Male–Male Competition in a Mixed‐Mating Fish

Mating systems that comprise a mixture of pure males and self‐fertilising hermaphrodites remain an evolutionary enigma. In particular, our understanding of the sexual selection pressures associated with such mating systems is nascent. Males can only reproduce by fertilising hermaphrodites’ eggs, but hermaphrodites can also fertilise their own eggs and gain a genetic advantage by doing so. Consequently, there should be intense competition among males to access hermaphrodites. Here, we test the importance of male size, colour and heterozygosity in predicting the outcome of male–male competition using the mangrove rivulus, which has a male‐hermaphrodite mixed‐mating system. We pitted males against one another in dyadic laboratory trials to develop a dominance score for each male. We then correlated these scores with male length, several components of male colour, and heterozygosity. Male size was the only significant correlate of dominance: larger males dominated smaller males, implying selection for large male size. However, male mangrove rivulus are similar in size to hermaphrodites, indicating that directional selection for large body size in males is no greater than it is in hermaphrodites. Across all trials, colour was unrelated to dominance, but contests between similarly sized males were usually won by more colourful individuals. As mangrove rivulus are dichromatic, we suspect that male colour may prove to be more important in mate choice than we found it to be in intrasexual competition. Heterozygosity did not explain dominance directly, but correlated strongly with male size, implying an indirect role in intrasexual competition.     

Sexual Selection in the Water Spider: Female Choice and Male–Male Competition

The water spider Argyroneta aquatica is the only spider spending its whole life under water, and one of the few spider species in which males are larger than females. Previous studies indicated that males can cannibalize females, which is uncommon among spiders. Here we aimed to further test for a potential influence of sexual selection on male body size. We examined the importance of female choice by testing whether females prefer the larger of two simultaneously presented males as mating partners. Further, we examined the influence of male–male competition by comparing the fighting behaviour between large and small males when alone or when together with a female, and we determined the outcome of fights. We found that females approach and choose large males as mating partners, despite the risk of male cannibalism. Additionally, males intensively compete for females, and large males clearly win against smaller ones. Hence sexual selection seems to be important for the evolution of the peculiar sexual size dimorphism of water spiders, as large size is beneficial for males in both the intra‐ and intersexual context. Previous studies have suggested an important role of natural selection in the sex‐specific body size of water spiders, but natural and sexual selection mechanisms apparently work in the same direction, favouring large male size.     

Pure Self‐Assessment of Size During Male–Male Contests in the Parasitoid Wasp Nasonia vitripennis

During contest competition, a competitor may persist in a given contest based on information regarding its own fighting ability (resource‐holding potential, RHP), or that of its opponent. Although a number of models formalize the ways in which competitors are hypothesized to use RHP‐related information to determine their persistence in contests, we focused on pure self‐assessment and mutual assessment models in this study. According to pure self‐assessment models, a competitor uses only information regarding its own RHP to determine its persistence in a contest. In contrast, according to mutual assessment models, persistence is based on information regarding a competitor's RHP relative to that of its opponent and therefore requires assessment between competitors. In this study, using size as a proxy for RHP, we tested whether the parasitoid wasp Nasonia vitripennis utilizes pure self‐assessment or mutual assessment during pairwise, male–male contests. When we examined competitors of varied sizes, we found that the losing male's size was positively related to contest duration, but the winning male's size was uncorrelated with contest duration. When we examined contests in which competitors were size‐matched, we found that the mean size of paired competitors was positively related to contest duration. These results suggest that male N. vitripennis engage in pure self‐assessment during contests.     

Clutch-size variation in the Pied Flycatcher Ficedula hypoleuca

Variation in clutch-size of the Pied Flycatcher Ficedula hypoleuca was studied simultaneously in relation to latitude, longitude, altitude, laying date and habitat, using data from 103 areas from north Africa to north Norway (latitudinal range about 3900 km). Clutch-size variation was significantly related to altitude and habitat, small clutches being laid at high altitude and in coniferous forests. In contrast to some earlier studies, latitudinal or longitudinal clutch-size trends were not found.     

