Sexual Dimorphism and Parental Care in Avian Cooperative Breeders

Abstract

Avian sexual dimorphism is divided into two categories: color dimorphism, describing the difference in plumage pigmentation between males and females of a species, and size dimorphism, describing the difference in size between males and females of a species, commonly expressed in terms of mass ratios. Sexual dimorphism is understood to be driven by sexual selection, where it communicates information about condition and fitness to potential mates. High levels of dimorphism on both axes have traditionally correlated with increased sexual selection pressure, or reproductive skew. In this study, I analyse sexual dimorphism in avian cooperative breeders, which often have complex and flexible mating systems. I developed my own categorical rating system for color dimorphism based on existing comparative methods. I then divide cooperative breeders into different categories according to different features of their breeding systems, and compare both size and color dimorphism. My analysis revealed that higher levels of color dimorphism are present in facultative cooperative breeders, compared to obligate cooperative breeders, that systems containing unrelated male helpers show higher levels of color dimorphism compared to related male helpers and systems with both male and female helpers. I also discovered that pair-based breeding systems showed higher levels of color dimorphism than species which breed in polygamous coalitions, and there was a greater degree of size dimorphism among polygynous compared to polyandrous breeding systems. I relate these findings to hypotheses considering reproductive skew, and discuss different alternative mating strategies as indications of more or less intense sexual selection pressure, while providing suggestions for further investigations based on the hypotheses my review has generated.