The dynamics of male brooding,mating patterns,and sex roles in pipefishes and seahorses (family Syngnathidae)

Modern theory predicts that relative parental investment of the sexes in their young is a key factor responsible for sexual selection. Seahorses and pipefishes (family Syngnathidae) are extraordinary among fishes in their remarkable adaptations for paternal care and frequent occurrences of sex-role reversals (i.e., female-female competition for mates), offering exceptional opportunities to test predictions of sexual selection theory. During mating, the female transfers eggs into or onto specialized egg-brooding structures that are located on either the male's abdomen or its tail, where they are osmoregulated, aerated, and nourished by specially adapted structures. All syngnathid males exhibit this form of parental care but the brooding structures vary, ranging from the simple ventral gluing areas of some pipefishes to the completely enclosed pouches found in seahorses. We present a molecular phylogeny that indicates that the diversification of pouch types is positively correlated with the major evolutionary radiation of the group, suggesting that this extreme development and diversification of paternal care may have been an important evolutionary innovation of the Syngnathidae. Based on recent studies that show that the complexity of brooding structures reflects the degree of paternal investment in several syngnathid species, we predicted sex-role reversals to be more common among species with more complex brooding structures. In contrast to this prediction, however, both parsimony- and likelihood-based reconstructions of the evolution of sex-role reversal in pipefishes and seahorses suggest multiple shifts in sex roles in the group, independent from the degree of brood pouch development. At the same time, our data demonstrate that sex-role reversal is positively associated with polygamous mating patterns, whereas most nonreversed species mate monogamously, suggesting that selection for polygamy or monogamy in pipefishes and seahorses may strongly influence sex roles in the wild.     

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.     

Mating systems and sexual selection in male-pregnant pipefishes and seahorses: insights from microsatellite-based studies of maternity

In pipefishes and seahorses (family Syngnathidae), the males provide all postzygotic care of offspring by brooding embryos on their ventral surfaces. In some species, this phenomenon of male "pregnancy" results in a reversal of the usual direction of sexual selection, such that females compete more than males for access to mates, and secondary sexual characteristics evolve in females. Thus the syngnathids can provide critical tests of theories related to the evolution of sex differences and sexual selection. Microsatellite-based studies of the genetic mating systems of several species of pipefishes and seahorses have provided insights into important aspects of the natural history and evolution of these fishes. First, males of species with completely enclosed pouches have complete confidence of paternity, as might be predicted from parental investment theory for species in which males invest so heavily in offspring. Second, a wide range of genetic mating systems have been documented in nature, including genetic monogamy in a seahorse, polygynandry in two species of pipefish, and polyandry in a third pipefish species. The genetic mating systems appear to be causally related to the intensity of sexual selection, with secondary sex characters evolving most often in females of the more polyandrous species. Third, genetic studies of captive-breeding pipefish suggest that the sexual selection gradient (or Bateman gradient) may be a substantially better method for characterizing the mating system than previously available techniques. Finally, these genetic studies of syngnathid mating systems have led to some general insights into the occurrence of clustered mutations at microsatellite loci, the utility of linked loci in studies of parentage, and the use of parentage data for direct estimation of adult population size.     

Brain size evolution in pipefishes and seahorses: the role of feeding ecology,life history and sexual selection

Brain size varies greatly at all taxonomic levels. Feeding ecology, life history and sexual selection have been proposed as key components in generating contemporary diversity in brain size across vertebrates. Analyses of brain size evolution have, however, been limited to lineages where males predominantly compete for mating and females choose mates. Here, we present the first original data set of brain sizes in pipefishes and seahorses (Syngnathidae) a group in which intense female mating competition occurs in many species. After controlling for the effect of shared ancestry and overall body size, brain size was positively correlated with relative snout length. Moreover, we found that females, on average, had 4.3% heavier brains than males and that polyandrous species demonstrated more pronounced (11.7%) female‐biased brain size dimorphism. Our results suggest that adaptations for feeding on mobile prey items and sexual selection in females are important factors in brain size evolution of pipefishes and seahorses. Most importantly, our study supports the idea that sexual selection plays a major role in brain size evolution, regardless of on which sex sexual selection acts stronger.     

Pipefishes and seahorses: Are they all sex role reversed?

The male pregnancy of pipefishes and seahorses has led to the inference that females compete most intensely for access to mates, because males limit female reproduction. However, recent work has shown that in different species either sex may be the predominant competitor for mates. In this family, there is an apparent association between the mating pattern and the sex roles: polygamous species show reversed sex roles whereas monogamous species exhibit 'conventional' sex roles. These studies emphasize that sex role reversal is not synonymous with male parental care.     

