Body size and elevation: do Bergmann's and Rensch's rule apply in the polytypic bushcricket Poecilimon veluchianus?

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Sexual selection explains Rensch's rule of allometry for sexual size dimorphism

In 1950, Rensch first described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespread and fundamental allometric relationship is now commonly referred to as 'Rensch's rule'. However, despite numerous recent studies, we still do not have a general explanation for this allometry. Here we report that patterns of allometry in over 5300 bird species demonstrate that Rensch's rule is driven by a correlated evolutionary change in females to directional sexual selection on males. First, in detailed multivariate analysis, the strength of sexual selection was, by far, the strongest predictor of allometry. This was found to be the case even after controlling for numerous potential confounding factors, such as overall size, degree of ornamentation, phylogenetic history and the range and degree of size dimorphism. Second, in groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch's rule. Taken together, these results provide the first clear solution to the long-standing evolutionary problem of allometry for sexual size dimorphism: sexual selection causes size dimorphism to correlate with species size.     

Body size evolution in South American Liolaemus lizards of the boulengeri clade: a contrasting reassessment

Bergmann's rule predicts larger body sizes in species living in higher latitudes and altitudes. This rule appears to be valid for endotherms, but its relevance to ectotherm vertebrates has largely been debated. In squamate reptiles (lizards and snakes), only one study, based on Liolaemus species of the boulengeri clade, has provided phylogenetic evidence in favour of Bergmann's clines. We reassessed this model in the same lizard clade, using a more representative measure of species body size and including a larger number of taxa in the sample. We found no evidence to support Bergmann's rule in this lineage. However, these non-significant results appear to be explained only by the inclusion of further species rather than by a different estimation of body size. Analyses conducted on the 16 species included in the previous study always revealed significant relationships between body size and latitude-altitude, whereas, the enlarged sample always rejected the pattern predicted by Bergmann's rule.     

Sexual selection, sexual size dimorphism and Rensch's rule in Odonata

Odonata (dragonflies and damselflies) exhibit a range of sexual size dimorphism (SSD) that includes species with male-biased (males > females) or female-biased SSD (males < females) and species exhibiting nonterritorial or territorial mating strategies. Here, we use phylogenetic comparative analyses to investigate the influence of sexual selection on SSD in both suborders: dragonflies (Anisoptera) and damselflies (Zygoptera). First, we show that damselflies have male-biased SSD, and exhibit an allometric relationship between body size and SSD, that is consistent with Rensch's rule. Second, SSD of dragonflies is not different from unit, and this suborder does not exhibit Rensch's rule. Third, we test the influence of sexual selection on SSD using proxy variables of territorial mating strategy and male agility. Using generalized least squares to account for phylogenetic relationships between species, we show that male-biased SSD increases with territoriality in damselflies, but not in dragonflies. Finally, we show that nonagile territorial odonates exhibit male-biased SSD, whereas male agility is not related to SSD in nonterritorial odonates. These results suggest that sexual selection acting on male sizes influences SSD in Odonata. Taken together, our results, along with avian studies (bustards and shorebirds), suggest that male agility influences SSD, although this influence is modulated by territorial mating strategy and thus the likely advantage of being large. Other evolutionary processes, such as fecundity selection and viability selection, however, need further investigation.     

Long-term paternity skew and the opportunity for selection in a mammal with reversed sexual size dimorphism

Most mammalian groups are characterized by male-biased sexual size dimorphism, in which size-dependent male-male competition and reproductive skew are tightly linked. By comparison, little is known about the opportunity for sexual selection in mammalian systems without male-biased dimorphism, where the traits under sexual selection might be less obvious. We examined 10 years of parentage data in a colony of greater horseshoe bats (Rhinolophus ferrumequinum) to determine the magnitude of male reproductive skew and the opportunity for sexual selection in a mammal in which females are the larger sex. Annual paternity success was weakly skewed but consistent patterns led to strong longitudinal paternity skew among breeders. Just three males accounted for a third of all paternity assignments, representing at least a fifth of all colony offspring born in a decade. Paternity success was in part determined by age but was not influenced by dispersal status. Our results show that paternity skew and the opportunity for sexual selection in a species with reversed sexual size dimorphism can approach levels reported for classical examples of species with polygyny and male-biased dimorphism, even where the traits under sexual selection are not known.     

The importance of phylogenetic scale in tests of Bergmann's and Rapoport's rules: lessons from a clade of South American lizards

We tested for the occurrence of Bergmann's rule, the pattern of increasing body size with latitude, and Rapoport's rule, the positive relationship between geographical range size and latitude, in 34 lineages of Liolaemus lizards that occupy arid regions of the Andean foothills. We tested the climatic-variability hypothesis (CVH) by examining the relationship between thermal tolerance breadth and distribution. Each of these analyses was performed varying the level of phylogenetic inclusiveness. Bergmann's rule and the CVH were supported, but Rapoport's rule was not. More variance in the data for Bergmann's rule and the CVH was explained using species belonging to the L. boulengeri series rather than all species, and inclusion of multiple outgroups tended to obscure these macroecological patterns. Evidence for Bergmann's rule and the predicted patterns from the CVH remained after application of phylogenetic comparative methods, indicating a greater role of ecological processes rather than phylogeny in shaping the current species distributions of these lizards.     

WHEN RENSCH MEETS BERGMANN: DOES SEXUAL SIZE DIMORPHISM CHANGE SYSTEMATICALLY WITH LATITUDE?

Bergmann's and Rensch's rules describe common large-scale patterns of body size variation, but their underlying causes remain elusive. Bergmann's rule states that organisms are larger at higher latitudes (or in colder climates). Rensch's rule states that male body size varies (or evolutionarily diverges) more than female body size among species, resulting in slopes greater than one when male size is regressed on female size. We use published studies of sex-specific latitudinal body size clines in vertebrates and invertebrates to investigate patterns equivalent to Rensch's rule among populations within species and to evaluate their possible relation to Bergmann's rule. Consistent with previous studies, we found a continuum of Bergmann (larger at higher latitudes: 58 species) and converse Bergmann body size clines (larger at lower latitudes: 40 species). Ignoring latitude, male size was more variable than female size in only 55 of 98 species, suggesting that intraspecific variation in sexual size dimorphism does not generally conform to Rensch's rule. In contrast, in a significant majority of species (66 of 98) male latitudinal body size clines were steeper than those of females. This pattern is consistent with a latitudinal version of Rensch's rule, and suggests that some factor that varies systematically with latitude is responsible for producing Rensch's rule among populations within species. Identifying the underlying mechanisms will require studies quantifying latitudinal variation in sex-specific natural and sexual selection on body size.