Walter Leutenegger

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.

Correlates of sexual dimorphism in primates: Ecological and size variables

The effects of a series of ecological and size factors on the degree of sexual dimorphism in body weight and canine size were studied among subsets of 70 primate species. Variation in body-weight dimorphism can be almost entirely attributed to body weight (83% of variance R2of weight dimorphism). Much smaller amounts of the variation can be attributed to mating system (R2 =6.8%,polygynous species being more dimorphic than monogamous ones) and diet (R2 = 2.5%,frugivorous species being more dimorphic than folivorous ones). Habitat (arboreal vs. terrestrial) and activity rhythm (nocturnal vs. diurnal) have only an indirect effect on weight dimorphism. Variation in canine-size dimorphism can be explained in terms of canine size (R2 =49%),activity rhythm (R2 = 20%,diurnal species being more dimorphic than nocturnal ones), and mating system (R2 = 10%).Habitat and diet do not play a significant role in canine-size dimorphism. The unexpectedly high contribution of size to sexual dimorphism coupled with the observation of increased sexual dimorphism with increased size leads us to formulate a new selection model for the evolution of sexual dimorphism. We suggest that if there is selection for size increase, whatever its cause, directional selection in both males and females will lead to an increase in sexual dimorphism based on differences in genetic variance between the sexes. Sexual selection, resource division between the sexes, or lopsided reproductive selection need not play a role in such a model.

Maternal-Fetal Weight Relationships in Primates

Analysis of maternal-fetal weight relationships in primates based on the regression equation F (fetal weight) = b · M (maternal weight)a indicates that prosimian and anthropoid primates are separated by an allometric shift which can be interpreted as a result of differences in placental structures. The regular occurrence of multiple births in both prosimians and anthropoids is limited to small-sized species only, and is considered to be the result of selective pressure against oversized dimensions of the fetus at term to guarantee successful delivery. Twinning in callithricids is demonstrated as having developed secondarily as a byproduct of evolutionary miniaturization, rather than as a retention of a primitive mammalian feature.

Functional aspects of pelvic morphology in simian Primates

Abstract In functional analyses of the Primate pelvis only selective forces resulting from locomotory functions have traditionally been taken into account, therefore neglecting possible selective forces resulting from obstetrical functions. This paper analyzes the nature, degree, and action of the selective forces acting on the pelvis of simian Primates which result from maternal-fetal size relationships and their interaction with selective forces resulting from locomotory functions. Based on the analysis of these factors in living Primates, an estimate of their degree and interaction in Australopithecus africanus is given to further elucidate the problem of the evolution of hominid bipedalism.

Sexual Dimorphism in Primates

Secondary sexual differentiation in sexually reproducing organisms leads to more or less pronounced sexual dimorphism in an array of interrelated morphological, physiological, and behavioral features. Among primates morphological sexual differences occur in a wide range of characteristics, including (1) size (body weight and linear body dimensions such as trunk, head, and tail length), (2) dentition (such as canine size), (3) cranial features (such as prognathism, or sagittal and nuchal cresting), (4) locomotor apparatus (dimensions of axial and appendicular skeleton, muscular development), (5) internal organs (such as brain and heart size), (6) external features (such as pelage color and markings, shoulder capes and manes, permanent skin ridges, and coloration, particularly on the face), and (7) maturational, seasonal, or periodic morphological changes associated with reproductive cycles (Leutenegger, 1982a).

Encephalization and Obstetrics in Primates with Particular Reference to Human Evolution

The existence of differences in pelvic morphology between Plio-Pleistocene hominids and modern humans is well documented (for a recent review see McHenry and Temerin, 1979). Pelvic remodeling during the evolution of the genus Homo has elicited two different interpretations: (1) it has been viewed as a reflection of increased efficiency in bipedalism (McHenry, 1975a; Zihlman, 1978), and (2) it has been interpreted in terms of rapid encephalization that would have generated selection pressure to increase birth canal dimensions (Lovejoy et al., 1973; Lovejoy, 1974, 1975, 1978). When the second hypothesis is scrutinized, the fossil evidence unambiguously indicates that the increase in brain size during human evolution was extraordinary both in magnitude and in rate (Pilbeam and Gould, 1974; Passingham, 1975; Sacher, 1975). This picture of encephalization from H. habilis to H. erectus to H. sapiens is based on estimated cranial capacities of a static series of adults. Obstetrical constraints and thus selection pressures on the pelvis are not, however, dependent on adult cranial dimensions but on those of the fetal cranium at term relative to birth canal dimensions. Moreover, evidence of differences between primate species in the growth rate of the brain and/or in the length of development during ontogeny (Schultz, 1941, 1956; Sacher and Staffeldt, 1974; Passingham, 1975) suggests that encephalization of adults cannot be expected to be proportionate to that of the neonate or to that of individuals at any particular developmental stage.

