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Odonates provide excellent model organisms for testing functional explanations of sexual size dimorphism (SSD) because of their wide variety of habitats, morphology, development, feeding behaviour, and mating strategies. This chapter discusses three major functional hypotheses of SSD and uses data on 133 odonate species to describe their patterns of SSD. It shows that SSD centres around monomorphism in dragonflies, whereas SSD is mostly male-biased in damselflies. Interestingly, phylogenetic comparative analyses suggest that damselflies — but not dragonflies — exhibit allometry consistent with Rensch's rule. Sexual selection acts mainly on males, whereas fecundity selection appears to influence female body size. Further tests, however, are essential, in particular of fecundity selection and the differential niche-utilization.

Sexual selection often favours sexual size dimorphism (SSD) in body size and fighting structures, since large males with massive weaponry achieve high reproductive success. However, sexual selection may be opposed by natural selection. This chapter describes a test of this hypothesis based on comparisons of sexual dimorphism, mating systems (sexual selection), and environmental variables (natural selection) among subspecies of hartebeest — a group of African savannah antelopes. The potential for polygyny explains dimorphism in fighting structures across hartebeest subspecies although it does not predict dimorphism in body size, suggesting that sexual selection toward large dimorphism is opposed by natural selection for smaller size. In addition to sexual selection, SSD in hartebeest may be influenced by antipredator advantages of small and agile males, intra-sexual competition for food and/or mates among female hartebeest, and fecundity selection.

This chapter summarizes the results of studies examining the adaptive changes in introduced populations of House Sparrows, including the classic study of natural selection by Hermon Bumpus. Clinal changes in size and sexual size dimorphism are some of the principal findings of these studies, along with the suggestion of reduced genetic variability in introduced populations (the founder effect). Genetic studies discussed include numerous isozyme studies, and more recently, the identification and characterization of several House Sparrow genes, including a period gene.

This chapter investigates sexual dimorphism in the dioecious, flowering plant, Silene latifolia. Results of comparisons among populations, a half-sib breeding experiment, and artificial selection on flower size indicate strong genetic integration such that selection for sexual dimorphism in any one trait affects the phenotype and extent of sexual dimorphism of many other traits. A flower size/number trade-off and the fact that larger flowers produce more ovules but not more pollen lead to selection for more (and smaller) flowers in males, but larger (and fewer) flowers in females. Strong genetic correlations between flower number and other traits are in accord with the hypothesis that the production of large numbers of flowers leads to a cost of reproduction. These results highlight the utility of taking a multi-trait, quantitative-genetic approach to understanding why the sexes differ from each other.

Rensch's rule is a common pattern of allometry for sexual size dimorphism among animal species. This chapter evaluates Rensch's rule in insects, using three levels of analysis. When comparisons are made among species, Rensch's rule is not more common than that which would be expected by chance: it occurs in Diptera (flies) and Heteroptera (Gerridae; water striders), but not in other insect groups. Comparisons among populations within species also show little evidence of Rensch's rule, although when the populations were ordered by latitude, Rensch's rule was more common than that which would be expected by chance. Within populations, body size tends to be more phenotypically plastic in females than in males, resulting in allometry opposite to Rensch's rule. Data on scathophagid and sepsid flies show that patterns across the three levels of comparison do not correspond well. Thus, in insects, neither the allometric patterns nor their causative processes can be generalized among taxa or among levels of analysis.

This chapter describes the sexual dimorphism of primates in terms of body weight, canine tooth, vocal anatomy, and cutaneous glands and scent-marking displays. It also discusses sexual skin and other secondary sexual traits in adult males. Some specific examples of these traits are capes of hair in male geladas and hamadry as baboons, bony paranasal swellings in mandrills and drills, and the impressive cheek flanges of the adult male orangutan. The chapter then examines the secondary sexual characteristics of adult female and addresses the swelling of female sexual skins. Female sexual skin swellings appear to be the result of runaway sexual selection. Sexual selection and human sexual dimorphism are also explained.

