A test of Rensch’s rule in varanid lizards PETRA FRÝDLOVÁ and DANIEL FRYNTA* Department of Zoology, Faculty of Science, Charles University in Prague, Vinic ˇná 7, CZ-12844 Praha 2, Czech Republic Received 21 October 2009; revised 27 December 2009; accepted for publication 27 December 2009 In a model group of giant reptiles, we explored the allometric relationships between male and female body size and compared the effects of sexual and fecundity selection, as well as some proximate causes, on macroevolutionary patterns of sexual size dimorphism (SSD). Monitor lizards are a morphologically homogeneous group that has been affected by extreme changes in body size during their evolutionary history, resulting in 14-fold differences among the body sizes of recent species. Here, we analysed data concerning the maximum and/or mean male and female snout–vent lengths in 42 species of monitor lizard from literary sources and supplemented these data with measurements made in zoos. There was a wide scale of SSD from nearly monomorphic species belonging mostly to the subgenus Odatria and Prasinus group of the Euprepriosaurus to apparently male-larger taxa. The variable best explaining SSD was the body size itself; the larger the species, the higher the SSD. This pattern agrees with the currently discussed Rensch’s rule, claiming that the relationship between male and female body size is hyperallometric, i.e. the allometric exponent of this relationship exceeds unity and thus SSD increases with body size in the case of male-larger taxa. All our estimates of the reduced major axis regression slopes of this relationship ranged from 1.132 to 1.155. These estimates are significantly higher than unity, and thus unequivo- cally corroborate the validity of Rensch’s rule in this reptilian group. In spite of our expectation that the variation in SSD can be alternatively explained by variables reflecting the strength of sexual selection (presence of male combat), fecundity selection (e.g. clutch size and mass) and/or proximate ecological factors (habitat type), none of these variables had consistent effects on SSD, especially when the data were adjusted to phylogenetic dependence and/or body size. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 293–306. ADDITIONAL KEYWORDS: allometry – body size – evolution of SSD – fecundity selection – monitor lizards – sexual selection – Varanidae. INTRODUCTION In many animal taxa, conspecific males and females differ in their body size. This phenomenon, referred to as sexual size dimorphism (SSD), has attracted con- siderable research effort since Darwin (1871). Recently, special attention has been paid to the rela- tionship between SSD and body size (e.g. Weckerly, 1998; Colwell, 2000; Smith & Cheverud, 2002; Lin- denfors, Gittleman & Jones, 2007; Székely, Lislevand & Figuerola, 2007). It has been demonstrated repeat- edly in various groups of animals that the relation- ship between male and female body size is not usually isometric. The slope of this allometric relationship usually exceeds unity, which means that larger species tend to exhibit higher ratios of male to female body size than do smaller species. This enigmatic empirical relationship is consistent with the hypoth- esis that the evolution of the female body size is more constrained than that of the male body size (Fair- bairn, 1997), and it is currently referred to as Ren- sch’s rule (Abouheif & Fairbairn, 1997). Several alternative mechanisms responsible for allometric patterns of SSD following Rensch’s rule have been hypothesized so far (cf. Fairbairn, 1997, 2007). For example, the sexual selection hypothesis suggests that Rensch’s rule is driven by a correlated evolution- ary change in female body size to directional sexual selection on increased body size in males. Currently, *Corresponding author. E-mail: frynta@centrum.cz; petra.frydlova@seznam.cz Biological Journal of the Linnean Society, 2010, 100, 293–306. With 3 figures © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 293–306 293