Intrasexual selection and phylogenetic constraints in the evolution of sexual canine dimorphism in strepsirhine primates

The goals of this study were to analyze the origin and function of sex differences in the size of canine teeth among Malagasy lemurs and other strepsirhine primates. These analyses allowed me to illuminate interactions between different mechanisms of sexual selection and to elucidate constraints on this sexually-selected trait. In contrast to central predictions of sexual selection theory, polygynous lemurs lack both sexual dimorphism in body size and male social dominance, but the degree of sexual dimorphism in the size of their canines is not known. A comparison of male and female canine size in 31 species of lemurs and lorises revealed significant male-biased canine dimorphism in only 6 of 13 polygynous lemur species. This result is in contrast to predictions of a hypothesis that would explain the lack of size dimorphism in lemurs as a result of high viability costs because canine teeth presumably have low maintenance costs and because they are used as weapons in male-male combat. Moreover, because females had significantly larger maxillary canines than males in only one lemur species, female dominance is not generally based on female physical superiority and selective forces favoring female dominance do not constrain sexual canine dimorphism in the sense of a pleiotropic effect. Contrary to predictions of sexual selection theory, species differences in canine dimorphism across strepsirhines were neither associated with differences in mating system, nor with the potential frequency of aggression. Variation in canine dimorphism was also unrelated to differences in body size, but there were significant differences among families, pointing to strong phylogenetic constraints. This study demonstrated that polygynous lemurs are at most subject to weak intrasexual selection on dental traits used in male combat and that traits thought to be under intense sexual selection are strongly influenced by phylogenetic factors.     

Numerical analysis of sexual dimorphism inSaguinus dentition

Among New World monkeys, more or less sexual dimorphism exists in the dentition, especially in the Cebidae. On the other hand, the Callitrichidae includingSaguinus are said to be characterized by a broad lack of sexual dimorphism with the exception of the reproductive organs. In the present article, sexual dimorphism in the dentition of someSaguinus species was reconfirmed using univariate and multivariate analytical methods. The results of the analysis were as follows: (1) there is no sexual dimorphism in the canine tooth size, except for the upper canine ofS. geoffroyi and lower canine ofS. mystax; (2) the overall tooth size difference between males and females is slight or none inS. geoffroyi, S. leucopus, andS. fuscicollis, relatively small inS. oedipus andS. mystax, and rather larger inS. midas; (3) an overall difference in shape factor between both sexes exists in all species ofSaguinus to a greater or lesser extent; (4) although only slight sexual dimorphism is recognized in the canine tooth itself, sexual dimorphism does exist in some adjacent teeth of the canine in a few species; and (5) there are some interspecific differences in the magnitude of the sexual dimorphism of theSaguinus dentition and these differences are more evident in species inhabiting the peripheral regions of the distribution areas of this genus. Taking all the evidence obtained into account, the sexual dimorphism in theSaguinus dentition must be re-investigated in comparison with other genera of the Callitrichidae.     

The ontogeny of cranial sexual dimorphism in two old world monkeys:Macaca fascicularis (Cercopithecinae) andNasalis larvatus (Colobinae)

Allometric and heterochronic approaches to sexual dimorphism have contributed much to our understanding of the evolutionary morphology of the primate skull and dentition. To date, however, extensive studies of sexual dimorphism have been carried out only on the great apes and a few cercopithecine monkeys. To fill this gap, representative dimensions of the skull were collected among ontogenetic series of two dimorphic Old World monkeys:Macaca fascicularis (Cercopithecinae) andNasalis larvatus (Colobinae). The ontogeny of cranial sexual dimorphism was evaluated with least-squares bivariate regression, analysis of covariance (ANCOVA), and analysis of variance (ANOVA). Results indicate that within each species the sexes typically exhibit nonsignificant differences in ANCOVAs of ontogenetic trajectories, except for bivariate comparisons with bicanine breadth. AmongMacaca fascicularis, ANOVAs between males and females of common dental ages show that adult, and frequently subadult, males are significantly larger than females, i.e., sexual dimorphism develops via time and rate hypermorphosis (males primarily grow for a longer time period as well as faster). AmongNasalis larvatus, however, comparisons between males and females of common dental ages indicate that only adult males are significantly larger than females, i.e., sexual dimorphism develops primarily via time hypermorphosis (males grow for a longer time period). Within both species, females appear to exhibit an early growth spurt at dental age 2; that is, many cranial measures for females tend to be larger than those for males. Measures of the circumorbital region (e.g., browridge height), body weight, and bicanine breadth exhibit typically the highest sexual dimorphism ratios. The fact that postcanine toothrow length and neurocranial volume (less so inNasalis) demonstrate very low dimorphism ratios generally supports assertions that postnatal systemic growth (and associated selective pressures thereon) exerts a greater influence on facial, but not neural, dental, or orbital, development (Cochard, 1985, 1987; Shea, 1985a,b, 1986; Shea and Gomez, 1988; Sheaet al., 1990). Additional consideration of ontogenetic differences between species generally supports previous functional interpretations of subfamilial differences in cranial form related to agonistic displays in cercopithecine monkeys (Ravosa, 1990).     

