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J M Plavcan

Publications and source records attributed to J M Plavcan.

14 recordsLinked to original sources
PubMed2001

Sexual dimorphism in primate evolution.

Sexual dimorphism is a pervasive phenomenon among anthropoid primates. Comparative analyses over the past 30 years have greatly expanded our understanding of both variation in the expression of dimorphism among primates, and the underlying causes of sexual dimorphism. Dimorphism in body mass and canine tooth size is familiar, as is pelage and "sex skin" dimorphism. More recent analyses are documenting subtle differences in the pattern of skeletal dimorphism among primates. Comparative analyses have corroborated the sexual selection hypotheses, and have provided a more detailed understanding of the relationship between sexual selection, natural selection, and mating systems in primates. A clearer picture is emerging of the relative contribution of various selective and nonselective mechanisms in the evolution and expression of dimorphism. Most importantly, recent studies have shown that dimorphism is the product of changes in both male and female traits. Developmental studies demonstrate the variety of ontogenetic pathways that can lead to dimorphism, and provide additional insight into the selective mechanisms that influence dimorphism throughout the lifetime of an animal. Evidence from the fossil record suggests that dimorphism probably evolved in parallel twice, and the dimorphism in some extinct hominoids probably exceeded that of any living primate. Our advances in understanding the behavioral/ecological correlates of dimorphism in living primates have not improved our ability to reconstruct social systems in extinct species on the basis of dimorphism alone, beyond the inference of polygyny or intense male-male competition. However, our understanding of the behavioral/ecological correlates of growth and development, and of the expression of dimorphism as a function of separate changes in male and female traits, offers great potential for inferring evolutionary changes in behavior over time.

Animals↗
PubMed2000

Inferring social behavior from sexual dimorphism in the fossil record.

Sexual dimorphism is commonly used as evidence of the behavior of extinct species. Even so, few analyses scrutinize whether extant comparative data support inferences of mating systems or behavior in extinct species. This analysis evaluates the relations between measures of dimorphism and several estimates of mating system and intrasexual competition. Dimorphism alone provides poor resolution for reconstructing behavior. Many behavioral inferences based on perceived dimorphism are not supported by extant comparative data. This reflects the large standard errors of relations between dimorphism estimates and behavioral classifications. Used with caution, dimorphism can provide a hint of the behavior of extinct species in some cases. However, in many cases inferred dimorphism allows little more than an inference of polygyny, without any indication of specific types of mating systems.

Animals↗
PubMed1999

Canine sexual dimorphism in Egyptian Eocene anthropoid primates: Catopithecus and Proteopithecus.

Two very small late Eocene anthropoid primates, Catopithecus browni and Proteopithecus sylviae, from Fayum, Egypt show evidence of substantial sexual dimorphism in canine teeth. The degree of dimorphism suggests that these early anthropoids lived in social groups with a polygynous mating system and intense male-male competition. Catopithecus and Proteopithecus are smaller in estimated body size than any living primates showing canine dimorphism. The origin of canine dimorphism and polygyny in anthropoids was not associated with the evolution of large body size.

Animals↗
PubMed1998

Correlated response, competition, and female canine size in primates.

Recently, comparative analyses of female canine tooth size in primates have yielded two hypotheses to explain interspecific variation in female relative canine size. Greenfield ([1992] Int. J. Primatol. 13:631-657; [1992] Yrbk. Phys. Anthropol. 35:153-184; [1996] J. Hum. Evol. 31:1-19) suggested that covariation in male and female canine size across species indicates that female canine size reflects correlated response (in which the expression of a trait in one sex causes the expression of the same trait in the other sex). Plavcan et al. ([1995] J. Hum. Evol. 28:245-276) noted that female canine size in primates is associated with variation in categorical estimates of the intensity of female-female agonistic competition, suggesting that selection favors large female canine size in many species. While it may seem that the two models are in conflict, they are not. To simultaneously evaluate these two models, this analysis examines the joint relations between male canine size, female canine size, and estimates of female-female competition in a sample of 108 primate species. Overall, female canine size is correlated with variation in male canine size. Controlling for variation in male canine size, female canine size is also correlated with estimates of the intensity of female-female agonistic competition. The relation between these variables differs strongly between anthropoid and strepsirhine primates. In anthropoids, the data suggest that selection for the development of large canines in females is not constrained by any affect of correlated response. In strepsirhines, the evidence suggests that sexual selection may affect male canine size but that correlated response affects female canine size, resulting in monomorphism for most species. These observations help reconcile the observations of Greenfield ([1992] Int. J. Primatol. 13:631-657; [1996] J. Hum. Evol. 31:1-19) and Plavcan et al. ([1995] J. Hum. Evol. 28:245-276) and provide a more precise model for understanding interspecific variation in female canine size and hence canine dimorphism.