The genetic mating system and tests for cuckoldry in a pipefish species in which males fertilize eggs and brood offspring externally

Highly variable microsatellite loci were used to study the mating system of Nerophis ophidion, a species of pipefish in which pregnant males carry embryos on the outside of their body rather than in an enclosed brood pouch. Despite this mode of external fertilization and brooding, otherwise rare in the family Syngnathidae, the genotypes of all embryos proved to be consistent with paternity by the tending male, thus indicating that cuckoldry by sneaker males is rare or nonexistent in this species. N. ophidion is a phylogenetic outlier within the Syngnathidae and its reproductive morphology is thought to be close to the presumed ancestral condition for pipefishes and seahorses. Thus, our genetic results suggest that the evolutionary elaboration of the enclosed brood pouch elsewhere in the family was probably not in response to selection pressures on pregnant males to avoid fertilization thievery. With regard to maternity assignments, our genotypic data are consistent with behavioural observations indicating that females sometimes mate with more than one male during a breeding episode, and that each male carries eggs from a single female. Thus, the polyandrous genetic mating system in this species parallels the social mating system, and both are consistent with a more intense sexual selection operating on females, and the elaboration of secondary sexual characters in that gender.     

Genetic evidence for extreme polyandry and extraordinary sex-role reversal in a pipefish.

Due to the phenomenon of male pregnancy, the fish family Syngnathidae (seahorses and pipefishes) has historically been considered an archetypal example of a group in which sexual selection should act more strongly on females than on males. However, more recent work has called into question the idea that all species with male pregnancy are sex-role reversed with respect to the intensity of sexual selection. Furthermore, no studies have formally quantified the opportunity for sexual selection in any natural breeding assemblage of pipefishes or seahorses in order to demonstrate conclusively that sexual selection acts most strongly on females. Here, we use a DNA-based study of parentage in the Gulf pipefish Syngnathus scovelli in order to show that sexual selection indeed acts more strongly on females than on males in this species. Moreover, the Gulf pipefish exhibits classical polyandry with the greatest asymmetry in reproductive roles (as quantified by variances in mating success) between males and females yet documented in any system. Thus, the intensity of sexual selection on females in pipefish rivals that of any other taxon yet studied.     

Sex Recognition via Chemical Cues in the Sex-Role-Reversed Gulf Pipefish (Syngnathus scovelli)

Sexual selection theory predicts that members of the choosy sex, usually females, should employ multiple sensory systems to obtain information about potential mates. Such predictions should also apply to systems in which sexual selection acts most strongly on females (i.e. sex-role-reversed species), and males of these taxa should be the individuals employing multiple cues to assess female attractiveness. Very little work has been directed toward mate choice involving multiple sensory systems in sex-role-reversed taxa, but fishes of the family Syngnathidae (pipefishes, seahorses, and sea dragons) provide an excellent opportunity to contribute to this research enterprise. While much is known about visual communication in pipefish, the role of chemical communication has not been investigated. Using dichotomous choice tests, we found that male, but not female, Gulf pipefish attended to chemical cues of opposite sex conspecifics. Given that males distinguished sex on the basis of chemical cues, we also tested whether males could assess female body size, an important trait with respect to mate choice in pipefish, on the basis of chemical cues alone. When given the choice between chemical cues produced by large vs. small females, males exhibited no preference. Our results suggest that male pipefish can use chemical cues to distinguish between males and females but not to differentiate females of different body size.     

POLYGYNANDRY IN THE DUSKY PIPEFISH SYNGNATHUS FLORIDAE REVEALED BY MICROSATELLITE DNA MARKERS

In the dusky pipefish Syngnathus floridae, like other species in the family Syngnathidae, ‘pregnant’ males provide all post-zygotic care. Male pregnancy has interesting implications for sexual selection theory and the evolution of mating systems. Here, we employ microsatellite markers to describe the genetic mating system of S. floridae, compare the outcome with a previous report of genetic polyandry for the Gulf pipefish S. scovelli, and consider possible associations between the mating system and degree of sexual dimorphism in these species. Twenty-two pregnant male dusky pipefish from one locale in the northern Gulf of Mexico were analyzed genetically, together with subsamples of 42 embryos from each male's brood pouch. Adult females also were assayed. The genotypes observed in these samples document that cuckoldry by males did not occur; males often receive eggs from multiple females during the course of a pregnancy (six males had one mate each, 13 had two mates, and three had three mates); embryos from different females are segregated spatially within a male's brood pouch; and a female's clutch of eggs often is divided among more than one male. Thus, the genetic mating system of the dusky pipefish is best described as polygynandrous. The genetic results for S. floridae and S. scovelli are consistent with a simple model of sexual selection which predicts that for sex role-reversed organisms, species with greater degrees of sexual dimorphism are more highly polyandrous.     

