Figure 1.
A. Rensch's rule as it is often represented schematically.
The dashed line is the line of isometry (male size = female size); the solid line represents Rensch's rule, with SSD increasing with size in species with MBSSD, decreasing with size in species with FBSSD. However, this representation also makes the assumption (not stated by Rensch) that in taxa with mixed SSD, those species with FBSSD will be smaller than species with MBSSD. B. An alternative representation of Rensch's rule, with SSD decreasing with size in FBSSD species, increasing in MBSSD species as before, but both MBSSD and FBSSD species can span the full range of sizes present in the group.
Table 1.
Family specific slopes of log(female) on log(male) size (with 95% CIs) for the 30 families in our dataset with ≥10 species.
Figure 2.
SSD (defined here as log(female size/male size)) against log(male size) for the 30 avian families in our dataset for which we had body size data for ≥ 10 species.
The dashed horizontal line indicates no scaling of SSD with size. Also shown are OLS estimates of the relationship between SSD and log(male size) for each family; families with >50% FBSSD species are shown as solid lines, those with >50% MBSSD species as dotted lines. Note that this plot is equivalent to plotting log(female size) against log(male size) with the slopes presented in table 1, but it is easier to see differences between families on the SSD scale.
Figure 3.
The family-specific slope estimates from table 1 plotted against PMB, the proportion of species within a family displaying MBSSD.
The dotted horizontal line indicates isometry. Circles indicate families with PMB <0.5, triangles those with PMB >0.5, the horizontal solid lines are the means of these two groups, and the shaded grey regions are their respective 95% C.I.s. Solid symbols are those families with PMB ≤0.2 or ≥0.8. The size of each symbol is proportional to the number of species in that family for which we had data. Error bars represent the standard error of the family-specific slope estimates, and for clarity are extended only towards a slope of 1. Three families with a PMB of 0.5 are indicated by crosses, and were not included in the above analysis. Also shown (dashed line) is the simple linear regression of family-specific slope on PMB (weighted by the reciprocal of the s.e. of the family-specific slope).
Figure 4.
SSD against log(male size) for four families with mixed SSD, A. Laridae, B. Tyrannidae, C. Fringillidae, D. Trochilidae.
In each case, species with FBSSD are shown as filled symbols, those with MBSSD as open symbols. We also include three regression lines on each figure. The solid line is the slope across all species, and is significantly negative in each case. The dashed line is the slope for species with FBSSD only, and differs from isometry only in D. The dotted line is for species with MBSSD only, which is significantly negative in all panels except D.
Table 2.
A summary of relationships obtained from the literature between log(male) and log(female) size from a wide range of taxa with predominantly FBSSD or with mixed MBSSD and FBSSD in which data were provided separately for FBSSD and MBSSD species.