Extra-pair Paternity Research Articles

From self-interest to cooperation: extra-pair mating as a driver of relaxed territorial aggression in social neighbourhoods

In socially monogamous bird species, males and females tailor their reproductive strategies to that of the other. Interactions are complex, and have elements of both conflict and cooperation. Breeding process is further complicated with the occurrence of matings outside the pair bond, which is a frequent phenomenon in these species. Extra-pair mating has clear benefits for males, but cuckolded males may withdraw care and resources at a cost to females, which produces an unbalanced costs-to-benefits ratios within the mating pair. We used an individual-based model with social networks approach, to study how female extra-pair mating strategies may affect male investment in offspring care and territorial defence. In our model, extra-pair copulation rate is a female-driven strategy; we use two adjustable male traits—care and territorial defence—that co-evolve with that strategy and control the number of extra-pair copulations initiated by a social female. The model utilises very simple rules of behaviours between individuals that lead to the emergence of evolved changes in mating strategies on a large scale. We show how extra-pair copulations initiated by females in their neighbourhood can reduce territorial defence and wasteful aggression between the males across the whole population. We propose that female mating behaviour and male responses are flexible traits that might serve as potential drivers of the evolution of cooperation.

A high level of extra-pair paternity in the Chestnut Thrush (Turdus rubrocanus)

Extra-pair copulation (EPC) can potentially maximize individual reproductive fitness, and this process may involve sexual selection of male and female traits that reflect individual quality. Previous studies have implied that adult characteristics are associated with the probability of extra-pair paternity (EPP), but it differs between species. Moreover, there are relatively few examples of the adaptive rationale for females’ engagement in EPCs based on an examination of these more traditionally recorded traits, in combination with female flight-mediated traits such as wing length. We investigated whether EPP existed in the wild Chestnut Thrush (Turdus rubrocanus) population during three breeding seasons (2019–2021), and whether paternity was related to morphological traits of males and females. Eight highly variable microsatellite loci were used to identify paternity, and generalized linear mixed models were used to analyze the relationship between paternity and morphological traits. We found that EPP existed in the Chestnut Thrush. 53.3% (N ​= ​41/77) of the broods contained at least one extra-pair offspring (EPO), and 34.6% (N ​= ​72/208) of the nestlings were EPO. We also found that male wing length was negatively associated with the probability of EPP and the proportion of EPO. Female body length was positively related to the probability of EPP. Both female body condition and mass were negatively associated with the proportion of EPO. While other traits of male and female did not relate to the probability of EPP or the proportion of EPO. Extra-pair males had better body condition compared to the males they cuckolded. EPO did not differ from their half-siblings in terms of body size or body condition. The results suggest that body size and body condition were associated with EPP in the Chestnut Thrush. This study provides fundamental information for further studies on the evolution and maintenance of EPP in the Chestnut Thrush, and it is also useful for the comparison of EPP among Turdus species.

Aberrant singing is associated with developmental stress and zero fitness in a male White-throated Sparrow ( Zonotrichia albicollis )

Oscine birdsong signals species identity, and individual quality, and functions as a cue for mate selection. Thus, producing an atypical song may have deleterious fitness effects. Nevertheless, learning during song development deems the vocal phenotype labile, creating the potential for errors, which may be elevated by developmental stress. Indeed, developmental stress has been shown to interfere with song learning in laboratory studies, but in the field, developmental conditions are rarely known, and knowledge of reproductive success is typically also lacking. We report on a male White-throated Sparrow (<em>Zonotrichia albicollis</em>) that we observed producing an atypical song comprised of a sparrow-like introductory phrase followed by a normal song. We banded this male as a nestling and observed potentially stressful developmental conditions that may have elevated the chances for learning errors. Specifically, this male hatched last in an asynchronous second clutch of three eggs, was structurally smaller than his siblings and fledged at nearly the latest date documented during that season. As an adult, he was unsuccessful at securing a social mate or extra-pair paternity, suggesting that females selected against his atypical vocal phenotype. Our observations of this unique individual support a connection between developmental stress, atypical song, and fitness outcomes.

The use and misuse of evolutionary psychology in online manosphere communities: The case of female mating strategies.

