Rensch’s rule in insects: patterns among and within species

Rensch’s rule (so termed by Abouheif and Fairbairn 1997; Fairbairn 1997) describes a widespread pattern in the animal kingdom that male body size diverges faster than female body size over evolutionarily time among related species, such that male-biased sexual size dimorphism (henceforth dimorphism) increases and femalebiased dimorphism decreases with body size (Rensch 1959). When first describing the phenomenon, Rensch (1950) did not offer an explanation, and its causes remain largely unclear to this day. Although it is unlikely that one single mechanism is responsible across the broad range of taxa in which it is observed (Fairbairn 1997, 2005), it has been suggested that Rensch’s rule may be driven primarily by sexual selection for large male size in combination with a typically high genetic correlation in body size between the sexes (Fairbairn and Preziosi 1994; Fairbairn 1997). However, to date evidence for the general importance of sexual selection in producing Rensch’s rule is equivocal at best (Fairbairn and Preziosi 1994; Fairbairn 1997, 2005; Kraushaar and Blanckenhorn 2002; Tamate and Maekawa 2005; Young 2005). Almost 10 years ago, Abouheif and Fairbairn (1997) reviewed the evidence for Rensch’s rule in animals and found support for it in a majority of taxa. However, arthropods, and especially insects, were greatly underrepresented in their survey and often displayed variation in dimorphism inconsistent with Rensch’s rule (see also Chapter 7 in this volume). Since then a number of new data-sets on insects have become available, which prompted the update provided in this chapter. The original formulation of Rensch’s rule refers to a systematic pattern of variation in dimorphism among closely related species (Rensch 1950, 1959; Abouheif and Fairbairn 1997; Fairbairn 1997). Statistically, Rensch’s rule is manifested in allometric slopes greater than 1 when male size is regressed on female size (described further below; Fairbairn 1997). Consequently, Rensch’s rule can also be studied among populations within species. Primarily with the aim of investigating the putative mechanisms causing Rensch’s rule, a number of studies have explored intraspecific variation in dimorphism in the past, but obtained mixed results (Fairbairn and Preziosi 1994; Fairbairn 1997, 2005; Kraushaar and Blanckenhorn 2002; Gustafsson and Lindenfors 2004; Tamate and Maekawa 2005; Young 2005; see also Chapters 8 and 14). Again, new intraspecific data have recently become available in connection with studies of Bergmann clines (Blanckenhorn et al. 2006), allowing a more comprehensive evaluation of the equivalent of Rensch’s rule at this taxonomic level. Patterns analogous to Rensch’s rule can further be studied within populations of a given species, for example when animals are reared in the laboratory under different environmental conditions and several such treatments or genetic groupings (e.g. families) are compared. In such cases, body size of males and females can be affected differentially to produce allometries in dimorphism. Such phenotypic plasticity in dimorphism has recently been investigated in a comparative study of insects by Teder and Tammaru (2005), and in a more mechanistic framework by Fairbairn (2005; see also Chapter 14).