Sexual dimorphisms in the overall proportions of primates

Morphometric analysis of Professor A.H. Schultz's data on the overall proportions of primates reveals differences between the sexes. Univariate examinations of these data confirm the existence of the spectrum of sexual dimorphism already well known. This spectrum relates mainly to differences in the proportions of the trunk. It has a differential expression with largest differences between the sexes in species such as orangutans and boboons, and smallest in species such as spider monkeys and douroucoulis. Multivariate statistical study of these same data reveal, however, further unsuspected sexual dimorphisms. Although differences between the sexes are only small when measures of the relative lengths of bodily parts are examined, they are big when bodily breadths are studied. Investigation of breadths alone reveals that the primates display two major patterns of sexual dimorphisms and seven unique sexual dimorphisms among the 18 genera examined. Such findings mean that sexual dimorphism of bodily structure is not a single phenomenon with differential expression, a concept widely noted in the literature and most recently associated with social organization. There are several different sexual dimorphisms and this suggests that their causation is likely to be multifactorial with multiple complex interactions among the factors. Some of the sexual dimorphisms must have evolved in parallel a number of times, and, given that chimpanzees, gorillas, and humans each display a different sexual dimorphism, at least some of the evolutionary changes in different sexual dimorphisms must be very recent. The findings even imply the possibility of further unique patterns of sexual dimorphism in some fossils. By further extension, the findings may have some implications for our understanding of nonstructural dimorphisms in humans.     

Seasonal and sexual dimorphisms in the green anole forebrain

During the breeding season, male green anole lizards extend a throat fan (dewlap) in courtship. This behavior is facilitated by testosterone (T). Females extend a much smaller dewlap less often, even with the same dose of T. During the nonbreeding season when T is low, dewlap extension is reduced. To determine if parallels exist between structure and function, we investigated neuron soma size and density in the preoptic area (POA) and ventromedial nucleus of the amygdala (AMY), which are involved in the display behavior, in breeding and nonbreeding males and females. Cells from breeding animals were larger than cells from nonbreeding animals, but they were not sexually dimorphic. No significant effects existed in neuron density. This experiment indicates that portions of the anole forebrain important for sexual behavior are plastic and might be influenced by seasonal changes in steroid hormones. To investigate whether T can reverse the seasonal difference in soma size in both sexes, gonadectomized nonbreeding anoles were implanted with an empty or T propionate-filled capsule; animals were also tested for male-typical courtship behavior. Males and females treated with T had higher rates of dewlap extension, but across treatment groups these rates were greater in males. Neuron soma size in the POA and AMY was larger in males than females, but no effects of treatment were detected. Taken together, the results indicate that T can stimulate behavior in the nonbreeding season and suggest that a dissociation exists between the regulation of the courtship display and soma size of relevant brain regions.     

Intrasexual selection and testis size in strepsirhine primates

The main objective of this study was to investigate two predictionsof sexual selection theory concerning interspecific variationin testis size among strepsirhine primates (Lemuriformes andLorisiformes). First, the unique evolutionary history of lemursprovides an opportunity for an independent test of the predictionsof sperm competition theory regarding the relationship betweenmating system and relative testis size. Second, I examined theevolutionary relationship between the morphological correlatesof pre- and postcopulatory competition (i.e., between sexualdimorphism and testis size) because polygamous lemurs, in contrastto other polygamous primates, lack sexual dimorphism. Basedon measurements from 174 captive strepsirhines from 24 species,I found that multi-male species had significantly larger testesthan pair-living ones, but that they did not differ significantlyfrom solitary species. This result deviates from theoreticalexpectations, but may be the result of yet-unknown heterogeneityin mechanisms of male-male competition in both multi-male andsolitary species. There was no difference in relative testissize between nonmonogamous lemurs and lorises, indicating thatpresumably lower levels of precopulatory competition are notnecessarily compensated by more intense sperm competition. Bodysize and phylogenetic effects were also found to considerablyaffect interspecific variability in testis size. Analyses ofindependent contrasts revealed that evolutionary changes inmating system, testis size, sexual size, and canine dimorphismwere not, or only weakly, associated in this monophyletic groupof primates. Additional comprehensive comparative studies ofsexual dimorphism, testis size, mating system, and copulatorybehavior in these and other taxa are indicated to illuminategeneral patterns and causes of covariation among these traits     

