No assortative mating based on size in black guillemots breeding in the Canadian Arctic

The Black Guillemot (Cepphus grylle) is a monomorphic, socially monogamous member of the Alcidae. Although aspects of their breeding and foraging ecology have been extensively studied, less is known about possible sex-based differences in morphology, nor whether Black Guillemots mate assortatively based on body size. Using molecular techniques, we identified the sex of 26 male and 21 female Black Guillemots captured in the Canadian Arctic, and measured six external body measurements: outer tarsus length, wing length, culmen length, bill depth, head plus bill length and body mass to test for sexual size dimorphism (SSD) and assortative mating. Overall, males were 1.7% and 8% larger than females in outer tarsus length and bill depth, respectively. Within breeding pairs, bill depth was the most dimorphic trait. Despite these morphological differences no evidence of assortative mating based upon body size was found. Thus, mate choice for body size does not appear to be an underlying mechanism of SSD in bill depth in Black Guillemots.

黑海鸠（学名Cepphus grylle）是海雀科（Alcidae）中一类形态单态、实行社会单配制的物种。尽管其繁殖与觅食生态学的多个方面已得到广泛研究，但学界对其形态学上可能存在的性别差异，以及黑海鸠是否会依据体型进行选型交配的认知仍较为匮乏。本研究通过分子技术，对在加拿大北极地区捕获的26只雄性与21只雌性黑海鸠完成了性别鉴定，并测量了跗跖外长、翼长、喙峰长、喙深、头喙总长与体重共6项外部体型指标，以此检验体型大小二态性（sexual size dimorphism, SSD）与选型交配现象。结果显示，雄性的跗跖外长与喙深分别较雌性高出1.7%与8%；在繁殖配对个体中，喙深是性别二态性最为显著的性状。尽管存在上述形态差异，本研究未发现任何基于体型的选型交配证据。由此可见，基于体型的配偶选择，似乎并非黑海鸠喙深相关体型二态性的潜在形成机制。