Search

Sexual selection determines the elaboration of morphological and behavioural traits and thus drives the evolution of phenotypes. Sexual selection on males and females can differ between populations, especially when populations exhibit different breeding systems. A substantial body of literature describes how breeding systems shape ornamentation across species, with a strong emphasis on male ornamentation and female preference. However, whether breeding system predicts ornamentation within species and whether similar mechanisms as in males also shape the phenotype of females remains unclear. Here, we investigate how different breeding systems are associated with male and female ornamentation in five geographically distinct populations of Kentish plovers Charadrius alexandrinus. We predicted that polygamous populations would exhibit more elaborate ornaments and stronger sexual dimorphism than monogamous populations. By estimating the size and intensity of male (n = 162) and female (n = 174) melanin-based plumage ornaments, i.e. breast bands and ear coverts, we show that plumage ornamentation is predicted by breeding system in both sexes. A difference in especially male ornamentation between polygamous (darker and smaller ornaments) and monogamous (lighter and larger) populations causes the greatest sexual dimorphism to be associated with polygamy. The non-social environment, however, may also influence the degree of ornamentation, for instance through availability of food. We found that, in addition to breeding system, a key environmental parameter, rainfall, predicted a seasonal change of ornamentation in a sex-specific manner. Our results emphasise that to understand the phenotype of animals, it is important to consider both natural and sexual selection acting on both males and females.

Melanin-based plumage ornaments have been shown to play an important role in male-male competition, but also influence inter-sexual communication. Consequently, ornaments may be associated with reproductive effort of both males and females. Females mated to males with larger melanin ornaments may acquire access to better territories or benefit from increased paternal care. Here we investigated whether the melanin-based breast-band of male and female Bar-throated Apalis Apalis thoracica is a signal of information about its bearer and is associated with male and female reproductive effort. Breast-band size was a highly variable morphometric trait in both sexes, but only in males was it associated with body mass. We then assessed whether male and female breast-band size predicted maternal and paternal investment. Egg mass increased with male breast-band size, but decreased with female breast-band size. Whether females adjust maternal hormone allocation in response to their partner's ornamentation remains a contentious issue. We found that yolk testosterone and androstenedione concentrations were not predicted by male ornamentation or body mass. Finally, males with larger breast-bands provided their mates with more food, allowing those females to spend more time incubating. Reproductive effort of both parents is therefore predicted by their own and their mate's ornamentation in Bar-throated Apalis, and thus breast-band size potentially acts as a signal of reproductive performance in both sexes. These results highlight the need for more comprehensive analyses of a relationship between melanin-based ornaments and fitness, incorporating multiple behavioural variables associated with reproductive effort.

Dispersal is a critical driver of gene flow, with important consequences for population genetic structure, social interactions and other biological processes. Limited dispersal may result in kin-structured populations in which kin selection may operate, but it may also increase the risk of kin competition and inbreeding. Here, we use a combination of long-term field data and molecular genetics to examine dispersal patterns and their consequences for the population genetics of a highly social bird, the sociable weaver (Philetairus socius), which exhibits cooperation at various levels of sociality from nuclear family groups to its unique communal nests. Using 20 years of data, involving capture of 6508 birds and 3151 recaptures at 48 colonies, we found that both sexes exhibit philopatry and that any dispersal occurs over relatively short distances. Dispersal is female-biased, with females dispersing earlier, further, and to less closely related destination colonies than males. Genotyping data from 30 colonies showed that this pattern of dispersal is reflected by fine-scale genetic structure for both sexes, revealed by isolation by distance in terms of genetic relatedness and significant genetic variance among colonies. Both relationships were stronger among males than females. Crucially, significant relatedness extended beyond the level of the colony for both sexes. Such fine-scale population genetic structure may have played an important role in the evolution of cooperative behaviour in this species, but it may also result in a significant inbreeding risk, against which female-biased dispersal alone is unlikely to be an effective strategy. This article is protected by copyright. All rights reserved.

In animal societies, individuals face the dilemma of whether to cooperate or to compete over a shared resource. Two intertwined mechanisms may help to resolve this enduring evolutionary dilemma by preventing conflicts and thereby mediating the costs of living in groups: the establishment of dominance hierarchies and the use of ‘badge-of-status’ for signalling dominance. We investigated these two mechanisms in the sociable weaver (Philetairus socius), a colonial and social passerine which cooperates over multiple tasks. We examined the sociable weavers’ dominance structure in 2 years by recording 2563 agonistic interactions between 152 individuals observed at a feeder at eight colonies. We tested which individual traits, including sex, age, relatedness and two melanin-based plumage traits, predicted variation in social status. First, using social network analysis, we found that colonies were structured by strongly ordered hierarchies which were stable between years. Second, medium-ranked birds engaged more in aggressive interactions than highly ranking individuals, suggesting that competition over food is most pronounced among birds of intermediate social status. Third, we found that colony size and kinship influenced agonistic interactions, so aggression was less pronounced in smaller colonies and among relatives. Finally, within- and between-individual variation in social status and the presence of an individual at the feeder were associated with variation in bib size, as predicted by the badge-of-status hypothesis. These results suggest that dominance hierarchies and bib size mediate conflicts in sociable weaver societies.

