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Extra‐pair paternity (EPP) is a widespread phenomenon, as EPP has been observed in 76% of the socially monogamous bird species. Many hypotheses try to explain the evolution of infidelity. While females may participate in extra‐pair copulations, for instance, to ensure the fertilisation of their eggs or to obtain potential genetic benefits for their offspring, unfaithful females face many potential costs too. As nestling provisioning is one of, if not the most energetically costly forms of parental care, the certainty of paternity hypothesis predicts that males with an unfaithful partner reduce their parental investment to avoid the fitness loss arising from rearing unrelated nestlings. We investigated the relationship between the presence and proportion of extra‐pair young (EPY) and the feeding rate of the social male to reveal whether males recognise and penalise unfaithfulness. We conducted the study in a Hungarian population of collared flycatchers (Ficedula albicollis) where the EPP rate had been reported to be high. We cross‐fostered nestlings so that each parent reared offspring from two foreign broods and none from their own. Thus, any relationship between paternal investment and paternity in the original brood of the male should be the direct consequence of the female's mating behaviour (as perceived by the male) and not the result of early maternal effects or different behaviour of extra‐ and within‐pair offspring. We found that 63.6% of the broods contained EPY, and 23% of the nestlings were sired by extra‐pair fathers. The only relationship we found was that males with larger broods fed their offspring more frequently. Neither the prevalence nor the proportion of EPY was related to the male feeding rate; thus, our results do not support the certainty of paternity hypothesis. This might be explained by the inability of the males to track their females' behaviour in a population with a high EPP rate. In an experimental study, we investigated the relationship between within‐pair paternity of the males and their feeding rates in a monogamous passerine with a high rate of infidelity to reveal whether males recognise and penalise the unfaithfulness of their social partners. Our results contradict those previously found in another population of the same species. We argue that the contradictory findings may be explained by the dissimilar rate of extra‐pair copulations in the two populations and the trade‐off between males' extra‐pair mating activity and ability to assess their within‐pair paternity reliably.

The theoretical literature predicts that parentage differences between the sexes, due to females mating with multiple males, select males to provide less parental care and females to care more for the offspring. We formulate simple evolutionary games to question the generality of this prediction. We find that the relationship between paternal care and fitness gained from extrapair matings is important. A trade-off between these two quantities is required for partial paternity and complete maternity to bias the evolutionary stable strategy (ESS) toward more female care. We argue that this trade-off has been implicitly or explicitly assumed in most previous theories. However, if there is no trade-off between paternal care and extra-pair matings, parentage differences do not influence the ESS sex roles. Moreover, it is also possible for these two quantities to have a positive relationship, in which case we predict selection for male care is possible. We support these predictions using agent-based simulations. We also consider the possibility that caring males have greater opportunities to guard their paternity, and find that this mechanism can also select for male-biased care. Hence, we derive the conditions under which male care may be selected despite partial paternity and complete maternity.

Despite keen interest in extra-pair mating in birds, its adaptive significance remains unresolved. Here, we use a multi-year dataset to test whether traits of a female's social mate influence her propensity to produce extra-pair offspring in a population of house wrens, and whether producing extra-pair young has consequences for a female's fitness through effects on offspring survival. Females were most likely to produce extra-pair offspring when paired with old males and when paired with males on poor-quality territories, although this latter effect was marginally nonsignificant. Among offspring, the cutaneous immunity of within-pair young decreased as the age of their sires increased, but cutaneous immunity of extra-pair young was not affected by the age of their extra-pair sires or by the age of the males rearing them. Extra-pair offspring were more likely than within-pair offspring to return as breeding adults to the local population, with extra-pair sons being more likely to return as as breeder for multiple years. Our findings support the hypothesis that females produce extra-pair offspring to enhance their inclusive fitness beyond what they are capable of given the male with which they are socially paired.

Whether females should prefer to mate with old males is controversial. Old males may sire offspring of low quality because of an aging germline, but their proven ability to reach an old age can also be an excellent indicator of superior genetic quality, especially in natural populations. These genetic effects are, however, hard to study in nature, because they are often confounded with direct benefits offered by old males to the female, such as experience and high territory quality. We, therefore, used naturally occurring extra‐pair young to disentangle different aspects of male age on female fitness in a natural population of collared flycatchers because any difference between within‐ and extra‐pair young within a nest should be caused by paternal genetic effects only. Based on 18 years of long‐term data, we found that females paired with older males as social partners experienced an overall reproductive advantage. However, offspring sired by old males were of lower quality as compared to their extra‐pair half‐siblings, whereas the opposite was found in nests attended by young males. These results imply a negative genetic effect of old paternal age, given that extra‐pair males are competitive middle‐age males. Thus, offspring may benefit from being sired by young males but raised by old males, to maximize both genetic and direct effects. Our results show that direct and genetic benefits from pairing with old males may act in opposing directions and that the quality of the germline may deteriorate before other signs of senescence become obvious.

