Only Half Right: Species with Female-Biased Sexual Size Dimorphism Consistently Break Rensch's Rule

Abstract

BackgroundMost animal species display Sexual Size Dimorphism (SSD): males and females consistently attain different sizes, most frequently with females being larger than males. However the selective mechanisms driving patterns of SSD remain controversial. ‘Rensch's rule’ proposes a general scaling phenomenon for all taxa, whereby SSD increases with average body size when males are larger than females, and decreases with body size when females are larger than males. Rensch's rule appears to be general in the former case, but there is little evidence for the rule when females are larger then males.Methodology/Principal FindingsUsing comprehensive data for 1291 species of birds across 30 families, we find strong support for Rensch's rule in families where males are typically larger than females, but no overall support for the rule in families with female-biased SSD. Reviewing previous studies of a broad range of taxa (arthropods, reptiles, fish and birds) showing predominantly female-biased SSD, we conclude that Rensch's conjecture is the exception rather than the rule in such species.Conclusions/SignificanceThe absence of consistent scaling of SSD in taxa with female-biased SSD, the most prevalent direction of dimorphism, calls into question previous general evolutionary explanations for Rensch's rule. We propose that, unlike several other ecological scaling relationships, Rensch's rule does not exist as an independent scaling phenomenon.

Relative sexual size dimorphism in Centrobolus fulgidus (Lawrence) compared to 18 congenerics

“…Unlike previous studies on SSD in invertebrates these results consistently corroborated Rensch's rule [18] . Figure 1 shows the finding for Centrobolus where mean volume ratios ranged from 0.63-2.72 with the regression of log male volume on log female volume was highly significant with a positive slope less than 1; showing females get larger than males with an increase in body size [4,7,9] .…”

Macroevolutionary patterns of sexual size dimorphism in copepods

“…1, and exceptions to this only occurred when females were the larger sex. An extensive comparison across birds found that RR is commonly broken where female-biased SSD occurs within a clade [7]. Recently, Blanckenhorn et al [31] evaluated the evidence for RR in a range of insect orders: the rule was found to apply consistently to Diptera and Heteroptera, but not to any of the other insect groups examined.…”

“…The demonstration that females are the more variant sex (or indeed that there is no difference in degree of variation, i.e. isometry) in a range of taxa has led to the generality and utility of the rule being questioned [7,8]. As an alternative, quantitative genetic theory predicts that greater interspecific variance in size occurs in the sex that has historically been under stronger directional selection [8,9].…”

Effects of Abiotic Factors on the Geographic Distribution of Body Size Variation and Chromosomal Polymorphisms in Two Neotropical Grasshopper Species (Dichroplus: Melanoplinae: Acrididae)

“…The latter has become to be considered the standard definition of Rensch's rule [53,54] but its interpretation is ambiguous. Although in male-biased SSD the rule is usually clearly demonstrated, in the opposite situation (female-biased SSD) the situation is far from clear [55]. This is most relevant because in a vast majority of insects and particularly in grasshoppers females are usually larger than males.…”