Abstract
Assortative mating is a potential outcome of sexual selection, and estimating its level is important to better understand local adaptation and underlying trait evolution. However, assortative mating studies frequently base their conclusions on small numbers of individuals sampled over short periods of time and limited spatial scales even though spatiotemporal variation is common. Here, we characterized assortative mating patterns over 10 years in four populations of the blue tit (Cyanistes caeruleus), a passerine bird. We focused on two plumage ornaments—the blue crown and the yellow breast patch. Based on data for 1,657 pairs of birds, we found large interannual variation: assortative mating varied from positive to negative. To determine whether there was nonetheless a general trend in the data, we ran a within‐study meta‐analysis. It revealed that assortative mating was moderately positive for both ornaments. It also showed that mating patterns differed among populations and especially between two neighboring populations that displayed phenotypic divergence. Our results therefore underscore that long‐term studies are needed to draw broad conclusions about mating patterns in natural populations. They also call for studying the potential role of assortative mating in local adaptation and evolution of ornaments in both sexes.
Highlights
Studying sexual selection in the wild poses several methodological challenges
The conclusions of such work may be unreliable be‐ cause key studies conducted on traits subject to sexual selection have clearly shown that mate choice can vary in space and time (Svensson & Gosden, 2007)
This study investigated spatiotemporal variability in assortative mat‐ ing in four blue tit populations in southern France; it focused on two ornaments, the blue crown and the yellow breast patch
Summary
Studying sexual selection in the wild poses several methodological challenges. In particular, in animal species with separate sexes, mea‐ suring the direction and force of intra- and intersexual selection re‐ quires collecting information on difficult-to-quantify behaviors from large numbers of individuals. The most common solution is to use the trait value for a given individual for 1 year only; frequently, researchers choose the value associated with the individual’s first appearance or with a randomly selected time period (Row & Weatherhead, 2011; Van Rooij & Griffith, 2012) This approach prevents pseudoreplication, it dramatically decreases sample size. Two previous studies on assortative mating in blue tits (Andersson, Örnborg, & Andersson, 1998; Garcia-Navas, Ortego, & Jose Sanz, 2009) focused on a single ornament, the blue crown, and obtained data from a single population (using 18 and 26 breeding pairs, respec‐ tively) over a single breeding season They found opposite results: Andersson et al (1998) found evidence for positive assortative mat‐ ing, whereas Garcia-Navas and colleagues did not. They present an ideal opportunity for investigating the link between assortative mating and local adaptation
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