The mode in which sexual organisms choose mates is a key evolutionary process, as it can have a profound impact on fitness and speciation. One way to study mate choice in the wild is by measuring trait correlation between mates. Positive assortative mating is inferred when individuals of a mating pair display traits that are more similar than those expected under random mating while negative assortative mating is the opposite. A recent review of 1134 trait correlations found that positive estimates of assortative mating were more frequent and larger in magnitude than negative estimates. Here, we describe the scale-of-choice effect (SCE), which occurs when mate choice exists at a smaller scale than that of the investigator's sampling, while simultaneously the trait is heterogeneously distributed at the true scale-of-choice. We demonstrate the SCE by Monte Carlo simulations and estimate it in two organisms showing positive (Littorina saxatilis) and negative (L. fabalis) assortative mating. Our results show that both positive and negative estimates are biased by the SCE by different magnitudes, typically toward positive values. Therefore, the low frequency of negative assortative mating observed in the literature may be due to the SCE's impact on correlation estimates, which demands new experimental evaluation.

The ability of sexually mature non-territorial floaters to sire offspring affects the success of floating as a breeding strategy. Red-winged Blackbirds (Agelaius phoeniceus) have second-year (SY) and after-second-year (ASY) floater males, and genetic studies suggest that floaters may gain paternity. Despite these studies, we still know little about the fitness costs and benefits of floating in this species. By presenting taxidermic models of females in soliciting, precopulatory postures in territories of experienced (previously attracted at least one mate in the study area) and inexperienced (did not previously defend a territory in the study area) males, I was able to examine the copulation behavior and success of floater male Red-winged Blackbirds as well as the effect of experience for territorial males. Floaters trespassed during 66.1% of presentations and 85.4% of trespassers were SY males. Experienced territorial males (92.5%) and neighbors (87.5%) were most successful in attempts to copulate with models, inexperienced territorial males (62.5%) and ASY floaters (50.0%) had intermediate success, and SY floaters (6.9%) were least successful. Experienced territorial males were more likely to approach models than inexperienced males, and floaters were more likely to approach models in territories of experienced than inexperienced males. These results provide further evidence that floaters trespass frequently, suggest that floaters sire offspring, and demonstrate that prior breeding experience affects the behavior and reproductive success of territorial male Red-winged Blackbirds. Floating appears to be a conditional strategy for ASY male Red-winged Blackbirds, but, because it is still not known if SY floaters sire offspring, these males may be trespassing to gain information or experience. La habilidad de individuos flotadores (sin territorio propio) sexualmente maduros, para dejar progenie afecta el éxito de los flotadores como estrategia reproductiva. Estudios genéticos sugieren que los machos flotadores de Mariquitas (Agelaius phoeniceus), de segundo año (SA) y de mayor edad (ME) pueden dejar progenie. Pese a estos estudios, sabemos muy poco del costo de aptitud y beneficios de los flotadores en esta especie. Mediante la presentación de montajes (por taxidermia) de hembras, solicitando a machos en posición pre-copulatriz, en territorios de machos con experiencia (que previamente atrajo al menos una pareja en el área de estudio), y sin experiencia (previamente no había defendido un territorio en el área de estudio), pude examinar la conducta de copular, el éxito de flotadores y el efecto de la experiencia en machos territoriales, en las mariquitas. Los flotadores se acercaron a los modelos en el 66.1% de los casos y el 85.4% de estos resultaron ser machos SA. El 92.5% de los machos con experiencia territorial y el 87.5% de los vecinos de estos, fueron los más exitosos en sus intentos de copular con los modelos, mientras que el 62.5% de los machos sin experiencia territorial y el 50% de los flotadores ME tuvieron éxito intermedio, siendo los menos exitosos los flotadores SA (6.9%). Los machos con experiencia territorial fueron mas adeptos a acercarse a los modelos que los que no tenían experiencia, y los flotadores fueron mas propensos de acercarse a los modelos que se colocaron en territorios con machos con experiencia, que de machos sin experiencia. Los resultados proveen evidencia adicional, de que los flotadores invaden territorios de otros con mucha frecuencia, lo que sugiere que estos dejan progenie y demuestra que la experiencia reproductiva previa, afecta la conducta y el éxito reproductivo de machos territoriales de la Mariquita. El \"flotar\" parece ser una estrategia condicionada de los machos ME, pero como todavía no se sabe si los flotadores SA dejan progenie, la transgresión territorial pudiera ser una forma de obtener información y la experiencia pertinente.

