectable. Based on the insignificant size sexual dimorphism present in the facial skeleton of P. paniscus

(Table 3.1), it follows that neither of the size correlated PCs identified (PCI and PC3) represent sexually dimorphic shape variation.

Although the shape of the facial skeleton as a whole is not sexually dimorphic, as determined in Analysis 3.2, the t-test result for the male and female adults is significant on PC4. This PC accounts for no more than 3.17% of the total shape variance in the sample, thus the significance of such a small degree of shape sexual dimorphism was likely overshadowed by the predominantly undimorphic shape variation in the rest of the adult facial skeleton. PC4 is uncorrelated with size amongst the juveniles and both male and female adults and as such the small degree of sexual dimorphism in P. paniscus is not the result of ontogenetic scaling. Given the lack of any significant difference in adult size between the sexes, it is not surprising that any sexual dimorphism in the facial skeleton of P. paniscus is not the product of ontogenetic scaling.

The shape changes associated with increasing size amongst the juveniles (as represented on PCI), are in a general sense as would be expected, a decrease in the orbital contribution to the facial skeleton relative to an increase in structures associated with mastication. There is a relative increase in palatal length and a relative decrease in palatal width, both clearly associated with the spatial requirements of the erupting dentition. The region of greatest relative increase in palatal length is that of the permanent molar dentition, posterior to the region of the deciduous molars and permanent premolars. The relative height and width of the facial skeleton also increase greatly with increasing centroid

size, thus increasing the overall relative size of the zygomatic and infraorbital region of the anterior face. The shape change associated with increasing size amongst the adults, which is represented on PC3 rather than an extension of the juvenile scaling trajectory is essentially a relative increase in the height of the subnasal region of the maxilla, with no noticeable changes to the proportions of the palate.

The shape differences associated with sexual dimorphism in the P.

paniscus face (represented on PC4) are twofold: a relative decrease in palatal width (the corollary of which is a further relative increase in the width of the zygomatic region) and a relative increase in the mesiodistal dimension of the canine alveolus. The implications of these sexually dimorphic shape differences being unrelated to size is discussed in Chapter 5.

In summary, the facial form in adult P. paniscus is the product of a shared male and female trajectory up until the eruption of the canines and third molars, at which point the adults of both sexes diverge from this trajectory. The size related shape changes are shared by both adult trajectories, but they differ in their non-allometric shape variation.

S.4.2.3 Pan troglodytes

The ontogenetic shape changes associated with increasing size amongst the juvenile P. troglodytes sample is represented by PC1 and PC3. These PCs have strong and moderate correlations, respectively, with centroid size, and as such the distribution of the juvenile sample across these two PCs approximates the scaling trajectory prior to adulthood. In addition, the male adults have a moderate correlation with size on PC1, the adult females however are uncorrelated. This lack of correlation amongst the adult females could be an artefact of the small size range they represent as a sample.

Based on t-test results (Table 3.13), PCs 3, 4 and 10 represent varying proportions of sexually dimorphic shape variation. Although PC3 is correlated with size amongst the juveniles, none of these PCs are

correlated with size amongst either the full (combined juvenile and adult) or adult samples, and as such there is no indication that any of the observed shape sexual dimorphism is the product of ontogenetic scaling of the juvenile scaling trajectory. This is in contrast to G. gorilla, where ontogenetic scaling of the juvenile PC1 scaling trajectory accounts for approximately 34% of the mean shape sexual dimorphism. From the strong size correlation on PC1 of the full sample and on viewing the graph of this PC against centroid size (Figure 3.15a), it appears that, even though there is no size correlation amongst the female adults, the adults loosely follow the juvenile scaling on this PC. It is conceivable that the lack of sexual dimorphism on this PC due to ontogenetic scaling is related to the proportionally smaller degree of size sexual dimorphism present in the facial skeleton of P. troglodytes. More specifically, as Table 3.1 demonstrates, the adult size dimorphism in P. troglodytes is approximately 25% of that observed in G. gorilla. Thus the sexually dimorphic shape variation amongst the P. troglodytes adults, represented by all three of these PCs is non-allometric.

The size related shape variations amongst the juveniles, as represented by PCI and PC3, are therefore summarised here. PCI generally refers to the a relative increase in regions associated with the masticatory apparatus and a relative decrease in the orbital contribution to the facial skeleton. There is an obvious relative increase in both the height of the face inferior to the orbit and the length of the premaxillary and maxillary alveolus. The relative increase in alveolar length contributes to a relative increase in the degree of lower facial prognathism. The infraorbital and zygomatic regions increase in relative width, which is accentuated by a relative decrease in palatal width. PC3 describes a concomitant change in the relative bony contributions of the infraorbital and zygomatic regions, such that there is a relative increase in the maxillary contribution with increasing size.

The sexually dimorphic shape variation amongst P. troglodytes

is represented over three PCs (PCs 3, 4 and 10) and combined below to give an overview of the major shape differences. The shape differences are presented in terms of the mean male shape relative to the mean female (the contributing PCs and their respective Figures are given in brackets):

• relative increase in mesiodistal dimension of canine alveolus (PC4 and PC10; Figure 3.18 and Figure 3.19, respectively)

• relative increase in maxillary contribution to infraorbital and zygomatic region (PC3 and PC4; Figure 3.17 and Figure 3.18, respectively)

• relative decrease in overall width of face (PC4; Figure 3.18) • relative decrease in palatal width (PC4; Figure 3.18)

• relative increase in the width of the zygomatic bone (PC10; Figure 3.19)

• relative increase in midfacial prognathism (PC10; Figure 3.19)

The implications of these sexually dimorphic shape differences being non-allometric is discussed in Chapter 5. To summarise, the facial form of P. troglodytes is the product of a shared male and female trajectory up until adulthood. After this point, the trajectories of the adults diverge such that sexual shape dimorphism is due to non-allometric shape variation.