CORRELATED EVOLUTION OF MIGRATION AND SEXUAL DICHROMATISM IN THE NEW WORLD ORIOLES (ICTERUS)

The evolution of sexual dimorphism has long been attributed to sexual selection, specifically as it would drive repeated gains of elaborate male traits. In contrast to this pattern, New World oriole species all exhibit elaborate male plumage, and the repeated gains of sexual dichromatism observed in the genus are due to losses of female elaboration. Interestingly, most sexually dichromatic orioles belong to migratory or temperate‐breeding clades. Using character scoring and ancestral state reconstructions from two recent studies in Icterus, we tested a hypothesis of correlated evolution between migration and sexual dichromatism. We employed two discrete phylogenetic comparative approaches: the concentrated changes test and Pagel's discrete likelihood test. Our results show that the evolution of these traits is significantly correlated (CCT: uncorrected P < 0.05; ML: LRT = 12.470, P < 0.005). Indeed, our best model of character evolution suggests that gains of sexual dichromatism are 23 times more likely to occur in migratory taxa. This study demonstrates that a life‐history trait with no direct relationship with sexual selection has a strong influence on the evolution of sexual dichromatism. We recommend that researchers further investigate the role of selection on elaborate female traits in the evolution of sexual dimorphism.     

Sexual dimorphism in the skull of the European mink<Emphasis Type="Italic">Mustela lutreola</Emphasis> from NW part of Russia

Morphometric variation in 22 characters of 86 skulls of the European minkMustela lutreola (Linnaeus, 1761), from the NW part of Russia, has been analysed. Stepwise discriminant analysis was used to estimate craniometric variables for sex determinations. Two characters (zygomatic breadth and interorbital width) are enough for the 96.5% correct classification. The male skull ofM. lutreola is characterised by a relatively high neurocranium, widely arranged zygomatic arches, a wide rostrum, and by wider auditory bullae and higher mandibles. Sexual size dimorphism ofM. lutreola is less than that of other similar-sized mustelids —Mustela putorius, M. eversmanii, M. sibirica, Neovison vison. The results are discussed in relation to the existing theories on sexual dimorphism in mustelids.     

Environmental stress increases skeletal fluctuating asymmetry in the moor frog Rana arvalis

Whether fluctuating asymmetry (FA) provides a useful metric indicator of the degree of environmental stress experienced by populations is still a contentious issue. We investigated whether the degree of FA in skeletal elements is useful in elucidating the degree of environmental stress experienced by frog populations, and further, tested the proposition that a trait’s sensitivity to stress—as reflected in the degree of FA—is related to the degree of directional selection experienced by the given trait. We compared the degree of FA in four bilateral skeletal elements of male and female moor frogs (Rana arvalis) originating from low (acidified) and neutral pH populations. While the degree of uncorrected FA was unrelated to the degree of acidity, the growth rate and age of the individuals, the size-corrected FA was significantly higher in low than in neutral pH populations and decreased with individual ages and growth rates. In addition, both measures of FA were significantly higher in males and in particular in traits presumably under high sexual selection as indicated by the degree of sexual size dimorphism. All in all, the results indicate that individuals from acidified localities are smaller, younger and exhibit a significantly higher degree of FA than individuals from neutral pH populations. These results constitute the first assessment of FA in amphibians and suggest that the degree of FA in skeletal traits can be a useful indicator of the degree of environmental stress experienced by amphibian populations.     