Alloparental care – care for unrelated young – is rare in animals, and its ecological or evolutionary advantages or, alternative maladaptive nature, remain unclear. We investigate alloparental care in the socially monogamous cichlid fish Perissodus microlepis from Lake Tanganyika that exhibits bi‐parental care. In a genetic parentage analysis, we discovered a surprisingly high percentage of alloparental care represented by brood mixing, extra‐pair paternity and extra‐pair maternity in all broods that we investigated. The percentage of nondescendant juveniles of other parents, i.e., brood mixing, ranged from 5% to 57% (mean = 28%). The distribution of genetic parentage also suggests that this socially monogamous species has, in fact, polygamous mating system. The prevalence of genetically mixed broods can be best explained by two, not mutually exclusive hypotheses on farming‐out and fostering behaviors. In the majority of broods, the sizes of the parents’ own (descendant) offspring were significantly larger than those of the adopted (nondescendant) juveniles, supporting the ‘selfish shepherd effect’ hypothesis, i.e., that foster parents preferentially accept unrelated “smaller or not larger” young since this would tend to lower the predation risks for their own larger offspring. There was also a tendency for larger parents particularly mothers, more so than smaller parents, to care predominantly for their own offspring. Larger parents might be better at defending against cuckoldry and having foreign young dumped into their broods through farming‐out behavior. This result might argue for maladaptive effects of allopatric care for the foster parents that only larger and possibly more experienced pairs can guard against. It needs to be determined why, apparently, the ability to recognize one's own young has not evolved in this species. We investigate alloparental care in the socially monogamous cichlid fish Perissodus microlepis from Lake Tanganyika that exhibits biparental care. In a genetic parentage analysis, we discovered a surprisingly high percentage of alloparental care represented by brood mixing, extra‐pair paternity and extra‐pair maternity in all broods that we investigated. The prevalence of genetically mixed broods can be best explained by two, not mutually exclusive hypotheses on farming‐out and fostering behaviours. We also find evidence supporting the ‘selfish shepherd effect’ hypothesis that foster parents preferentially accept unrelated “smaller or not larger” young since this would tend to lower the predation risks for their own larger offspring.

Extra-pair copulations (EPCs) are the poorly known antecedents of extra-pair fertilizations (EPFs) in birds. EPFs occur in most bird species that have been examined, but sexual conflict will generally reward females hiding their EPCs from males attempting to protect their paternity. EPCs will be difficult for researchers to document, and necessarily underestimated, in that case. We measured EPC behaviors and EPF frequency in a colonial seabird, the Nazca booby Sula granti , in which all copulations occur in a visually open setting with numerous possible copulatory partners readily available. Females are larger and more physically powerful than males, and are the numerically limiting sex, perhaps limiting options for males to control females. We found that all copulations were voluntary, and females’ sexual activities were wholly unconstrained by male coercion. Most females had multiple copulatory partners in the weeks preceding egg-laying. Despite the commonness of EPC, EPFs did not occur. The different schedules of EPC and within-pair copulation (WPC) provided a sufficient explanation for this outcome: during the ovulation window days before laying, WPC rate increased and EPC rate approached zero. To our knowledge, this is the first robust evidence of complete sexual agency in a female bird aside from lek-mating species, contributing a valuable exemplar to the literature on sexual conflict over reproduction.

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.

Population decline among Asian horseshoe crabs in Asia is increasingly reported, but knowledge of their population and ecological status in China is limited. We conducted community interviews in 30 fishing villages around Beibu Gulf in Guangxi, China, to collect distribution information about the potential spawning/nursery grounds of Tachypleus tridentatus and Carcinoscorpius rotundicauda, and any imminent threats to their populations. Based on the results from 400 respondents we identified 45 potential spawning/nursery grounds distributed widely along the shores of Beibu Gulf. We visited 10 of these sites and verified the presence of juvenile horseshoe crabs by field surveys. Nearly all respondents reported an overall depletion in horseshoe crab populations from these 45 sites, which they attributed mainly to unsustainable fishing practices. Respondents who reported having seen horseshoe crab mating pairs on shores were mostly older people, which may suggest a considerable reduction in horseshoe crabs coming to the shores to spawn in recent years. The mean daily harvest of adult T. tridentatus offshore, as indicated by fishers, has declined from c. 50–1,000 in the 1990s to 0–30 individuals during 2011–2016. Our Wisdom of Crowds approach, supported by confirmatory field surveys, is a cost-effective method for assessing the population status of horseshoe crabs, and the level of threat they face. Similar approaches with other species are likely to be particularly valuable in the Asia–Pacific region, where well-structured population monitoring is largely unaffordable.