Animals↗
PubMed1998

A simulation test of hominoid species number at Lufeng, China: implications for the use of the coefficient of variation in paleotaxonomy.

High dental metric variation in the large hominoid sample from the late Miocene site of Lufeng, China has been interpreted in two ways: (1) there are two morphologically similar species that broadly overlap in size, and (2) there is one species that is more highly sexually dimorphic in dental size, and perhaps in body size, than any extant primate. It has been claimed that the high levels of dental metric variation falsify the single-species hypothesis, which has been viewed implicitly as corroboration of the two-species hypothesis. However, the two-species hypothesis has not been subjected to testing. Here we test the two-species hypothesis using computer simulations to attempt to reproduce the unusual pattern of intrasexual and intersexual dental metric variation observed in the Lufeng postcanine dentition. Conditions of the simulation experiments were optimized to favor the two-species hypothesis. It was found that, although the Lufeng pattern of metric variation could be reproduced by sampling two species, the likelihood of this occurrence was very low even when the conditions were optimized to the point of improbability. We conclude that the likelihood is very high that the Lufeng sample is composed of one species that is more highly sexually dimorphic in the postcanine dentition than any extent primate species. If so, then the high levels of sexual dimorphism and intraspecific dental metric variation in this species violate the central assumption of methods that employ the coefficient of variation (CV) for paleotaxonomy, namely, that neither can lie outside the ranges observed among extant species. Thus, we further conclude that the CV must be used with caution when evaluating the taxonomic composition of fossil samples and, further, that it cannot be used to falsify a single-species hypothesis in any meaningful way. Other fossil hominoid samples with high measures of dental variation may indicate that considerable sexual size dimorphism typified many Eurasian middle-late Miocene hominoids.

Animals↗
PubMed1997

Intrasexual competition and body weight dimorphism in anthropoid primates.

Body weight dimorphism in anthropoid primates has been thought to be a consequence of sexual selection resulting from male-male competition for access to mates. However, while monogamous anthropoids show low degrees of weight dimorphism, as predicted by the sexual selection hypothesis, polygynous anthropoids show high variation in weight dimorphism that is not associated with measures of mating system or sex ratio. This observation has led many to debate the role of other factors such as dietary constraints, predation pressure, substrate constraints, allometric effects, and phylogeny in the evolution of anthropoid weight dimorphism. Here, we re-evaluate variation in adult body weight dimorphism in anthropoids, testing the sexual selection hypothesis using categorical estimates of the degree of male-male intrasexual competition ("competition levels"). We also test the hypotheses that interspecific variation in body weight dimorphism is associated with female body weight and categorical estimates of diet, substrate use, and phylogeny. Weight dimorphism is strongly associated with competition levels, corroborating the sexual selection hypothesis. Weight dimorphism is positively correlated with increasing female body weight, but evidence suggests that the correlation reflects an interaction between overall size and behavior. Arboreal species are, on average, less dimorphic than terrestrial species, while more frugivorous species tend to be more dimorphic than folivorous or insectivorous species. Several alternative hypotheses can explain these latter results. Weight dimorphism is correlated with taxonomy, but so too are competition levels. We suggest that most taxonomic correlations of weight dimorphism represent "phylogenetic niche conservatism"; however, colobines show consistently low degrees of weight dimorphism for reasons that are not clear.

Animals↗
PubMed1997

Interpreting hominid behavior on the basis of sexual dimorphism.

Numerous studies use estimates of sexual dimorphism in canine tooth size and body weight to support speculation about the behavior of australopithecines. However, the range of mating systems inferred for australopithecines encompasses virtually the entire spectrum of mating systems seen among extant anthropoid primates, from monogamy to polygyny characterized by intense male male competition. This variety of opinion can be attributed partly to the unusual combination of high body size dimorphism and reduced canine dimorphism in australopithecines. Here we provide a joint comparison of recent models for the behavioral correlates of both canine dimorphism and body size dimorphism, and apply this to published estimates of dimorphism in body size and canine tooth size in hominids. Among extant species, body weight dimorphism and canine dimorphism are strongly correlated with estimates of intrasexual competition. Canine crown height dimorphism provides the best discrimination between taxa that show high degrees of male-male competition, and those that do not. Relative male maxillary canine tooth size offers additional evidence about male-male competition. On the other hand, canine occlusal dimorphism offers little discrimination among species of different male-male competition levels. Estimates of canine dimorphism, relative canine size, and body weight dimorphism in australopithecines provide little definitive information about male-male competition or mating systems. Dimorphism of Australopithecus africanus and Australopithecus robustus can be reconciled with a mating system characterized by low-intensity male-male competition. The pattern of dimorphism and relative canine size in Australopithecus afarensis and A. robustus provides contradictory evidence about mating systems and male-male competition. We review a number of hypotheses that may explain the unusual pattern of dimorphism of A. afarensis and Australopithecus boisei, but non-satisfactorily resolves the problem given current data.

