Patterns Of Size Dimorphism Research Articles

Breaking the cliché: sex reversal in size dimorphism and mobility in South American Allocosinae (Lycosidae) spiders

Abstract Typically, females and males are expected to have characteristic sexual strategies and patterns of size dimorphism, but these generalizations are subject to exceptions. The occurrence of atypical cases has been related to species or populations from environments under strong physical, ecological and/or social constraints. Allocosa marindia and Allocosa senex are two coastal spiders (Lycosidae: Allocosinae) with reversal in sex roles and sexual size dimorphism. Males are larger than females, and females are the mobile sex that initiates courtship. It is unclear whether the occurrence of non-typical sexual traits in Allocosinae spiders is correlated with coastal habitats. Our aim was to study sexual size dimorphism and surface mobility in Allocosinae spiders from different habitats throughout South America. We revised specimens from scientific collections and performed 3-day samplings to collect individuals and determine nocturnal surface mobility. We analysed a total of 1071 Allocosinae adult individuals from 18 species and/or morphotypes. Our results revealed new species inhabiting coastal habitats with reversal in sexual size dimorphism and higher nocturnal surface activity in females; however, not all coastal species shared those characteristics. Future studies will focus on studying other ecological, physiological and/or phylogenetic factors that could be shaping the origin and maintenance of sex role reversal in Allocosinae.

Do Large Females Lay Small Eggs? Sexual Dimorphism and the Allometry of Egg and Clutch Volume

This paper describes the use of comprehensive models of the relations between egg volume and body weight and clutch volume and body weight to provide insight into hypotheses relating to the evolution and adaptive significance of sexual dimorphism in body size. Mean egg dimensions and adult body weights were obtained for 326 species, mainly of raptors (including owls), shorebirds, frogmouths (including nightjars and allies) and storks (including the New World vultures); groups in which there is a range of dimorphism, in both direction and degree. In addition to female body weight, sexual dimorphism (D) in body size, phylogenetic relationship, and clutch size all added significantly to the overall model of egg volume, which was curvilinear in form and accounted for 98.3% of the variation. Approximately, egg volume was proportional to D026. For clutch volume our model accounted for 90% of the variation. After controlling for body weight, dimorphism accounted for a significant proportion of the residual variation and clutch volume was proportional to D-)22 approximately. Amongst species exhibiting reversed sexual dimorphism (RSD: female the larger sex) in body size, the more dimorphic species had relatively small eggs and a small clutch volume for the weight of the female. Conversely, NRSD (nonreversed dimorphism; male larger) species had relatively large egg and clutch volume. The geometric mean weight of the sexes, rather than female weight or male weight and dimorphism, was the best descriptor of egg volume. We argue that these results are compatible with the hypothesis that patterns of size dimorphism in predatory birds are determined by sexual selection.

Factors Influencing Sexual Size Dimorphism in Temperate Waterstriders

Sexual size ratios (mean female length divided by mean male length) of 12 species of temperate waterstriders in the subfamily Gerrinae (Hemiptera, Gerridae) are analyzed to test both potentially nonadaptive and adaptive hypotheses for the evolution of sexual size dimorphism. Females tend to be larger than males, and regression of female length on male length (both log-transformed) reveals significant hypoallometry for size dimorphism. Both adaptive and nonadaptive explanations for the evolution of this allometry are discussed. Of these, only a nonadaptive model is consistent with the available data. Two closely related genera, Limnoporus and Gerris, are included in the data set, but covariance analysis reveals no significant effect of phylogeny on patterns of variation in size dimorphism. It is suggested, however, that the basic female bias may reflect descent from a common, highly dimorphic ancestor, rather than adaptation within this taxon. Four adaptive hypotheses are considered. Comparisons of the development times of males and females reveal that the female-biased size dimorphism is not associated with protandry, and the developmental-constraints hypothesis is therefore rejected. The hypothesis that selection for migration by flight tends to reduce sexual size dimorphism is supported by intraspecific comparisons of long-winged and short-winged morphs. A positive correlation between mating duration and residuals from the body-size regression supports the hypothesis that loading constraints associated with prolonged pairing select for increased size dimorphism. The available data on mating systems and size-selective mating support the hypothesis that sexual selection for large males promotes reduced or even reversed size dimorphism in some species, whereas sexual selection for large females increases size dimorphism in other species. Thus, patterns of size dimorphism in this taxon may be influenced by at least three, potentially interacting, selective processes: selection for dispersal by flight, selection for reduced loading during prolonged pairing, and sexual selection. The relative influence of these factors in three groups of ecologically similar species is discussed. The general conclusions of this study are consistent with the results of recent comparative studies of size dimorphism in primates. In both taxa, sexual size dimorphism is most strongly influenced by potentially nonadaptive factors, and the residual variation reflects the action of several selective factors. The complexity of size-ratio evolution revealed by these studies suggests that patterns of sizeratio variation within and among taxa, or size ratios typical of single species, are unlikely to be satisfactorily explained by reference to single hypotheses or to selective hypotheses only.

Sexual selection on male body size and mating behaviour in American toads, Bufo americanus

Mating behaviour of B. americanus was observed from 1985 to 1987. The population contained 38–45 males and 11–26 females, depending on the year. The breeding season of this ‘explosive breeder’ usually encompassed less than 48 h. Male reproductive success varied from zero to two matings per season and zero to an estimated 15 126 zygotes per season. All females mated once per season and variation in their zygote production was estimated to be 4017–11 624 zygotes per season. Body length explained 76% of the variation in zygote production of females. However, male body length was only weakly correlated with mating success in two of three seasons, and with zygote production in one of three seasons. Male arm length was predicted to correlate with male mating success because longer arms should facilitate remaining clasped to females when challenged by rival males. However, mating males did not differ from non-mating males in arm length, and the relationship between arm length and body length was the same for the sexes. Various male behaviours were measured using focal-animal sampling but only call rate correlated with male mating success. The pattern of size dimorphism (females larger than males) is consistent with the observed sex-specific relationships between reproductive success and body size.