While early evolutionary accounts of female sexuality insisted on coyness and monogamous tendencies, evidence from the field of primatology started challenging those assumptions in the 1970s. Decades later, there exist many competing and overlapping hypotheses stressing the potential fitness benefits of female short-term and extra-pair mating. Female mammals are now seen as enacting varied and flexible reproductive strategies. This is both a victory for science, with a better fit between theory and reality, and for feminism, with the downfall of narrow stereotypes about female sexuality. However, evolutionary hypotheses on female mating strategies are routinely invoked among the antifeminist online communities collectively known as 'the manosphere'. Based on extensive qualitative analysis of manosphere discourse, this study shows how these hypotheses are sometimes interpreted in misogynistic online spaces. Indeed, evolutionary scholars might be surprised to see sexist worldviews reinforced by the 'dual mating strategy' and 'sexy son' hypotheses, or by the latest research on the ovulatory cycle. The manosphere has its own version of evolutionary psychology, mingling cutting-edge scientific theories and hypotheses with personal narratives, sexual double standards and misogynistic beliefs. After analysing this phenomenon, this article suggests ways to mitigate it.

Low Extra-Pair Paternity and Polygamy in the Common Kingfisher Alcedo atthis

La monogamia es considerada el sistema de apareamiento común en las aves. Sin embargo, análisis moleculares han revelado que, en la mayoría de las aves, pueden ocurrir casos de emparejamientos extrapareja, así como de intercambios de parejas durante o entre las temporadas de reproducción. En este estudio nos centramos en una población de martín pescador común localizada en el río Danubio (Eslovaquia) durante tres temporadas consecutivas de reproducción (2016-2018), analizando un total de 648 individuos correspondientes a 71 adultos y 577 polluelos de 92 nidadas. Los análisis mostraron que seis pollos (1,04%) de tres crías diferentes (3,26%) tenían un padre diferente, aunque ninguno de ellos tuvo una madre distinta. Además, detectamos dos machos polígamos simultáneamente (5,13%). Mientras que un macho se apareó con dos hembras, otro tuvo cuatro parejas. En siete casos (9,6%) encontramos evidencias de intercambio de pareja entre distintas crías, de las que en tres ocasiones (4,11%) se intercambiaron hembras y cuatro veces (5,48%) machos. En general, estos resultados indican que el martín pescador común es una especie social, genéticamente monógama, con una rara ocurrencia de pollos procedentes de emparejamientos extrapareja y poligamia, y en ningún caso procedentes de parasitismo de nido intraespecífico. Este sistema de emparejamiento viene en parte apoyado por el hecho de que, en esta especie, el cuidado parental es llevado a cabo tanto por parte de la hembra como del macho durante un período de reproducción generalmente extenso.—Cepková, M., Melišková, M. y Rubáčová, L. (2023). Baja paternidad extrapareja y poligamia en el martín pescador común Alcedo atthis. Ardeola, 70: 41-58.

Patterns of extra-territorial nest-box visits in a songbird suggest a role in extrapair mating.

Many animals make visits outside of their territory during the breeding period, but these are typically infrequent and difficult to observe. As a consequence, comprehensive data on extra-territorial movements at the population-level are scarce and the function of this behavior remains poorly understood. Using an automated nest-box visit tracking system in a wild blue tit population over six breeding seasons, we recorded all extra-territorial nest-box visits (n = 22 137) related to 1195 individual breeding attempts (761 unique individuals). Sixty-two percent of breeders made at least one extra-territorial visit between the onset of nest building and the day of fledging of their offspring, and individuals visited another nest-box on average on 11% of the days during this period. Visit behavior differed clearly between the sexes, with males making over three times as many extra-territorial forays as females. There was a strong overall seasonal decline in visit behavior, but this was sex dependent, with females showing a strong reduction in the number of extra-territorial visits before the onset of egg laying and males showing a strong and sudden reduction on the day their offspring hatched. The likelihood of visiting a particular nest-box declined sharply with the distance to that box, and blue tits almost exclusively visited direct neighbors. Individuals were more likely to have extrapair offspring with an individual whose box they visited, but they were not more likely to disperse to a box they had visited. Thus, our results are inconsistent with the hypothesis that extra-territorial nest-box visits serve to inform dispersal decisions, but suggest that such visits are linked to extrapair mating opportunities.