Structures built by animals, such as nests, mounds and burrows, are often the product of cooperative investment by more than one individual. Such structures may be viewed as a public good, since all individuals that occupy them share the benefits they provide. However, access to the benefits generated by the structure may vary among individuals and is likely to be an important determinant of social organisation. Here we use the massive, communal nests of sociable weavers Philetairus socius, to investigate whether their thermoregulatory function varies in relation to the size of communal nests, and the position of individual nest chambers within the communal structure. We then examine whether this spatial variation in thermoregulatory function predicts the social organisation of colonies. First, we show that the sociable weavers’ communal nests buffer variation in ambient temperature, and reduce temperature variability within nest chambers. The extent of this buffering effect depends significantly on the position of nest chambers within the communal structure, and on the depth to which chambers are embedded within the nest mass. We detected no effect of nest volume on thermoregulatory benefits, suggesting that there are likely to be additional, non-thermoregulatory benefits leading to communal nests. Finally, our results indicate that there may be competition for access to the benefits of the public good, since older birds occupied the chambers with the highest thermoregulatory benefits, where breeding activity was also more common. We discuss how the spatial structure of the benefits of the public good might influence social organisation in the unique communal lifestyle of sociable weavers.

Parental care is costly, and in many organisms, the male or the female parent benefits from reducing its own care which may be compensated for by its mate. One of the parents may even face all costs of parental care if its mate deserts and leaves him/her to care for the offspring alone. Theoretical models have generated contrasting predictions as to how parents negotiate a resolution of this sexual conflict over care, although empirical tests are largely lacking. We investigated pre-desertion behaviour (nest attendance) of a highly polygamous passerine, the Eurasian penduline tit ( Remiz pendulinus) that exhibits intense sexual conflict over care culminating in clutch desertion by the male, by the female or by both parents. We conjectured that nest attendance of parents should predict desertion, so that the lack of care provided by the deserting parent may be compensated for. By analysing over 200 000 video frames (2.50 +/- 1.36 d per pair, 302 +/- 170 min/d) of 20 pairs, we show that nest desertion is not a gradual process and cannot be predicted by nest attendance. These results are consistent with the argument that the predictable desertion may not be evolutionarily stable, and suggest that male and female penduline tits do not negotiate clutch desertion.

Spatial and temporal distributions of resources and habitats often influence breeding systems. These influences are particularly relevant in those species that exhibit variable breeding systems. We studied such a species, the Penduline Tit Remiz pendulinus. This small passerine bird has sequential polygamy by both sexes, and evidence suggests intense sexual conflict between males and females over care. We estimated habitat structure by scoring the vegetation important for nest building and foraging in the immediate surroundings of the nest. Using four principal components we show that sites with more abundant vegetation are occupied earlier than sites with sparse vegetation. However, habitat structure does not predict mating success or reproductive success, and it neither predicts which parent (the male, the female or both) deserts the clutch. We therefore suggest that habitat structure does not have a direct effect on reproductive success or on the resolution of sexual conflict in Penduline Tits. Specific aspects of habitats, such as food and nest material availability, remain to be tested.

Do the two parents at a nest make simultaneous decisions whether to care for their offspring or to desert? If a single parent is sufficient for rearing young, one parent (typically, the male) may desert and reproduce with a new mate within the same breeding season, leaving the other parent with the brunt of care. As each parent is expected to maximize its own reproductive success, the interests of the two parents do not necessarily coincide, and a sexual conflict over care may emerge. Here we investigate the process of clutch desertion in a small passerine bird, the Penduline Tit Remiz pendulinus . Among birds, this species has a remarkably variable breeding system, because a single parent (either the male or the female) may provide the full care of the young, whereas about 30% of clutches are abandoned by both parents. First, we show that biparental desertion occurs within a single day in 73.7% of the clutches ( n = 14), whereas desertion decisions are sequential in 26.3% of the clutches ( n = 5) (male first: 10.5% ( n = 2); female first: 15.8% ( n = 3); n = 19 clutches in total). Secondly, we observed the behaviour of both parents before desertion, and investigated whether desertion can be predicted from their behaviour. However, neither singing nor nest-building behaviour predicted whether the male or the female would desert. We therefore suggest that biparental desertion may be simultaneous by male and female in our population of Penduline Tits. Furthermore, the parents do not appear to signal their intention to desert their mate. We argue that the parents’ interest may be actually to disguise their intention to desert.

