*Corresponding author. E-mail: rics@bio.usyd.edu.au Biological Journal of the Linnean Society, 2002, 77, 113–125. With 7 figures © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 113–125 113 Sex-specific niche partitioning and sexual size dimorphism in Australian pythons (Morelia spilota imbricata) D. PEARSON 1,2 , R. SHINE 2 * and R. HOW 3 1 Department of Conservation and Land Management, PO Box 51, Wanneroo, WA 6065, Australia 2 School of Biological Sciences A08, University of Sydney, NSW 2006, Australia 3 Western Australian Museum, Francis Street, Perth, WA 6000, Australia Received 2 January 2002; accepted for publication 20 March 2002 Sexual dimorphism is usually interpreted in terms of reproductive adaptations, but the degree of sex divergence also may be affected by sex-based niche partitioning. In gape-limited animals like snakes, the degree of sexual dimorphism in body size (SSD) or relative head size can determine the size spectrum of ingestible prey for each sex. Our studies of one mainland and four insular Western Australian populations of carpet pythons (Morelia spilota) reveal remarkable geographical variation in SSD, associated with differences in prey resources available to the snakes. In all five populations, females grew larger than males and had larger heads relative to body length. However, the populations differed in mean body sizes and relative head sizes, as well as in the degree of sexual dimorphism in these traits. Adult males and females also diverged strongly in dietary composition: males consumed small prey (lizards, mice and small birds), while females took larger mammals such as possums and wallabies. Geo- graphic differences in the availability of large mammalian prey were linked to differences in mean adult body sizes of females (the larger sex) and thus contributed to sex-based resource partitioning. For example, in one population adult male snakes ate mice and adult females ate wallabies; in another, birds and lizards were important prey types for both sexes. Thus, the high degree of geographical variation among python populations in sexually dimorphic aspects of body size and shape plausibly results from geographical variation in prey availability. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 77, 113–125. ADDITIONAL KEYWORDS: intraspecific variation – sexual size dimorphism – insular populations – snakes. INTRODUCTION In many species of animals, adult males and adult females differ considerably in body size and body shape (e.g. Darwin, 1871). In some cases, variations in the degree of sexual size dimorphism (SSD) occur even between different populations within a single species. Such cases of intraspecific variation in SSD offer pow- erful opportunities to identify the evolutionary forces affecting this trait (Harvey & Ralls, 1985; Andersson, 1994). Nonetheless, interpretation is difficult even in such apparently straightforward cases, because the degree of SSD within a population reflects the end result of a complex series of selective forces and direct (proximate) environmental pressures. For example, geographical variation in mating systems may gen- erate among-population differences in the intensity of sexual selection and/or fecundity selection on the body sizes of the two sexes (Andersson, 1994; Shine & Fitzgerald, 1995). In such cases, geographical vari- ation in SSD may reflect adaptive responses of mating ‘tactics’ in each sex to local conditions. Although reproductive correlates of SSD have attracted considerable scientific attention, another set of forces can also modify SSD. Even if selective forces related to reproduction strongly influence the direc- tion and degree of SSD, the local environment (and especially, the spectrum of available prey sizes) may influence the body sizes attained by organisms. In such a situation the degree of SSD may vary geo- graphically either (a) because local prey resources Downloaded from https://academic.oup.com/biolinnean/article/77/1/113/2639715 by guest on 17 June 2022