Monogamous pair bonds and mate switching in the Western Australian seahorse Hippocampus subelongatus

Apparently monogamous animals often prove, upon genetic inspection, to mate polygamously. Seahorse males provide care in a brood pouch. An earlier genetic study of the Western Australian seahorse demonstrated that males mate with only one female for each particular brood. Here we investigate whether males remain monogamous in sequential pregnancies during a breeding season. In a natural population we tagged males and sampled young from two successive broods of 14 males. Microsatellite analyses of parentage revealed that eight males re‐mated with the same female, and six with a new female. Thus, in this first study to document long‐term genetic monogamy in a seahorse, we show that switches of mates still occur. Polygynous males moved greater distances between broods, and tended to have longer interbrood intervals, than monogamous males, suggesting substantial costs associated with the breaking of pair bonds which may explain the high degree of social monogamy in this fish genus.     

Sex allocation in the sexually monomorphic fairy martin

Offspring sex ratios were examined at the population and family level in the sexually monomorphic, socially monogamous fairy martin Petrochelidon ariel at five colony sites over a 4-year period (1993–1996). The sex of 465 nestlings from 169 broods was determined using sex-specific PCR at the CHD locus. In accordance with predicted sex allocation patterns, population sex ratios at hatching and fledging did not differ from parity in any year and the variance in brood sex ratios did not deviate from the binomial distribution. Further, brood sex ratio did not vary with hatching date during the season, brood number, brood size or colony size. The sex ratio of broods with extra-pair young did not differ from those without, while the sex ratio of broods fathered by males that gained extra-pair fertilizations did not differ from broods fathered by other males. Extra-pair chicks were as likely to be male as female. Neither the total number of feeding visits to the brood nor the relative feeding contribution by the sexes varied significantly with brood sex ratio. Brood sex ratios were also unrelated to paternal size, condition and breeding experience or maternal condition and breeding experience. However, contrary to our prediction, brood sex ratio was negatively correlated with maternal size. Generally, these results were consistent with our expectations that brood sex ratios would not vary with environmental factors or parental characteristics, and would not influence the level of parental provisioning. However, the finding that females with longer tarsi produced an excess of daughters is difficult to reconcile with our current understanding of fairy martin life history and breeding ecology.     

Local Mating,Dispersal and Sex Ratio in a Gregarious Parasitoid Wasp

Parasitoid sex ratios can be greatly influenced by mating and dispersal behaviour. Many sex ratio models assume that mating is strictly local (only mated females disperse from the natal patch) and that a single male is sufficient to inseminate all females in a brood. Bethylids (aculeate parasitoids) have been used to test predictions of these models, but less attention has been paid to testing their underlying assumptions. We investigated the timing of eclosion, mating and dispersal in mixed-sex and single-sex broods of the bethylid wasp Goniozus nephantidis. In mixed-sex broods, almost all females mate before dispersal and a single male is sufficient to inseminate virtually all females, even when brood sizes are large. Males disperse from both mixed-sex and all-male broods, but males in all-male broods disperse more slowly. Virgin females disperse from all-female broods, which are common. Virgin females can produce a brood, mate with their own sons and subsequently produce mixed-sex broods, but their success rate is very low. Virgin females could potentially circumvent sex allocation constraints by superparasitizing mixed-sex broods, but when presented with hosts bearing mixed-sex broods they destroy all members of the initial brood before ovipositing. Because of the high prevalence of single-sex broods and dispersal of both sexes, the mating structure of G. nephantidis is unlikely to conform to the assumption of strict local mating.     

No evidence of skewed secondary sex ratios in nestlings of the Common Raven (Corvus corax)

Many bird species adjust their offspring sex ratio as a response to environmental conditions or sexual dimorphism in size and dispersal. Offspring sex ratios may therefore vary among populations depending on the different demographic and ecological trajectories. We sampled Common Raven Corvus corax nestlings close to the fledging stage from three Central European regions to test for skewed secondary sex ratios and to investigate differences in sex ratios between populations that differ in recent recolonization history and breeding densities. Between 2005 and 2007, a total of 108 broods with 335 nestlings were sampled and their sex determined using molecular methods. We observed a mean of 3.1 (±1.2) nestlings per brood with no differences among nesting sites, years or regions. Nestling sex ratios were independent of the number of siblings. The overall secondary sex ratio was close to parity and did not differ between the variably structured populations.     