Sexual size dimorphism and sexual selection in primates

1. In most animals, females are larger than males. Paradoxically, sexual size dimorphism is biased towards males in most mammalian species. An accepted explanation is that sexual dimorphism in mammals evolved by intramale sexual selection. I tested this hypothesis in primates, by relating sexual size dimorphism to seven proxies of sexual selection intensity: operational sex ratio, mating system, intermale competition, group sex ratio, group size, maximum mating percentage (percentage of observed copulations involving the most successful male), and total paternity (a genetic estimate of the percentage of young sired by the most successful male).
2. I fitted phylogenetic generalised least squares models using sexual size dimorphism as the dependent variable and each of the seven measures of intensity of sexual selection as independent variables. I conducted this comparative analysis with data from 50 extant species of primates, including Homo sapiens, Pan troglodytes, and Gorilla spp.
3. Sexual dimorphism was positively related to the four measures of female monopolisation (operational sex ratio, mating system, intermale competition, and group sex ratio) and in some cases to group size, but was not associated with maximum mating percentage or total paternity. Additional regression analyses indicated that maximum mating percentage and total paternity were negatively associated with group size.
4. These results are predicted by reproductive skew theory: in large groups, males can lose control of the sexual behaviour of the other members of the group or can concede reproductive opportunities to others. The results are also consistent with the evolution of sexual size dimorphism before polygyny, due to the effects of natural, rather than sexual, selection. In birds, the study of molecular paternity showed that variance in male reproductive success is much higher than expected by behaviour. In mammals, recent studies have begun to show the opposite trend, i.e. that intensity of sexual selection is lower than expected by polygyny.
5. Results of this comparative analysis of sexual size dimorphism and sexual selection intensity in primates suggest that the use of intramale sexual selection theory to explain the evolution of polygyny and sexual dimorphism in mammals should be reviewed, and that natural selection should be considered alongside sexual selection as an evolutionary driver of sexual size dimorphism and polygyny in mammals.

     

Sexual Dimorphism and Mortality Bias in a Small Miocene North American Rhino, <Emphasis Type="Italic">Menoceras arikarense</Emphasis>: Insights into the Coevolution of Sexual Dimorphism and Sociality in Rhinos

Rhinos are the only modern perissodactyls that possess cranial weapons similar to the horns, antlers and ossicones of modern ruminants. Yet, unlike ruminants, there is no clear relationship between sexual dimorphism and sociality. It is possible to extend the study of the coevolution of sociality and sexual dimorphism into extinct rhinos by examining the demographic patterns in large fossil assemblages. An assemblage of the North American early Miocene (∼22 million years ago) rhino, Menoceras arikarense, from Agate Springs National Monument, Nebraska, exhibits dimorphism in incisor size and nasal bone size, but there is no detectible dimorphism in body size. The degree of dimorphism of the nasal horn is greater than the degree of sexual dimorphism of any living rhino and more like that of modern horned ruminants. The greater degree of sexual dimorphism in Menoceras horns may relate to its relatively small body size and suggests that the horn had a more sex-specific function. It could be hypothesized that Menoceras evolved a more gregarious type of sociality in which a fewer number of males were capable of monopolizing a larger number of females. Demographic patterns in the Menoceras assemblage indicate that males suffered from a localized risk of elevated mortality at an age equivalent to the years of early adulthood. This mortality pattern is typical of living rhinos and indicates that young males were susceptible to the aggressive behaviors of dominant individuals in areas conducive to fossilization (e.g., ponds, lakes, rivers). Menoceras mortality patterns do not suggest a type of sociality different from modern rhinos although a group forming type of sociality remains possible. Among both living and extinct rhinos, the severity of socially mediated mortality seems unrelated to the degree of sexual dimorphism. Thus, sexual dimorphism in rhinos is not consistent with traditional theories about the co-evolution of sexual dimorphism and sociality.     

Females make tough neighbors: sex-specific competitive effects in seedlings of a dioecious grass

If males and females of a species differ in their effect on intraspecific competition then this can have significant ecological and evolutionary consequences because it can lead to size and mortality disparities between the sexes, and thus cause biased population sex ratios. If the degree of sexual dimorphism of competitive effect varies across environments then this variation can generate sex ratio variation within and between populations. In a California population of Distichlis spicata, a dioecious grass species exhibiting extreme within-population sex ratio variation (spatial segregation of the sexes), I evaluated the intraspecific competitive effects of male and female D. spicata seedlings in three soil types. The sex of seedlings was determined using a RAPD-PCR marker co-segregating with female phenotype. Distichlis spicata seedlings, regardless of sex, were six times larger when grown with male versus female conspecific seedlings in soil from microsites where the majority of D. spicata plants are female, and this sexual dimorphism of competitive effect was weaker or did not occur in other soil types. This study suggests that it is not just the higher costs of female versus male reproduction itself that cause spatial segregation of the sexes in D. spicata, but that differences in competitive abilities between the sexes—which occur as early as the seedling stage—can generate sex ratio variation.     

