Group sex ratio and sex reversal

In hermaphroditic fishes, the initiation of sex reversal by male removal explains the replacement of lost males but does not explain how the number of males in a group may increase. Since numerous species apparently cannot produce primary males, a second means of initiating sex reversal must exist. In the present study we formulate a model which suggests the existence of an additional mechanism governing sex change: as soon as the ratio of adult females to males within a group exceeds a certain threshold value, a female changes sex even though no male has been removed. This process is inferred from comparison of data collected in the Red Sea and the western Indian Ocean with the model's predictions concerning size at sex reversal and the sex ratio of groups. The results suggest how several ecological factors may influence the occurrence rate of sex reversal and the development and growth of social groups.     

Size‐selective fishing affects sex ratios and the opportunity for sexual selection in Alaskan sockeye salmon Oncorhynchus nerka

Selective exploitation can cause adverse ecological and evolutionary changes in wild populations and also affect sex ratios but few studies have empirically documented skewed sex ratios in exploited fishes (other than species with extreme sexual size dimorphism, SSD). To investigate the possibility of sex‐selective fishing on Alaskan sockeye salmon Oncorhynchus nerka, we assessed sex ratios in fish at two spatial scales: within each of five fishing districts and among 13 breeding populations in one of these districts. We predicted that populations’ sex ratios would vary based on the average size of fish and SSD because size affects vulnerability to fishing. At the larger scale, we found a small but significant bias in fish returning to four of the five fishing districts (average = 52% females), and in four of the five districts males were caught at significantly higher rates than females. At the finer scale there was marked variation in sex ratio on the breeding grounds, ranging from 36% to 47% males. Populations with fish of intermediate sizes experienced the greatest sex ratio biases; the greater vulnerability of males than females to fishing resulted from a combination of larger SSD and different harvest rates between the sexes associated with the fishery size‐selectivity curve shape. Skewed sex ratios may change competition and behavior on the breeding grounds, relaxing selection on male traits associated with mate choice by females or intra‐sexual competition and altering demographic and evolutionary pressures on the fish. Assessment of the size selectivity of fishing gear and the population's SSD can help to illuminate if and how exploitation can affect sex ratios. Future studies examining size‐selective fishing should also evaluate the consequences for sex ratios, as this might help explain changes in harvested population structure and sustainability.     

Female-biased operational sex ratio of sexual host fish in a gynogenetic complex of Carassius auratus

To study the coexistence of sexual and gynogenetic forms, we examined the population structure of a gynogenetic complex of the Japanese crucian carp, Carassius auratus Temminck et Schlegel, during the April–June reproductive season by collecting 1225 mature fish that migrated from Lake Suwa to a tributary river for spawning. There were more sexual fish (about 80%) than gynogenetic fish in this complex, and the operational sex ratio in the sexual form was female biased (males were about 20%). Mean standard length and body weight of sexual females were larger than those of sexual males. Sex ratio was male biased in smaller fish (standard length, <8.5 cm) but female biased in larger fish (standard length, ≥8.5 cm). We determined age by scale ring marks; the average age of sexual females was higher than that of males, but there was no significant difference in the average age between sexual and gynogenetic females. Sex ratio in the sexual form was more female biased for old than for young fish, and the mean size of sexual females was larger than that of males of the same age. The clear female-biased sex ratio and age difference between sexual females and males can be explained either by (1) higher mortality of males or by (2) female-biased sex allocation. The latter process reduces the disadvantage of sex and contributes to the coexistence of sexual and gynogenetic forms. Received: November 24, 2000 / Accepted: March 6, 2001     

Sex ratios in offspring of sex-reversed sea bass and the relationship between growth and phenotypic sex differentiation

