Disentangling the causes of age‐assortative mating in bird populations with contrasting life‐history strategies

Abstract

Age shapes fundamental processes related to behaviour, survival and reproduction, where age influences reproductive success, non‐random mating with respect to age can magnify or mitigate such effects. Consequently, the correlation in partners' age across a population may influence its productivity. Despite widespread evidence for age‐assortative mating, little is known about what drives this assortment and its variation. Specifically, the relative importance of active (same‐age mate preference) and passive processes (assortment as a consequence of other spatial or temporal effects) in driving age assortment is not well understood. In this paper, we compare breeding data from a great tit and mute swan population (51‐ and 31‐year datasets, respectively) to tease apart the contributions of pair retention, cohort age structure and active age‐related mate selection to age assortment in species with contrasting life histories. Both species show age‐assortative mating and variable assortment between years. However, we demonstrate that the drivers of age assortment differ between the species, as expected from their life histories and resultant demographic differences. In great tits, pair fidelity has a weak effect on age‐assortative mating through pair retention; variation in age assortment is primarily driven by fluctuations in age structure from variable juvenile recruitment. Age‐assortative mating is, therefore, largely passive, with no evidence consistent with active age‐related mate selection. In mute swans, age assortment is partly explained by pair retention, but not population age structure, and evidence exists for active age‐assortative pairing. This difference is likely to result from shorter life‐spans in great tits compared with mute swans, leading to fundamental differences in their population age structure, whereby a larger proportion of great tit populations consist of a single age cohort. In mute swans, age‐assortative pairing through mate selection may also be driven by greater age‐dependent variation in fitness. The study highlights the importance of considering how different life histories and demographic differences arising from these affect population processes that appear congruent across species. We suggest that future research should focus on uncovering the proximate mechanisms that lead to variation in active age‐assortative mate selection (as seen in mute swans); and the consequences of variation in age structure on the ecological and social functioning of wild populations.

Coming of age in your local mating market: Just a numbers game?

“…Data underlying Figure 1 are available from the Dryad Digital Repository https://doi.org/10.5061/dryad.m63xs j44s (Woodman et al, 2022a).…”

“…Positive (or negative) assortative mating occurs when members of a mating pair are more similar (or dissimilar) to each other with respect to a given trait than would be expected from chance. This form of non‐random mating is widespread among animal taxa, where pairs form on the basis of (dis)similarities in a host of traits including size (Arnqvist et al, 1996; Wang et al, 2019), ornamentation (Jawor et al, 2003), immunogenes (Andreou et al, 2017) and, perhaps not unexpectedly, breeding age (Woodman et al, 2022b). Positive age‐assortative mating is among those traits exhibiting the strongest forms of assortment in animals (Jiang et al, 2013), with profound consequences for the genetic structure of populations, speciation, and rates of adaptive evolution.…”

“…A pattern of age‐based assortment remains even after decomposing the dataset to newly formed unique pairs. Thus, assortative mating in mute swans is not merely a consequence of interannual mate fidelity (data from Woodman et al, 2022a).…”

“…-R. M. Pirsig Positive (or negative) assortative mating occurs when members of a mating pair are more similar (or dissimilar) to each other with respect to a given trait than would be expected from chance. This form of non-random mating is widespread among animal taxa, where pairs form on the basis of (dis)similarities in a host of traits including size (Arnqvist et al, 1996;Wang et al, 2019), ornamentation (Jawor et al, 2003), immunogenes (Andreou et al, 2017) and, perhaps not unexpectedly, breeding age (Woodman et al, 2022b).…”

Age-specificity in territory quality and spatial structure in a wild bird population

“…Such variation might affect how age is arranged in space through passive mechanisms whereby clusters of territories occupied by same-age individuals will be more likely to arise when the age distribution is skewed towards this age-cohort. For example, recent work demonstrates how fluctuations in population age structure determine how age is structured within breeding pairs in a short-lived bird population, where there is greater age-assortative pairing when the proportion of yearlings is higher [19]. Thus, between-year age distribution is likely to passively affect spatial age structure as clusters of territories occupied by same-age individuals are more likely to arise when much of the population exists in a single age-cohort (Figure 1a).…”