Magnitude Of Sexual Dimorphism Research Articles

Obstetrical and Ecogeographical Impacts on Sexual Dimorphism and Intraspecific Variation in the Modern Human Thorax

The growing fetus compresses maternal abdominal organs against the diaphragm, constraining diaphragmatic respiration and increasing dependence on intercostal muscles during pregnancy. In conjunction with females’ smaller thoracic cavities, this fetus‐induced abdominal displacement and its theorized impact on respiration has been cited as the explanation behind females’ relatively longer and more angled ribs and transverse thoracic processes. Arguably, this morphology should enable females’ intercostal muscles to produce greater ribcage elevation and a more powerful inhale than the more horizontal intercostal muscles and ribs of males. This “abdominal displacement” hypothesis is often used to explain human thorax dimorphism. However, intraspecific variation in ribcage dimorphism due to ecogeographic rules (e.g., Allen’s rule) demonstrates that human populations adapted to low latitudes have narrower trunks, which in turn may display different patterns or degrees of dimorphism than populations adapted to temperate climates with wider trunks. This study’s objective is to explore how the human thorax has responded to obstetric and thermoregulatory demands to assess the underlying premise of the “abdominal displacement” hypothesis that ribcage dimorphism is uniform across populations. Focusing on thorax volume and rib morphology, the following predictions were tested: 1) the pattern of sexual dimorphism (i.e., female‐biased) remains the same across human populations and conforms with that of the “abdominal displacement” hypothesis, yet 2) the degree of sexual dimorphism (i.e., the extent of differences between the sexes) varies with geographic ancestry due to ecogeographic rules. Using 3D scans of articulated torsos and isolated ribs, size‐adjusted thorax volume, size‐adjusted rib arc and chord lengths, and rib angles were calculated for low‐latitude adapted (represented here by 23 African American individuals) and moderate climate adapted (represented by 22 European American individuals) adults. Factors were compared using ANOVA (rib angle) and ANCOVA (all other variables) using height as the covariate. Despite admixture, African Americans have 11% smaller size‐adjusted total thoracic volumes than European Americans, as expected given their descent from low‐latitude adapted individuals. Given females’ 12% smaller size‐adjusted thoracic volumes, the “abdominal displacement” hypothesis would predict female‐biased dimorphism in rib morphology to ameliorate shifting organs into this smaller space during pregnancy. This prediction is not met as the size‐adjusted arc and chord lengths show 4 to 10% significant male‐biased dimorphism across ribs. Dimorphism in rib angles is variably female‐ or male‐ biased. The magnitude of sexual dimorphism in these features does vary among the populations, following predictions. Cumulatively, these results suggest that the angles and lengths of isolated ribs do not follow expectations of the “abdominal displacement” hypothesis. It is possible that the transverse processes of thoracic vertebrae, and not the ribs, may be the primary driver of the observed thoracic dimorphism reported by this hypothesis. Future work will explore sexual dimorphism of the isolated vertebrae in conjunction with the ribs while considering how ecogeographic forces may alter the degree of sexual dimorphism across populations.

Temporal dynamics of sexual dimorphism in a dioecious species.

Sexual dimorphism for floral traits is common in dioecious plant species. Beyond its significance for understanding how selection acts on plant traits through male vs. female reproductive function, sexual dimorphism has also been proposed as a possible risky characteristic for insect-pollinated plants, as it could drive pollinators to forage mostly on male plants. However, even though most flowering plant species spread their flowering across several weeks or months, the temporal variation of floral phenotypes and sexual dimorphism have rarely been investigated. We performed a survey of male and female plants from the dioecious generalist-pollinated Silene dioica (Caryophyllaceae) in a common garden experiment, over two consecutive flowering seasons. Flower number and floral size were measured each week, as well as pollen quantity and viability in male plants. Sexual dimorphism was found for all investigated floral traits, with males showing an overall higher investment in flower production and flower size. Males and females showed a similar temporal decline in flower size. The temporal dynamics of daily flower number differed between sexes, with males showing a peak in the middle of their flowering season, whereas flower production by females was quite stable over time. At the scale of the experimental population, both individual and floral sex ratios appeared to vary across the flowering season. Moreover, because the onset of flowering varied among plants, the magnitude of sexual dimorphism in floral size also fluctuated strongly through time. Capturing male/female differences with only one temporal measurement per population may not be informative. This opens stimulating questions about how pollinator behaviour and resulting pollination efficiency may vary across the flowering season.