Groups of sexually undifferentiated sea bass Dicentrarchus labrax were fed with the androgen 17α-methyltestosterone (MT) during sex differentiation. MT treatment increased males from 79±3% in the controls (the usual 3:1 male:female sex ratio of cultured sea bass) to 100±0%, implying that in the treated groups one out of each five resulting males was a masculinized female (neomale). Thirteen males from the MT treated groups were taken as the parental generation and their sperm used to individually fertilize a pool of eggs from unrelated females. The probability of having at least one neomale was 95% and most probably two or three of the males used were neomales. The offspring from each family were reared separately under the same environmental conditions. Samples were taken at 11 and 15 months of age, during and after sex differentiation, respectively. Results showed that females predominated among the larger fish whereas males and undifferentiated fish predominated among the smaller ones. Intersexes exhibited an intermediate size. All fish with a body length smaller than 12 cm were undifferentiated. These results suggest that sex differentiation is more dependent on length than on age. At 15 months, sex ratios were male-biased in all families, except one (females ranged from 5 to 50%) and only two families had sex ratios not significantly different from 1:1, suggesting that the mechanism of sex determination in the sea bass is not of a XX/XY or ZW/ZZ type since no family exhibited a female-biased progeny, as would be expected from both types. Results support the hypothesis that factors other than genetic, i.e., environmental, may act epigenetically on the sex determination mechanisms of sea bass, as has been demonstrated in other fishes.     

Hatching sex ratio and sex specific chick mortality in common terns Sterna hirundo

Bias in sex ratios at hatching and sex specific post hatching mortality in size dimorphic species has been frequently detected, and is usually skewed towards the production and survival of the smaller sex. Since common terns Sterna hirundo show a limited sexual size dimorphism, with males being only about 1–6% larger than females in a few measurements, we would expect to find small or no differences in production and survival of sons and daughters. To test this prediction, we carried out a 2-year observational study on sex ratio variation in common terns at hatching and on sex specific post hatching mortality. Sons and daughters hatched from eggs of similar volume. Post hatching mortality was heavily influenced by hatching sequence. In addition, we detected a sex specific mortality bias towards sons. Overall, hatching sex ratio and sex specific mortality resulted in fledging sex ratios 8% biased towards females. Thus, other reasons than body size may be influencing the costs of rearing sons. Son mortality was not homogeneous between brood sizes, but greater for two-chick broods. Since adults rearing two-chick broods were younger, lighter and bred consistently later than those rearing three-chick broods, it is suggested that lower capacity of two-chick brood parents adversely affected offspring survival of sons. Though not significantly, two-chick broods tended to be female biased at hatching, perhaps to counteract the greater male-biased nestling mortality. Thus, population bias in secondary sex ratio is not limited to strongly size dimorphic species, but species with a slight sexual size dimorphism can also show sex ratio bias through a combination of differential production and mortality of sons and daughters.     

Induced sex change in black sea bass

Experiments were conducted to identify factors involved in sex change in the protogynous black sea bass Centropristis striata . Black sea bass maintained in the ratio of 8 females (F):0 males (M) for 9 months reversed sex while those kept at the ratios of 6F:2M or 4F:4M did not. Female black sea bass implanted with 1·0 mg 11-ketotestosterone (11-KT) or 10 mg fadrozole (FAD) changed sex and began spermiating while those implanted with 0·1 mg 11-KT or 1·0 mg FAD underwent incomplete sex reversal. One fish implanted with 1·0 mg FAD initiated sex change but was not spermiating at the end of the study. One fish in the control group, the largest fish in the study, initiated sex change. These results suggest that the presence of males may restrict sex reversal in black sea bass and that high 17β-oestradiol:11-KT is required for maintaining ovarian function.     

Predicting the effects of exploitation on male-first sex-changing fish

Sex change is widespread among tropical marine fishes, many of which are targeted by fisheries. Conservation concerns have been raised that sex-changing species may be particularly prone to overexploitation by size-selective fishing. In the case of male-first sex-changers, populations may become egg limited if large females are disproportionately killed. However, if males reduce the size at which they change sex in response to higher female mortality, the population may still be sufficiently productive. We develop an age-based model to explore the effects of fishing on two types of male-first sex-changing fish: one with flexibility in size-at-sex-change and one without. These effects were compared with those of non-sex-changing populations with similar life-history and population characteristics. The model predicts that if male-first sex-changers cannot respond to elevated female mortality by adjusting their size-at-sex-change, the population will be more prone to recruitment limitation and extinction than non-sex-changers. These effects will be amplified as smaller individuals become susceptible to fishing mortality. However, if size-at-sex-change is flexible, sex-changers may be as resilient to fishing as non-sex-changers. Knowledge of a species' size-at-sex-change, and the mechanisms controlling it, should be fundamental to the selection of fisheries conservation strategies.     