Sexual selection determines parental care patterns in cichlid fishes

Despite a massive research effort, our understanding of why, in most vertebrates, males compete for mates and females care for offspring remains incomplete. Two alternative hypotheses have been proposed to explain the direction of causality between parental care and sexual selection. Traditionally, sexual selection has been explained as a consequence of relative parental investment, where the sex investing less will compete for the sex investing more. However, a more recent model suggests that parental care patterns result from sexual selection acting on one sex favoring mating competition and lower parental investment. Using species-level comparative analyses on Tanganyikan cichlid fishes we tested these alternative hypotheses employing a proxy of sexual selection based on mating system, sexual dichromatism, and dimorphism data. First, while controlling for female reproductive investment, we found that species with intense sexual selection were associated with female-only care whereas species with moderate sexual selection were associated with biparental care. Second, using contingency analyses, we found that, contrary to the traditional view, evolutionary changes in parental care type are dependent on the intensity of sexual selection. Hence, our results support the hypothesis that sexual selection determines parental care patterns in Tanganyikan cichlid fishes.     

Greater opportunities for sexual selection in male than in female obligate brood parasitic birds

Females are expected to have evolved to be more discriminatory in mate choice than males as a result of greater reproductive investment into larger gametes (eggs vs. sperm). In turn, males are predicted to be more promiscuous than females, showing both a larger variance in the number of mates and a greater increase in reproductive success with more mates, yielding more intense sexual selection on males vs. females (Bateman's Paradigm). However, sex differences in costly parental care strategies can either reinforce or counteract the initial asymmetry in reproductive investment, which may be one cause for some studies failing to conform with predictions of Bateman's Paradigm. For example, in many bird species with small female‐biased initial investment but extensive biparental care, both sexes should be subject to similar strengths of sexual selection because males and females are similarly restricted in their ability to pursue additional mates. Unlike 99% of avian species, however, obligate brood parasitic birds lack any parental care in either sex, predicting a conformation to Bateman's Paradigm. Here we use microsatellite genotyping to demonstrate that in brood parasitic brown‐headed cowbirds (Molothrus ater), per capita annual reproductive success increases with the number of mates in males, but not in females. Furthermore, also as predicted, the variance of the number of mates and offspring is greater in males than in females. Thus, contrary to previous findings in this species, our results conform to predictions of the Bateman's Paradigm for taxa without parental care.     

Molecular markers reveal reproductive strategies of non‐pollinating fig wasps

1. Fig wasps have proved extremely useful study organisms for testing how reproductive decisions evolve in response to population structure. In particular, they provide textbook examples of how natural selection can favour female‐biased offspring sex ratios, lethal combat for mates and dimorphic mating strategies. 2. However, previous work has been challenged, because supposedly single species have been discovered to be a number of cryptic species. Consequently, new studies are required to determine population structure and reproductive decisions of individuals unambiguously assigned to species. 3. Microsatellites were used to determine species identity and reproductive patterns in three non‐pollinating Sycoscapter species associated with the same fig species. Foundress number was typically one to five and most figs contained more than one Sycoscapter species. Foundresses produced very small clutches of about one to four offspring, but one foundress may lay eggs in several figs. 4. Overall, the data were a poor match to theoretical predictions of solitary male clutches and gregarious clutches with n ? 1 females. However, sex ratios were male‐biased in solitary clutches and female‐biased in gregarious ones. 5. At the brood level (all wasps in a fig), a decrease in sex ratio with increasing brood size was only significant in one species, and sex ratio was unrelated to foundress number. In addition, figs with more foundresses contain more wasp offspring. 6. Finally, 10–22% of females developed in patches without males. As males are wingless, these females disperse unmated and are constrained to produce only sons from unfertilised eggs.     

Mate quality influences multiple maternity in the sex-role-reversed pipefish Syngnathus typhle

In the pipefish Syngnathus typhle , pregnant males provide all parental care. Females are able to produce more eggs than males can brood, and consequently females compete more intensely for mates than do males, a phenomenon defined as sex-role reversal. As the genetic mating system influences the operation of sexual selection, we investigate variation in one phenotypic component of mate quality, female body size, as a possible proximate influence on mating system variation in S. typhle . Breeding trials were employed, each consisting of a single receptive male with four adult females. In each replicate, a focal male was paired either with a set of small or with a set of large females. Males were allowed to mate freely, and after several weeks of brood development, maternity of the progeny was resolved using three microsatellite loci. Males with access either to small or to large females successfully mated with a mean of 2.1 or 1.3 females, respectively, a significant difference. Results indicate that variation in female size can affect the mating system and thereby influence sexual selection in pipefish. Thus, the high rate of multiple mating by S. typhle males in the wild may be explained in part by the extensive size variation in naturally occurring, sexually mature females.     