Metric variation and sexual dimorphism in the dentition of Ouranopithecus macedoniensis

The fossil sample attributed to the late Miocene hominoid taxon Ouranopithecus macedoniensis is characterized by a high degree of dental metric variation. As a result, some researchers support a multiple-species taxonomy for this sample. Other researchers do not think that the sample variation is too great to be accommodated within one species. This study examines variation and sexual dimorphism in mandibular canine and postcanine dental metrics of an Ouranopithecus sample. Bootstrapping (resampling with replacement) of extant hominoid dental metric data is performed to test the hypothesis that the coefficients of variation (CV) and the indices of sexual dimorphism (ISD) of the fossil sample are not significantly different from those of modern great apes. Variation and sexual dimorphism in Ouranopithecus M(1) dimensions were statistically different from those of all extant ape samples; however, most of the dental metrics of Ouranopithecus were neither more variable nor more sexually dimorphic than those of Gorilla and Pongo. Similarly high levels of mandibular molar variation are known to characterize other fossil hominoid species. The Ouranopithecus specimens are morphologically homogeneous and it is probable that all but one specimen included in this study are from a single population. It is unlikely that the sample includes specimens of two sympatric large-bodied hominoid species. For these reasons, a single-species hypothesis is not rejected for the Ouranopithecus macedoniensis material. Correlations between mandibular first molar tooth size dimorphism and body size dimorphism indicate that O. macedoniensis and other extinct hominoids were more sexually size dimorphic than any living great apes, which suggests that social behaviors and life history profiles of these species may have been different from those of living species.     

Canine reduction in the miocene hominoid Oreopithecus bambolii: behavioural and evolutionary implications

The degree of canine size sexual dimorphism and relative canine size, which have been related to levels of agonistic behaviour amongst living anthropoid primates, together with relative molar size, are evaluated in the fossil hominoid Oreopithecus bambolii from the Late Miocene of Italy. Although Oreopithecus displays a significant degree of canine height sexual dimorphism, using allometric techniques and body mass estimates for fossil species, it is shown that Oreopithecus males are microdont (smaller postcanine as well as canine teeth than expected) when compared to most living hominoids and its putative ancestor Dryopithecus. Canine reduction in Oreopithecus includes both crown height and, especially, basal area, and most closely resembles the condition found in the pygmy chimpanzee Pan paniscus. Interestingly, it had been previously proposed that Oreopithecus displays, like pygmy chimpanzees, a paedomorphic cranial morphology resulting in a reduction of facial prognathism, which could be related to microdontia in both taxa. Independent canine reduction in several anthropoid lineages (including hominids and P. paniscus) has been related to a relaxation of the selection pressure favouring canine use as a weapon. Although changes in socio-sexual behaviour, as documented in P. paniscus, cannot be currently discarded in Oreopithecus, canine reduction could be also alternatively (although not exclusively) interpreted as an aspect of generalized microdontia. The latter is best considered an adaptive readjustment required by the paedomorphic reduction of prognathism and the resulting lack of space to accommodate the adult dentition. This mechanism of canine reduction highlights the significance of developmental constraints in evolution and had not been previously suggested for any anthropoid primate.     

Intrasexual competition and canine dimorphism in anthropoid primates.

