Data from: Domestic chickens defy Rensch's rule: sexual size dimorphism in chicken breeds

Sexual size dimorphism (SSD), i.e., the difference in sizes of males and
females, is a key evolutionary feature that is related to ecology,
behaviour and life histories of organisms. Although the basic patterns of
SSD are well documented for several major taxa, the processes generating
SSD are poorly understood. Domesticated animals offer excellent
opportunities for testing predictions of functional explanations of SSD
theory because domestic stocks were often selected by humans for
particular desirable traits. Here we analyze SSD in 139 breeds of domestic
chickens Gallus gallus domesticus, and compare them to their wild
relatives (pheasants, partridges and grouse; Phasianidae, 53 species). SSD
was male-biased in all chicken breeds, since males were 21.5 ± 0.55% (mean
± SE) heavier than females. The extent of SSD did not differ among breeds
(cock fighting, ornamental, and breeds selected for egg and meat
production). SSD of chicken breeds was not different from wild pheasants
and allies (23.5 ± 3.43%), although the wild ancestor of chickens, the red
jungle fowl Gallus gallus, had more extreme SSD (male 68.8% heavier) than
any domesticated breed. Male mass and female mass exhibited positive
allometry among pheasants and allies, consistently with the Rensch’s rule
reported from various taxa. However, body mass scaled isometrically across
chicken breeds. The latter results suggest that sex-specific selection on
males versus females is necessary to generate positive allometry, i.e.,
the Rensch's rule, in wild populations.