Sex differences in the patterning of age-related bone loss in the human hallucal metatarsal in rural and urban populations.

Age-degenerative features of the metatarsals are poorly known despite the importance of metatarsal bone properties for investigating mobility patterns. We assessed the role of habitual activity in shaping the patterning and magnitude of sexual dimorphism in age-related bone loss in the hallucal metatarsal. Cross-sections were extracted at midshaft from micro-computed tomography scan models of individuals from medieval rural (Abingdon Vineyard) and early industrial urban (Spitalfields) settings (n = 71). A suite of cross-sectional geometry dimensions and biomechanical properties were compared between populations. The rural group display generally stronger and larger metatarsals that show a greater capacity to resist torsion and that have comparatively greater bending strength along the medio-lateral plane. Men in both groups show greater values of cortical area than women, but only in the urban group do men show lower magnitudes of age-related decline compared to females. Women in rural and urban populations show different patterns of age-related decline in bone mass, particularly old women in the urban group show a marked decline in cortical area that is absent for women in the rural group. Lifetime exposure to hard, physical activity in an agricultural setting has contributed to the attainment of greater bone mass and stronger bones in young adults. Furthermore, over the life-course, less of this greater amount of bone is lost, such that sustained activity levels may have acted to buffer against age-related decline, and this is most pronounced for women, who are expected to experience greater bone loss later in life than men.

The Allometry of Sexual Dimorphism in Sceloporus formosus (Squamata: Phrynosomatidae)

In many animal species, adult females and males often differ in characteristics other than primary sexual traits. Thus, body size tends to be related to reproductive success through different pathways in females and males, sex-specific foraging or dispersal strategies, and/or adaptations to reduce intersexual trophic competition. As result of these differences, the body size that conveys maximal fitness often differs between the sexes, favoring the evolution of sexual size dimorphism. In this study, we use an allometric approach to evaluate the changes in magnitude of sexual dimorphism in a population of Sceloporus formosus. Allometry refers to the relationships observed between body size and other organismal traits. Natural and sexual selection on morphological traits may modify allometric relationships. Traits under selection can show positive allometry, with slopes steeper than 1 (hyperallometry). We performed allometric major axis regressions of seven morphological traits on the snout–vent length, a standard measure used as a proxy for lizard size. The major axis regressions of female trunk length and width, and male trunk length showed hyperallometric slopes, suggesting that in S. formosus these traits could be under natural selection.

Population specificity in the estimation of skeletal age and sex: case studies using a Western Australian population

ABSTRACTThe expertise of the forensic anthropologist is grounded in the study of skeletal morphology relative to normal state; the latter is variously defined according to the complex interrelationship between biological function, genetics and intra-specific variation (amongst others). In relation to sex estimation, there are multiple dimorphic attributes in the adult skeleton that facilitate accurate sex estimations. It is, however, well established that there are population specific influences in not only skeletal growth, but also the expression and magnitude of sexual dimorphism (notwithstanding relative consistency in ‘universally dimorphic traits’ that relate to biological functioning – e.g., pelvic shape in relation to female functional requirements for childbirth).The recognition of the need for statistical quantification of forensic assessment is mandated by judicial requirement. Application of reference standards ‘foreign’ to that of the local jurisdiction, however, introduces an unknown variation and by association possible misclassifications. The present paper aims to empirically demonstrate the effect of the latter in a Western Australian population using multivariate classification statistics. We report here the analysis of a sample comprising three-dimensionally reconstructed cranial (n = 400) and pelvic (n = 400) multi-detector computed tomography (MDCT) thin-slice scans. In the morphometric analysis of cranial dimorphism, we demonstrate an unacceptably large sex bias; foreign standards thus frequently misclassify females.