A number of factors, including sexual selection, body weight, body-weight dimorphism, predation, diet, and phylogenetic inertia have been proposed as influences on the evolution of canine dimorphism in anthropoid primates. Although these factors are not mutually exclusive, opinions vary as to which is the most important. The role of sexual selection has been questioned because mating system, which should reflect its strength, poorly predicts variation in canine dimorphism, particularly among polygynous species. Kay et al. (1988) demonstrate that a more refined estimate of intermale competition explains a large proportion of the variation in canine dimorphism in platyrrhine primates. We expand their analysis, developing a more generalized measure of intermale competition based on the frequency and intensity of male-male agonism. We examine the relative influences of predation (inferred by substrate use), female body weight, body-weight dimorphism, diet, and sexual selection on the evolution of anthropoid canine dimorphism. Intermale competition is very strongly associated with canine dimorphism. Predation also has a marked effect on canine dimorphism, in that savanna-dwelling species consistently show greater canine dimorphism than other species, all other factors being held equal. Body-weight dimorphism is also strongly associated with canine dimorphism, though apparently through a common selective basis, rather than through allometric effects. Body weight seems to play only a minor, indirect role in the evolution of canine dimorphism. Diet plays no role. Likewise, we find little evidence that phylogenetic inertia is a constraint on the evolution of canine dimorphism.     

Craniometric variation in a population of mantled howler monkeys (Alouatta palliata): evidence of size selection in females and growth in dentally mature males

A large body of work on monkey cranial metrics (involving conclusions about interspecific variation, sexual dimorphism, and ontogeny) depends on the assumptions that growth effectively ceases with dental maturity and that intraspecific variation is negligible. We test these assumptions by examining variation in 39 measurements of 166 dentally mature Alouatta palliata skulls from animals found dead on Barro Colorado Island (BCI), Panama. We also investigate whether this population is under size-based selection, since our found-dead sample reflects the natural mortality in this population. The sample was divided into three age stages by occlusal wear (A-C, least to most wear). Female stage A means are significantly smaller than female stage B means for three cranial measures. Female stage B means are significantly smaller than female stage C means for five cranial measures. Male stage A means are significantly smaller than male stage B means for 21 cranial measures. Multivariate analyses confirm this trend of expansion between adult age stages. The dental metric and suture closure data suggest that the cranial expansion in females is due to size-based selection, while the cranial expansion in males is due to significant growth after dental maturity. Sexual dimorphism ratios are highly variable across different samples of A. palliata, indicating that dimorphism varies between populations of this species. These results provide insight into the selective forces operating on the BCI howlers and challenge the validity of the many studies which pool subspecies and assume growth ceases with maturity.     

Sexual dimorphism in the canines and skulls of carnivores: effects of size, phylogency, and behavioural ecology

Sexual dimorphism in craniodental features is investigated in a sample of 45 carnivore species in relation to allometry, phylogeny, and behavioural ecology. Dimorphism is more pronounced in both upper and lower canine size and strength than in carnassial size, skull dimensions and biomechanical features, but all dimorphism indices covary. As with most morphological characters, differences in canine sexual dimorphism are significantly related to phylogeny, estimated from either taxonomic rankings or a limited matrix of molecular distances; in particular, mustelids, felids and procyonids are more dimorphic than other carnivore families. Thus, because of problems related to species dependence in comparative data, remaining analyses are based on phylogenetically transformed values using a spatial autoregressive method.
In contrast to other mammals, sexual dimorphism in carnivore canines is not correlated with differences in body weight, skull length or basicranial axis length. Nor is it correlated with categorical variables of activity pattern, habitat, or diet. In our Carnivore sample, canine dimorphism is related only to breeding system: uni-male, group-living (harem) species have significantly greater canine dimorphism than multi-male, multi-female groups and monogamous pair-bonding species. By contrast, dimorphism in carnassial size is related to dietary differences, specifically greater dimorphism in meat-eating species, and not breeding patterns. Dimorphism in estimates of jaw muscle size suggest functional demands from both diet and breeding type. It is concluded that, befitting patterns of heterodont dentition, sexual selection influences variation in canine dimorphism while feeding ecology is related to carnassial dimorphism.     

Multimale mating and absence of canine tooth dimorphism in woolly spider monkeys (Brachyteles arachnoides)

The relatively low degree of canine tooth dimorphism in Australopithecus afarensis has been used as “primary evidence” to support the concept of a mating system of monogamous pair-bonding and male provisioning. A recent field study of woolly spider monkeys shows that these large primates, which lack canine tooth (and body size) dimorphism, are characterized by apolygynous mating system. Male parental care of infants is absent in this species. These data support the view that a lack of canine tooth dimorphism in an anthropoid species does not necessarily imply either a monogamous, pair-bonded mating system or male parental care.     

