Extra-pair paternity and sexual dimorphism in birds

Differences in the strength of sexual selection between males and females can lead to sexual dimorphism. Extra-pair paternity (EPP) can increase the variance in male reproductive success and hence the opportunity for sexual selection. Previous research on birds suggests that EPP drives the evolution of dimorphism in plumage colour and in body size. Because EPP increases the intensity of sexual selection in males, it should lead to increased dimorphism in species with larger or more colourful males, but decreased dimorphism in species with larger or more colourful females. We explored the covariation between EPP and sexual dimorphism in wing length and plumage colouration in 401 bird species, while controlling for other, potentially confounding variables. Wing length dimorphism was associated positively with the frequency of EPP, but also with social polygamy, sex bias in parental behaviour and body size and negatively with migration distance. The frequency of EPP was the only predictor of plumage colour dimorphism. In support of our prediction, high EPP levels were associated with sexual dichromatism, positively in species in which males are more colourful and negatively in those in which females are more colourful. Contrary to our prediction, high EPP rates were associated with increased wing length dimorphism in species with both male- and female-biased dimorphism. The results support a role for EPP in the evolution of both size and plumage colour dimorphism. The two forms of dimorphism were weakly correlated and predicted by different reproductive, social and life-history traits, suggesting an independent evolution.     

Why change mates?

In some wading birds (Charadrii) each adult rears a brood alone. The female leaves her first clutch with her first mate before herself rearing a clutch fertilized by a second male, who is already tending its first mate's first clutch. We here develop a simple model to account for the exchange of mates between clutches, and relate it to reported field studies. We suggest that females are attempting to be polygamous, rather than simply bigamous. Provided a female has a chance of obtaining a second, unmated male and that the costs of leaving the first male before remating are not high, matechanging will be favoured over monogamy. The implications of this model to the evolution of the more usual forms of polyandry and of male-like females are discussed.     

The mating system of the Sichuan snub‐nosed monkey (Rhinopithecus roxellana)

This article reports the first genetic study of the mating system of the Sichuan snub‐nosed monkey (Rhinopithecus roxellana), an endemic and endangered species in China. The investigation was carried out in a population (WRT) in the Qinling Mountains using data from both field observation and paternity analysis through microsatellite DNA profiling. During a mating season, a male on an average copulated with 5.7 females. Approximately 18% of the females were observed to copulate with more than one male over the study period. The majority of copulations (94.5%) were initiated by females. Twenty‐eight of 430 observed matings were extra‐unit copulations. Eight polymorphic microsatellite loci were used for paternity analysis. The number of alleles at each locus ranged from 3 to 7 (mean=4.3). Observed heterozygosity ranged between 0.32 and 0.79. None of the loci showed significant deviation from Hardy–Weinberg equilibrium. Results from paternity exclusion showed that 12 of 21 (57.1%) immature individuals were sired by extra‐unit males. Although the basic social unit of snub‐nosed monkeys is consistent with a polygynous mating system, both field observation and genetic data suggests that their mating system is polygamous. Infanticide and inbreeding avoidance are the most likely explanations for the promiscuity of female snub‐nosed monkeys. Am. J. Primatol. 72:25–32, 2010. © 2009 Wiley‐Liss, Inc.     

Do female pied flycatchers seek extrapair copulations with familiar males? A test of the incomplete knowledge hypothesis

In birds with biparental care, great variation exists in thefrequency of extrapair paternity. Several hypotheses have beenproposed to account for this variation. We tested the incompleteknowledge hypothesis, which states that females are constrainedin their knowledge of male quality and that this influencestheir willingness to engage in extrapair copulations (EPC).By selective removal and release of female pied flycatchersFicedula hypoleuca, we created a situation where females finallysettled with a social mate close to the site where a formersocial mate was breeding. According to the incomplete knowledgehypothesis, this would lower the threshold for females to seekextrapair copulations in cases where their former social matewas of higher quality than the one finally chosen. The hypothesiswas not supported because manipulation of female settlementdid not increase frequency of extrapair paternity, not evenin cases where the female nested close to the previous mateand the current mate apparently was of lower quality becausehe was younger and more dull colored. However, we found that when extrapair paternity did occur, the cuckolder tended tobe a familiar male (i.e., the female's initial social mate).     