Evidence for protogynous hermaphroditism in some lethrinid fishes

An examination of the size/sex relationships and gonadial structure and histology of eight species of the commercially important family Lethrinidae from the North West Shelf of Australia and the Gulf of Carpentaria suggests that protogynous hermaphroditism is the typical mode of sexuality in these fishes. A linear relationship between size and sex ratio, in which females predominate at smaller sizes and males at larger sizes, was demonstrated in five species for which sufficient information was available. All five of these species showed a considerable overlap in the size distribution of the sexes but there was no evidence for the occurrence of primary males in the populations sampled. The tests of all species examined showed typical 'secondary male' morphology and the presence of atretic ovarian material ('brown bodies'). Individuals with intersex gonads were observed in five species. It is suggested that the effects of fishing on such protogynous fish stocks will depend on the precise mechanims, as yet unknown in lethrinids, that control the onset of sex change.     

Terminal phase males stimulate ovarian function and inhibit sex change in the protogynous wrasse Thalassoma duperrey

Under experimental conditions, the probability of sex change in the protogynous wrasse Thalassoma duperrey is determined largely by an individual's relative size within a social group. Natural populations, however, contain two distinct male phenotypes that may also play a role in regulating sex change. To investigate potential effects of male phenotype, the ability to change sex, ovarian histology and serum estradiol-17 beta levels were examined in females maintained under controlled social settings. Large females housed with smaller or larger terminal phase males had significantly larger gonadosomatic indices than females housed singly, with other females or with smaller initial phase males. Similarly, ovaries of females housed with terminal phase males showed no histological evidence of sex change, whereas large females from other social groupings were in advanced stages of sex change. These results demonstrate terminal phase males inhibit sex change regardless of their size relative to the female. Furthermore, gonadosomatic indices, ovarian histology, and serum estradiol-17 beta levels of females housed with terminal phase males indicate normal ovarian function whereas ovaries of other treatment groups appear quiescent or are undergoing sex change. Consequently, terminal phase males may be required for normal ovarian development which may, in turn, inhibit sex change in T. duperrey.     

Brood sex ratio in the Kentish plover

How and why do the mating opportunities of males and femalesdiffer in natural population of animals? Previously we showedthat females have higher mating opportunities than males inthe Kentish plover Charadrius alexandrinus. Both parents incubatethe eggs, and males provide more brood care than females; thusit is not obvious why the females find new mates sooner thanthe males. In this study we investigated whether the sex-biasedmating opportunities stem from biased offspring sex ratios.We determined the sex of newly hatched, precocial chicks usingCHD gene markers. Among fully sexed broods, 0.461 ± 0.024(SE) of chicks (454 chicks in 158 broods) were male, and thissex ratio was not significantly different from unity. The proportionof males at hatching decreased significantly over the breedingseason, which occurred consistently in all 3 years of the study.Large chicks were more likely to be males than females. Neitherparental age nor body size of male and female parents was relatedto brood sex ratio. We also sexed a number of chicks that werecaught after they left their nest (range of estimated ages 0–17days) and found that the proportion of males increased withbrood age. This relationship remained highly significant whencontrolling statistically for hatching date. As brood size decreaseddue to mortality after the chicks left their nest, these resultssuggest that the mortality of daughters was higher than thatof the sons shortly after hatching. Taken together, our resultsshow that the female-biased mating opportunities in the Kentishplover are not due to biased brood sex ratio at hatching but,at least in part, are due to female-biased chick mortality soonafter hatching.     