Early Life Conditions and Physiological Stress following the Transition to Farming in Central/Southeast Europe: Skeletal Growth Impairment and 6000 Years of Gradual Recovery.

Early life conditions play an important role in determining adult body size. In particular, childhood malnutrition and disease can elicit growth delays and affect adult body size if severe or prolonged enough. In the earliest stages of farming, skeletal growth impairment and small adult body size are often documented relative to hunter-gatherer groups, though this pattern is regionally variable. In Central/Southeast Europe, it is unclear how early life stress, growth history, and adult body size were impacted by the introduction of agriculture and ensuing long-term demographic, social, and behavioral change. The current study assesses this impact through the reconstruction and analysis of mean stature, body mass, limb proportion indices, and sexual dimorphism among 407 skeletally mature men and women from foraging and farming populations spanning the Late Mesolithic through Early Medieval periods in Central/Southeast Europe (~7100 calBC to 850 AD). Results document significantly reduced mean stature, body mass, and crural index in Neolithic agriculturalists relative both to Late Mesolithic hunter-gatherer-fishers and to later farming populations. This indication of relative growth impairment in the Neolithic, particularly among women, is supported by existing evidence of high developmental stress, intensive physical activity, and variable access to animal protein in these early agricultural populations. Among subsequent agriculturalists, temporal increases in mean stature, body mass, and crural index were more pronounced among Central European women, driving declines in the magnitude of sexual dimorphism through time. Overall, results suggest that the transition to agriculture in Central/Southeast Europe was challenging for early farming populations, but was followed by gradual amelioration across thousands of years, particularly among Central European women. This sex difference may be indicative, in part, of greater temporal variation in the social status afforded to young girls, in their access to resources during growth, and/or in their health status than was experienced by men.

Complex Population Patterns of Eunica tatila Herrich-Schäffer (Lepidoptera: Nymphalidae), with Special Emphasis on Sexual Dimorphism.

The species Eunica tatila (Herrich-Schäffer) is present in the Neotropical region and comprises three subspecies. In Mexico, only one subspecies is reported: E. t. tatila (Herrich-Schäffer). The Yucatan Peninsula, in southeastern Mexico, is located in a transitional geographical position, between southern Florida, the West Indies and Central America. It is part of a transitional region, important for the dispersion of insects from southern Florida via Cuba and the Yucatan Peninsula. Considering the possibility of the overlapping and delimitation of described subspecies, we sampled different populations in the Yucatan Peninsula to possibly assign a subspecies name and evaluate the magnitude of sexual dimorphism. We collected 591 individuals (♀284, ♂307) in conserved areas. The study of male genitalia led to the identification of Eunica tatila tatilista (Kaye) as a subspecies; however, hypandrium structure and wing pattern analysis suggest a mix of E. t. tatila and E. t. tatilista characteristics. The analysis of sexual dimorphism provided evidence of more complex wing morphs for females, with 12 patterns instead of four as previously described. Our results demonstrate the complexity of characterizing E. tatila and suggest that the Yucatan Peninsula is a transitional zone for subspecies of some butterflies.

Variation in the ontogeny of sex steroid levels between latitudinal populations of the medaka.