Sexual dichronism and intersexual phoresy in gall-forming coccoids

Summary Coccoid gall-formers from the genus Cystococcus exhibit sexual dichronism, producing the males of a brood well before their sisters. This ensures the coexistence in the maternal gall of winged adult males and their first instar wingless sisters. Females disperse by clinging to the attenuate abdomens of their brothers, and the efficiency of this dispersal is reflected in an even dispersion on host trees. Evolution of this form of phoresy may be associated with the paternal genome loss prevalent in the Coccoidea.     

Ecological dimorphisms: An introduction to the symposium

A growing body of literature is devoted to understanding differencesbetween males and females in numerous behavioral, morphological,and physiological variables that do not appear to play a rolein sexual or fecundity selection. Despite the increasing attentionpaid to such ecological dimorphisms, there has thus far beenno attempt to consolidate or review studies in this area. Thissymposium comprises a series of papers that examine ecologicaldimorphisms from several diverse perspectives, spanning a rangeof ecological fields from functional morphology to behaviorand evolution of life-history. These studies show that genderexerts considerable effects on individual ecology, even outsideof the context of reproduction.     

Intrasexual selection predicts the evolution of signal complexity in lizards

Sexual selection has often been invoked in explaining extravagant morphological and behavioural adaptations that function to increase mating success. Much is known about the effects of intersexual selection, which operates through female mate choice, in shaping animal signals. The role of intrasexual selection has been less clear. We report on the first evidence for the coevolution of signal complexity and sexual size dimorphism (SSD), which is characteristically produced by high levels of male male competition. We used two complementary comparative methods in order to reveal that the use of complex signals is associated with SSD in extant species and that historical increases in complexity have occurred in regions of a phylogenetic tree characterized by high levels of pre-existing size dimorphism. We suggest that signal complexity has evolved in order to improve opponent assessment under conditions of high male male competition. Our findings suggest that intrasexual selection may play an important and previously underestimated role in the evolution of communicative systems.     

Functional and ecological correlates of ecologically-based dimorphisms in squamate reptiles

Sexual dimorphism in phenotypic traits associated with the useof resources is a widespread phenomenon throughout the animalkingdom. While ecological dimorphisms are often initially generatedby sexual selection operating on an animal's size, natural selectionis believed to maintain, or even amplify, these dimorphismsin certain ecological settings. The trophic apparatus of snakeshas proven to be a model system for testing the adaptive natureof ecological dimorphisms because head size is rarely undersexual selection and it limits the maximum ingestible size ofprey in these gape-limited predators. Significantly less attentionhas been paid to the evolution of ecological dimorphisms inlizards, however, which may be due to the fact that lizards’feeding apparatus can be under both sexual and natural selectionsimultaneously, making it difficult to formulate clear-cut hypothesesto distinguish between the influences of natural and sexualselection. In order to tease apart the respective influencesof natural selection and sexual selection on the feeding apparatusof squamates, we take an integrative approach to formulate twohypotheses for snakes and lizards, respectively: (1) For gape-limitedsnakes, we predict that natural selection will act to generatedifferences in maximum gape, which will translate into differencesin maximum ingestible prey size between the sexes. (2) For lizardswhich mechanically reduce their prey, we predict that the degreeof dimorphism in head size should be positively correlated tothe degree of dimorphism in bite force which, in turn, shouldbe correlated to dimorphism in aspects of size or hardness ofprey. Finally, we predict that functional differences in thefeeding apparatus of these animals will also be linked withdifferences in sex-based feeding behavior and with selectionof prey.     