Paternity Loss in Relation to Male Age, Territorial Behaviour and Stress in the Pied Flycatcher

For sexual selection to operate in monogamous species, males of poor quality in some factor like age, ornamentation, condition or aggressiveness, should lose paternity compared with higher quality males. We tested this idea in an Iberian population of pied flycatchers ( Ficedula hypoleuca ). Microsatellite analysis of 67 broods revealed moderate levels of extra-pair paternity (22.4% of broods, 7.5% of young). In a sample of 58 broods for which the caring male was identified, a higher paternity loss was associated with younger males, males that were less aggressive during territorial intrusion tests performed before the commencement of laying, and with males that showed higher levels of corticosterone metabolites in faecal samples collected at the end of the nestling period. Plumage darkness, forehead patch size and condition were not related to paternity loss. Paternity loss is more related to behavioural or physiological traits than to morphological ones in this population.     

Association between haematozoan infections and reproduction in the Pied Flycatcher

1. Parasites may affect breeding success of their host since they compete for the same resources as their hosts. Reproduction may also increase the susceptibility of a host to parasite infections owing to lowered resistance to parasites during breeding.
2. We studied the association between breeding performance and haematozoan parasite infection in the Pied Flycatcher ( Ficedula hypoleuca ) by using both natural data on reproduction and data from clutch size manipulations.
3. The most frequent blood parasites of the Pied Flycatcher in central Finland were Haemoproteus pallidus , Haemoproteus balmorali and Trypanosoma avium complex.
4. We did not find evidence that these haematozoan parasites have any debilitating effects on either reproduction or survival. The variation in reproductive effort did not seem to influence susceptibility to new blood parasite infections.
5. The intensity of Haemoproteus balmorali tended to increase in infected males as the brood size was artificially enlarged. Also, in females intensity of H. pallidus infection tended to increase with the level of clutch size manipulation. Thus, increased reproductive effort seems to debilitate the ability of Pied Flycatcher to control chronic infections.
6. Individuals with enlarged clutches/broods increased their reproductive effort at the expense of defence towards parasites. The cost of current reproduction may then be at least partly mediated by haematozoan infections.     

Determinants of the Mating Success of Polyterritorial Pied Flycatcher Males

Much attention has been paid to the polyterritorial mating system of some passerine birds. Here we report how a male's mating success is related to the behavioral traits of polyterritorial pied flycatcher (Ficedula hypoleuca) males. We found no evidence that the timing of polyterritoriality in relation to egg laying in the primary nest or the singing behavior of males have any influence on mating success. However, results show clearly that male mating success was improved with an increase in the distance between territories up to a distance of about 200–300 m whereupon there was no further enhancement of mating success. This finding is crucial for both the deception hypothesis and female-female aggression hypothesis which have been put forward to explain polyterritorial polygyny. Males who establish a distant second territory seemed to allocate more time to singing there. The association, although weak, between singing activity and distance between territories makes it more difficult for females to use song rate as a cue to discriminate males with a secondary territory far from the primary territory, and these are the males that are least likely to feed the young of a second female. Males who established a second territory late in relation to egg laying in the primary nest did not take over a close second territory as might be predicted from female-female aggression hypothesis.     

Geographic variation in breeding parameters of the Pied Flycatcher Ficedula hypoleuca

Variation in laying date, clutch-size and number of fledglings of the Pied Flycatcher Ficedula hypoleuca was studied in relation to altitude, habitat and latitude, using data from 99 breeding areas from 35̀ N to 70̀ N. Laying date significantly increased with altitude and showed a significant quadratic relationship with latitude, with the earliest values at about 50̀ N. This quadratic relationship shows the same pattern as the relationship between arrival time of adults on the breeding areas and latitude. Clutch-size variation was significantly related to altitude and habitat, small clutches being laid at high altitude and in coniferous forests. Number of fledglings was significantly affected by habitat type, with lower values in the coniferous than in the deciduous forest. Mean clutch-size and mean number of fledglings showed the same quadratic relationship with latitude. In contrast to some earlier studies, an increased clutch-size with latitude was found, mainly because of new data from areas in the southern part of the species' distribution.     

Heterogeneous Maturation of Retina of the Pied Flycatcher Ficedula hypoleuca

A method is developed to determine the moment of establishment of functional maturity of cones by the appearance of their lipid droplet. Using this method, the process of differentiation of photoreceptor layer cells is traced and general regularities are elucidated of growth and maturation of the bifoveal retina of chicks of the pied flycatcher Ficedula hypoleuca. Development of retina occurs heterochronously; the first colored lipid droplets determining mature photoreceptors are photooptically revealed locally in the temporal retina area at the 5th day; at the 7th day, the lipid droplets appear within the limits of the central fovea; then the maturation wave moves from the temporal and central fovea towards periphery of retina. By the 14th day the entire area of retina turns out to be filled with mature photoreceptors. It is suggested that the predominant early maturation of photoreceptor cells of the temporal area is due to a necessity of the forestalling development of the retina parts providing the short-focus binocular vision that is the most important for establishment of chick alimentary behavior and start of hunting on their own after flight from the nest.     