Maternal correlates of brood sex ratio variation in the lekking lance-tailed manakin Chiroxiphia lanceolata

Theory predicts that overall population sex ratios should be around parity. But when individual females can receive higher fitness from offspring of one sex, they may benefit by biasing their brood sex ratios accordingly. In lekking species, higher variance in male reproductive success relative to that of females predicts that male offspring gain disproportionately from favorable rearing conditions. Females should therefore produce male-biased broods when they are in a position to raise higher quality offspring: i.e., in better body condition or when they reproduce earlier in the breeding season. To investigate these hypotheses, we studied brood sex ratios of lance-tailed manakins Chiroxiphia lanceolata . We found that overall sex ratios and mean brood sex ratios were not different from random expectation. Brood sex ratios were not related to laying date or female body condition. However, we detected a quadratic relationship between brood sex ratios and maternal age: both young (1–2 years) and old (8+ years) females produced female-biased brood sex ratios. This relationship was most clear in a year also distinguished by early rainy and breeding seasons. We suggest that breeding inexperience in young females and senescence in older females is the most plausible explanation for these results, and that the relationship between female age and brood sex ratio is mediated by environmental conditions.     

Evidence for Strategic Sex Allocation in the Guppy (Poecilia reticulata): Brood Sex Ratio and Size Predict Subsequent Adult Male Mating Success

Sex allocation theory predicts that females should produce more sons when the reproductive success of sons is expected to be high, whereas they should produce more daughters, not daughters when the reproductive success of sons is expected to be low. The guppy (Poecilia reticulata) is a live‐bearing fish, and female guppies are known to produce broods with biased sex ratios. In this study, we examined the relationship between brood sex ratio and reproductive success of sons and daughters, to determine whether female guppies benefit from producing broods with biased sex ratios. We found that sons in male‐biased broods had greater mating success at maturity than sons in female‐biased broods when brood sizes were larger. On the other hand, the reproductive output of daughters was not significantly affected by brood sizes and sex ratios. Our results suggest that female guppies benefit from producing large, male‐biased brood when the reproductive success of sons is expected to be high.     

Genetic monogamy despite social promiscuity in the pot-bellied seahorse (Hippocampus abdominalis)

Sexual selection theory predicts a positive correlation between relative parental investment and mate choice. In syngnathid fishes (seahorses and pipefish), males brood offspring in specialized brooding structures. While female-female mating competition has been demonstrated in some pipefishes, all seahorses (genus Hippocampus) studied to date have been found to have conventional sex roles with greater male-male competition for access to mates despite possessing the most complex brood structures in the family. Although multiple mating is common in pipefish, seahorses are again exceptional, exhibiting strict genetic monogamy. Both demographic and behavioural explanations have been offered to explain the lack of multiple mating in seahorse species, but these hypotheses have not yet been explicitly addressed. We investigated mating systems and brood parentage of the pot-bellied seahorse, Hippocampus abdominalis, a temperate-water species that is socially promiscuous with conventional sex roles in laboratory populations. We observed promiscuous courtship behaviour and sex-role reversal in high density, female-biased field populations of H. abdominalis. We hypothesize that sex roles are plastic in H. abdominalis, depending on local population density and sex ratio. Despite promiscuous courtship behaviour, all assayed male seahorses were genetically monogamous in both laboratory and wild populations. Physiological limitations associated with embryo incubation may explain the absence of multiple mating in seahorses and may have played an important role in the development of the unique reproductive behaviour typical in these species.     

Monogamy and sex role reversal in the pipefish Corythoichthys haematopterus

We observed the mating pattern and social behaviour of the pipefish Corythoichthys haematopterus in temperate waters of Japan during three successive breeding seasons. Males cared for a clutch in their brood pouch for 9-19 days until hatching and had several broods in the season with nonbrooding intervals of only 1 or 2 days. The population sex ratio was female biased and some females were always excluded from reproduction. Although males were sometimes courted by unmated females together with their regular partners, they always mated with the latter. The pair bond was maintained until the next season if both members survived. When males lost their partners, they remated with neighbouring unmated females within a few days. In contrast, widowed females remained unmated for a long time. Females had larger home ranges and were more active in courtship displays than males. This pipefish provides the first example of sex role reversal among monogamous syngnathid fish. We suggest that mate guarding by females is a primary proximate factor for maintenance of monogamy in this fish. Copyright 2001 The Association for the Study of Animal Behaviour.     

