Selection on females can create 'larger males'

Area of Science:

• Evolutionary biology
• Zoology
• Comparative anatomy

Background:

• Sexual dimorphism, where males are larger than females, is common in birds and mammals.
• The prevailing hypothesis attributes this to sexual selection favoring larger male size.
• The degree of dimorphism is often assumed to reflect the strength of selection on male size.

Purpose of the Study:

• To challenge the assumption that sexual dimorphism solely results from selection for increased male size.
• To investigate alternative explanations for male-biased size dimorphism in a large avian clade.
• To determine the role of selection on female size in driving sexual dimorphism.

Main Methods:

• Reconstruction of ancestral morphology for males and females within a large avian clade.
• Phylogenetic comparative analysis to infer selective pressures.
• Testing hypotheses related to sexual selection, ecological niche partitioning, and breeding benefits.

Main Results:

• Selection for decreased female size explains male-biased sexual dimorphism in approximately 50% of cases studied.
• Findings contradict the assumption that dimorphism strength directly indicates selection for male size.
• Explanations involving ecological niche partitioning and breeding benefits for reduced female size were inconsistent with the data.

Conclusions:

• It is incorrect to assume sexual dimorphism arises from a single selective factor, like directional sexual selection on males.
• The selective forces driving sexual dimorphism can vary significantly between species.
• A case-by-case investigation using phylogenetic approaches is necessary to understand the evolution of size dimorphism.