Dental sexual dimorphism and morphology in african colobus monkeys as related to diet

We considered the relationship between dental sexual dimorphism and diet in 542 specimens of olive, red, and black-and-white colobus. Using univariate statistical techniques, we examined 41 measurements of the maxillary and mandibular dentitions. The results reveal two trends of dental sexual dimorphism in black-and-white colobus wherein (i) maleColobus guereza andC. angolensis are generally larger than females throughout the dentition and (ii)C. satanas and, to a lesser degree,C. polykomos exhibit reduced sexual differences in the canine base and females are slightly larger than males in noncanine dimensions. Females of the red colobus,Procolobus (Piliocolobus) badius, are slightly larger than males in most noncanine measurements but canine sex differences are more pronounced than those of black-and-white colobus.Procolobus (Procolobus) verus, the olive colobus, is characterized by some of the largest canine sex differences, yet the sexes do not differ much in noncanine mean values. When patterns of sexual dimorphism are considered in terms of specific ecology and behavior, it is possible to relate sex differences, in part, to known dietary differences. For example, overall dental morphology and the trend of sexual differences inC. satanas andC. polykomos seem to be associated with the consumption of a diet rich in seeds. The pattern of dental sexual dimorphism inC. badius may also be influenced by dietary factors in that their patrilineal social organization could restrict female access to certain foods thereby affecting rates of attrition and creating selection pressure for larger teeth. Relatively less is known of the ecology and social organization ofP. verus but their dental sexual dimorphism is possibly less related to dietary factors than is the case for red or black-and-white colobus.     

Sexual dimorphism in Laccopithecus robustus, a late miocene hominoid from China

Laccopithecus robustus is a siamang-sized fossil ape from the Miocene site of Lufeng, China. The species is known from a partial cranium, numerous mandibles, and scores of isolated teeth. This species shows striking dental similarities to Pliopithecus from the Miocene of Europe and a number of cranial similarities to extant gibbons. Laccopithecus differs from extant gibbons and resembles other fossil and extant apes in showing marked sexual dimorphism in the size and shape of the canines and anterior lower premolars. Evidence for sexual differences in either the size or shape of other teeth is less clear. There is some evidence for a sexual size dimorphism based on the variability of molar teeth.     

Relationship of sexual dimorphism in canine size and body size to social,behavioral, and ecological correlates in anthropoid primates

Among anthropoid primates there are interspecific differences in the degree of sexual dimorphism in both body size and canine size. Within the suborder body size dimorphism and canine size dimorphism are positively correlated,r=0.76. This correlation suggests that the two dimorphisms are equally developed in some species, while in other species there is a differential degree of sexual dimorphism. An analysis of these results and their relation to social organization and other ecological variables reveals: (1) the degree of canine size dimorphism is closely related to the amount of male intrasexual selection in a given mating system; and (2) the degree of body size dimorphism is also related to male intrasexual selection, but may be modified (either enhanced or diminished) by selection pressure from factors such as habitat, diet, foraging behavior, antipredator behavior, locomotory behavior, and female preference.     

Ontogenetic bases of canine dimorphism in anthropoid primates

This study tests hypotheses regarding the ontogeny of canine tooth size dimorphism in five anthropoid primate species (Saguinus fuscicollis, Macaca mulatta, Cercocebus atys, Papio hamadryas, and Mandrillus sphinx). Canine measurements and chronological age data are analyzed to determine if bimaturism, a sex difference in the age at which eruption ceases, accounts for canine tooth sexual dimorphism. Canine height measurements are evaluated through a variety of regression techniques. Results show a lack of sexual dimorphism in Saguinus. While size dimorphism is absent in the deciduous teeth of all species analyzed, the adult teeth in cercopithecines become increasingly dimorphic through ontogeny. Female adult tooth eruption regularly precedes male tooth eruption, and regression-based eruption trajectories for both sexes intersect at about the age at which the female tooth reaches adult size. Males erupt the tooth later and more rapidly than females. Males also reach a larger adult size than females by erupting the tooth for much longer periods of time. Bimaturism is primary in the production of dimorphism, but rates of eruption show modest variation. These results point to the scheduling of sexual selection through intermale competition as a primary factor determining male eruption timing, rates of eruption, and adult size. Life history factors may play a role in determining the relations between the scheduling of intrasexual competition and canine eruption. Female contributions to sexual dimorphism are apparent in these species, suggesting that similar levels of dimorphism can be attained through diverse ontogenetic pathways.     