Evolution of mating systems in coral reef gobies and constraints on mating system plasticity

Social and mating systems can be influenced by the distribution, abundance, and economic defendability of breeding partners and essential resources. Polygyny is predicted where males can economically defend multiple females or essential resources used by females. In contrast, monogamy is predicted where neither sex can monopolise multiple partners, either directly or through resource control, but where one mate is economically defendable. The mating system and reproductive behaviour of five species of coral reef goby were investigated and contrasted with population density and individual mobility. The two most abundant species (Asterropteryx semipunctatus and Istigobius goldmanni) were polygynous. In contrast, the less populous and more widely dispersed epibenthic species (Amblygobius bynoensis, Amblygobius phalaena and Valenciennea muralis) were pair forming and monogamous. All five species had low mobility, mostly remaining within metres (3 epibenthic species) or centimetres (2 cryptobenthic species) of a permanent shelter site. Interspecific differences in the mating system may have been shaped by differences in population density and the ability of reproductive individuals to economically defend breeding partners/sites. However, in a test of mating system plasticity, males of the three monogamous species did not mate polygynously when given the opportunity to do so in experimental manipulations of density and sex ratio. Mate guarding and complex spawning characteristics, which have likely co-evolved with the monogamous mating system, could contribute to mating system inflexibility by making polygynous mating unprofitable for individuals of the pair forming species, even when presented with current-day ecological conditions that usually favour polygyny.     

Does the female seahorse still prefer her mating partner after a period of separation?

For species showing sexual monogamy, once one male and one female form a mating pair bond, they will be faithful to each other in subsequent breeding events. However, if their pair bond is broken for some reason, do they continue to prefer their partner when they come together again for mating? In other words, can the broken pair bond of sexually monogamous species be repaired? This is an interesting question but not yet well answered. To address this question, in the present study we used the lined seahorse (Hippocampus erectus), a typical sexually monogamous species, to study the partner preference of a female individual who experienced a complete separation followed by a reunion with her partner. Our main findings are as follows: (i) The female seahorse no longer prefers her partner after a separation, whether it is a former partner or a recent partner. No preference for partner-males may indicate that the broken pair bond cannot be repaired. (ii) The female seahorse maintains sexual fidelity to her partner in the absence of separation. However, once the health of her partner decreases, the female will switch mate, and her courtship with the new partner can take place during the pregnancy of her original partner. The first finding may provide insight into whether monogamous species still have an opportunity to reselect a new partner in the future to correct their poor choice once they have mated with a low-quality partner. The answer is that they can still gain an opportunity as long as the pair bonds with their current partners are broken. The second finding may reveal the conditions and timing at which a female seahorse switches her mate. These findings help us better understand the mating system of the seahorse H. erectus.     

Correlates of complete brood failure in blue tits: could extra‐pair mating provide unexplored benefits to females?

Behavioural ecologists have for decades investigated the adaptive value of extra‐pair copulation (EPC) for females of socially monogamous species. Despite extensive effort testing for genetic benefits, there now seems to be a consensus that the so‐called ‘good genes’ effects are at most weak. In parallel the search for direct benefits has mostly focused on the period surrounding egg laying, thus neglecting potential correlates of EPC that might be expressed at later stages in the breeding cycle. Here we used Bayesian methods to analyse data collected over four years in a population of blue tits Cyanistes caeruleus, where no support was previously found for ‘good genes’ effects. We found that broods with mixed paternity experienced less brood failure at the nestling stage than broods with single paternity, and that females having experienced complete brood failure in their previous breeding attempt had higher rates of mixed paternity than either yearling or previously successful females. To better understand these observations we also explored relationships between extra‐pair mating, male and female phenotype, and local breeding density. We found that in almost all cases the sires of extra‐pair offspring were close neighbours, and that within those close neighbourhoods extra‐pair sires were older than other males not siring extra‐pair offspring. Also, females did not display consistent EPC status across years. Taken together our results suggest that multiple mating might be a flexible female behaviour influenced by previous breeding experience, and motivate further experimental tests of causal links between extra‐pair copulation and predation.     

Genetic monogamy in the Common Crossbill ( Loxia curvirostra )

Extrapair paternity seems to be common in socially monogamous passerines, but the genetic mating system of most species is currently unknown. Here, we report the first study of paternity in the socially monogamous Common Crossbill (Loxia curvirostra). We found no evidence of extrapair paternity among 96 offspring in 34 examined broods. An upper 95% confidence limit of 3.1% suggests that extrapair fertilizations were truly infrequent in our study population. Common Crossbills thus seem to represent an exception to the rule of extrapair mating among socially monogamous passerine bird species. A potentially important selective pressure preventing promiscuity in Common Crossbills is the harsh environmental conditions experienced during breeding at wintertime, which may increase the importance of paternal care and limit the time available for seeking extrapair copulations.     