The importance of extrapair paternity (EPP) as an aspect of mixed reproductive strategies is currently the focus of many studies. Since females have at least some control over the occurrence of inseminations, they are expected to engage in extrapair copulations only if they benefit, for instance through gaining high-quality or compatible genes for their offspring, or insurance against infertility. To distinguish between these benefits, we studied characteristics related to intermale variation in fertilization success as well as differences in fitness between half-siblings in the reed bunting, Emberiza schoeniclus, a socially monogamous passerine with high levels of EPP (50% of young). We found that older males were more successful at siring offspring in other broods and a nonsignificant tendency for them to be less cuckolded in their own broods. The success of older males does not support the fertility insurance hypothesis: a male's sperm storage capacity did not predict his fertilization success, the occurrence of infertile eggs was unrelated to the occurrence of EPP in a nest and older males had more infertile eggs in nests with no EPP. Extrapair young had longer tarsi at fledging than their maternal half-siblings, but we found no other differences between maternal or paternal half-siblings in several presumed quality traits. Owing to the absence of long-term fitness data, we are unable to determine whether the difference in tarsus length is related to a difference in fitness benefits. At this time, we cannot exclude the possibility that females do gain genetic benefits through EPP. (c) 2006 The Association for the Study of Animal Behaviour Published by Elsevier Ltd. All rights reserved.

Diverse patterns of parental care, including uniparental care by either the male or the female, provide excellent opportunities to investigate how variation in social traits is maintained in wild populations. Coexistence of different parental strategies within the same population is expected when they exhibit similar cost-benefit ratios. We investigated one of the most diverse avian breeding systems and compared parental behavior and reproductive output between nests that are uniparentally cared for by male or female Eurasian Penduline Tits (Remiz pendulinus). In this small passerine bird, full care (incubation and brood care) is provided by the male only (7-18% of nests) or the female only (48-65% of nests). Additionally, a third of all nests are deserted by both parents as a result of intense sexual conflict over care. We show that parental effort is similar between male and female parents, and they achieve similar offspring survival after controlling for clutch-size differences. Females care for larger clutches than males (5.7 +/- 0.1 [SE] and 3.5 +/- 0.2 eggs, respectively), and they produce more nestlings (3.6 +/- 0.2 and 2.4 +/- 0.3 nestlings, respectively), although this difference is not significant at fledging of the chicks (3.1 +/- 0.2 and 2.4 +/- 0.3 young, respectively). Therefore, sex difference in care does not explain the bias toward female-only care in Eurasian Penduline Tits. We propose that the differential benefits of desertion and/or the differential costs of care for males and females facilitate the existence of both uniparental care types in the population. Received 18 April 2012, accepted 2 July 2012.

Eurasian penduline tits ( Remiz pendulinus ) have an unusually diverse breeding system consisting of frequent male and female polygamy, and uniparental care by the male or the female. Intriguingly, 30 to 40 per cent of all nests are deserted by both parents. To understand the evolution of this diverse breeding system and frequent clutch desertion, we use 6 years of field data to derive fitness expectations for males and females depending on whether or not they care for their offspring. The resulting payoff matrix corresponds to an asymmetric Snowdrift Game with two alternative evolutionarily stable strategies (ESSs): female-only and male-only care. This, however, does not explain the polymorphism in care strategies and frequent biparental desertion, because theory predicts that one of the two ESSs should have spread to fixation. Using a bootstrapping approach, we demonstrate that taking account of individual variation in payoffs explains the patterns of care better than a model based on the average population payoff matrix. In particular, a model incorporating differences in male attractiveness closely predicts the observed frequencies of male and female desertion. Our work highlights the need for a new generation of individual-based evolutionary game-theoretic models.

Engagement in extra-pair copulations is an example of the abundant conflicting interests between males and females over reproduction. Potential benefits for females and the risk of cuckoldry for males are expected to have important implications on the evolution of parental care. However, whether parents adjust parental care in response to parentage remains unclear. In Eurasian penduline tits Remiz pendulinus, which are small polygamous songbirds, parental care is carried out either by the male or by the female. In addition, one third of clutches is deserted by both male and female. Desertion takes place during the egg-laying phase. Using genotypes of nine microsatellite loci of 443 offspring and 211 adults, we test whether extra-pair paternity predicts parental care. We expect males to be more likely to desert cuckolded broods, whereas we expect females, if they obtain benefits from having multiple sires, to be more likely to care for broods with multiple paternity. Our results suggest that parental care is not adjusted to parentage on an ecological timescale. Furthermore, we found that male attractiveness does not predict cuckoldry, and we found no evidence for indirect benefits for females (i.e., increased growth rates or heterozygosity of extra-pair offspring). We argue that male Eurasian penduline tits may not be able to assess the risk of cuckoldry; thus, a direct association with parental care is unlikely to evolve. However, timing of desertion (i.e., when to desert during the egg-laying phase) may be influenced by the risk of cuckoldry. Future work applying extensive gene sequencing and quantitative genetics is likely to further our understanding of how selection may influence the association between parentage and parental care.