Sexual selection and canine dimorphism in New World monkeys

Social and ecological factors are important in shaping sexual dimorphism in Anthropoidea, but there is also a tendency for body-size dimorphism and canine dimorphism to increase with increased body size (Rensch's rule) (Rensch: Evolution Above the Species Level. London: Methuen, 1959.) Most ecologist interpret Rensch's rule to be a consequence of social and ecological selective factors that covary with body size, but recent claims have been advanced that dimorphism is principally a consequence of selection for increased body size alone. Here we assess the effects of body size, body-size dimorphism, and social structure on canine dimorphism among platyrrhine monkeys. Platyrrhine species examined are classified into four behavioral groups reflecting the intensity of intermale competition for access to females or to limiting resources. As canine dimorphism increases, so does the level of intermale competition. Those species with monogamous and polyandrous social structures have the lowest canine dimorphism, while those with dominance rank hierarchies of males have the most canine dimorphism. Species with fission-fusion social structures and transitory intermale breeding-season competition fall between these extremes. Among platyrrhines there is a significant positive correlation between body size and canine dimorphism However, within levels of competition, no significant correlation was found between the two. Also, with increased body size, body-size dimorphism tends to increase, and this correlation holds in some cases within competition levels. In an analysis of covariance, once the level of intermale competition is controlled for, neither molar size nor molar-size dimorphism accounts for a significant part of the variance in canine dimorphism. A similar analysis using body weight as a measure of size and dimorphism yields a less clear-cut picture: body weight contributes significantly to the model when the effects of the other factors are controlled. Finally, in a model using head and body length as a measure of size and dimorphism, all factors and the interactions between them are significant. We conclude that intermale competition among platyrrhine species is the most important factor explaining variations in canine dimorphism. The significant effects of size and size dimorphism in some models may be evidence that natural (as opposed to sexual) selection also plays a role in the evolution of increased canine dimorphism.     

The differential expression of dental sexual dimorphism in subspecies ofColobus guereza

Data on dental sex differences in seven of the eight currently recognized subspecies of Colobus guerezareveals a range of expression of sexual dimorphism. Males of most subspecies are larger than females throughout the dentition and this is especially pronounced for the canines and P ₃ For C. g. guerezaand C. g. gallarum,however, sex differences in the canines and P ₃ are less pronounced and females are often slightly larger than males in noncanine dental measurements. C. guerezasspp. occupying comparable habitats express similarities in the degree of maxillary canine dimorphism. In addition, for those subspecies distributed above the equator, there is also a cline of decreasing maxillary canine dimorphism from west to east in a progressively more northern direction. This cline corresponds to the occupation of increasingly more arid habitats, and reduced dimorphism is the result of larger maxillary canine size in females. We propose that this pattern of sexual dimorphism is related to differences in the relative intensity of predation pressure, guereza social organization, and energetic considerations. That the mandibular canine does not exhibit a similar trend of sexual dimorphism suggests that larger maxillary canines in females may function as weapons.     

Phylogenetic analyses of dimorphism in primates: evidence for stronger selection on canine size than on body size

Phylogenetic comparative methods were used to analyze the consequences of sexual selection on canine size and canine size dimorphism in primates. Our analyses of previously published body mass and canine size data revealed that the degree of sexual selection is correlated with canine size dimorphism, as well as with canine size in both sexes, in haplorhine but not in strepsirrhine primates. Consistent with these results, male and female canine size was found to be highly correlated in all primates. Since canine dimorphism and canine size in both sexes in haplorhines were found to be not only related to mating system but also to body size and body size dimorphism (characters which are also subject to or the result of sexual selection), it was not apparent whether the degree of canine dimorphism is the result of sexual selection on canine size itself, or whether canine dimorphism is instead a consequence of selection on body size, or vice versa. To distinguish among these possibilities, we conducted matched-pairs analyses on canine size after correcting for the effects of body size. These tests revealed significant effects of sexual selection on relative canine size, indicating that canine size is more important in haplorhine male-male competition than body size. Further analyses showed, however, that it was not possible to detect any evolutionary lag between canine size and body size, or between canine size dimorphism and body size dimorphism. Additional support for the notion of special selection on canine size consisted of allometric relationships in haplorhines between canine size and canine size dimorphism in males, as well as between canine size dimorphism and body size dimorphism. In conclusion, these analyses revealed that the effects of sexual selection on canine size are stronger than those on body size, perhaps indicating that canines are more important than body size in haplorhine male-male competition.     