杂色山雀的婚外父权与配偶间的遗传相容性无关

婚配制度作为一种进化稳定对策是动物对某一环境包括种群内部环境适应的结果。据统计,约92%的鸟类为社会性单配制,但单配制鸟类中很多都存在婚外父权现象,表现出社会性婚配制度与遗传性婚配制度的不一致性。杂色山雀(Parus varius)是一种社会性单配制的小型森林洞巢鸟,其是否存在婚外父权现象至今尚未见报道。通过对杂色山雀进行亲权鉴定以确定其有无婚外父权现象及婚外父权的比例,结果显示:45.45%(20/44)的巢存在婚外父权,14.39%(38/264)的后代为婚外子代,说明杂色山雀具有较高的婚外父权水平。进一步探究其婚外父权的发生原因,结果如下:(1)有、无婚外父权巢的社会性亲本之间的遗传相似性无显著差异(P=0.504);(2)有婚外父权巢中婚内子代和无婚外父权巢中子代的杂合度(P=0.118)以及有婚外父权巢中婚外子代与婚内子代的杂合度(P=0.206)均无显著差异。(3)有婚外父权巢中的婚内子代与婚外子代间8项体征指标比较,差异均不显著(Ps0.05)。综上,社会性单配制杂色山雀婚外父权的发生与配偶间的遗传相容性无关,还有待从其他角度进行探究。     

Effects of maternal quality and mating status on female reproductive success in the polygynous spotless starling

Female mating status may affect reproductive success in avian polygynous mating systems through a combination of differences in female parental quality and status-dependent male assistance in parental care. Traditionally the literature has emphasized male assistance, neglecting evidence for consistency in female parental quality independent of mating status or repeatability in status. We studied the effects of male assistance on breeding success and its association with female mating status in a population of the polygynous spotless starling, Sturnus unicolor, during 3 years. Nestling provisioning by males improved the fledging success of late (mostly second) but not early (mostly first) broods. Reproductive success of females was affected mainly by female maternal quality: (1) primary and secondary females did not differ in output for early broods and in seasonal output despite a greater male assistance at primary nests; (2) monogamous females were more successful despite receiving no more help than other females; and (3) primary and secondary females had different clutch sizes before male assistance in parental care could operate. Female mating status was not significantly repeatable within seasons or between years. Females who changed status between years were as successful when rearing broods as secondary females as they were as primary or monogamous females. Breeding success (proportion of eggs resulting in fledglings) and clutch size were significantly repeatable between years for late but not for early clutches. Performance showed no strong association with female age or condition. Female breeding success seems to be weakly affected by male assistance in this sedentary, colonial species, and more dependent on inherent differences between females. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Non-seasonal changes in the intensity of female mate preference and offspring sex ratio in the wild guppy Poecilia reticulata

It has been demonstrated that the exaggeration of male sexual ornaments and the intensity of female mate preferences of a wild guppy population change over a period of several months. However, the factors that determine the short-term changes in male ornaments and female preferences remained unclear. In this study, we examined the effect of season on these short-term changes by measuring these traits in the same seasons of different years for a wild guppy population in Okinawa, Japan. We also compared the characteristics of the offspring in each collection term, as female guppies are known to have the ability to control offspring characteristics, such as brood size and sex ratios, depending on their mates' attractiveness. Results showed that the total lengths of the males changed seasonally; males in the summer were larger than those in the spring. In contrast, the size of orange spots in males and the intensity of female mating preferences differed in the same seasons of different years. Brood size and offspring body size in each term showed seasonal changes. However, offspring sex ratios exhibited different patterns in the same seasons of different years. Females produced female-biased broods when attractive males with large orange spots were rare. These results suggest that short-term changes in some traits of adult male and female guppies as well as offspring sex ratios may be not determined by seasonal factors, and that these traits may be interrelated.     

A Comparative Study of Sex Ratio and Clutch Size in Gregarious Ichneumonoid Wasps

Some convincing support for sex ratio theory comes from the cross-species relationship between sex ratio and brood size in gregarious bethylid wasps (Hymenoptera: Bethylidae), in which the proportion males declines as brood size increases as predicted under local mate competition. It is unknown how widely such relationships hold within parasitoid wasps as a whole. We assemble a dataset on sex ratio and brood size for gregarious Braconidae and Ichneumonidae. Their sex ratios deviate substantially from those of bethylids; sex ratios differ widely across species; and they are not significantly related to brood size across species. Several factors explain the heterogeneity in sex ratios including across-species differences in mating system, sex determining mechanism, and sexual asymmetries in larval competition and polyembryony leading to single-sexed broods.     

