A Comparative Test of Adaptive Hypotheses for Sexual Size Dimorphism in Lizards

Abstract

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.

Natural history of Micrablepharus maximiliani (Squamata: Gymnophthalmidae) in a Cerrado region of northeastern Brazil

“…Our data suggest that females and males of M. maximiliani may also differ in the proportional lengths of trunk, hind limbs and head. The presence of longer trunks in females may be associated with a fertility advantage conferred by a larger space in the peritoneal cavity for egg development (OLSSON et al 2002, COX et al 2003. A larger trunk length in females is recurrent in many species of Gymnophthalmidae (VITT 1982, VITT & ÁVILA-PIRES 1998, BALESTRIN et al 2010.…”

Fecundity selection theory: concepts and evidence

“…The idea that fecundity selection targets female body size has implicitly prevailed in the literature given the overwhelming volume of empirical evidence revealing a positive relationship between female size and fecundity (Shine, 1988;Reiss, 1989;Stearns, 1992;Roff, 2002;Blanckenhorn, 2005;Fairbairn et al, 2007), especially amongst ectotherms, e.g. insects (Honek, 1993;Preziosi et al, 1996), fish (Wootton, 1979;Morita & Takashima, 1998;Foster & Vincent, 2004;Wilson, 2009), amphibians (Shine, 1979;Kupfer, 2007Kupfer, , 2009, and reptiles (Cox et al, 2003;Cox, Butler & John-Alder, 2007;Stephens & Wiens, 2009;Pincheira-Donoso & Tregenza, 2011;Meiri, Brown & Sibly, 2012). This relationship, however, is less robust in birds and mammals (Boyce, 1988;Shine, 1988;Purvis & Harvey, 1995;Lindenfors, Gittleman & Jones, 2007;Szekely, Lislevand & Figuerola, 2007).…”

“…Despite the above distinction between the three main mechanisms of selection as the engine of adaptation, it is essential to stress that evolution is the multivariate response of whole organisms' components to these forms of selection operating in coordination (Fisher, 1930;Williams, 1966). The hypothesis of fecundity selection was originally formulated by Darwin (1874) to explain the evolution of large body size in females, and in particular, the widespread evolution of female-biased sexual size dimorphisms (SSDs) in which females are larger than males (Shine, 1988;Cox, Skelly & John-Alder, 2003). The mechanistic basis of fecundity selection is that larger female size provides a greater body space to accommodate more offspring (Williams, 1966), and additionally, a higher capacity for energy storage to be subsequently invested into reproduction (Calder, 1984).…”

“…Olsson et al, 2002;Parker et al, 2011;Winkler, Stolting & Wilson, 2012). In addition, it has been suggested that although fecundity selection can explain female-biased SSD, the expression and magnitude of this intersexual size asymmetry does not necessarily reflect the strength of fecundity selection (Zamudio, 1998;Olsson et al, 2002;Cox et al, 2003;Pincheira-Donoso & Tregenza, 2011;Soulsbury, Kervinen & Lebigre, 2014). Therefore, female-biased SSD can evolve in the absence of fecundity selection (e.g.…”

Reflectance of sexually dichromatic UV-blue patches varies during the breeding season and between two subspecies ofGallotia galloti(Squamata: Lacertidae)

“…That their habitats differ markedly suggests that ecological factors may have driven the morphological divergence between them (see below). Differential growth , mortality rates, and food-resource use also affect body size differences between sexes and among populations (Cox et al, 2003Kaliontzopoulou, Carretero & Llorente, 2010). Male and female growth trajectories are different in the larger species Gallotia simonyi (Rodríguez-Domínguez et al, 1998) and this is also probably the case in G. galloti (Castanet & Báez, 1988).…”