Sex roles and mutual mate choice matter during mate sampling

The roles of females and males in mating competition and mate choice have lately proven more variable, between and within species, than previously thought. In nature, mating competition occurs during mate search and is expected to be regulated by the numbers of potential mates and same-sex competitors. Here, we present the first study to test how a temporal change in sex roles affects mating competition and mate choice during mate sampling. Our model system (the marine fish Gobiusculus flavescens) is uniquely suitable because of its change in sex roles, from conventional to reversed, over the breeding season. As predicted from sex role theory, courtship was typically initiated by males and terminated by females early in the breeding season. The opposite pattern was observed late in the season, at which time several females often simultaneously courted the same male. Mate-searching females visited more males early than late in the breeding season. Our study shows that mutual mate choice and mating competition can have profound effects on female and male behavior. Future work needs to consider the dynamic nature of mating competition and mate choice if we aim to fully understand sexual selection in the wild.     

Evolution of Classical Polyandry: Three Steps to Female Emancipation

Abstract In classical polyandry, sex roles are reversed and a female reproduces with several males, each of whom raises his offspring with little or no help from her. This mating system occurs in some fishes and birds, and it is of great interest in relation to parental investment, sex role and sexual selection theory. The evolution of classical polyandry, however, is debated and not well understood. It is here suggested to generally take place in three main steps. (1) First evolves male care for eggs, for reasons that differ between fishes and birds. (2) Second, a female becomes able to lay more eggs than a male can accommodate. This can happen, for example, by evolution of male pregnancy or smaller body size, or by female production of more or larger eggs, made possible by larger female body size or more food. Polyandry in several taxa is associated with shift to a habitat rich in food during the breeding season, to novel specialised foraging methods, or to both. A favourable food situation may be crucial for evolution of classical polyandry. (3) In step three, females compete to lay two or more clutches in sequence for different males. Successful polyandrous females obtain more offspring, spreading traits that enhance success in competition over males. Step three may be most likely in species with small body size, for reasons of reproductive constraints and seasonality. Evolution of classical polyandry appears to have followed these steps in shorebirds, coucals and pipefishes, but the reasons why certain species differ from their close phylogenetic relatives in being polyandrous are far from clear. Behavioural and ecological studies of additional species, and detailed phylogenies of taxa with diverse mating systems including polyandry, are needed for testing these ideas.     

Social polyandry, parental investment, sexual selection, and evolution of reduced female gamete size

Abstract Sexual selection in the form of sperm competition is a major explanation for small size of male gametes. Can sexual selection in polyandrous species with reversed sex roles also lead to reduced female gamete size? Comparative studies show that egg size in birds tends to decrease as a lineage evolves social polyandry. Here, a quantitative genetic model predicts that female scrambles over mates lead to evolution of reduced female gamete size. Increased female mating success drives the evolution of smaller eggs, which take less time to produce, until balanced by lowered offspring survival. Mean egg size is usually reduced and polyandry increased by increasing sex ratio (male bias) and maximum possible number of mates. Polyandry also increases with the asynchrony (variance) in female breeding start. Opportunity for sexual selection increases with the maximum number of mates but decreases with increasing sex ratio. It is well known that parental investment can affect sexual selection. The model suggests that the influence is mutual: owing to a coevolutionary feedback loop, sexual selection in females also shapes initial parental investment by reducing egg size. Feedback between sexual selection and parental investment may be common.     

Mate choice, operational sex ratio, and social promiscuity in a wild population of the long-snouted seahorse Hippocampus guttulatus

Mate competition and mate choice are not mutually exclusivebehaviors. Both behaviors may drive sexual selection in oneor both sexes of a population. One of several factors affectingwhich behavior is exhibited by which sex is the operationalsex ratio (OSR) in the study population. The present study combinesbehavioral observations in the field with controlled experimentsin aquaria to investigate social interactions and mate choicein both male and female long-snouted seahorses Hippocampus guttulatusin the context of the population OSR. Compared with the morereadily studied pipefishes, data on OSR and mate choice in seahorsesare scarce in the published literature. Our field data providenovel evidence of social promiscuity, size-assortative mating,and an OSR that varies from being unbiased early and midseasonto male biased at the end of the breeding season. Our mate choiceexperiments revealed intersexual differences in mate preferencewith males significantly preferring larger females to familiarones. Taken together, our field and experimental results suggestthat mate choice rather than intrasexual competition could drivesexual selection in seahorses.     