IntroductionSex steroids mediate the expression of sexual dimorphism during ontogeny, and populations that differ in the magnitudes of sexual dimorphism may accordingly differ in the ontogenetic patterns of their sex steroid levels. The medaka, Oryzias latipes species complex, shows geographic variation in the magnitude of sexual dimorphism with respect to the lengths of their anal and dorsal fins; dimorphism is greater in low-latitude populations than in high-latitude populations. However, sexual differences in the ontogenetic dynamics of sex steroids, and its interpopulation variation, have not been examined.ResultsWe measured testosterone (T), estradiol-17β (E2), and 11-ketotestosterone (11-KT) concentrations throughout ontogeny of laboratory-reared fish from two latitudinal populations: Aomori (northern) and Okinawa (southern). In both populations, the levels of all three steroids were high during early ontogenetic stages and decreased with growth. After reaching about 15 mm in standard length, when sexual dimorphisms in fin lengths became apparent, steroid levels increased and tended to plateau. Sexual differences in the steroid levels were observed only in the later ontogenetic stages; T and 11-KT levels were higher in males, while E2 levels were higher in females. Accordingly, interpopulation differences also became clearer; the southern fish tended to show higher T levels and lower E2 levels than the northern fish.ConclusionsThe ontogenetic patterns of sex steroid levels paralleled the ontogeny of anal and dorsal fins in the two latitudinal populations, suggesting that interpopulation variation in the degree of sexual dimorphisms in fin lengths is mediated by sex steroid-dependent regulation of fin elongation.

Sexual size dimorphism in ground squirrels (Rodentia: Sciuridae: Marmotini) does not correlate with body size and sociality

IntroductionSexual size dimorphism (SSD) is a widespread phenomenon in animals including mammals. It has been demonstrated that across species, the direction and magnitude of sexual dimorphism in body size often corresponds to social systems. Moreover, many animal lineages conform to “Rensch’s rule”, which states that male-biased SSD increases with body size. We tested whether considerable differences in sociality and large variation in body size were connected with the evolution of SSD in the structural body size of ground squirrels, an otherwise ecologically relatively homogenous group of terrestrial rodents.ResultsWe found the general trend of male-biased SSD in ground squirrels, however, male size increases nearly perfectly isometrically with female size among species and sociality does not explain departures from this relationship. Species with different sociality grades significantly differ in body size, with the most social species tending to be the largest.ConclusionsWe suggest that lack of conformity with Rensch´s rule in ground squirrels may be attributed to their low variation in SSD, and briefly discuss three potential causes of small magnitude of SSD in the structural size in rodents: low selection on SSD in structural dimensions, ontogenetic and genetic constraints and the existence of ecological/selection factors preventing the evolution of extensive SSD.

Sexual dimorphism in floral traits of gynodioecious Lobelia siphilitica (Lobeliaceae) is consistent across populations

In angiosperms, the evolution of gender dimorphism is often correlated with the evolution of sexual dimorphism in floral traits. The magnitude of sexual dimorphism will reflect both sex-specific selection and constraints on evolution in response to this selection. If the evolution of sexual dimorphism is constrained by genetic correlations, then trait means are predicted to evolve more quickly than the magnitude of sexual dimorphism in the same traits. To test whether mean floral traits evolve more quickly than sexual dimorphism in these traits, I measured flower size and flower number of gynodioecious Lobelia siphilitica L. from 10–14 populations across the species’ range. Females produced more flowers than hermaphrodites, but neither mean flower number nor the magnitude of dimorphism in flower number varied among populations. Pistillate flowers were smaller than perfect flowers, and mean flower size varied among populations. However, the magnitude of dimorphism in flower size did not vary. My results suggest that the evolution of sexual dimorphism in flower size, but not necessarily flower number, could be constrained by within- or between-sex genetic correlations in L. siphilitica.

Divergence in androgen sensitivity contributes to population differences in sexual dimorphism of electrocommunication behavior

