A test of Rensch's rule in varanid lizards

Abstract

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 unequivocally 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.

Fecundity Selection and the Evolution of Reproductive Output and Sex-Specific Body Size in the Liolaemus Lizard Adaptive Radiation

“…However, our data failed to support the derived prediction that fecundity selection is associated with female-biased SSD in this lineage, where male-biased SSD is the norm. These observations are consistent with previous studies involving lizard lineages from different areas of the world (Zamudio 1998;Frýdlová and Frynta 2010;see Shine 1994, for similar results in snakes), but contrast with some other findings (Fitch 1978(Fitch , 1981Braña 1996;Cox et al 2003; see Stephens and Wiens 2009, for similar results in turtles). Thus, our study contributes to a growing body of evidence suggesting that female-biased SSD cannot reliably be predicted by means of the effect of fecundity selection on female size inferred from the association between brood size and SSD.…”

“…Despite the significance of these hypotheses for our understanding of the evolution of fecundity and its implications for the evolution of female-biased sexual size dimorphism (SSD) among terrestrial vertebrates (where male-biased SSD predominates), and despite fecundity being a primary component of fitness (Losos 2009;Charlesworth and Charlesworth 2010), only a few studies, scattered across animal lineages, have explored these predictions (Fairbairn and Shine 1993;Shine 1994;Head 1995;Cox et al 2007;Stephens and Wiens 2009;Frýdlová and Frynta 2010). Indeed, although most research on these hypotheses comes from lizards, only six studies on these reptiles have appeared in three decades (Cox et al 2007), two of which (Fitch 1978(Fitch , 1981 were published when phylogenetic analyses were not incorporated into evolutionary research.…”

“…Based on a largescale approach involving 302 species from different lineages and areas of the world, this study showed a significant covariation between fecundity and female-biased SSD, but weak associations between fecundity and environmental constraints. The remaining three studies (Braña 1996, Zamudio 1998Frýdlová and Frynta 2010) focus on the interaction between female-biased SSD and reproductive output, resulting in contradictory findings. However, it seems surprising that the use of prominent adaptive radiations as model systems to investigate these predictions has largely been neglected.…”

A Test of Rensch’s Rule in Greater Horseshoe Bat (Rhinolophus ferrumequinum) with Female-Biased Sexual Size Dimorphism

“…In other words, Rensch’s rule predicts that male size varies more than female size, and male is purportedly the driver of size divergence whereas female size co-varies passively with that of males, thereby generating a pattern of allometry in SSD [5], [6]. Rensch’s rule has been confirmed by observations in various animal taxa including insects, fishes, reptiles, birds and mammals [3], [7]–[12]. However, Rensch’s rule does not apply universally; while it is well supported for taxa that exhibit male-biased SSD or mixed SSD ([2], [3], [8], [10], for the exception see [13]), patterns of allometry among taxa with female-biased size dimorphism are less clear and there is evidence both for [11], [14], [15] and against [4], [16] the rule.…”

Wandering drunks and general lawlessness in biology: does diversity and complexity tend to increase in evolutionary systems?

“…In the twentieth century, the use of the word ''law'' seemed to give way to the slightly less dogmatic ''rule'', such as Rensch's rule (that sexual size dimorphism increases with increasing body size in species where males are larger than females, but decreases with increasing body size in species where the female is the larger sex: e.g. Frydlova and Frynta 2010;Stuart-Fox 2009), Cope's rule (the tendency for the average body size of species to increase toward the poles: e.g. Hone et al 2005;Monroe and Bokma 2010;Novak-Gottshall and Lanier 2008) and the island rule (large animal species tend to evolve smaller body size on islands, but smaller animals get larger: e.g.…”