Intrasexual competition and eating restriction in heterosexual and homosexual individuals

Restrictive eating attitudes and behaviors have been hypothesized to be related to processes of intrasexual competition. According to this perspective, within-sex competition for status serves the adaptive purpose of attracting mates. As such, status competition salience may lead to concerns of mating desirability. For heterosexual women and gay men, such concerns revolve around appearing youthful and, thus, thinner. Following this logic, we examined how exposure to high-status and competitive (but not thin or highly attractive) same-sex individuals would influence body image and eating attitudes in heterosexual and in gay/lesbian individuals. Results indicated that for heterosexuals, intrasexual competition cues led to greater body image dissatisfaction and more restrictive eating attitudes for women, but not for men. In contrast, for homosexual individuals, intrasexual competition cues led to worse body image and eating attitudes for gay men, but not for lesbian women. These findings support the idea that the ultimate explanation for eating disorders is related to intrasexual competition.     

Discovery of sexual dimorphisms in metabolic and genetic biomarkers

Metabolomic profiling and the integration of whole-genome genetic association data has proven to be a powerful tool to comprehensively explore gene regulatory networks and to investigate the effects of genetic variation at the molecular level. Serum metabolite concentrations allow a direct readout of biological processes, and association of specific metabolomic signatures with complex diseases such as Alzheimer's disease and cardiovascular and metabolic disorders has been shown. There are well-known correlations between sex and the incidence, prevalence, age of onset, symptoms, and severity of a disease, as well as the reaction to drugs. However, most of the studies published so far did not consider the role of sexual dimorphism and did not analyse their data stratified by gender. This study investigated sex-specific differences of serum metabolite concentrations and their underlying genetic determination. For discovery and replication we used more than 3,300 independent individuals from KORA F3 and F4 with metabolite measurements of 131 metabolites, including amino acids, phosphatidylcholines, sphingomyelins, acylcarnitines, and C6-sugars. A linear regression approach revealed significant concentration differences between males and females for 102 out of 131 metabolites (p-values<3.8×10(-4); Bonferroni-corrected threshold). Sex-specific genome-wide association studies (GWAS) showed genome-wide significant differences in beta-estimates for SNPs in the CPS1 locus (carbamoyl-phosphate synthase 1, significance level: p<3.8×10(-10); Bonferroni-corrected threshold) for glycine. We showed that the metabolite profiles of males and females are significantly different and, furthermore, that specific genetic variants in metabolism-related genes depict sexual dimorphism. Our study provides new important insights into sex-specific differences of cell regulatory processes and underscores that studies should consider sex-specific effects in design and interpretation.     

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.     

Sexually dimorphic distribution of galanin in the preoptic area of red salmon, Oncorhynchus nerka

A sexually dimorphic distribution of galanin in the preoptic region of the molly and goldfish has previously been demonstrated. Females of these species lack galanin-immunoreactive perikarya in the preoptic nucleus. In contrast, we have found, in female red salmon, galanin-immunoreactive neurons in the magnocellular preoptic nucleus, located far lateral to the preoptic recess, whereas many immunoreactive fibers are present in the preoptic area in both genders. In addition, many immunoreactive neurons have been seen in the nucleus preopticus periventricularis and nucleus lateralis tuberis, also in both sexes. These findings support the notion that galanin may play a gender-specific role in red salmon.     

Matters of scale: positive allometry and the evolution of male dimorphisms

The developmental independence of alternative phenotypes is key to evolutionary theories of phenotypic plasticity and the origins of diversity. Male dimorphisms associated with alternative reproductive tactics are widely cited examples of such facultative expression of divergent fitness optima. Current models for the evolution of male dimorphisms invoke a size-dependent threshold at which the phenotype is reprogrammed. We use predictions derived from allometric modeling to test for the existence of reprogramming thresholds in two species of beetle, Onthophagus taurus and Onthophagus binodis, and the European earwig Forficula auricularia. We also compare the allometry of a number of morphological traits to determine whether minor males suppress their secondary sexual traits. The intercept of the horn allometry was suppressed, but there was no evidence of reprogramming of horn growth in either beetle species. There was reprogramming in the earwig. In the beetles, the horn length in all males can be explained largely in terms of exponential horn growth following an extraordinarily steep power function. The asymptote in O. taurus can be explained by exponential growth meeting the constraint of resource exhaustion. These findings question the currently held view that beetle horn dimorphisms showcase the importance of developmental independence in the evolution of diversity.