Weakly-electric fish (Apteronotidae) produce highly diverse electrocommunication signals. Electric organ discharges (EODs) vary across species, sexes, and in the magnitude and direction of their sexual dimorphism. Gonadal steroid hormones can modulate EODs, and differences in androgen sensitivity are hypothesized to underlie variation in the degree of sexual dimorphism across species. In this study, we asked whether variation in androgen sensitivity explained variation in sexual dimorphism of EODs within species, at the population level. We examined two populations of black ghost knifefish (Apteronotus albifrons), one from the Orinoco and the other from the Amazon River Basin. EOD frequency (EODf) and chirp rates were measured to characterize diversity in sexual dimorphism across populations. The magnitude of sexual dimorphism in EODf differed significantly across populations, and was more pronounced in the Orinoco population than in the Amazon population. Chirp rates were sexually monomorphic in both populations. 11-Ketotestosterone (11-kT) was administered over a two-week period to assess population differences in sensitivity to androgens. 11-kT masculinized EODf significantly more in the population with the greater degree of sexual dimorphism. 11-kT had no effect on the sexually monomorphic chirping rates. We conclude that population divergence in androgen sensitivity contributes to variation in sexual dimorphism of EODf in A. albifrons.

Reduction of sexual dimorphism in stream-resident forms of three-spined stickleback Gasterosteus aculeatus.

Sexual dimorphism in geometric body shape and external morphology was compared between marine and stream-resident forms of three-spined stickleback Gasterosteus aculeatus collected from North America and Japan. Some aspects of sexual dimorphism were shared between ecotypes: males had larger heads than females with no significant effect of ecotype on the magnitude of sexual dimorphism. By contrast, a significant sex-by-ecotype interaction was found for body depth. Males tended to have deeper bodies than females in both forms, but the magnitude of sexual dimorphism was reduced in stream-resident forms. Although females were generally larger in standard length and had larger pelvic girdles, significant sexual dimorphism in these traits was not consistently found across populations or ecotypes. These results suggest that some aspects of sexual dimorphism were shared between ecotypes, while others were unique to each population. The results further suggest that ecology may influence the evolution of sexual dimorphism in some external morphological traits, such as body depth.

Sexual Size and Shape Dimorphism Variation in Caesar's Lizard (Gallotia caesaris, Lacertidae) from Different Habitats

We compared sexual dimorphism of body and head traits from adult lizards of populations of Gallotia caesaris living in ecologically different habitats of El Hierro and La Gomera. Males had larger body sizes than females, and sexual size and shape dimorphisms were greater in a population from La Gomera than in three populations from El Hierro. Multivariate analyses of variance, using linear and shape-adjusted traits, showed that the populations differed significantly in body and head traits, with pileus (head) width, snout–vent length (SVL), and body mass the main traits contributing to the differences. Males had larger SVL, heads, and limbs than females in all populations, but SVL relative to a shape index (calculated as the geometric mean of several body parameters) was larger in females than in males. Moreover, shape-adjusted hind-limb lengths were significantly shorter in lizards from the more densely vegetated habitats than in those from the less vegetated ones. The magnitude of sexual dimorphism was larger for relative limb length and head depth in the populations with less vegetation than in those with more vegetation. Our data suggest that morphological differences between populations reflect local adaptation to habitat structure.

Odontometric sex assessment in Indians

Sex determination of unidentified skeletal remains is important and various hard-tissue parameters have been evaluated towards this end. The dentition is considered as a useful adjunct in skeletal sex determination, particularly since teeth are resistant to postmortem destruction and fragmentation. Sex dimorphism in tooth size and the accuracy of odontometric sex prediction, is found to vary in different regions and researchers have advocated the need for population-specific data. No odontometric standards exist for Indians for use in forensic sex prediction. Therefore, this study examined sexual dimorphism in Indians using univariate and multivariate statistics. Mesiodistal (MD) and buccolingual (BL) measurements of all teeth, except third molars, were obtained from 105 Indians (53 females, 52 males). The mandibular first molar was found to be the most dimorphic tooth, followed by the canines and the BL dimension of maxillary first and second molars. While canines have, conventionally, shown the greatest degree of sexual dimorphism across populations, first molars have also been reported as the most or among the most sexually dimorphic teeth in some studies. Hence, apart from canines, the first molar would be the tooth most likely to exhibit the greatest univariate sexual dimorphism. In general, the mandibular teeth and BL dimensions showed greater tendency to be larger in males. However, nine tooth variables exhibited reverse dimorphism, i.e. female dimensions were larger than those of males. Overall, using univariate analysis, merely 37.5% of all tooth variables were statistically larger in males ( p < 0.05); the magnitude of sexual dimorphism in Indians is reduced when compared to other populations but similar to South Asian groups. Stepwise discriminant function analysis revealed moderate accuracy in sex prediction: the mandibular teeth were able to determine sex to higher levels (75.2%) compared to teeth of both jaws taken together (74.3%) or using only maxillary teeth (62.9%). Hence, for optimum odontometric sex prediction in Indians, recovery of the mandible alone suffices. Interestingly, a few teeth that showed reverse dimorphism entered the stepwise discriminant analysis, implying that larger female teeth can contribute to sex assessment and teeth being larger in males may not be a prerequisite.