Animals↗
PubMed1996

Evaluating the "dual selection" hypothesis of canine reduction.

A recently proposed model for canine reduction in hominid evolution (the "dual selection" model) suggests that canine reduction occurs as a result of selection for incorporation of the canines into a functional incisal field. Among the evidence used to support this model are patterns of wear and occlusion of the canine teeth, particularly in female anthropoid primates. We examined wear and occlusal patterns of the canine teeth of 311 male and female anthropoid primates. We find no evidence that the canines are typically occluded tip-to-tip, or that they show wear patterns indicating a "gripping and pulling" function during food ingestion and processing. Furthermore, we do not find compelling evidence that the development of the mesial cristid is associated with canine reduction. While we agree that the mechanisms of selective pressures underlying canine reduction need to be investigated, the "dual selection" hypothesis is unsupported by comparative data.

Animals↗
PubMed1995

Quantitative evaluation of craniofacial growth in the third trimester human.

A detailed understanding of fetal craniofacial growth is necessary for understanding the mechanisms underlying the growth process and how altered growth patterns may give rise to specific craniofacial anomalies. Although descriptive studies of human fetal craniofacial growth are abundant, quantitative studies are less common; most employ only lateral cephalograms or tracings of sectioned specimens. In this study, we compared growth rates in the individual bones of the facial and basicranial regions in a large sample of spontaneously aborted, third trimester fetuses. Growth in the individual dimensions of most bones is not significantly different from isometry when compared with body weight, but comparisons of dimensions within bones show clear changes in shape. In particular, the mandible is characterized by a deepening of the corpus, probably in association with development of the teeth. The maxilla also shows a relatively rapid height increase associated with dental development. The temporal bone grows faster in superoinferior height as opposed to anteroposterior width. The lateral and inferior margins of the zygomatic bone grow faster than the orbital margin. The basioccipital bone, cited in radiographic studies as growing substantially slower than the facial region, actually changes in shape more rapidly than any other skeletal component examined in this study, with widths growing much faster than lengths. Our findings demonstrate that craniofacial development in the third trimester is characterized by localized variation in the rate of growth within and between individual skeletal components, and that gross characterizations of regional rates of growth inadequately characterize craniofacial development.

Body Weight↗
PubMed1994

Comparison of four simple methods for estimating sexual dimorphism in fossils.

Estimating sexual dimorphism in skeletal and dental features of fossil species is difficult when the sex of individuals cannot be reliably determined. Several different methods of estimating dimorphism in this situation have been suggested: extrapolation from coefficients of variation, division of a sample about the mean or median into two subsamples which are then treated as males and females, and finite mixture analysis (specifically for estimating the maximum dimorphism that could be present in a unimodal distribution). The accuracy of none of these methods has been thoroughly investigated and compared in a controlled manner. Such analysis is necessary because the accuracy of all methods is potentially affected by fluctuations in either sample size, sex ratio, or the magnitude of intrasexual variability. Computer modeling experiments show that the mean method is the least sensitive to fluctuations in these parameters and generally provides the best estimates of dimorphism. However, no method can accurately estimate low to moderate levels of dimorphism, particularly if intrasexual variability is high and sex ratios are skewed.

Animals↗
PubMed1993

Canine size and shape in male anthropoid primates.

While studies of canine dimorphism in primates are common, only a few have examined canine tooth size independently within each sex. Recently, Greenfield and Washburn (Am. J. Phys. Anthropol. 84:17-34, 1991) proposed that there are two types of male canines which reflect different allometric scaling patterns of canine crown height against canine occlusal dimensions. They also suggest that proportional canine shape, measured as canine crown height (or projection) relative to the occlusal dimensions, provides an estimate of the canine's function as a weapon, though they provide no test of this hypothesis. This analysis critically examines the claim that there are two types of male canines among anthropoids. It then tests the hypothesis that relative male canine size (measured against body weight) and proportional canine shape are related to estimates of intermale competition, diet, and substrate (used as a surrogate measure of predation pressure). While there is strong taxonomic variation in canine size and shape among male anthropoids, no evidence is found for two discrete canine types. Rather, within families and subfamilies, canine dimensions scale isometrically against body weight and against each other, with a continuum of canine shapes among different taxa. While variation in male canine size is associated with intermale competition and substrate, even when taxonomic variation is controlled, variation in proportional canine shape is not. Neither canine size nor shape are generally associated with variation in diet.

Analysis of Variance↗
PubMed1992

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.

Analysis of Variance↗
PubMed1988

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.

Animals↗