Introduction

Sexual size dimorphism (SSD) is a common pattern in nature and has been a major focus of research since the origins of evolutionary biology (Darwin, 1874). Numerous potential hypotheses for the ecological and evolutionary origins of SSD have been proposed (reviews in Hormiga et al., 2000; Cox et al., 2003; Stephens & Wiens, 2009), and a large body of research has clarified these mechanisms into three primary categories. These are (i) sexual selection on male size through mechanisms such as male–male combat (Cox et al., 2003), (ii) selection related to fecundity on females, driven by a relationship between reproductive capacity and size (Fairbairn & Shine, 1993), and (iii) ecological divergence in size mediated by intraspecific competition due to niche partitioning (Shine, 1989). Relatively few studies have considered all three explanations simultaneously, and those that have found mixed evidence for the various mechanisms (Hormiga et al., 2000; Cox et al., 2003; Stephens & Wiens, 2009).

To complicate matters further, SSD often scales allometrically and in different directions depending on which sex is larger (Rensch, 1960). Rensch's rule states that when males are larger (male‐biased SSD, MBSSD hereafter), SSD is expected to increase with increasing body size, resulting in a hyperallometric relationship of male body size to female body size. Correspondingly, in taxa with larger females (female‐biased SSD, FBSSD hereafter), SSD would be expected to decrease with increasing body size. This has been attributed to directional sexual selection for larger male body sizes, the ‘Fairbairn–Preziosi hypothesis’ (Fairbairn & Preziosi, 1994; Abouheif & Fairbairn, 1997; Dale et al., 2007; Stephens & Wiens, 2009). However, support for Rensch's rule and the Fairbairn–Preziosi hypothesis is not universal, and few studies have made links between the expected patterns of allometry and any mechanistic explanation (Kratochvíl & Frynta, 2002; Stephens & Wiens, 2009; Starostová et al., 2010).

All of these processes can be addressed most powerfully in a comparative phylogenetic context. Previous studies have used such frameworks for testing the three classes of mechanisms for promoting the evolution of SSD (Hormiga et al., 2000; Cox et al., 2003; Stephens & Wiens, 2009). However, these previous studies were limited in some ways by (i) not including a majority of species in the groups examined, (ii) not using a time‐calibrated phylogeny to place the evolutionary history of SSD in an explicit temporal context or (iii) lacking key information about the ecological correlates of intraspecific divergence.

We test here for the effects of sexual, fecundity and ecological selection on SSD, using size and ecological data and a well‐sampled phylogenetic tree for New World (NW) pitvipers (Crotalinae [part]). Intraspecific combat has been shown to select for larger males and to be significantly associated with degree of SSD in snakes (Shine, 1978, 1994). Whether sexual selection acts in this manner is established by quantifying the relationship between male body size and SSD. We then examine the effects of fecundity selection by testing for a relationship between female body size and SSD, as larger females typically exhibit higher fecundity (Shine, 1994). Finally, differential resource availability and partitioning between macrohabitats or along gradients in latitude and elevation might be expected to result in ecological character displacement for body size (Bolnick & Doebeli, 2003). This results in SSD when different physiological optima are selected for according to different constraints acting upon each sex. We therefore examine geographical, elevational and macrohabitat data to quantify the influence of ecological selection on SSD to address the third established hypothesis for the evolution of SSD, the presence of ecological divergence.

New World crotalines are distributed throughout temperate North America, tropical Central America and tropical South America, with major elevational gradients in species richness, and major macrohabitat differentiation between arboreal and terrestrial species, in addition to frequent male combat and massive differences in female fecundity related to body size (Campbell & Lamar, 2002). Maximum adult total length (TL) in this group varies widely, from 50 cm in Mixcoatlus barbouri (Jadin et al., 2011) to 360 cm in Lachesis stenophrys (Köhler, 2008). Given these attributes, NW crotalines present an excellent opportunity to study the correlates and drivers of SSD evolution. This study considers the effects of sexual, fecundity and ecological selection on the origins of SSD in the group. For the purposes of this study, SSD is measured as the difference in maximum adult size between conspecific males and females.

In snakes, SSD can reach extremes: in multiple groups of colubroids, males can be up to 30% larger than conspecific females, whereas at the other end of the spectrum, some female boids can exceed conspecific males by up to 50% (Cox et al., 2007). The NW crotalines exhibit the full spectrum of SSD, from FBSSD through monomorphism to MBSSD (Campbell & Lamar, 2002), which makes them a suitable candidate group with which to test the validity of Rensch's rule and the Fairbairn–Preziosi hypothesis. While the validity of Rensch's rule has been tested in snakes previously (Abouheif & Fairbairn, 1997), support was found to be mixed, no mechanistic hypotheses were tested, and crotalines were grouped with Old World viperines.

We assess allometric scaling of SSD in NW crotalines using phylogenetic regression to determine whether or not Rensch's rule is supported in the group. We use reconstruction of body size shifts along branches to assess support for the Fairbairn–Preziosi hypothesis. Directional selection for increasing male size may be expected to occur differentially between groups of lineages, for instance those that show opposing patterns of SSD or occupy different habitats, and therefore, examining the behaviour of body size in the context of the Fairbairn–Preziosi hypothesis may still provide key insights into the evolution of SSD in NW crotalines, even if Rensch's rule is not supported for the group overall.

Finally, we use recently developed algorithms linking continuous traits to diversification rates (FitzJohn, 2010) to determine whether SSD affects speciation. If sexual selection on male size or fecundity selection on female size drives SSD, we might expect a significant relationship between SSD and speciation, as high fecundity could be expected to influence speciation rate, or differential mate choice may lead to increased population segregation (West‐Eberhard, 1983), and thus higher speciation. If ecological selection in different habitats is responsible for SSD, we would expect little relationship between SSD and speciation, as divergence between sexes would not be directly linked to divergence between populations. Finally, SSD may represent a trade‐off between sex‐specific and population‐level selection pressures relating to resource partitioning, and thus, speciation rates may be reduced at extremes of SSD (Bolnick & Doebeli, 2003).