Female bias in the adult sex ratio of African annual fishes: interspecific differences,seasonal trends and environmental predictors

Many populations have consistently biased adult sex ratios with important demographic and evolutionary consequences. However, geographical variation, the mechanisms, temporal dynamics and predictors of biased sex ratios are notoriously difficult to explain. We studied 334 wild populations of four species of African annual fish (Nothobranchius furzeri, N. kadleci, N. orthonotus, N. rachovii) across their ranges to compare their adult sex ratio, its seasonal dynamics, interpopulation variation and environmental predictors. Nothobranchius populations comprise a single age cohort and inhabit discrete isolated pools, with wide-ranging environmental conditions (habitat size, water turbidity, structural complexity, predators), making them ideal to study adult sex ratio variation. In captivity adult sex ratios were equal. In natural populations, adult sex ratios were biased 1:2 toward females in three study species while N. kadleci had sex ratios at unity. There was a decline in the proportion of males with age in one species, but not in the other species, implying most severe male mortality early after maturation, declining later perhaps with a decrease in male abundance. In general, the populations at vegetated sites had relatively more males than populations at sites with turbid water and little vegetation. Selective avian predation on brightly coloured male fish likely contributed to female dominance and vegetation cover may have protected males from birds. In addition, an aquatic predator, a large belastomid hemipteran, decreased the proportion of males in populations, possibly due to greater male activity rather than conspicuous colouration. Alternative explanations for a sex ratio bias, stemming from male–male contests for matings, are discussed. We conclude that the effect of environmental conditions on adult sex ratio varies dramatically even in closely related and ecologically similar sympatric species. Therefore, difficulties in explaining the ecological predictors of sex ratio biases are likely due to high stochasticity rather than limited sample size.     

The reproductive cycle of yellowfin bream, Acanthopagms australis (Günther), with particular reference to protandrous sex inversion

Yellowfin bream, Acanthopagrus australis , of all age classes were collected from Moreton Bay, Australia. The species possessed typical sparid ovotestes in which the testis and ovary occur in separate zones. During the spawning period (June-August) juveniles, functional males and functional females could be distinguished by the macroscopic appearance of the gonad. The sex ratio of males to females decreases with age, indicating protandrous sex inversion.
Histological and structural study of the ovotestis showed all fish have previtellogenic cells in the ovarian zone but only juvenile and male fish have developing spermatogenic cells in the testis. Most juveniles become functional males by the age of two years but a small proportion of juveniles develop directly into functional females (primary females). Protandrous sex inversion commences after the spawning period when male fish appear with spermatozoa and no other spermatogenic cells in the testis. During the period November-January male fish with no spermatogenic cells are common and a reduction in size of the testis occurs so that by March-April the ovotestis becomes structurally and histologically similar to the female ovotestis. Some fish remain functional males during their whole life-history (primary males). In functional females vitellogenic cells are present in the ovary only during the spawning period and the testis remains very small in size.     

Facultative and non‐facultative sex ratio adjustments in a dimorphic bird species

If parental allocation to each offspring sex has the same cost/benefit ratio, Fisher's hypothesis predicts a sex ratio biased towards the cheaper sex. However, in dimorphic birds there is little evidence for this, especially at hatching. We investigated the pre‐fledgling 1) sex ratio, 2) body condition and 3) sex‐differential mortality in a population of the glossy ibis Plegadis falcinellus, in southern Spain between 2001 and 2011. We defined two age groups for the period between hatching and fledging. We also compared pre‐fledgling with the autumn sex ratio. Metabolic rates were estimated by the doubly labeled water (DLW) technique to establish that sons (the bigger sex) were 18% more energy demanding than daughters, and to compute the predicted Fisher's sex ratio (0.465). As population size increased between years, body condition decreased in both sexes, and mortality increased more for daughters than sons prior to fledging. At the same time, the proportion of males among chicks close to fledging increased (average sex ratio: 0.606) while the proportion close to hatching decreased (average sex ratio: 0.434, in line with Fisher's prediction). Furthermore, the proportions of males at fledging and the following autumn were negatively correlated across years. We suggest that, as population density increased and conditions worsened the larger sex had relatively higher survival. These differences in survival produce a shift from a facultative female‐biased sex ratio at hatching into a non‐facultative male‐biased sex ratio of fledglings. Additionally, the excess of males at fledging was counterbalanced by sex‐related dispersal during the autumn. Overall, glossy ibis sex ratio is a product of a combination of facultative and non‐facultative adjustments triggered by environmental conditions, driven by rapid population growth, and mediated by highly interrelated life‐history traits such as body condition, mortality, and dispersal.     

