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Proximate causes of Rensch's rule in arthropods: Does sexual size dimorphism imply a similar development time difference of the sexes?

Wolf U. Blanckenhorn, Anthony F. G. Dixon, Daphne J. Fairbairn, Matthias W. Foellmer, Patricia Gibert, Kim van der Linde, Rudolf Meier, Sören Nylin, Scott Pitnick, Christopher Schoff, Martino Signorelli, Tiit Teder & Christer Wiklund (2007).

American Naturalist, 169(2): 245-257. Online,

A prominent inter-specific pattern of sexual size dimorphism (SSD) is Rensch's rule (RR), according to which male body size is more variable or evolutionarily divergent than female body size. A reasonable working hypothesis is that males and females should have equal growth rates; SSD would then be mediated, and RR proximately caused, by sexual differences in development times, or sexual bimaturism (SBM), with the larger sex developing for 5 proportionately longer time. Using several analytical methods, we compared SSD and SBM among species for 7 arthropod groups. Our study shows that RR accurately describes the pattern of SSD in some arthropod lineages but not others. Furthermore, we found but a weak positive relationship between SSD and SBM overall, suggesting that growth rate differences between the sexes are more important than development time differences in proximately mediating SSD in a 10 wide but by no means comprehensive range of arthropod taxa. Except when protandry is of selective advantage (as in many butterflies, Hymenoptera and spiders), male development time was equal to (water striders and beetles) or even longer (two groups of flies: Drosophilidae and Sepsidae) than that of females. As all taxa show female-biased SSD, this implies faster growth of females in general, a pattern markedly different from primates and birds (analyzed here for 15 comparison). We discuss three potential explanations for this pattern based on life history tradeoffs and sexual selection.