Background The evolutionary interests of males and females rarely coincide (sexual conflict), and these conflicting interests influence morphology, behavior and speciation in various organisms. We examined consequences of variation in sexual conflict in two closely-related passerine birds with contrasting breeding systems: the Eurasian penduline tit Remiz pendulinus (EPT) exhibiting a highly polygamous breeding system with sexually antagonistic interests over parental care, and the socially monogamous Cape penduline tit Anthoscopus minutus (CPT). We derived four a priori predictions from sexual conflict theory and tested these using data collected in Central Europe (EPT) and South Africa (CPT). Firstly, we predicted that EPTs exhibit more sexually dimorphic plumage than CPTs due to more intense sexual selection. Secondly, we expected brighter EPT males to provide less care than duller males. Thirdly, since song is a sexually selected trait in many birds, male EPTs were expected to exhibit more complex songs than CPT males. Finally, intense sexual conflict in EPT was expected to lead to low nest attendance as an indication of sexually antagonistic interests, whereas we expected more cooperation between parents in CPT consistent with their socially monogamous breeding system. Results Consistent with our predictions EPTs exhibited greater sexual dimorphism in plumage and more complex song than CPTs, and brighter EPT males provided less care than duller ones. EPT parents attended the nest less frequently and less simultaneously than CPT parents. Conclusions These results are consistent with sexual conflict theory: species in which sexual conflict is more manifested (EPT) exhibited a stronger sexual dimorphism and more elaborated sexually selected traits than species with less intense sexual conflict (CPT). Our results are also consistent with the notion that EPTs attempt to force their partner to work harder as expected under sexual conflict: each member of the breeding pair attempts to shift the costs of care to the other parent. More brightly colored males benefit more from desertion than dull ones, because they are more likely to remate with a new female. Taken together, the comparison between two closely related species with contrasting breeding systems suggest that sexual conflict over care has influenced the evolution of behavior and morphology in penduline tits.

Animals have diverse strategies that improve the success of their offspring after mating (i.e. post-fertilization reproductive strategies); the most common ones are parental care and offspring provisioning. The type, mode and duration of parental care exhibited by males and females depend on social and nonsocial environment, and on genetic and phylogenetic constraints. We overview five rapidly developing areas of parental care research, and conclude that sexual conflict between parents, social interactions and environmental conditions play important roles in determining post-fertilization reproductive strategies. Keywords: parental care; offspring desertion; sexual conflict; cooperation

Despite the growing literature on facultative sex-ratio adjustment in chromosomal sex-determining vertebrate taxa (birds, mammals), the consistency of results is often low between studies and species. Here, we investigate the primary and secondary offspring sex ratio of a small passerine bird, the Eurasian Penduline Tit (Remiz pendulinus) in three consecutive years. This species has a uniquely diverse breeding system, in which the male (and/or the female) abandons the nest during egg-laying, and starts a new breeding attempt. This allowed us to test (1) whether patterns of parental care, i.e., male-only care, female-only care or biparental desertion, influence offspring sex ratio, and (2) whether the offspring sex ratio is repeatable between successive clutches of males and females. Using molecular markers to sex 497 offspring in 176 broods, we show that (1) offspring sex ratio does not depend on which parent provides care, and (2) the offspring sex ratio is not repeatable between clutches of a given individual. The overall primary and secondary offspring sex ratio at a population level is not different from parity (54 +/- 6% males, and 50 +/- 3% (mean +/- SE), respectively). We suggest that ecological and phenotypic factors, rather than individual traits of parents, may influence offspring's sex, and conclude that there is currently no evidence for a facultative adjustment of offspring sex ratio in the Penduline Tit.

Maternal effects can influence offspring phenotype with short- and long-term consequences. Yet, how the social environment may influence egg composition is not well understood. Here, we investigate how laying order and social environment predict maternal effects in the sociable weaver, Philetairus socius, a species that lives in massive communal nests which may be occupied by only a few to 100+ individuals in a single nest. This range of social environments is associated with variation in a number of phenotypic and life-history traits. We investigate whether maternal effects are adjusted accordingly. We found no evidence for the prediction that females might benefit from modifying brood hierarchies through an increased deposition of androgens with laying order. Instead, females appear to exacerbate brood reduction by decreasing the costly production of yolk mass and antioxidants with laying order. Additionally, we found that this effect did not depend on colony size. Finally, in accordance with an expected increased intensity of environmental stress with increasing colony size, we found that yolk androgen concentration increased with colony size. This result suggests that females may enhance the competitive ability of offspring raised in larger colonies, possibly preparing the offspring for a competitive social environment.