Sexual selection is the advantage that certain individuals have over others of the same sex and species, solely in respect of reproduction. The biology of anoles opens the door to the existence of varied ways in which sexual selection may occur, such as sperm competition and cryptic female choice. This chapter explores the behavior and reproductive biology of anoles. It examines social behavior with regard to sexual selection by considering what an anole does with its time: how much of their day do anoles allocate to different activities, and how do such allocations differ between the sexes and across the seasons? The chapter assesses the importance of sexual selection and the forms it takes, both within anole populations and potentially over evolutionary time spans, and examines variation in sexual dimorphism in both size and shape in anoles.

This chapter uses data for 489 spider species from fifteen families to describe patterns of variation in sexual size dimorphism (SSD), and to evaluate hypotheses explaining these patterns. The direction and magnitude of SSD is found to depend strongly on the size measure chosen, and the use of carapace width is recommended because it is less affected by condition than body mass or length. Comparative analyses reveal that spiders do not exhibit allometry consistent with Rensch's rule. Instead, females appear to have diverged more than males over evolutionary time, and male and female body size show uncorrelated co-evolution, which is unusual for animals. Only two adaptive hypotheses — fecundity selection favouring large size in females and gravity selection favouring small size in males — have general explanatory power for patterns of SSD in spiders. However, processes may differ among species and comprehensive studies of selection within given species are needed.

This chapter presents data that is in line with the notion that averageness, sexual dimorphism, and symmetry may all advertise qualities in human faces and are, hence, found attractive. Individual differences in preferences for some traits will prove adaptive and so can be consistent with evolutionary theory. The chapter also documents several potentially adaptive individual differences in human face preferences. For humans, as with other species, there is no optimal strategy for mate-choice and parenting that applies to all individuals. Indeed the range of personal circumstances(physical, environmental, social)will guarantee that what is a good or adequate strategy, and, therefore, what is attractive, will depend on the individual. In this way facial beauty can be said to be both in the face of the beheld and in the eye of beholder.

Differences in the duration development between males and females is one of the major proximate mechanisms mediating sexual size dimorphism. This chapter reviews evidence for such differences in insects. Using the concept of developmental rate isomorphy, the slopes of male and female rates of development on temperature are compared for 122 insect species from eleven orders. On average, males develop significantly faster than females but there is large variation within insect orders, suggesting little phylogenetic inertia. The faster male relative to female development is more pronounced in heterometabolous insects (with no pupal stage) than in holometabolous insects, perhaps related to pre-imaginal development of male gonads being more costly than that of female gonads in the latter group. In contrast, the pattern was not affected by other life history traits such as a parasitoid life history or the existence of quiescent stages in insects lacking the true pupal stage.

Females and males share the same genome, which places a significant constraint on the evolution of sex differences. This chapter begins with a review of current theory explaining the initial evolution of anisogamy and subsequent differentiation of the sexes. It then describes four mechanisms that relieve constraints on sexual differentiation: (i) genetic differences between the sexes; (ii) sex-limited or differential expression of autosomal loci; (iii) trans-generational epigenetic effects; and (iv) phenotypic plasticity for sexual traits (i.e., environmental influences on sexual development). All four mechanisms have evolved convergently in different evolutionary lineages. The chapter closes by advocating research programmes that integrate evolutionary and mechanistic approaches to discover how sex-specific selection interacts with genetic (and physiological) variation to produce sexual dimorphism.

Qualitative sexual dimorphisms in cognition and behavior may afford each sex with the requisite tools that provide an edge for their mutual survival and for the survival of their progeny. This chapter reviews cognitive function in rodents within this context of qualitative and quantitative sex differences in performance. It examines whether these sex differences impact day-to-day functioning, contribute to the successful evolution of a species, and can be accounted for by mechanisms underlying cognitive function.

This chapter examines the general trends in health that accompanied increasing reliance on agriculture and a later increase in social complexity experienced by human communities of the Yellow and Wei River Valleys of north-central China, comparing them to those documented for other parts of East Asia in earlier studies. While trajectories of human health differed considerably in specific environmental settings, we identify a few commonalities. Sexual dimorphism in stature tended to increase following the development of social complexity. Local reliance on different staple cereals, in conjunction with varied cooking and food processing techniques, apparently resulted in differing distributions of carious lesions on dental crowns. Cooked millet is apparently quite cariogenic, but its adverse effects on dental health can be ameliorated, provided that other components of the diet are fairly abrasive. An expected increase in non-specific stress markers with the transition to agriculture in East Asia seems clearly evident only in the case of the Chinese Central Plains.