Willow tit Parus montanus extrapair offspring are more heterozygous than their maternal half‐siblings

Through extrapair matings, males can sire additional offspring with low cost and females may look for direct benefits in form of food or additional paternal care or gain genetic benefits that increase offspring fitness. We studied the patterns of female mate choice and frequency of extrapair paternity in the socially monogamous willow tit Parus montanus using microsatellites. We also examined the effect of heterozygosity on the growth rate and survival of the chicks. We found 25 mixed‐paternity broods out of 117 broods of which both parents were sampled. Altogether, 6.7% of sampled chicks were classified as extrapair young. The pairwise relatedness of social pairs did not correlate with the percentage of extrapair young in the brood and there was no difference in heterozygosity between promiscuous and monogamous parents. However, the extrapair young were more heterozygous than the within‐pair young in the mixed‐paternity broods. The maternal half‐siblings in mixed paternity broods were similar in body size. Thus, there was no indication for different growth rate between the siblings, but there were indications that heterozygosity affects survival.     

Males achieve greater reproductive success through multiple broods than through extrapair mating in house wrens

In polygynous species, it is unclear whether extrapair matings provide a better reproductive payoff to males than additional social mates. Male house wrens, Troglodytes aedon, show three types of social mating behaviour: single-brooded monogamy, sequential monogamy (two broods) and polygyny. Thus, male reproductive success can vary depending on the number of mates, the number of broods and the number of extrapair fertilizations. We used microsatellite markers to determine the realized reproductive success (total number of young sired from both within-pair and extrapair fertilizations) of males in these three categories. We found that polygynous males were more likely to be cuckolded than monogamous males; however, half of the polygynous males had a third brood, which resulted in similar reproductive success for sequentially monogamous and polygynous males. Despite the paternity gained from extrapair fertilizations by single-brooded males, males were more successful when they produced multiple broods during a season, either sequentially (monogamy) or simultaneously (polygyny). In our population, multibrooded males were more likely to have prior breeding experience and arrived earlier in the season, which provided a better opportunity to obtain more than one brood and, thus, produce more young.     

Phenotypic variation and promiscuity in a wild population of pure dingoes (Canis dingo)

Phenotypic diversity occurs in natural populations as a result of the interaction between an individual's genotype and the environment. Nevertheless, individual variation in phenotypic traits such as coat colour and body size is routinely used to differentiate between “pure” dingoes Canis dingo and dingo‐dog hybrids. Extensive anthropogenic impacts and widespread hybridization with domestic dogs has hindered our ability to study intact dingo populations and, therefore, most of our basic understanding of dingo biology (e.g., phenotypic variation, mating systems, genetic diversity) stems from observational studies on perturbed populations. We sampled a relatively undisturbed population of dingoes, from arid Australia, to determine their purity and genetic diversity. We explored their mating strategy using a pedigree built from genetic data and examined how phenotypic variation was influenced by age, sex, heterozygosity, and relatedness. Coat colour was our measure of phenotype and our population displayed four types (sandy, black & tan, white, and sable). All dingoes (n = 83) possessed a high level of dingo ancestry (mean purity > 90%) and were closely related to each other; with all but one individual related as full‐sibling or parent–offspring. Our pedigree shows both monogamous and promiscuous mating strategies exist within an undisturbed population. Variation in coat colour or body size cannot be used to infer a dingo's level of purity because the phenotype of pure dingoes is intrinsically variable. The breeding system of dingoes was long thought to be monogamous, but we provide genetic evidence for numerous mating strategies including both long‐term monogamy and extreme promiscuity.     

Same school,different conduct: rates of multiple paternity vary within a mixed‐species breeding school of semi‐pelagic cichlid fish (Cyprichromis spp.)

Mating system variability is known to exist between and within species, often due to environmental influences. An open question is whether, vice versa, similar environmental conditions entail congruent mating behavior, for example in terms of multiple paternity, in species or populations sharing largely comparable breeding modes. This study employed microsatellite markers to investigate the incidence of multiple paternity in Cyprichromis coloratus and Cyprichromis leptosoma, two sympatric, closely related, mouthbrooding Lake Tanganyika cichlids with similar ecological and behavioral characteristics including the formation of open‐water schools. Mouthbrooding females of both species were collected from the same mixed‐species breeding school at the same time, minimizing environmental variation during courtship and mating. In C. coloratus, four of 12 broods had more than one sire, with a mean of 1.33 reconstructed sires per brood. C. leptosoma exhibited multiple paternity in 18 of 22 broods, with a mean of 2.59 or 2.86 reconstructed sires per brood according to the programs gerud and colony , respectively. In addition, two broods were found to contain offspring transplanted from another brood. There was no significant difference in brood size between species, but mean sire number did differ significantly. Hence, substantial similarity in reproductive behavior along with shared environmental conditions during courtship and spawning did not lead to equal rates of polyandry or sneaking in the two species.