Lack of Mate-guarding in a Territorial Passerine Bird with a Low Intensity of Sperm Competition,the Pied Flycatcher; (Ficedula hypoleuca)

We examined a Norwegian population of pied flycatchers (Ficedula hypoleuca) for behaviour associated with the paternity-protection tactic of mate-guarding and found that this tactic was absent. The absence of mate-guarding and the fact that pied flycatchers do not have a high rate of copulation make this population unusual to the extent that the current literature implies that, in birds, one or other paternity guard should be present. Comparison of our population with a Swedish population that does exhibit mate-guarding lends some support to the notion that differences in nesting density may drive the relative risk of extra-pair copulation, leading to the need for paternity guards such as mate-guarding. However, we conclude that, whilst the results of both our study and the Swedish study are consistent with the nesting-density hypothesis, there is at least one other plausible hypothesis that fits the available data. Specifically, we direct attention to the role of the female in determining overall patterns of pair behaviour and, potentially, the rate of extra-pair paternity, challenging the primacy of male—male competition as the driving force behind the sperm-competition dynamics of this species.     

Experimentally induced clutch size enlargements affect reproductive success in the Pied Flycatcher

We performed a food provisioning experiment in a population of Pied Flycatchers Ficedula hypoleuca breeding at high altitude in central Spain to test if food availability before and during laying determines clutch size. Food was provided to one of two pairs with the same date of initiation of nest-building (15 dyads of subsequently reproducing pairs were thus created). Food provisioning began on the day of initiation of nest-building and ended on the day after the last egg was laid. Although laying date was unaffected by the experiment, clutch size in the experimental treatment was significantly larger. This result could indicate that food availability at laying (1) proximately constrained clutch size or (2) that females evaluated future conditions for incubating eggs and feeding nestlings based on food availability at laying. Reproductive success (proportion of eggs that resulted in fledged young) was significantly reduced in the experimental treatment. This effect suggest that supplemented females were tricked by the experiment into laying more eggs than the number of eggs they were able to incubate with success and the number of nestlings they were able to feed, a source of error in clutch size adjustment which could be common in non-experimental situations.     

Extra‐Pair Paternity Declines with Female Age and Wing Length in the Pied Flycatcher

Despite many studies of how male characteristics affect paternity in predominantly monogamous birds, relatively little attention has been given to the traits of females that may influence extra‐pair paternity (EPP). However, the occurrence of EPP may be the result of behavioural interactions in which both male and female traits are important for determining the outcome. If EPP is driven mainly by female choice of extra‐pair sires, older, more experienced or larger females would be better able to evade mate guarding tactics and more capable of selecting extra‐pair mates and resisting unwanted suitors. This would be especially noticeable in females paired with unattractive mates. On the other hand, if EPP is driven mainly by male pursuit, we should expect that young, inexperienced or small females would be more exposed to coercive male approaches independently of social mate traits. In a study of an Iberian population of the pied flycatcher Ficedula hypoleuca, we found that EPP affected 38% of the broods and 17% of the nestlings. These values are relatively high, allowing a relatively large number of affected within‐pair mates to be included. We found that EPP is related to both female and male traits although not to any interaction between male and female traits. EPP was higher at nests tended by both younger and short‐winged females and by browner males. Older females may be more experienced and dominant while long‐winged females may be faster fliers, these traits enabling them to avoid extra‐pair copulations, while brown males are less aggressive towards male intruders. In our study population, EPP appears to be caused by male pursuit, which in some cases may overwhelm female attempts to avoid extra‐pair copulations and their social partner's ability to prevent them.     