Gynodioecy in Lobelia Siphilitica and L. Spicata (Lobeliaceae) from Western Massachusetts1

Abstract Gynodioecy is a dimorphic breeding system in which male-sterile individuals (i.e., females) coexist with hermaphroditic individuals in populations. Previous studies of two species of Lobelia in North America have documented gynodioecy in parts of their ranges and here, we document gynodioecy for a single population of L. siphilitica and in two populations of L. spicata from western Massachusetts. Our objectives were to (1) determine sex ratios in natural populations of these species, (2) use controlled pollinations to investigate the capacity and extent of self fertilization in hermaphrodites and (3) compare the relative fertility and magnitude of sexual dimorphism in floral morphology between females and hermaphrodites. The frequency of females was only 3% in L. siphilitica, whereas both populations of L. spicata contained 12% females. Hermaphrodites in both species were self-compatible and estimates of mating system parameters confirmed mixed mating in L. spicata. Females of L. spicata had high...

Costal process of the first sacral vertebra: Sexual dimorphism and obstetrical adaptation

The human sacrum is sexually dimorphic, with males being larger than females in most dimensions. Previous studies, though, suggest that females may have a longer costal process of the first sacral vertebra (S1) than males. However, these studies neither quantified nor tested statistically the costal process of S1. This study compares S1 with the five lumbar vertebrae (L1 to L5) for a number of metric dimensions, including costal process length. Four issues are addressed, the: 1) hypothesis that females have a longer costal process of S1 than males; 2)hypothesis that homologous structures (i.e., costal processes of L1 to S1) differ in their direction of sexual dimorphism; 3) importance of the costal process of S1 to the obstetrical capacity of the pelvis; and 4) evolution of sexual dimorphism in costal process length of S1. One hundred ninety-seven individuals, including males and females of American blacks and whites, from the Hamann-Todd and Terry Collections were studied. Results show that males are significantly larger than females for most vertebral measurements, except that females have a significantly longer costal process of S1 than males. Costal process length of S1 is positively correlated with the transverse diameter and circumference of the pelvic inlet. The magnitude of sexual dimorphism in costal process length of S1 ranks this measure among the most highly dimorphic of the pelvis. Compared with the humans in this study, australopithecines have a relatively long costal process of S1, but their broad sacrum was not associated with obstetrical imperatives.

The Evolution of Condition‐Dependent Sexual Dimorphism

Theory suggests that the net benefit of allocating resources to a sexual trait depends both on the strength of sexual selection on that trait and on individual condition. This predicts a tight coevolution between sexual dimorphism and condition dependence and suggests that these patterns of within-sex and between-sex variation may share a common genetic and developmental basis. Although condition-dependent expression of sexual traits is widely documented, the extent of covariation between condition dependence and sexual dimorphism remains poorly known. I investigated the effects of condition (larval diet quality) on multivariate sexual dimorphism in the fly Telostylinus angusticollis (Neriidae). Condition determined the direction of sexual size dimorphism and modulated sexual shape dimorphism by affecting allometric slopes and/or intercepts of sexually homologous traits in both sexes. Although the greatest responses to condition manipulation were observed in male sexual traits, both sexual and nonsexual traits exhibited substantial variation in the nature and magnitude of condition effects. Nonetheless, condition dependence and sexual dimorphism were remarkably congruent: variation in the strength of condition effects on male traits explained more than 90% of the variation in the magnitude of sexual dimorphism, whether quantified in terms of trait size or allometric slope. The genetic mechanisms that give rise to multivariate sexual dimorphism in body shape thus function in a strongly condition-dependent manner in this species, suggesting a common genetic basis for body shape variation within and between sexes.