This chapter reviews aspects of neuroimmune interactions shown to be sexually dimorphic. This includes interactions between immunogenic substances, cytokines, and neuroendocrine hormones; CNS abnormalities associated with immune activation; the relationship between stress, development and immunity; and behavioral changes associated with immune responding.

Scientific attempts to measure beauty in bodies have not been as extensive as studies of beauty in faces. However, some of the principles that make a face beautiful, such as symmetry, averageness, and sexual dimorphism, also apply to bodies.

The frameworks of modern economics demonstrate how stature and sexual dimorphism can be used to model inequality among human populations. The authors synthesize a wide range of data from archaeological and historic contexts to characterize stature variation during the Neolithic and the Industrial revolutions. First, they find that the shift from foraging to farming widely introduced inequalities significant enough to affect the distribution of health and stature and was fundamentally linked to the invention of coercive sociopolitical mechanisms. Second, a rise in sexual dimorphism that accompanies intensive agriculture and may often reflect both a society’s more efficient allocation of nutrition and a drop in female bargaining power related to increased sexual division of labor and gendered inequalities. Third, political structures deeply shape nutritional outcomes. As economists, they engage a literature and measures of inequality that are foreign to most archaeologists. Aside from the substance of their findings, this chapter represents a valuable cross-disciplinary contribution.

Oceanic epipelagic fishes live most or all of their lives in blue water environments, far from continental coasts. Many oceanic epipelagic species are of major commercial importance and provide a significant proportion of harvested fish biomass. However, the inaccessibility of their environment has made studies of their biology—and in particular their reproductive biology—extremely difficult. This chapter focuses on the reproduction and development of oceanic epipelagic fishes. It describes four orders of true oceanic epipelagic fishes: Lampriformes (Lampridae, opahs), Beloniformes, three suborders of the Perciformes (Xiphioidei, Percoidei, and Scombroidei); and Tetraodontiformes (Molidae, ocean sunfishes). The chapter examines selected species in terms of distribution; maximum size; all-tackle game fish record; longevity; sexual dimorphism; size at first maturity; spawning location, season, and temperature; migrations; breeding habits; fecundity; egg characteristics; and sources of larval illustrations, followed by comments on fishery importance and threat status of the species using the Red List categories of the International Union for the Conservation of Nature.

This chapter discusses the areas of human sexuality in relation to different primatological evidence. It describes the connection between sexual selection and human evolution that triggers the evolution of human copulatory patterns. Sexual selection and natural selection may have favoured sexual dimorphism in several physical traits in the early members of the genus Homo. Also, sperm competition commonly occurs in humans, affecting the human evolution significantly. The chapter examines the development of human sexual behaviour, including the issues of homosexuality and bisexuality among humans. The menstrual cycle and sexual behaviour of women are also given some explanations. The chapter concludes that new discoveries on the central actions of ovarian and adrenal hormones during sexual behaviour in non-human primates may advance the understanding of human behavioural endocrinology.

Sex-determination systems have profound consequences on the biology of organisms, affecting many aspects of their life histories. Certain sex-determination systems may affect the evolution of polyploidy and parthenogenesis. Sex allocation, the relative investment of resources to the male versus female function, bears direct link with sex determination; sex chromosomes play an important role in this context because they can constrain adaptive sex allocation, harbour meiotic drive elements, or protect against the invasion of cytoplasmic sex-ratio distorters. Sex chromosomes and patterns of heterogamety may also affect sexual selection: there is extensive theory about the evolution of sex linkage of sexually dimorphic traits, including sexually antagonistic genes, but more empirical evidence is required. Finally, sex-determination systems can interfere with reproductive isolation and speciation processes. Sex chromosomes largely contribute to the several ‘rules of speciation’ including Haldane’s rule (higher sensitivity of the heterogametic sex to hybridization), Darwin’s corollary (asymmetric hybrid sensitivity), and large X-effects (disproportionate effect of X chromosomes on hybrid sterility or unviability). These reciprocal interactions between sex-determination systems and processes of sexual selection and speciation are illustrated with specific examples.