New data on sexual dimorphism and reproductive biology of Alaska skate <Emphasis Type="Italic">Bathyraja parmifera</Emphasis> from the northwestern Pacific Ocean

New data on sexual dimorphism and features of reproductive biology of Alaska skate Bathyraja parmifera from the western Bering Sea and the northern Sea of Okhotsk are presented. Based on comparative analysis of 11 external morphological characters, the males differ from the females in the length of the disc and snout length (to the eyes, to the mouth, and to the nostrils). In the western Bering Sea, 50% of the females and 50% of the males reach the first sexual maturity at 84.5 cm TL and 85.2–90.2 cm TL, respectively; in the Sea of Okhotsk, 50% of the males reach their first sexual maturity at 80.8–83.8 cm TL (the assessment depends on the features of the method). Among smaller skates (less than 45 cm TL), the males prevail, but the proportion of the females increases in larger size groups; the sex ratio becomes equal in the exemplars 60–80 cm TL, but the males prevail again among the largest fishes (more than 80 cm TL).     

Ontogenetic approaches to sexual dimorphism in anthropoids

Studies of sexual dimorphism have traditionally focused on the static differences in size and shape between adult males and females. In this paper, I suggest that an investigation of the ontogenetic bases of sexual dimorphism can provide new insights and information unobtainable from studies concerned only with adult endpoints. While growth is often viewed as simply the developmental pathway utilized to attain final adult size and shape, we must recognize that it is the entire pattern of sex-differentiated growth, and not merely the adult endpoints, which is adaptive and the target of natural selection. The importance of an ontogenetic approach to the analysis of sexual dimorphism is also demonstrated by the fact that a given morphologicalresult (e.g., a certain degree of adult weight dimorphism) may be attained by very different developmentalprocesses, signalling selection for quite different factors. The need to analyze the ontogenetic bases of sexual dimorphism in size and shape has recently been recognized by Jarman, in his study of dimorphism in large terrestrial herbivores. Here I combine aspects of Jarman’s approach with those of allometry and heterochrony in an analysis of sexual dimorphism in selected anthropoid primates. It is demonstrated that although all dimorphic anthropoids appear to be characterized by somebimaturism, the degree varies significantly. Marked weight dimorphism in certain species is primarily produced by an increased differentiation of female and male growthrates, while in other species the primary change involves differences in thetime or duration of growth between the sexes. These variations are illustrated with anthropoid genera such asMiopithecus, Cercopithecus, Erythrocebus, Macaca, Papio, Pan, andGorilla. It is suggested that additional ontogenetic investigations of other anthropoids will help clarify some of the socioecological bases of this variation in the ways of attaining an adult dimorphic state. This will contribute to our understanding of the complex factors underlying and producing sexual dimorphism in primates and other mammals.     

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.     

Sexual dimorphism and mating systems: jumping to conclusions

Previous studies have suggested that there is a strong relationship between a high degree of aggressive competition among males for access to fertile females and large body and canine size in males. It has further been suggested that such a relationship among living primates can be used to infer the social organization of extinct primate species from the degree of sexual dimorphism exhibited. Our field studies of patas (Erythrocebus patas) and blue monkeys (Cercopithecus mitis), two species which had previously been characterized as having one-male ‘harem’ group structures, indicate considerable variability in mating systems. We suggest, on the basis of our observations of these species, that factors other than male-male competition (e.g., predation) may also have influenced the degree of dimorphism in primates.     

