DO BIRDS SHOW UNIQUE MACROEVOLUTIONARY PATTERNS OF SEXUAL SIZE DIMORPHISM COMPARED TO OTHER AMNIOTES?
Authors: Saitta Evan Thomas 
Published: April 2024

Body size is undoubtedly one of the most useful measures of sexual dimorphism and, by proxy, sexual selection. Here, I examine large, published datasets of average sexual size dimorphism (SSD) in four clades of amniotes: birds, mammals, squamates, and turtles. Most sexual variation is of subtle magnitude; attempts to discretely categorize species as monomorphic may overlook genuine and common sexual variations of small magnitude (e.g., <10–20% difference). Mammals, squamates, and turtles have unimodal SSD distributions centered close to zero that vary in skew. Mammals skew towards a preponderance of taxa with larger males than females, and mammals with the most extreme SSD have larger males than females. Turtles, however, skew strongly towards a preponderance of taxa with larger females than males, and turtles with the most extreme SSD have larger females than males. Squamates are intermediate to these two clades. Birds are unique in that they 1) are noticeably deficient in taxa near monomorphism, 2) have a bimodal distribution with peaks closely and roughly equidistantly straddling either side of monomorphism, and 3) have a high preponderance of taxa with larger males than females. This suggests stronger disruptive selection or constraints against monomorphism in birds compared to other amniotes. Bird data from Dunning (2007) yields bimodality, while other datasets do not, possibly due to data artefacts/errors. Although Rensch’s rule (RR) is difficult to apply to broad clades, scaling patterns were nevertheless examined here. While turtles and squamates show full adherence to RR, mammals show weaker adherence. Mammal scaling is comparatively less male-biased with increased size than scaling in squamates and turtles, and sex-role reversed mammals instead approach isometry between male and female size. Although bird taxa with larger males than females follow RR, sex-role reversed birds show the converse RR pattern. In birds, increasing size leads to increased dimorphism magnitude regardless of the direction of dimorphism, even though regression of the entire clade deceptively suggests they scale isometrically. This paradoxical scaling explains their unusual bimodal SSD distribution, as shown here through simulation. Equidistant bimodality from monomorphism might suggest disruptive selection where both mating systems have mirrored sexual selection dynamics of comparable effect. Scaling patterns between dimorphism magnitude and overall taxon size in non-reversed and reversed systems might not be readily apparent when examining the whole clade. Large mammals have disproportionately male-biased and more extreme SSD magnitudes. In comparison, large birds have relatively numerous sex-role reversed taxa as well as more extreme SSD magnitudes. These results deserve further testing with tighter phylogenetic controls and comparison of data sources. Additional ecological, physiological, and behavioral variables should also be examined in relation to SSD (e.g., altriciality vs. precociality, oviparity vs. viviparity, clutch size, neonate mass).