Trading up: the fitness consequences of divorce in monogamous birds

Abstract

Social and genetic mating systems play an important role in natural and sexual selection, as well as in the dynamics of populations. In socially monogamous species different genetic mating patterns appear when individuals mate outside the breeding pair within a breeding season (extra-pair mating) or when they change partners between two breeding seasons (widowing or divorce). Divorce can be defined as having occurred when two previously paired individuals are alive during the next breeding season and at least one of them has re-mated with a new partner. In socially monogamous birds divorce is widespread, but it is not clear whether it is a behavioural adaptation to improve the quality of a mating decision or whether, alternatively, it results as a non-selected consequence of other processes: existing studies suggest a heterogeneous set of results with respect to this central question. This heterogeneity could result from a number of factors, ranging from the methodological approaches used, to population- or species-specific characters. In this review we use phylogenetic meta-analyses to assess the evidence that divorce is adaptive (in terms of breeding success) across 64 species of socially monogamous birds. Second, we explore biological and methodological reasons for the heterogeneity in the results of previous studies. Results of our analyses supported the hypothesis that divorce is, in general, an adaptive behavioural strategy as: (1) divorce is triggered by relatively low breeding success; (2) there is a positive change in breeding success as a result of divorce. More specifically, while controlling for methodological moderators, we show that: (i) earlier stages of breeding are better predictors of divorce than later stages (r = 0.231; 95% CI: 0.061-0.391 for clutch size; similar for laying date); (ii) females benefited from divorce more than males in terms of increasing breeding success between successive breeding attempts, with different stages of the breeding cycle improving at different rates (e.g. r = 0.637; 95% CI: 0.328-0.817 for brood-level measures). We show that the effect size was dependent on the methodological approach used across studies and argue that research on the adaptive nature of divorce should be cautious when designing the study and interpreting the results. Altogether, by providing strong evidence that divorce is an adaptive strategy across monogamous birds, the results of our analysis provide a firm ground for further exploration of external covariates of divorce (e.g. demographic factors) and the mechanisms underlying the differences in the effect sizes of the proximal fitness causes and consequences of divorce.

Sibling conflict and dishonest signaling in birds

“…Divorce is the rate at which pairs mate with different partners when both original partners are still present in the population (51). In nine species where data on divorce rates were unavailable, we used mating system (lifetime vs. seasonal monogamy) to estimate the likelihood that parents will breed together again.…”

Carry-over effects of the social environment on future divorce probability in a wild bird population

“…For example, the frequency of extra-pair fertilizations in a house finch population was found to be largely determined by the availability of suitable extra-pair mates [43], and is thus specific for each individual in the population. Our study demonstrates that divorce, which can be considered as a form of mate choice [10], and a way to correct for a suboptimal mating situation [15], may also depend on aspects of the social environment in which initial mate choice happens. Our finding that males with a lower proportion of female associates during the period of pair formation divorced more often after breeding indicates that the intensity of competition (which we expressed as the proportion of the opposite sex associates) may limit the possibilities available for preferred pairing, regardless of the absolute number of female associates.…”

“…In this way, the social environment may not only impact breeding success through the immediate effects of the quality of the social partnership, but also through later fitness effects of secondary mating strategies. For example, across socially monogamous birds, divorce has been shown to lead to an increase in reproductive success with a new partner [15].…”

Honest signaling and the double counting of inclusive fitness

“…In contrast to this, Culina et al. () found that divorce increased fitness by an average of only 37% more nestlings or fledglings, in a representative sample of 15 species from that analysis with data on the number of offspring produced before and after divorce. We investigated the possibility that the fitness consequences of divorce might eliminate the effect of divorce on honesty with an exploratory analysis on 15 species where there are data on both honesty and the fitness consequences of divorce.…”