An odontometric assessment of variability inAustralopithecus afarensis

Odontometric data are utilized to investigate both the extent of variation in the Pliocene hominid remains from Hadar and Laetoli and whether this variation is best explained as resulting from sexual dimorphism or from the presence of more than one species in the sample. Coefficients of variation for the Hadar/Laetoli dental elements are compared with those from other established Plio-Pleistocene hominid taxa and extant pongids. Results indicate that while CVs for the central cheek teeth (M1/1 and M2/2) tend to be rather high, the variability does not consistently exceed ranges of variability for extant anthropoids and other primate species. Thus odontometric data do not disprove the null hypothesis that the Hadar/Laetoli sample can be accommodated within a single species. Therefore, although the Hadar/Laetoli sample tends to exhibit less canine variability than is found among sexually dimorphic apes, odontometric variation in this sample is more likely due to sexual dimorphism than the presence of multiple taxa in the sample.     

Sexual dimorphism and dental variability in platyrrhine primates

Leutenegger and Cheverud (1982, 1985) propose a hypothesis to explain why larger primates are more sexually dimorphic in body weight and canine size. Their hypothesis states that any factor selecting for an evolutionary increase in body size will produce an increase in sexual dimorphism in any character if either heritability or phenotypic variability is greater in males than in females for that character. They cite no evidence for heritability but give some data to suggest that males are, in fact, more variable than females. We test the latter proposition more fully using measurements on the dentitions of platyrrhine primates. Male and female phenotypic variances are not significantly different in most cases. Cases of greater male phenotypic variance are not limited to sexually dimorphic species. We conclude that the hypothesis of Leutenegger and Cheverud does not explain the observed patterns of dental sexual dimorphism, at least in platyrrhines.     

Sexual dimorphism and interspecific cranial form in two capuchin species: Cebus albifrons and C. apella

Ontogenetic patterns of sexual dimorphism and cranial form in two capuchin monkeys, Cebus albifrons and C. apella, are investigated by means of univariate, bivariate, and multivariate statistics. The analyses are based on 23 linear variables. Univariate analyses indicate that similar ontogenetic patterns of cranial sexual dimorphism are present; however, interspecific differences exist in timing. Ontogenetic scaling is present in both species' crania; however, it is more prevalent in C. albifrons. Several departures are present in cranial regions associated with orbital shape, the dental arcade, and the muscles of mastication. The latter two indicate that sexual differences in diet and/or foraging strategies may exist. Sexual selection is suggested as being the primary selective regime underlying the observed patterns of cranial sexual dimorphism in each species. Interspecific comparisons confirm that C. apella possesses a more dimorphic cranium than C. albifrons and that sexual dimorphism in C. apella begins earlier in development. Although interspecific ontogenetic scaling is present in some cranial variables, C. apella is not just a scaled-up version of C. albifrons. These sympatric congeners seem to be differentiated by variables related to the orbital region and the masticatory apparatus, as indicated by both departures from ontogenetic scaling and results of the discriminant function analysis. Ecological selection, rather than varying degrees of sexual selection, is likely to be responsible for this finding given that C. apella is known to consume hard-object foods. This is consistent with the predicted outcome of the competitive exclusion principle. Am J Phys Anthropol 104:487–511, 1997. © 1997 Wiley-Liss, Inc.     

Sexual dimorphism of Dall's porpoise and harbor porpoise skulls

To evaluate and quantify sexual dimorphism of skull shape and assess the ontogenetic background for differences, samples of 134 harbor porpoise (Phocoena phocoena) and 85 Dall's porpoise (Phocoenoides dalli) were compared in terms of cranial shape and shape ontogeny using three-dimensional geometric morphometrics. After correction for allometry, no sexual differences were detected in harbor porpoise, while Dall's porpoise showed statistically significant sexual dimorphism of skull shape. Since no sex-specific differences were detected in the directionalities of the ontogenetic vectors, we cannot reject that the dimorphism is innate. Based on the different mating systems of the two species and the lack of sexual dimorphism in the harbor porpoise, the dimorphism in Dall's porpoise is most likely a result of sexual selection in relation physical competition for mates given that male skulls provide room for larger neck muscles with a more favorable lever arm.