Male aggressiveness during the female fertile phase in relation to extra-pair paternity, plumage ornaments and female traits

In many bird species, physical aggression between males become more frequent during the female’s fertile period, as female encounters with extra-pair males are more frequent and can entail paternity losses. Male aggressiveness during this stage has been proposed as crucial for ensuring male reproductive success. Thus, plumage ornaments could represent honest signals of individual quality that could reflect the aggressiveness of paired territorial males. Furthermore, male aggressiveness could be related to mate quality or defensive capacity. We quantified extra-pair paternity in the broods and investigated the association of male and female traits with the aggressive behaviour of territorial paired males in a Spanish population of Pied Flycatchers (Ficedula hypoleuca), where territorial intrusions were simulated during the female fertile period by placing a taxidermic male mount close to the nest. We predicted that (1) more aggressive males should better protect their mates from intruding males and thereby reduce their paternity losses, (2) males with larger white patches and higher UV reflectance of wing patches should respond more strongly to intrusions, and (3) that males should be more aggressive when mated with higher quality females. We found evidence that males that responded less intensely to a territorial intrusion suffered a higher paternity loss, which offers strong support to the basic tenet of the theory of territoriality as paternity defence. Moreover, both the level of male aggressiveness and control of the territory increased with male UV reflectance of wing patches. Finally, we found, contrary to our prediction, that males were less aggressive when mated with more ornamented females.

Observation of an Attempted Forced Copulation within a Captive Flock of Greater Flamingos (Phoenicopterus roseus)

Flamingos (Phoenicopteriformes) are obligate colonial species that nest in large colonies, with monogamous pairs rearing a single chick following a synchronised group courtship display. Within this relatively simplistic behavioural description, deviations from these social and reproductive norms are apparent. Same sex pairings, multi-bird relationships and extra pair copulations are documented in the literature. Flamingos display highly sexually selected characteristics of plumage colour, carotenoid accumulation and diversity of display movements that underpin mate choice decisions. The brightest birds in best body condition are more successful at breeding. Therefore, documented mate guarding of female birds by male partners, is a relevant response to maximise investment in a pair bond. Limited information that describes the action of forced copulation by the male flamingo and the response of the female bird is available in the literature. This paper describes an occurrence of an attempted forced copulation by an older male Greater Flamingo to a younger female bird. Such behaviour may be an artefact of the captive environment, and limited mate choice when compared to the sizes of wild flocks, or it could be regularly apparent in the wild and therefore worthy of more scrutiny and empirical study.

Combined roles for breeding synchrony, habitat and scale as predictors of extrapair paternity

Despite extensive research on passerine reproduction, it remains difficult to predict which traits of individuals, nesting populations and landscapes drive extrapair paternity (EPP). Two broad categories of drivers, habitat structure and population level factors, have been considered separately or at a single spatial or temporal scale. We used a 6-year nesting data set of scissor-tailed flycatcher, Tyrannus forficatus , which have high rates of cuckoldry (73% of nests, 59% of nestlings), to explore how nest density, breeding synchrony, habitat structure and clutch initiation date affect EPP rates. We further considered synchrony at two temporal and habitat structure at two spatial scales. We predicted that visual occlusion from vertical habitat structuring on breeding territories would allow extrapair males to go unnoticed and provide cover for extrapair copulations. Predictions for effects of nest density and fertility synchrony were double-edged: EPP may either increase as extrapair mate availability and ease of comparison with social mates increases or decrease as social mates increase their efforts to assure paternity. Using Bayesian techniques, we found a combination of population level factors and habitat structure, including interactions among the latter, at different scales best accounted for variation in EPP. EPP declined with increasing population synchrony. Variation in EPP was also explained by fine-scale habitat measures, decreasing with increasing nest tree diameter at breast height and woody vegetation cover but increasing with tree density. Notably, EPP increased with a coarse-scale habitat measure, linear extent of fence or powerline, suggesting a role for human alteration of habitat. Fences are used as communal perches by neighbours and floater males, potentially increasing interactions between asynchronous individuals. Our study demonstrates that breeding synchrony influences the probability of EPP and that habitat structure on individual territories is a strong predictor of paternity that acts independently of breeding density or synchrony at our scale of measurement. • We explore factors influencing cuckoldry in scissor-tailed flycatchers across 6 years. • Cuckoldry increased with populationwide fertility synchrony. • Cuckoldry was not related to spatial distribution of nests. • Cuckoldry increased with tree number and powerline/fence length and low shrub cover. • These correlates may impact cuckoldry via their effects on male mate guarding.