Latitude and intersocietal variation of human sexual dimorphism of stature

In a recent article Alexander et al.(1979) argue that intersocietal variation in human sexual dimorphism of stature can be explained by differing degrees of male- male competition for mates found in different types of societal marriage systems. In this paper we reexamine their data and suggest that their results may be more simply explained by reference to the latitude of the societies in their sample. Our investigation indicates that latitude has a significant influence on the degree of societal sexual dimorphism of stature. Peoples of the arctic region and the short peoples of the equatorial regions are less dimorphic in stature than midlatitude peoples. Thus, we conclude that the sociobiological hypothesis presented by Alexander et al.cannot alone explain the intersocietal variation in sexual dimorphism of stature and that environmental factors must be taken into account in the explanation of such variation.This research was supported by a Faculty Research Grant from North Texas State University.     

Contrasting growth strategies of pond versus marine populations of nine-spined stickleback (Pungitius pungitius): a combined effect of predation and competition?

Gigantism in isolated ponds in the absence of sympatric fish species has previously been observed in nine-spined sticklebacks (Pungitius pungitius). Patterns in sexual size dimorphism suggested that fecundity selection acting on females might be responsible for the phenomenon. However, the growth strategy behind gigantism in pond sticklebacks has not been studied yet. Here, we compared von Bertalanffy growth parameters of four independent nine-spined stickleback populations reared in a common laboratory environment: two coastal marine (typical size) and two pond (giant size) populations. We found that both pond populations had larger estimated final size than marine populations, which in turn exhibited higher intrinsic growth rates than the pond populations. Female growth strategies were more divergent among marine and pond populations than those of males. Asymptotic body size and intrinsic growth rate were strongly negatively correlated. Hence, pond versus marine populations exhibited different growth strategies along a continuum. Our data suggest that quick maturation—even with the cost of being small (low fecundity)—is favoured in marine environments. On the contrary, growth to a giant final size (high fecundity)—even if it entails extended growth period—is favoured in ponds. We suggest that the absence (ponds) versus presence (marine environment) of sympatric predatory fish species, and the consequent change in the importance of intraspecific competition are responsible for the divergence in growth strategies. The sex-dependence of the patterns further emphasizes the role of females in the body size divergence in the species. Possible alternative hypotheses are also discussed.     

Human sexual dimorphism in size may be triggered by environmental cues

Evolutionary biologists mostly assume that polygyny increases sexual dimorphism in size because, under polygyny, larger males monopolize mating opportunities and pass on their 'large male' genes to their sons. Available data on parent-child correlations in height among humans (Homo sapiens) do not support the crucial assumption that height is transmitted along sex lines. This paper instead suggests that human sexual dimorphism in size emerged, not because men got taller, but because women got shorter by undergoing early menarche in response to polygyny. It further speculates that, rather than genetically transmitted, the sexual dimorphism may emerge anew in each generation in response to the degree of polygyny in society. The analysis of comparative data supports the prediction that polygyny reduces women's height, but has no effect on men's, and is consistent with the speculation that the origin of human sexual dimorphism in size may be cultural, not genetic.     

Sexual dimorphism in the Atapuerca-SH hominids: the evidence from the mandibles

The pattern of sexual dimorphism in 15 mandibles from the Atapuerca-SH Middle Pleistocene site, attributed to Homo heidelbergensis, is explored. Two modern human samples of known sex are used as a baseline for establishing sexing criteria. The mandible was divided for analysis into seven study regions and differential expression of sexual dimorphism in these regions is analysed. A total of 40 continuous and 32 discrete variables were scored on the mandibles. The means method given in Regh & Leigh (Am. J. phys. Anthrop.110, 95-104, 1999) was followed for evaluating the potential of correct sex attribution for each variable.On average, the mandibles from the Atapuerca-SH site present a degree of sexual dimorphism about eight points higher than in H. sapiens samples. However, mandibular anatomy of the European Middle Pleistocene hominid records sexual dimorphism differentially. Different areas of the Atapuerca-SH mandibles exhibit quite distinct degrees of sexual dimorphism. For instance, variables of the alveolar arcade present very low or practically no sexual dimorphism. Variables related to overall size of the mandible and symphysis region present a medium degree of sex differences. Finally, ramus height, and gonion and coronoid process present a high degree of sexual dimorphism (indexes of sexual dimorphism are all above 130%). Whether this marked sexual dimorphism in specific anatomical systems affects sexual differences in body size is not completely clear and further studies are needed.Sexual differences detected in the mandible of modern humans have at least two components: differences related to musculo-skeletal development and differences related to a different growth trajectory in males and females (relative development of some of the basal border features). The Atapuerca-SH mandibles display little variation in the basal border, however. The limited variation of this mandibular region may indicate that the pattern of sexual variation in H. heidelbergensis is different enough to that of H. sapiens to caution against simple extrapolation of criteria from one pattern to the other.     

