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Sexual Selection and Sex Ratios in Anolis Lizards Pavitra Muralidhar

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Sexual Selection and Sex Ratios in Anolis Lizards Pavitra Muralidhar CORE Metadata, citation and similar papers at core.ac.uk Provided by Trinity University Trinity University Digital Commons @ Trinity Biology Faculty Research Biology Department 7-2017 Sexual Selection and Sex Ratios in Anolis Lizards Pavitra Muralidhar Michele A. Johnson Trinity University, [email protected] Follow this and additional works at: https://digitalcommons.trinity.edu/bio_faculty Part of the Biology Commons Repository Citation Muralidhar, P., & Johnson, M. A. (2017). Sexual selection and sex ratios in Anolis lizards. Journal of Zoology, 302, 178-183. doi:10.1111/jzo.12446 This Article is brought to you for free and open access by the Biology Department at Digital Commons @ Trinity. It has been accepted for inclusion in Biology Faculty Research by an authorized administrator of Digital Commons @ Trinity. For more information, please contact [email protected]. Journal of Zoology. Print ISSN 0952-8369 Sexual selection and sex ratios in Anolis lizards P. Muralidhar1 & M. A. Johnson2 1 Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA 2 Department of Biology, Trinity University, San Antonio, TX, USA Keywords Abstract reptiles; adaptive radiation; sexual size dimorphism; sex ratio; sexual selection; Anolis Biased sex ratios, or unequal numbers of males and females in a population, can lizards; phylogenetic comparison. alter the intensity of sexual selection by enhancing competition for mates, and thus may affect the evolution of reproductive strategies. Studies of sex ratio variation Correspondence across a clade provide an opportunity to examine the morphological or behavioral Pavitra Muralidhar, Museum of Comparative consequences of different levels of sexual selection. We examined sex ratio varia- Zoology, Laboratory Building 214, 26 Oxford tion, using phylogenetic comparative methods, across a diverse clade of terrestrial Street, Cambridge, MA 02138, USA. Tel: (+1) vertebrates, the Anolis lizards. Across a sample of 14 species in 21 localities, we 405 778 4591; Fax: (+1) 617 495 5667 found remarkable bidirectional variation in sex ratios across the Anolis clade: males Email: [email protected] are more common in some populations and females in others. However, we find no evidence that sex ratio bias is associated with sexual size dimorphism, a proxy Editor: Mark-Oliver Rodel€ for sexual selection. Nor do we find an association of sex ratio bias with ecological niches (i.e. ecomorphs), which vary in sexual selection pressures and mating sys- Received 1 July 2016; revised 28 October 2016; tems. The observed inter-specific variation in sex ratio suggests that a balance of accepted 10 January 2017 different, possibly opposing, factors, including sex-specific dispersal and mortality rates, may play a role in determining population sex ratio. doi:10.1111/jzo.12446 € Introduction amphibians (Prohl, 2002; Jones, Arguello & Arnold, 2004), fish (Grant, Bryant & Soos, 1995; Mills & Reynolds, 2003), The population sex ratio reflects both the distribution and birds (Pruett-Jones & Lewis, 1990) and mammals (Mitani, availability of potential mates, and thus affects sexual selection Gros-Louis & Richards, 1996). Studying the causes and conse- within a species (Emlen & Oring, 1977; Kvarnemo & Ahnesjo,€ quences of biased sex ratios may therefore uncover a frame- 1996). Skew in adult sex ratio often corresponds with differ- work for the evolution of mating systems, and by extension, ences in the variation in average reproductive success within reflect the strength of sexual selection in different species each sex (Emlen & Oring, 1977; Donald, 2007). If the sexes (Emlen & Oring, 1977; Pipoly et al., 2015). Specifically, if differ in their variance in reproductive success, divergent mat- one sex is limited in a population, the other, more common, ing strategies for each sex are favored; this phenomenon is sex is expected to compete more for mating opportunities and exemplified by polygynous mating systems, in which females thus be under stronger sexual selection (Emlen & Oring, 1977; discriminate between possible mates, while males compete for Kvarnemo & Ahnesjo,€ 1996; Jones et al., 2004; Kokko & Jen- access to mating opportunities (Trivers, 1972; Emlen & Oring, nions, 2008). Despite the important role that sex ratio may 1977; Kokko & Jennions, 2008). play in mating systems and patterns of sexual selection, the Variation in adult sex ratio can be observed across geo- consequences of variation in sex ratio among diverse members graphically distinct populations within a single species, while of a clade remain unclear (Pipoly et al., 2015). the sex ratio of a single population can also change across Here, we