Seasonal variation in cuckoldry rates in the socially monogamous cichlid fish Variabilichromis moorii

Mating patterns in animal populations can respond to environmental conditions and consequently vary across time. To examine this variation in nature, studies must include temporal replicates from the same population. Here, we report temporal variation in genetic parentage in the socially monogamous cichlid Variabilichromis moorii from Lake Tanganyika, using samples of broods and their brood-tending parents that were collected across five field trips from the same study population. The sampled broods were either spawned during the dry season (three field trips) or during the rainy season (two trips). In all seasons, we detected substantial rates of extra-pair paternity, which were ascribed to cuckoldry by bachelor males. Paternity shares of brood-tending males were consistently higher, and the numbers of sires per brood were consistently lower, in broods that were spawned in the dry seasons compared to broods from the rainy seasons. In contrast, the strength of size-assortative pairing in our V. moorii population did not vary temporally. Seasonal fluctuations in environmental conditions, such as water turbidity, are proposed as a mechanism behind variable cuckolder pressure. Our data demonstrate the utility of long-term monitoring to improve our understanding of animal mating patterns.

Extra‐pair paternity correlates with genetic diversity, but not breeding density, in a Neotropical passerine, the black catbird

The frequency of extra‐pair paternity (EPP) varies widely across socially monogamous birds, but the proximate mechanisms driving this variation remain unclear. In this study, we tested two major factors hypothesized to influence extra‐pair mating – breeding density and genetic diversity – by comparing genetic mating patterns in two populations of black catbirdsMelanoptila glabrirostris. This Neotropical songbird is endemic to the Yucatán Peninsula, including eastern Mexico, and its offshore islands. We sampled one mainland (Sian Ka'an Biosphere Reserve) and one island (Isla Cozumel) population and used single‐nucleotide polymorphisms (SNPs) to quantify heterozygosity and genetic parentage over two breeding seasons. Moderate levels of EPP occurred in both populations (9.5–35% of offspring and 17–45% of nests). Contrary to predictions, breeding density did not affect EPP: although breeding densities were much higher on the mainland than on the island, EPP rates did not differ between populations, and local breeding density was not correlated with EPP at individual nests. In contrast, partial support emerged for the hypothesis that genetic diversity influences EPP: extra‐pair offspring were more heterozygous than within‐pair offspring. However, the two populations did not differ in genetic diversity, and neither the heterozygosity of social fathers nor within‐pair relatedness predicted EPP. These results are consistent with recent comparative studies suggesting that breeding density is not a critical driver of EPP rates, and that not all tropical songbirds exhibit low rates of EPP.

Extra-pair paternity in the Saffron Finch is related to song peak frequency and body condition

ABSTRACT In many avian species, age, body size, and song could signal quality or individual breeding experience and these traits are often correlated with reproductive success. The relationship between sexually selected signals and avian mating systems is a topic of active research, but still remains poorly investigated in species from southern latitudes. Here, we explored the relationships between body size, condition, age, song and social and genetic reproductive success in Sicalis flaveola pelzelni, a Neotropical thraupid. This socially monogamous species has both age-related changes in song and plumage colouration and extra-pair paternity. When measuring male song we found that a lower peak frequency may be associated with patterns of extra-pair paternity gain by males. We also found that male age may not have a marked effect on paternity assurance. In addition, we found that female body condition appears to influence on the proportion of extra-pair nestlings in females’ nests. We contribute novel data to help bridge the geographic and taxonomic gap in this research field.

Extra-pair paternity drives plumage colour elaboration in male passerines.

The elaborate ornamental plumage displayed by birds has largely been attributed to sexual selection, whereby the greater success of ornamented males in attaining mates drives a rapid elaboration of those ornaments. Indeed, plumage elaboration tends to be greatest in species with a high variance in reproductive success such as polygynous mating systems. Even among socially monogamous species, many males are extremely colourful. In their now-classic study, Møller and Birkhead (1994) suggested that increased variance in reproductive success afforded by extra-pair paternity should intensify sexual selection pressure and thus an elaboration of male plumage and sexual dichromatism, but the relatively few measures of extra-pair paternity at the time prevented a rigorous test of this hypothesis. In the nearly three decades since that paper's publication, hundreds of studies have been published on rates of extra-pair paternity and more objective measures of plumage colouration have been developed, allowing for a large-scale comparative test of Møller and Birkhead's (1994) hypothesis. Using an analysis of 186 socially monogamous passerine species with estimates of extra-pair paternity, our phylogenetically controlled analysis confirms Møller and Birkhead's (1994) early work, demonstrating that rates of extra-pair paternity are positively associated with male, but not female, colouration and with the extent of sexual dichromatism. Plumage evolution is complex and multifaceted, driven by phylogenetic, ecological, and social factors, but our analysis confirms a key role of extra-pair mate choice in driving the evolution of ornamental traits.

