Temporal patterns in prey size between sexes in a raptor with extreme size dimorphism: testing the intersexual competition hypothesis using web-sourced photographs

In most vertebrates, males are larger than females. For raptors, sexual size dimorphism is reversed, with females being larger. Reversed sexual dimorphism (RSD) in raptors is strongly linked to diet, with species feeding on the most agile prey, for example bird-eating raptors, showing the greatest size differences between the sexes. Hypotheses for reversed sexual dimorphism (RSD) include the ‘intersexual competition’ hypothesis, which proposes that RSD evolved to enable pairs to expand their dietary niche (taking a wider range of prey sizes) during the nestling period when both sexes occupy and hunt within the same territory, and thereby reduce competition between the sexes. If intersexual competition is responsible for the evolution of RSD, we predict that sex-related differences in prey size and dietary niche breadth will be particularly pronounced during the nestling period (cf. the non-nestling period). We explore this prediction in the highly dimorphic Eurasian Sparrowhawk (Accipiter nisus), which displays the largest degree of RSD of all raptors, using web-sourced photographs to identify diet throughout the entire year. We analysed 666 photographs of sparrowhawks on their prey over time. In contrast to our predictions, sex-specific prey sizes were most similar during the nestling period compared to any other time of the year. Both males and females reduced the size of their prey during the nestling period which may be a result of the ‘ingestion rate’ hypothesis, or a strategy employed to prevent hunting-related injuries during this critical period of the year.

Flexibility of nuptial colouration in a unique ecotype of Threespine Stickleback (Gasterosteus aculeatus)

Nuptial colouration in animals may serve as a signal of competitor and/or mate quality during breeding. In many temperate fishes, nuptial colouration develops during discrete breeding seasons and is a target of sexual selection. We examine nuptial colouration and behaviour of a unique ecotype of Threespine Stickleback (Gasterosteus aculeatus Linnaeus, 1758), wherein males turn from dull brown-grey to pearlescent white during the breeding season. The main goal of this work was to determine the relative role of white colouration in intersexual competition and mate choice. In a combination of a field and laboratory work, we found that males are brightest white when engaging in courtship activities in the presence of a female; this indicates that white colouration may be primarily related to enhancing signalling during mate attraction. White colouration intensity increased as the breeding season progressed and may be related to an influx of conspecifics. Colour change from cryptic grey to bright white occurred rapidly (< 90 seconds) and may be deployed to enhance behavioural signals. We conclude that bright white colouration in the white ecotype is a potential signal of mate quality and may have evolved from a previously existing capacity for colour plasticity in common Threespine Stickleback.

Intersexual and Intrasexual Competition and Their Relation to Jealousy

This chapter discusses sex differences in intersexual competition and describes particularly the consequences of such competition for conflict between the sexes, as well as for sex differences in mate guarding and, relatedly, in the types of infidelity that evoke jealousy, including online infidelity. It also discusses individual differences in jealousy as related to attachment styles and describes the effects of height, hormones, and the menstrual cycle on jealousy. Next, the chapter moves on to intrasexual competition and discusses, among other topics, intrasexual competition among men and among women, the role of sex differences in rival characteristics in evoking jealousy, the role of attachment styles and hormones, and individual differences in intrasexual competitiveness.

Intersexual niche divergence in northern map turtles (Graptemys geographica): the roles of diet and habitat

Sexual dimorphism in body size and in trophic morphology are common in animals and are often concordant with patterns of habitat use and diet. Proximate factors leading to intersexual differences in habitat use, however, are challenging to unravel because these differences may stem from sexual dimorphism or may be caused by intersexual competition. Intersexual differences in diet and habitat use are common in size dimorphic reptiles. In this study, we investigated factors contributing to intersexual differences in diet and habitat use in a population of northern map turtles ( Graptemys geographica (Le Sueur, 1817)) from Ontario, Canada. Using radiotelemetry, we showed that in a lake map turtles do not exhibit intersexual differences in habitat use, in contrast to river populations. Patterns of habitat use were also inconsistent with prey distribution. The lack of intersexual habitat use differences in our lake population, despite marked differences in prey distribution, also indicated that intersexual habitat use differences documented in river populations are a consequence of sexual dimorphism in swimming capacity. Using stable isotope analysis and fecal analysis, we found a large dietary overlap between males and females, indicating no intersexual competition for food. Patterns of prey selection in females, however, were concordant with the reproductive role hypothesis.

Disproportionate bill length dimorphism and niche differentiation in wintering western sandpipers (Calidris mauri)

Western sandpipers ( Calidris mauri (Cabanis, 1857)) exhibit slight female-biased sexual size dimorphism (5%) but disproportionate bill length dimorphism (15.9%). We test two predictions of the niche differentiation hypothesis at two wintering sites in Mexico with uniform western sandpiper densities, and use sex ratio as an index of intersexual competition. First, to test whether bill length dimorphism is larger at sites where sex ratios are strongly male-biased, we develop a migrant-based null model to represent dimorphism (12%, based on the average of males and females) in the absence of competition. Relative to the null model, bill length dimorphism was significantly larger at the large site (Santa María: 13.4%) but not at the small site (Punta Banda: 12.7%). Second, we tested whether bill length dimorphism increases as sex ratio approaches 1:1. Although the sex-ratio difference between sites was only 5%, bill length dimorphism increased marginally in the predicted direction. Additional comparisons suggest a cline in bill length dimorphism that mirrors a latitudinal gradient in prey burial depth. While sexual size dimorphism in the western sandpiper likely derived from selection for different body size optima, intersexual competition for food on the wintering grounds appears to have promoted further divergence in bill length.