AbstractThis chapter reviews patterns of sexual dimorphism in amphibians and discusses their proximal causes and possible adaptive significance. Amphibians are diverse organisms that live in aquatic or terrestrial ecosystems. Female-biased sexual size dimorphism (SSD) is the common pattern in frogs and salamanders, and male-biased SSD is only present in few lineages. Preliminary SSD data for caecilians indicate that many are monomorphic in body size, while others exhibit female-biased dimorphism. The typical female-biased SSD may be partly explained by sex-specific growth trajectories and delayed maturity of females. Male-biased SSD is associated with sexual selection for large males through territoriality and male-male combat. In comparison with other vertebrates, our understanding of SSD in amphibians is still incomplete. Phylogenetic comparative analyses are needed to describe more fully the evolutionary patterns of amphibian SSD and to test hypotheses based on fecundity and sexual selection, life history theory, and ecological divergence.

The theory of sexual selection is the most widely accepted theory explaining the evolution of mating systems and secondary sexual characters. Polygyny is the most common mating system in mammals, and there is a strong correlation between the degree of polygyny and the degree of sexual size dimorphism skewed towards males. Sexual selection theory posits that polygyny in mammals has evolved through direct, precopulatory, intrasexual selection in males, and that sexual size dimorphism is a result of male competition for mates. New results that are being obtained with the use of molecular techniques and with comparative phylogenetic methods do not appear to support predictions from this classical model in full. In this article, an expansion of the classical model is presented that combines the effects of at least four forms of selection: natural, precopulatory intrasexual, postcopulatory intrasexual, and intersexual selection. This mixed model consists of an initial phase in which natural selection operates on body size, followed by a second phase dominated by sexual selection and involving increases in sexual dimorphism and coercive behaviour of males towards females. Sexual harassment induces female aggregation, thus creating social potential for polygyny. Males compete for access to the groups of females, following two possible evolutionary scenarios, directional or equilibrium sexual selection, both producing similar behavioural polygyny, but with differences in the intensity of intra‐male precopulatory sexual selection. Predictions of the mixed model are as follows: 1) polygyny can exist without high variance in male reproductive success (a fundamental requirement in the classical model); 2) extra‐group fertilisation can be common; 3) sexual size dimorphism evolved prior to polygyny; 4) sexual coercion is widespread; and 5) females reduce levels of sexual coercion by joining groups.

AbstractFemales 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.

Size differences between males and females are common across the tree of life (termed sexual size dimorphism; SSD), and have fundamental implications for ecology, life history and behaviour of both sexes. Conventionally, SSD is thought to evolve in response to sex-specific sexual selection but more recent work suggests that ecological processes can also promote sex-differences in size. Here, we provide a global test for the role of sexual selection in the evolution of sexual size dimorphism using data from 77 comparative studies spanning the major classes of the animal kingdom. We show that intense sexual selection typically correlates with male-biased SSD across species. Importantly, pre-copulatory but not post-copulatory sexual selection predicts SSD, suggesting a pervasive role of premating male-male competition and female choice to drive sex differences in body size. Collectively, our findings suggest that pre-copulatory sexual selection plays a major role in the evolution of male-biased SSD.

Many of these examples seem to be attributable to sexual selection, but others reflect adaptations for niche divergence between the sexes. For example, dwarf non-feeding males without functional mouthparts have evolved independently in many taxa. In other cases, males and females differ in trophic structures apparently because of differences in diets. Such divergence may often reflect specific nutritional requirements for reproduction in females, or extreme (sexually selected?) differences between males and females in habitats or body sizes. Ecological competition between the sexes may be responsible for intersexual niche divergence in some cases, but the independent evolution of foraging specializations by each sex may be of more general importance. If ecological causation for dimorphism can be demonstrated in so many cases, despite the inadequacies of the available criteria, the degree of sexual size dimorphism in many other animal species may well also have been influenced by ecological factors. Hence, it may be premature to dismiss this hypothesis, despite the difficulty of testing it. Published in The Quarterly review of biology (1989)

Sexual selection and canine dimorphism in New World monkeys. Social and ecological factors are important in shaping sexual dimorphism in Anthropoidea, but there is also a tendency for body-size dimorphism and canine dimorphism to increase with increased body size (Rensch's rule) (Rensch: Evolution Above the Species Level. London: Methuen, 1959.) Most ecologist interpret Rensch's rule to be a consequence of social and ecological selective factors that covary with body size, but recent claims have been advanced that dimorphism is principally a consequence of selection for increased body size alone. Here we assess the effects of body size, body-size dimorphism, and social structure on canine dimorphism among platyrrhine monkeys. Platyrrhine species examined are classified into four behavioral groups reflecting the intensity of intermale competition for access to females or to limiting resources. As canine dimorphism increases, so does the level of intermale competition. Those species with monogamous and polyandrous social structures have the lowest canine dimorphism, while those with dominance rank hierarchies of males have the most canine dimorphism.

In this study we investigated the developmental basis of adult phenotypes in a non-model organism, a polymorphic damselfly (Ischnura elegans) with three female colour morphs. This polymorphic species presents an ideal opportunity to study intraspecific variation in growth trajectories, morphological variation in size and shape during the course of ontogeny, and to relate these juvenile differences to the phenotypic differences of the discrete adult phenotypes; the two sexes and the three female morphs. We raised larvae of different families in individual enclosures in the laboratory, and trace