Male social niche conformance? Effects of manipulated opportunity for extra-pair mating on behavior and hormones of male zebra finches

Success in sperm competition is an important determinant of male fitness in mating systems with female multiple mating. Thus, sperm competition risk represents a key dimension of the male social environment to which individual males are expected to adaptively adjust their reproductive phenotype. Such adaptive phenotypic adjustment we here refer to as male social niche conformance. In this pre-registered study, we investigated how male zebra finches, Taeniopygia guttata, adjust their behavior to sperm competition risk. We experimentally manipulated the opportunity for extra-pair mating to create two levels of sperm competition risk: 1) Single-pair, no sperm competition risk; 2) Double-pair, sperm competition risk. We compared male courtship, mate guarding, copulation rates, and aggression between the treatment groups. To identify hormonal correlates of male behavioral adjustment, we measured plasma testosterone and corticosterone levels before and after the social treatment started. Contrary to our pre-registered predictions, males from the Double-pair treatment group decreased courtship rates compared to those from the Single-pair group, and Double-pair males responded less aggressively towards intruders than Single-pair males. Testosterone levels decreased over the breeding cycle, but social treatment had no effect on either testosterone or corticosterone levels. Our results indicate that male zebra finches do not intensify courtship or competitive reproductive behaviors, or upregulate key hormones when another breeding pair is present. Although we found no evidence for the predicted adaptive behavioral responses to sperm competition risk, we show that male zebra finches plastically adjust their behavior to their social environment.

Modulation of dear enemy effects by male dusky warblers (Phylloscopus fuscatus) at different reproductive stages

Territorial animals show less aggression during intrusion by neighbours than by strangers, a phenomenon known as the “dear enemy effect”. Recent studies have shown that the dear enemy effect is variable in some birds throughout the breeding season. However, there is limited research on changes in dear enemy effects in songbirds during the breeding season. In this study, we monitored dusky warblers (Phylloscopus fuscatus) throughout their breeding stages, conducting playback experiments simulating incursions by neighbours and strangers during their egg-laying and incubation/nestling periods, and recording physical responses and song responses from territory owners. The results showed that male dusky warblers responded similarly to neighbours and strangers during the female egg-laying period, but there was a significantly stronger response to strangers than to neighbours during the incubation/nestling period. This suggests that male dusky warblers adjust the intensity of their defences against neighbours according to the reproductive status of their own mate. This may be because neighbouring males threaten the paternity of territorial males during the egg-laying period, so territorial males demote the 'dear enemy' friendly relationships with their neighbours to defend their mate and paternity. In contrast, during the non-reproductive period of females, when neighbours are less of a threat to the paternity of territorial males, lowering defences against neighbouring males allow them to devote more time and energy to activities that improve their fitness, such as foraging and seeking extra-pair mating.

Extrapair mating opportunity explains the evolutionary transition between the presence and absence of tail ornamentation in swallows

Extrapair mating opportunity explains the evolutionary transition between the presence and absence of tail ornamentation in swallows

Experimental extra-pair copulations provide proof of concept for fertility insurance in a socially monogamous bird.

Extra-pair paternity is common among socially monogamous birds, but whether females benefit from having extra-pair copulations remains unresolved. In this study, I staged single extra-pair copulations in captive pairs of socially monogamous Japanese quail (Coturnix japonica) and used extra-pair males of a different colour to identify extra-pair fertilizations. This eliminated among-female variation in extra-pair copulations as a source of variation in extra-pair fertilizations and tested whether a single extra-pair copulation would insure against infertility of the social male. Overall, the probability of extra-pair fertilization was 0.46 for the first egg that was fertilized after extra-pair copulation, but this rapidly declined over consecutive eggs in the laying sequence. However, a single extra-pair copulation was effective in ensuring fertilization of the majority of a typical clutch in the few cases where the social male was completely infertile. Hence, my results show that variation in extra-pair paternity can be independent from variation in extra-pair copulation behaviour and that a single, strategically timed, extra-pair copulation can largely insure against social male infertility. Among-female variation in extra-pair fertilizations, and their declining probability over the laying sequence, as typical for many bird species, can thus, in principle, be parsimoniously explained by a uniform female fertility insurance strategy.