Sexual differences in European Neanderthal crania with special reference to the Krapina remains

Problems relating to the identification of sex in Neanderthal specimens are discussed. Three morphological features—morphology of the mastoid process and surrounding area, form of the supraorbital torus at glabella and the superciliary region, and the rugosity of the nuchal plane—were selected as most indicative of sex in Neanderthal crania based on observations from the Krapina collection and on those European specimens with pelvicly determined sex. Thirteen Neanderthal crania (eight males, five females) are sexed on the basis of these criteria, and the pattern and degree of sexual dimorphism determined for this sample is compared to those exhibited by other samples of more recent European hominids. It is concluded that the degree of sexual dimorphism in Neanderthal crania, as defined by this study, is consistent with that observed in the other fossil samples and that Neanderthals exhibit slightly more cranial sexual dimorphism than more recent Europeans. Models for explaining this are discussed as is the difference in pattern of change and degree of sexual dimorphism between the cranium and postcranium during later prehistoric hominid evolution in Europe.     

Degrees of sexual dimorphism in Cebus and other new world monkeys

Sexual dimorphism in primate species expresses the effects of phylogeny, life history, behavior, and ontogeny. The causes and implications of sexual dimorphism have been studied in several different primates using a variety of morphological databases such as body weight, canine length, and coat color and ornamentation. In addition to these different patterns of dimorphism, the degree to which a species is dimorphic results from a variety of possible causes. In this study we test the general hypothesis that a species highly dimorphic for one size-based index of dimorphism will be equally dimorphic (relative to other species) for other size-based indices. Specifically, the degree and pattern of sexual dimorphism in Cebus and several other New World monkey species is measured using craniometric data as a substitute for the troublesome range of variation in body weight estimates. In general, the rank ordering of species for dimorphism ratios differs considerably across neural vs. non-neural functional domains of the cranium. The relative degree of sexual dimorphism in different functional regions of the cranium is affected by the independent action of natural selection on those regions. Regions of the cranium upon which natural selection is presumed to have acted within a species show greater degrees of dimorphism than do the same regions in closely related taxa. Within Cebus, C. apella is consistently more dimorphic than other Cebus species for facial measurements, but not for neural or body weight measurements. The pattern in C. apella indicates no single best measurement of the degree of dimorphism in a species; rather, the relative degree of dimorphism applies only to the region being measured and may be enhanced by other selective pressures on morphology. Am J Phys Anthropol 107:243–256, 1998. © 1998 Wiley-Liss, Inc.     

Cranial variability in East African ‘Robust’ hominis

Among Plio-Pleistocene hominins the East African ‘robust’ group [Australopithecus (Paranthropus) boisei sensu lato] has the largest sample. This makes it an important test case for examining within-group variability and its implications for early hominin systematics. When using the CV to test for mixed-species samples, sexual dimorphism and diachronic variation are important additional (confounding) sources of variability. After examining nineteen variables, five craniometric variables in the East African ‘robust’ group are identified that have low sexual dimorphism. All are characterised by CVs from a combined KNM-WT 17000 andA. (P.) boisei sample less than the CV of the bonobo. Diachronic variation is found to be an important source of variability for cranial capacity. This form of variation cannot be detected in other variables studied here, but its possible presence cannot be ignored (owing mostly to small sample sizes). It is concluded that there is insufficient evidence to refute the hypothesis that all East African ‘robust’ fossils belong toA. (P.) boisei (Walker and Leakey, 1988).