Scrutinizing assortative mating in birds

Abstract

It is often claimed that pair bonds preferentially form between individuals that resemble one another. Such assortative mating appears to be widespread throughout the animal kingdom. Yet it is unclear whether the apparent ubiquity of assortative mating arises primarily from mate choice (“like attracts like”), which can be constrained by same-sex competition for mates; from spatial or temporal separation; or from observer, reporting, publication, or search bias. Here, based on a conventional literature search, we find compelling meta-analytical evidence for size-assortative mating in birds (r = 0.178, 95% CI 0.142–0.215, 83 species, 35,591 pairs). However, our analyses reveal that this effect vanishes gradually with increased control of confounding factors. Specifically, the effect size decreased by 42% when we used previously unpublished data from nine long-term field studies, i.e., data free of reporting and publication bias (r = 0.103, 95% CI 0.074–0.132, eight species, 16,611 pairs). Moreover, in those data, assortative mating effectively disappeared when both partners were measured by independent observers or separately in space and time (mean r = 0.018, 95% CI −0.016–0.057). Likewise, we also found no evidence for assortative mating in a direct experimental test for mutual mate choice in captive populations of Zebra finches (r = −0.020, 95% CI −0.148–0.107, 1,414 pairs). These results highlight the importance of unpublished data in generating unbiased meta-analytical conclusions and suggest that the apparent ubiquity of assortative mating reported in the literature is overestimated and may not be driven by mate choice or mating competition for preferred mates.

Assortative mating in space and time: patterns and biases

“…There is an ongoing discussion in the literature on the potential problems of neglecting ecological variations in space and time on patterns of mate assortment (Rolán-Alvarez et al 2015;Ng et al 2016;Indykiewicz et al 2017). In fact, recent meta-analyses have detected biases resulting from pooling data from assortments in amphibians (Green 2019) and birds (Wang et al 2019). However, a broader quantitative assessment of these problems is still necessary.…”

Why do we pick similar mates, or do we?

“…For example, a population may be segregated into phenotypically (and genetically) differentiated clusters [4,7,9,23], creating mating pools of similar individuals. We call this 'stratification' [4,[24][25][26], but elsewhere it is referred to as 'social homogamy' [27][28][29][30]), 'propinquity' [7,9] or 'segregation' [31]. PAM may also reflect mate choice based on preferences [1,32], either for similarity itself [1,5,[33][34][35] or other traits/characteristics that produce PAM indirectly.…”

Telomere lengths correlate with fitness but assortative mating by telomeres confers no benefit to fledgling recruitment

“…Assortative mating, or a positive correlation between the phenotypes or genotypes of mates, is a common phenomenon among animal species, including assortative mating by size, age, and visual cues 1 – 3 , although the evidence for assortment by size has recently been questioned 4 . Yet both proximate and ultimate questions remain, including what types of traits assortative mating is observed in and whether assortative mating is adaptive.…”

“…Hence, animals are expected to prefer mates with long telomeres, independent of which cues they use in mate assessment, and in species where both sexes exercise mate choice assortative mating for telomere length could result. Assortative mating by telomere length has been reported in several bird species in which it is unlikely to be confounded with age 25 – 28 , but assortment by telomere length should be assessed for confounds with age and methodological similarities 4 . It could also be a by-product of exposure to shared environments if pair bond durations are long 29 .…”