Nest‐dwelling ectoparasites reduce begging effort in Pied Flycatcher Ficedula hypoleuca nestlings

Parasitized nestlings might be expected to increase begging effort to obtain additional resources to compensate for those sequestered by their parasites. However, begging is costly and chicks harbouring parasites may find it more difficult to attain high begging levels. Consequently, we predicted that, for the same level of nutritional need, nestlings that are parasitized will invest less in begging than those that are not parasitized. We tested this prediction by measuring begging in Pied Flycatcher Ficedula hypoleuca nestlings parasitized with haematophagous mites Dermanyssus gallinoides and Dermanyssus gallinae and blowfly larvae Protocalliphora azurea, and subjected to different levels of food deprivation in order to control for short‐term nutritional need. Nestlings from nests with ectoparasites spent less time begging than those from nests without parasites, especially when very hungry, although there was no association with latency to beg or begging intensity. Our results suggest that time invested in begging may indicate not only the level of need, but also nestling parasitism status.     

Divergent character displacement in the song of two allospecies: the Pied Flycatcher Ficedula hypoleuca, and the Collared Flycatcher Ficedula albicollis

Songs of the three flycatcher forms, Ficedula hypoleuca hypoleuca, Ficedula albicollis albicollis and Ficedula albicollis semitorquata , were distinguishable from one another using a multivariate analysis. F. albicollis semitorquata , which has an intermediate song character, is considered to be a primary allopatric form while the nominate subspecies of albicollis is assumed to be dispersed into sympatry with F. h. hypoleuca . The differences in song between the nominate albicollis and F. hypoleuca are accentuated in their zone of sympatry and are mostly due to divergence in the albicollis song. The song is primarily an epigamic vocalization in the two species and conforms to the pattern characteristic of divergent character displacement associated with reproductive behaviour. Two alarm calls are quite similar in the two species and are the main vocalizations associated with territorial combat. Their interspecific territorial behaviour has apparently not been affected in areas of sympatry.     

Male‐Biased Mate Competition in King Penguin Trio Parades

Darwin devised sexual selection theory to explain sexual dimorphisms. Further developments of the theory identified the operational sex‐ratio (OSR) as one of its cornerstones, and it was commonly admitted that an OSR biased toward one sex would lead to stronger selection pressures toward that sex. Recent theoretical developments have challenged this view and showed that the OSR alone does not determine the direction of sexual selection, more particularly in mutually ornamented species exhibiting high and similar parental investment by both sexes. These developments, however, focused on mutual intersexual selection, and little is known about intrasexual selection of both males and females in species exhibiting such characteristics. The first aim of our study was to test the relative involvement of males and females in same‐sex contest over mates in the king penguin, a species exhibiting mutual ornamentation of the sexes, high parental investment by both sexes, and a male‐biased OSR. We investigated the sex composition of trio parades, which are groups of three individuals that compete for mates during pair formation. We found that these trios consist of a female trailed by two fighting males in 19 of 20 cases; the 20th trio was all male. The second aim of our study was to investigate the existence of within‐sex differences in colour ornaments between individuals involved in such trios and individuals already paired. While limited sample sizes precluded detection of statistically significant differences between trios vs. pairs, reflectance measurements suggested that the beak spot of males in trios were more strongly ultraviolet than the beak spot of males in pairs. We concluded that intrasexual selection in our colony follows the typical pattern of mate competition observed in species in which sexual dimorphisms and OSR are male biased, and discussed the ultraviolet difference within the framework of the king penguins' colour perception.     

Female Contests for Nest Sites and Mates in the Pied Flycatcher Ficedula hypoleuca

Predictions of game theory models about the outcome of animal conflicts have most often been tested using male contests for mates, territories or food. We studied female contests for nest sites and mates in the pied flycatcher, Ficedula hypoleuca, and asked which factors affect the outcome and duration of the contests. Conflicts over residency were experimentally induced, but natural cases in which both females regarded themselves as owners were also included in the analyses. The ensuing escalated contests involved physical fighting, defence of the nestbox of the male and alarm calling. Contrary to expectations from game theory, fighting seemed to occur most often at the start of conflicts, whereas alarm calling occurred towards the end and nestbox visits throughout the conflict. The outcome of the contests was not determined by asymmetries in body size or age, nor by a simple previous-present owner asymmetry. Instead, it depended on the relative residence times of the opponents. The duration of the contests tended to increase with decreasing asymmetry in the residence times of females, whereas body size asymmetries had no influence. Because there is intense competition between females for a mate in the pied flycatcher, we suggest that females do not respect asymmetries in residency and body size, but fight in relation to the value of the mating opportunity. In particular, we point to the possibility that the value of a mating opportunity may increase with residence time because knowledge of other mating options may become outdated.