Sexual dimorphism in the antennae of terrestrial isopods: a result of male contests or scramble competition?

In Oniscidea (terrestrial crustaceans), males are known to have longer antennae than females. This sexual dimorphism may result from a variety of selection pressures. However, some species are well known for their highly aggressive males, which use their antennae as weapons. We tested the hypothesis that longer antennae in males have been selected for by means of antennal contests. Morphological analysis of the antennae and behavioral analysis of male dyads were performed in parallel on 7 species. We demonstrate significant sexual dimorphism of the antennae in 6 of the 7 species, and various forms of male aggressiveness depending on the species. Our hypothesis was rejected because we found a negative correlation between the use of the antennae in contests and the magnitude of sexual dimorphism. Furthermore, some species are sexually dimorphic but the males never compete using their antennae. We propose and argue that scramble competition to be the first to find receptive females could explain why males have longer chemoreceptive antennae.

Sexual dimorphism in the antennae of terrestrial isopods: a result of male contests or scramble competition?

In Oniscidea (terrestrial crustaceans), males are known to have longer antennae than females. This sexual dimorphism may result from a variety of selection pressures. However, some species are well known for their highly aggressive males, which use their antennae as weapons. We tested the hypothesis that longer antennae in males have been selected for by means of antennal contests. Morphological analysis of the antennae and behavioral analysis of male dyads were performed in parallel on 7 species. We demonstrate significant sexual dimorphism of the antennae in 6 of the 7 species, and various forms of male aggressiveness depending on the species. Our hypothesis was rejected because we found a negative correlation between the use of the antennae in contests and the magnitude of sexual dimorphism. Furthermore, some species are sexually dimorphic but the males never compete using their antennae. We propose and argue that scramble competition to be the first to find receptive females could explain why males have longer chemoreceptive antennae.

Morphological variation in great ape and modern human mandibles.

Adult mandibles of 317 modern humans and 91 great apes were selected that showed no pathology. Adult mandibles of Pan troglodytes troglodytes, Pongo pygmaeus pygmaeus and Gorilla gorilla gorilla and from 2 modern human populations (Zulu and Europeans from Spitalfields) were reliably sexed. Thirteen measurements were defined and included mandibular height, length and breadth in representative positions. Univariate statistical techniques and multivariate (principal component analysis and discriminant analysis) statistical techniques were used to investigate interspecific variability and sexual dimorphism in human and great ape mandibles, and intraspecific variability among the modern human mandibles. Analysis of interspecific differences revealed some pairs of variables with a tight linear relationship and others where Homo and the great apes pulled apart from one another due to shape differences. Homo and Pan are least sexually dimorphic in the mandible, Pan less so than Homo sapiens, but both the magnitude of sexual dimorphism and the distribution of sexually dimorphic measurements varied both among and between modern humans and great apes. Intraspecific variation among the 10 populations of modern humans was less than that generally reported in studies of crania (74.3% of mandibles were correctly classified into 1 of 10 populations using discriminant functions based on 13 variables as compared with 93% of crania from 17 populations based on 70 variables in one extensive study of crania). A subrecent European population (Poundbury) emerged as more different from a recent European population (Spitalfields) than other more diverse modern populations were from each other, suggesting considerable morphological plasticity in the mandible through time. This study forms a sound basis on which to explore mandibular variation in Neanderthals, early Homo sapiens and other more ancient fossil hominids.