Code and data for: Females pair with males larger than themselves in a socially monogamous songbird

Abstract:   

Mate choice is a key driver of evolutionary phenomena such as sexual dimorphism. Social mate choice is studied less often than reproductive mate choice, but for species that exhibit biparental care, choice of a social mate may have important implications for offspring survival and success. Many species make pairing decisions based on size that can lead to population-scale pairing patterns such as assortative and disassortative mating by size. Other size-based pairing patterns, such as females pairing with males larger than themselves, have been commonly studied in humans, but less often studied in nonhuman animal systems. Here we show that sexually size-dimorphic mountain chickadees, Poecile gambeli, appear to exhibit multiple self-referential pairing patterns when choosing a social mate. Females paired with males that were larger than themselves more often than expected by chance, and they paired with males that were slightly larger than themselves more often than they paired with males that were much larger than themselves. Preference for slightly larger males versus much larger males did not appear to be driven by reproductive benefits as there were no statistically significant differences in reproductive performance between pairs in which males were slightly larger and pairs in which males were much larger than females. Our results indicate that self-referential pairing beyond positive and negative assortment may be common in nonhuman animal systems.