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.

摘要：择偶是性二态性（sexual dimorphism）等进化现象的核心驱动因素。相较于繁殖配偶选择（reproductive mate choice），社会配偶选择（social mate choice）的研究相对较少。但对于存在双亲抚育（biparental care）的物种而言，社会配偶的选择对后代存活与繁衍成功率可能具有关键影响。诸多物种会基于体型做出配对决策，这可能形成种群层面的配对模式，例如基于体型的选型交配（assortative mating）与负选型交配（disassortative mating）。另一类基于体型的配对模式——雌性选择体型大于自身的雄性——在人类中已被广泛探讨，但在非人类动物类群中的相关研究较为匮乏。
本研究针对具有体型性二态性（sexually size-dimorphic）的甘贝利山雀（Poecile gambeli）展开分析，结果显示其在选择社会配偶时，似乎存在多种自我参照式配对模式（self-referential pairing patterns）。雌性选择体型大于自身的雄性的概率，高于随机配对的预期概率；且相较于体型远大于自身的雄性，雌性更倾向于选择体型略大于自身的雄性。
针对这一偏好体型略大于自身雄性的现象，其背后似乎并非由繁殖收益所驱动：雄性体型略大于雌性的配对组，与雄性体型远大于雌性的配对组，二者的繁殖表现（reproductive performance）并无统计学显著性差异（statistically significant differences）。本研究结果表明，除正向与负向选型交配之外的自我参照式配对模式，在非人类动物类群中或许普遍存在。