The effects of sexual selection on life‐history traits: an experimental study on guppies

Sexual selection is often prevented during captive breeding in order to maximize effective population size and retain genetic diversity. However, enforcing monogamy and thereby preventing sexual selection may affect population fitness either negatively by preventing the purging of deleterious mutations or positively by reducing sexual conflicts. To better understand the effect of sexual selection on the fitness of small populations, we compared components of female fitness and the expression of male secondary sexual characters in 19 experimental populations of guppies (Poecilia reticulata) maintained under polygamous or monogamous mating regimes over nine generations. In order to generate treatments that solely differed by their level of sexual selection, the middle‐class neighbourhood breeding design was enforced in the monogamous populations, while in the polygamous populations, all females contributed similarly to the next generation with one male and one female offspring. This experimental design allowed potential sexual conflicts to increase in the polygamous populations because selection could not operate on adult‐female traits. Clutch size and offspring survival showed a weak decline from generation to generation but did not differ among treatments. Offspring size, however, declined across generations, but more in monogamous than polygamous populations. By generation eight, orange‐ and black‐spot areas were larger in males from the polygamous treatment, but these differences were not statistically significant. Overall, these results suggest that neither sexual conflict nor the purging of deleterious mutation had important effects on the fitness of our experimental populations. However, only few generations of enforced monogamy in a benign environment were sufficient to negatively affect offspring size, a trait potentially crucial for survival in the wild. Sexual selection may therefore, under certain circumstances, be beneficial over enforced monogamy during captive breeding.     

Animal‐borne video systems provide insight into the reproductive behavior of the Asian black bear

Previous studies on the mating system of the Asian black bear (Ursus thibetanus) have been limited to observations of captive populations and estimations of multiple paternities. Hence, the mating system of wild bears remains poorly understood. Animal‐borne camera systems (i.e., cameras mounted on animals) provide novel tools to study the behavior of elusive animals. Here, we used an animal‐borne video system to record the activities of wild bears during the mating season. Video camera collars were attached to four adult Asian black bears (male “A” and “B,” and female “A” and “B”) captured in Tokyo, central Japan, in May and June 2018. The collars were retrieved in July 2018, after which the video data were downloaded and analyzed in terms of bear activity and mating behavior. All the bears were found to interact with other uniquely identifiable bears for some of the time (range 9–22 days) during the deployment period (range 36–45 days), and multiple mating in males was documented. Both males and females exhibited different behaviors on social days (i.e., days when the bear interacted with conspecifics) compared with solitary days (i.e., days with no observed interactions with conspecifics). Compared with solitary days, the bears spent a lower proportion of time on foraging activities and higher proportion of time on resting activities on social days. Our results suggest that Asian black bears have a polygamous mating system, as both sexes consort and potentially mate with multiple partners during a given mating season. Furthermore, bears appeared to reduce their foraging activities on social days and engaged more in social interactions.     

Preliminary study on female choice of mates versus sites in a wrasse, Cirrhilabrus temminckii

Female choice of mates versus sites was studied in a wrasse, Cirrhilabrus temminckii. Males had territories within a restricted area on a rocky slope at which females visited and pair-spawned pelagic gametes. Females visited several males or territories before spawning, suggesting the opportunity of female choice. Of the four characteristics of territorial males examined—body size, ratio of pelvic fin length to body size, courtship, frequency and territory depth—only territory depth was significantly correlated with daily mating success of males. The former three male characters were not related to territory depth. These results suggest that female C. temminckii chooses deep sites rather than specific mates in mating.     

Population genetic models of male and mutual mate choice

Examples of male mate choice are becoming increasingly common, even in polygynous species. We create a series of population genetic models to examine the evolutionary equilibria and dynamics resulting from male mate choice during polygyny, alone and in the context of mutual mate choice by both sexes. We find that unless males with a preference are able to increase their overall courtship output, male preference will be lost. This loss can be counteracted if males choose females not based on arbitrary traits, but based on a trait that indicates high fertility or viability. We also conclude that if male and female preferences and traits are all controlled by different loci, the male and female mate choice systems are decoupled; the presence of a male preference then has no influence on the equilibria or dynamics of female mate choice. If male and female traits are coupled by pleiotropy, it becomes possible for a male preference to be maintained, regardless of whether preferences between the sexes are pleiotropic or controlled by separate loci.     

The effects of familiarity and group size on mating preferences in the guppy,Poecilia reticulata

Abstract In recent years, it has become evident that frequency dependence in the attractiveness of a particular phenotype to mates can contribute to the maintenance of polymorphism. However, these preferences for rare and unfamiliar male phenotypes have only been demonstrated in small, controlled experiments. Here, we tested the preference for unfamiliar mates in groups of six to 96 individuals over 13 days, in the guppy (Poecilia reticulata). We observed individual behaviour in situ to test whether fish discriminate two unfamiliar individuals among many familiar ones. We found that unfamiliar males and females were preferred over the familiar fishes in all groups and that this effect decayed over time. Increasing group sizes and levels of sexual activity did not hamper the preference for unfamiliar mates, providing further support for the role of frequency dependent mate choice in the maintenance of trait polymorphism in natural populations.