Assessing behavioral sex differences to chemical cues of predation risk while provisioning nestlings in a hole-nesting bird.

Birds can assess nest predation risk and adjust their parental activity accordingly. Risk taking behavior should be related to investment in reproduction as well as to confidence in parenthood that often differ between sexes. In those cases, sexual differences in risk taking behavior may be expected. For example, in blue tits, females invest more time and energy than males in nest-building, egg laying and incubation. Furthermore, confidence in parenthood is supposed to be higher for females, as extrapair paternity is common in this species. Therefore, the reproductive value of nestlings may be higher for females than for males and the former may assume greater risks to ensure nestling growth and maximize their reproductive success. We examined potential sexual differences in the risk assumed by parents in relation to perceived risk of predation inside the nest cavity, where predation risk perception may be higher. We increased perceived predation risk by adding predator chemical cues inside blue tit (Cyanistes caeruleus) nest-boxes, and we tested whether female and male parents differed in the risk assumed when taking care of nestlings. Females and males did not differ in the risk assumed in response to perceived predation risk. However, females reduced time devoted to nest sanitation activities when predator chemical cues were detected inside the nest-box, likely as an anti-predatory strategy to minimize their own risk of predation. Therefore, these results add to the evidence that birds can detect chemical cues of predators inside the nest cavity and suggest that the behavioral response to an increase in risk of predation perceived through olfactory cues is not sex-dependent in blue tit.

Random mating and the lack of sex-biased kin clustering in an island population of the bull-headed shrike, Lanius bucephalus

The encounter with relatives at the time of mating can be reduced if one or both sexes breed away from the natal site, and it is often assumed that this occurs to a lesser extent on islands where only short dispersal is possible. However, endemic insular populations may have evolved fine-tuned dispersal patterns enabling effective avoidance of inbreeding even in a small habitat, and the influence of spatial constraint should be more apparent for non-endemic than endemic populations. Recently established island populations of originally migratory birds can provide insights into the direct effect of habitat area on inbreeding risk. We studied spatial distribution of kin and inbreeding in a population of the bull-headed shrike, Lanius bucephalus, that became established on small oceanic islands separated from the mainland by > 360 km, Minami-daito and Kita-daito Islands, early in the 1970s. Sex-specific spatial genetic structure that could reduce inbreeding risk was absent within the focal Minami-daito Island. This result remained unchanged after the neighbouring Kita-daito Island was incorporated into the analysis. Average relatedness of breeding pairs was not different from that expected under random mating, and FIS was no greater than zero. The estimates of extra-pair paternity frequency were not high (4.5–16% of offspring, depending on the estimation method) compared to the mainland. Inbreeding coefficient of offspring was variable among nests, indicating some occurrence of inbreeding. We found no clear evidence of recent population bottleneck, suggesting that the purging of deleterious recessive alleles did not occur upon colonisation. These results suggest the absence of obvious inbreeding avoidance despite the potentially negative effects of inbreeding. Future studies should quantify inbreeding depression and also formally rule out the possibility that inbreeding is not avoided as an adaptive strategy as has been suggested for endemic island species.

No evidence of immediate fitness benefits of within-season divorce in monogamous birds.

Individuals of socially monogamous species can correct for suboptimal partnerships via two secondary mating strategies: divorce and extra-pair mating, with the former potentially providing both genetic and social benefits. Divorcing between breeding seasons has been shown to be generally adaptive behaviour across monogamous birds. Interestingly, some pairs also divorce during the breeding season, when constraints on finding a new partner are stronger. Despite being important for a comprehensive understanding of the evolution of social monogamy, whether within-season divorce is adaptive and how it relates to extra-pair mating remains unknown. Here, we meta-analysed 90 effect sizes on within-season divorce and breeding success, extracted from 31 studies on 24 species. We found no evidence that within-season divorce is adaptive for breeding success. However, the large heterogeneity of effect sizes and strong phylogenetic signal suggest social and environmental factors—which have rarely been considered in empirical studies—may play an important role in explaining variation among populations and species. Furthermore, we found no evidence that within-season divorce and extra-pair mating are complementary strategies. We discuss our findings within the current evidence of the adaptiveness of secondary mating strategies and their interplay that ultimately shapes the evolution of social monogamy.