Patterns of sexual size dimorphism in stingless bees: Testing Rensch’s rule and potential causes in highly eusocial bees (Hymenoptera: Apidae, Meliponini)

Abstract

Eusocial insects offer a unique opportunity to analyze the evolution of body size differences between sexes in relation to social environment. The workers, being sterile females, are not subject to selection for reproductive function providing a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other kinds of natural selection. Patterns of sexual size dimorphism (SSD) and testing of Rensch's rule controlling for phylogenetic effects were analyzed in the Meliponini or stingless bees. Theory predicts that queens may exhibit higher selection for fecundity in eusocial taxa, but contrary to this, we found mixed patterns of SSD in Meliponini. Non‐ Melipona species generally have a female‐biased SSD, while all analyzed species of Melipona showed a male‐biased SSD, indicating that the direction and magnitude of the selective pressures do not operate in the same way for all members of this taxon. The phylogenetic regressions revealed that the rate of divergence has not differed between the two castes of females and the males, that is, stingless bees do not seem to follow Rensch's rule (a slope >1), adding this highly eusocial taxon to the various solitary insect taxa not conforming with it. Noteworthy, when Melipona was removed from the analysis, the phylogenetic regressions for the thorax width of males on queens had a slope significantly smaller than 1, suggesting that the evolutionary divergence has been larger in queens than males, and could be explained by stronger selection on female fecundity only in non‐ Melipona species. Our results in the stingless bees question the classical explanation of female‐biased SSD via fecundity and provide a first evidence of a more complex determination of SSD in highly eusocial species. We suggest that in highly eusocial taxa, additional selection mechanisms, possibly related to individual and colonial interests, could influence the evolution of environmentally determined traits such as body size.

Body size affects specialization and modularity in the global resin foraging meta-network of stingless bees

“…Because biological data from different species might be statistically non-independent due to a hierarchically structured phylogeny (Felsenstein 1985), we included the most comprehensive phylogeny of Meliponini in our statistical analyses (Quezada-Euán et al 2019). The original tree includes 258 tips with representatives of all current genera.…”

Altitudinal variation of sexual size dimorphism in ground beetle Carabus odoratus Shill.

“…Though originally formulated to explain interspecific variation in sexual size dimorphism, a few studies have addressed the Rensch's rule by examining intraspecific, among-population variation in dimorphism, thereby testing the basic assumption that macroevolutionary patterns should be grounded in equivalent microevolutionary mechanisms. The results suggest, however, that Rensch's rule may or may not hold among populations within species [32,33], depending on the taxon studied. The main idea of this paper was to analyze the data in relation to sexual size dimorphism (SSD).…”