report variation in sex ratios across and within multiple generations. If such changes in sex ratio are persistent species using a diverse clade of reptiles: lizards in the genus across multiple generations in their bias toward a single sex, Anolis. Anoles are well-known for their ecological niche diver- they can lead to alterations in patterns of sexual selection sification, and species vary in territoriality, sexual dimorphism within a population, modifying the intensity of both inter-sex- and mating systems (reviewed in Losos, 2009). Greater Antil- ual and intra-sexual selection. This phenomenon of increased lean anole species are grouped into six main ecological niches, sexual selection due to sex ratio bias has been observed across or ecomorphs (trunk-crown, trunk-ground, trunk, grass-bush, multiple taxa in both natural populations and experimental twig and crown-giant), that have independently evolved multi- manipulations: within-sex competition and mate selection ple times on different islands (Williams, 1972; Losos et al., strategies respond to changes in sex ratio skew in plants (San- 1998). These ecomorphs are also characterized by different derson et al., 2016), insects (Linklater et al., 2007), degrees of sexual size dimorphism (Butler, Schoener & Losos, Journal of Zoology (2017) – ª 2017 The Zoological Society of London 1 Sex ratios in Anolis lizards P. Muralidhar and M. A. Johnson 2000; Butler, Sawyer & Losos, 2007), a measure that often level of sexual dimorphism and (3) to examine the level of sex corresponds to the degree of sexual selection in a lineage ratio variation within a species. (Andersson, 1994; Mitani et al., 1996). We would therefore expect that a population will exhibit a bias in sexual size Materials and methods dimorphism toward the more common sex, as a result of increased sexual selection pressure. In addition, as both sexual We chose four sites on Jamaica, Puerto Rico, Dominican size dimorphism and mating systems diverge between eco- Republic and South Bimini, Bahamas, with each study site morphs, we expect an association between biased sex ratios containing multiple anole species. We also chose a site in and ecomorph class. To date, this hypothesis has not been southeast Texas to sample multiple localities of the North examined, and few comparative data are available on anole sex American mainland anole, Anolis carolinensis, as a comparison ratios. Understanding these correlations between sex ratio and to the island populations. All anole species have male sexual selection provides additional information on whether heterogametic sex determination and so their sex ratios at con- biased sex ratios, and the opportunity they provide for sexual ception will not be directly affected by different environments selection, can influence the evolution and divergence of anole (Gamble et al., 2014). At each site, we set up two or three species. 500–1000 m2 plots and captured every individual found within We examined sex ratios of 14 species of Anolis lizards in each plot. In cases where we set-up paired plots to determine 21 localities. These species occupy four different islands and variation in sex ratios within a species (Dominican Republic four ecomorph types in the Caribbean, with one species in and Jamaica), we chose two localities that were similar in spe- mainland North America. For six of these species, we also cies richness, population density, habitat availability and levels examined the sex ratio in multiple localities to determine the of direct sunlight to minimize environmental variation between extent of intra-specific variation in sex ratio. Our aims were the plots. In this way, we determined the level of sex ratio (1) to quantify sex ratios in species across the Anolis clade variation between localities without confounding habitat differ- and determine if these ratios varied across species, (2) to test ences. At our mainland site, we sampled three localities from a the hypothesis that sex ratios converge by ecomorph class or single forested area that differed in microhabitat type. Addi- tional details on the study plots are available in Johnson, Rev- ell & Losos (2010) and Dill et al. (2013). Every individual within a plot was captured and permanently 0.7 tagged with beads sewn into the tail (Fisher & Muth, 1989) or temporarily marked with bee marking tags (Johnson, 2005). We recorded snout–vent length (SVL) and sex for each indi- vidual, and released it at its site of capture for subsequent behavioral observations over the next several weeks. The sex 0.6 of each individual was determined by a combination of evert- ing the hemipenes of males, palpating large eggs in the abdo- mens of females and visual inspections of dewlap size in species with sexually dimorphic dewlaps. We tagged any new 0.5 anoles entering the plots during the observation period
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