Scaling of Sexual Dimorphism in Body Weight and Canine Size in Primates

Among primates sexual dimorphism in both body weight and canine size increases exponentially with increasing body size. This suggests that a full explanation of the variance in sexual dimorphism can only be attained if, in addition to sexual selection, parental investment, and various ecological factors, the influence of body size is taken into account. Positive allometry in both body weight dimorphism and canine size dimorphism is demonstrated to be associated with polygyny. In monogamous species the two dimorphisms not only remain constant throughout the size range but actually are minimal or lacking at any given body size.

Variability and sexual dimorphism in canine size of Australopithecus and extant hominoids

Abstract Among extant hominoids degrees of sexual dimorphism and combined-sex coefficients of variation of canine teeth dimensions are highly correlated. Based on this relationship and coefficients of variation of four species of the genus Australopithecus , we predict degrees of canine dimorphism for these extinct hominids. The estimates show that A. afarensis is as dimorphic as the pygmy chimpanzee, A. boisei slightly less dimorphic than the pygmy chimpanzee, A. robustus slightly more dimorphic than the lar gibbon, while A. africanus overiaps with the lar gibbon as well as a modern human sample. These estimates represent degrees of canine dimorphism substantially lower than results based upon prior sexing of individual specimens. The relationship between canine dimorphism and body weight dimorphism is also analyzed. All four species of Australopithecus are considerably less dimorphic in canine size for their body weight dimorphism than expected. This dissociation of canine size dimorphism and body weight dimorphism is shared with modern humans, and thus represents a unique hominid trait. We interpret the moderate to strong body weight dimorphism in australopithecines as the result of intra- and intersexual selection typical of a polygynous mating structure, while the rather mild canine dimorphism is interpreted in terms of the “developmental crowding” model for reduction in canine size.

Sexual dimorphism in the postcranial skeleton of New World primates

This study examines sexual dimorphism in 24 dimensions of the postcranial skeleton of four platyrrhine species: Callithrix jacchus, Saguinus nigricollis, Saimiri sciureus, and Cebus albifrons. The two callitrichid species show a relatively small amount of variation in the degree of sexual dimorphism among the different dimensions. Variation is considerably higher in the two cebid species as reflected by a mosaic pattern of sexual dimorphisms with males being significantly larger than females in some dimensions, and females significantly larger than males in others. In dimensions of the pectoral girdle and limb bones, males and females in each of the two cebid species are essentially scaled versions of each other, with males being peramorphic compared to females. This pattern is primarily the result of time hypermorphosis, i.e. an extension of the growth period in time in males. Rate hypermorphosis, i.e. an increase in the rate of growth in time in males, appears to play an additional role, however, in S. sciureus. By contrast, in dimensions of the true pelvis, sex differences in shape are dissociated from those in size. They are interpreted as the result of acceleration, i.e. increase in rate of shape change in females, as an adaptation to obstetrical functions. Interspecific analyses indicate positive allometry of mean degree of postcranial dimorphism with respect to body size. This coincides with previous findings by Leutenegger and Cheverud [1982, 1985] on the scaling of sexual dimorphism in body weight and canine size, and thus supports their model which posits selection on body size as the prime mover for the evolution of sexual dimorphism.

Neonatal brain size and neurocranial dimensions in Pliocene hominids: implications for obstetrics

Recent obstetrical analyses of the pelvis of A.L. 288-I have assumed equivalency in neonatal neurocranial dimensions of Australopithecus afarensis and Pan troglodytes, and argued for more or less severe cephalo-pelvic constraints. Here I reexamine the validity of the chimpanzee model. Based on a strong correlation between neonatal brain mass and adult female body mass in extant hominoids (r = 0·99) in conjunction with estimates of body mass for A.L. 288-I and STS 14, I estimate neonatal brain mass in A. afarensis and A. africanus to be substantially smaller than that of chimpanzees. Since neurocranial dimensions would have been correspondingly smaller, it is argued that (1) the chimpanzee model is no longer tenable, and that (2) they also would have been smaller than corresponding female pelvic dimensions, i.e. obstetrical constraints were absent in Pliocene hominids.