Sexual selection explains Rensch's rule of allometry for sexual size dimorphism

Abstract

In 1950, Rensch first described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespread and fundamental allometric relationship is now commonly referred to as 'Rensch's rule'. However, despite numerous recent studies, we still do not have a general explanation for this allometry. Here we report that patterns of allometry in over 5300 bird species demonstrate that Rensch's rule is driven by a correlated evolutionary change in females to directional sexual selection on males. First, in detailed multivariate analysis, the strength of sexual selection was, by far, the strongest predictor of allometry. This was found to be the case even after controlling for numerous potential confounding factors, such as overall size, degree of ornamentation, phylogenetic history and the range and degree of size dimorphism. Second, in groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch's rule. Taken together, these results provide the first clear solution to the long-standing evolutionary problem of allometry for sexual size dimorphism: sexual selection causes size dimorphism to correlate with species size.

Unexpected diversity in socially synchronized rhythms of shorebirds

“…For each population, body size was defined as mean female wing length 52 , either for individuals measured at the breeding area or at the wintering area. In case no individuals were measured, we used the mean value from the literature (see open access data for specific values and references 53 ).…”

The evolution of sexual dimorphism in New Zealand giant moa (Dinornis) and other ratites

“…Allometry consistent with Rensch's Rule has been confirmed in a range of avian taxa, but significant examples are generally associated with male-biased SSD, suggesting that sexual selection acting on male size drives the evolution of this pattern of allometry [32]. Although Rensch's Rule has also been demonstrated in Charadriiformes [33], other studies have found no evidence for it in Falconiformes, Strigiformes or Tinamiformes, suggesting that it is not generally supported in birds that exclusively exhibit RSD [3,28].…”

A phylogenetic approach to the study of sexual size dimorphism in Felidae and an assessment of Rensch´s rule.

“…Esto se debe a varios factores entre los que figura en forma prominente el hecho de que ninguno de los mecanismos evolutivos propuestos para explicar el escalamiento de SSD con el tamaño corporal, resulta satisfactorio. Habitualmente, se ha considerado a la selección sexual como el factor predominante detrás del escalamiento entre el SSD y el tamaño corporal (Hedrick y Temeles 1989;Fairbairn 1997Fairbairn , 2005Székely et al 2004;Dale et al 2007;Lindenfors et al 2007). El SSD puede ser explicado de forma relativamente simple por la selección sexual (Maynard Smith 1982Reiss 1989) sin embargo, la relación alométrica entre SSD y tamaño requiere una mayor variabilidad en tamaño en machos, y una respuesta diferencial a la selección de tamaño corporal en machos y hembras aún cuando ambos sexos comparten la mayor parte de los genes responsables por este carácter (Fairbairn 1997).…”

“…A partir de la publicación de "The Descent of Man and Selection in Relation to Sex" por Darwin (1871), el SSD ha sido considerado esencialmente como producto de la acción de la selección sexual en el sentido darwiniano de competición intrasexual por el acceso a la pareja en uno de los sexos, y la elección o preferencia por parte del otro (Trivers 1972;Andersson 1994;Isaac 2005;Dale et al 2007). Sin embargo, la selección natural clásica también puede explicar el SSD si favoreciese diferentes tamaños corporales para adecuar los sexos a diferentes papeles ecológicos o de utilización de nicho, o si competencia alimentaria ocurre entre machos y hembras (Hedrick y Temeles 1989;Shine 1989;Andersson 1994;Mysterud 2000;Isaac 2005).…”