HOMAGE TO BATEMAN: SEX ROLES PREDICT SEX DIFFERENCES IN SEXUAL SELECTION

Classic sex role theory predicts that sexual selection should be stronger in males in taxa showing conventional sex roles and stronger in females in role reversed mating systems. To test this very central prediction and to assess the utility of different measures of sexual selection, we estimated sexual selection in both sexes in four seed beetle species with divergent sex roles using a novel experimental design. We found that sexual selection was sizeable in females and the strength of sexual selection was similar in females and males in role‐reversed species. Sexual selection was overall significantly stronger in males than in females and residual selection formed a substantial component of net selection in both sexes. Furthermore, sexual selection in females was stronger in role‐reversed species compared to species with conventional sex roles. Variance‐based measures of sexual selection (the Bateman gradient and selection opportunities) were better predictors of sexual dimorphism in reproductive behavior and morphology across species compared to trait‐based measures (selection differentials). Our results highlight the importance of using assays that incorporate components of fitness manifested after mating. We suggest that the Bateman gradient is generally the most informative measure of the strength of sexual selection in comparisons across sexes and/or species.     

Competition for access to mates predicts female-specific ornamentation and male investment in relative testis size

Sexually selected ornaments are highly variable and the factors that drive variation in ornament expression are not always clear. Rare instances of female-specific ornament evolution (such as in some dance fly species) are particularly puzzling. While some evidence suggests that such rare instances represent straightforward reversals of sexual selection intensity, the distinct nature of trade-offs between ornaments and offspring pose special constraints in females. To examine whether competition for access to mates generally favors heightened ornament expression, we built a phylogeny and conducted a comparative analysis of Empidinae dance fly taxa that display female-specific ornaments. We show that species with more female-biased operational sex ratios in lek-like mating swarms have greater female ornamentation, and in taxa with more ornate females, male relative testis investment is increased. These findings support the hypothesis that ornament diversity in dance flies depends on female receptivity to mates, which is associated with contests for nutritious nuptial gifts provided by males. Moreover, our results suggest that increases in female receptivity lead to higher levels of sperm competition among males. The incidence of both heightened premating sexual selection on females and postmating selection on males contradicts assertions that sex roles are straightforwardly reversed in dance flies.     

Parental investment, sexual selection and sex ratios

Conventional sex roles imply caring females and competitive males. The evolution of sex role divergence is widely attributed to anisogamy initiating a self‐reinforcing process. The initial asymmetry in pre‐mating parental investment (eggs vs. sperm) is assumed to promote even greater divergence in post‐mating parental investment (parental care). But do we really understand the process? Trivers [Sexual Selection and the Descent of Man 1871–1971 (1972), Aldine Press, Chicago] introduced two arguments with a female and male perspective on whether to care for offspring that try to link pre‐mating and post‐mating investment. Here we review their merits and subsequent theoretical developments. The first argument is that females are more committed than males to providing care because they stand to lose a greater initial investment. This, however, commits the ‘Concorde Fallacy’ as optimal decisions should depend on future pay‐offs not past costs. Although the argument can be rephrased in terms of residual reproductive value when past investment affects future pay‐offs, it remains weak. The factors likely to change future pay‐offs seem to work against females providing more care than males. The second argument takes the reasonable premise that anisogamy produces a male‐biased operational sex ratio (OSR) leading to males competing for mates. Male care is then predicted to be less likely to evolve as it consumes resources that could otherwise be used to increase competitiveness. However, given each offspring has precisely two genetic parents (the Fisher condition), a biased OSR generates frequency‐dependent selection, analogous to Fisherian sex ratio selection, that favours increased parental investment by whichever sex faces more intense competition. Sex role divergence is therefore still an evolutionary conundrum. Here we review some possible solutions. Factors that promote conventional sex roles are sexual selection on males (but non‐random variance in male mating success must be high to override the Fisher condition), loss of paternity because of female multiple mating or group spawning and patterns of mortality that generate female‐biased adult sex ratios (ASR). We present an integrative model that shows how these factors interact to generate sex roles. We emphasize the need to distinguish between the ASR and the operational sex ratio (OSR). If mortality is higher when caring than competing this diminishes the likelihood of sex role divergence because this strongly limits the mating success of the earlier deserting sex. We illustrate this in a model where a change in relative mortality rates while caring and competing generates a shift from a mammalian type breeding system (female‐only care, male‐biased OSR and female‐biased ASR) to an avian type system (biparental care and a male‐biased OSR and ASR).