Patterns of variation in female-biased colony sex ratios in a social spider

Colonies of a social spider Achaearanea wau (Theridiidae) from Papua, New Guinea have adult and juvenile sex ratios that are biased towards females, and this probably represents a primary bias at the egg stage. Adult sex ratios are less female-biased than are juvenile sex ratios, and both vary significantly among colonies. Adult sex ratios covary with colony size: small colonies have a larger proportion of males than large ones. The pattern of variation in adult sex ratio may be due to greater mortality of females than of males during maturation. Juvenile sex ratios do not covary with colony size, nor do they differ among populations. Colony size, however, does have a significant effect on survival and dispersal in colonies. I conclude, therefore, that a conditional sex ratio strategy, in which the primary sex ratio of the colony is adjusted to changing demographic patterns, does not occur in A. wau. I suggest that environmental heterogeneity acting on individual reproductive output may be responsible for the observed variation among colonies in juvenile sex ratios.     

Seasonal sex ratio and unbalanced investment sex ratio in the Banksia bee Hylaeus alcyoneus

Abstract 1. Hylaeus alcyoneus is an endemic solitary bee common on coastal heaths of Western Australia. The bee is unusual in that males are larger than females. This size dimorphism presents an opportunity to test the theory of resource-dependent sex allocation, in which theory predicts that when resources are low the sex ratio should be biased towards the smaller sex. In most bees, females are larger than males and, in line with theoretical prediction, sex ratios are male biased when resources are scarce.
2. The emerging sex ratio and brood mass from a natural population of H. alcyoneus using trap nests was studied over two seasons (1999, 2000). A switch from a male- to a female-biased sex ratio through the season was found, which was related to a reduced floral resource.
3. Fisherian sex ratio theory predicts that total investment in each sex throughout a season should be equal and that the sex ratio should be biased towards the smaller sex. By measuring the mass of the emerging progeny, the total investment was found to favour males. Possible explanations for this bias in investment are discussed.     

Sexual dimorphism and intraspecific aggression,and their relationship to sex ratios in Caprella gorgonia Laubitz & Lewbel (Crustacea: Amphipoda: Caprellidae)

The amphipod Caprella gorgonia Laubitz & Lewbel is an obligate commensal on gorgonian octocorals. Its primary host is Lophogorgia chilensis (Verrill), found below 20 m.C. gorgonia breeds throughout the year, with wide fluctuations in abundance. Mating and oviposition follow molting. Sex reversal does not occur; two distinct sexes are present from the first instar after emergence from the brood pouch.Young males and females grow at approximately the same rate, but males are larger by a relatively constant increment. Males continue to grow at their original rate to a maximum size (about twice that of females). The growth rate of females is not limited by the onset of reproduction and brooding, but rather by an approach to maximum size when the rate is greatly reduced. Fecundity of females is not affected by size.The population sex ratio is about 1:3 (males:females), and about 1:4 among adults. The secondary sex ratio is 1:1. The post-emergence sex ratio bias is a result of heavier mortality among males. Sex ratios drop from 50% at emergence to 25% as females approach maximum size, then rise to 100% in larger size classes.Differential predation on males did not appear to be a source of any sex ratio bias. Adult males possess a “poison spine”, a puncturing weapon on the large second gnathopod, which functions in mating-related intraspecific combat with other males. Intraspecific male aggression during mating is a major cause of sex ratio bias. In the laboratory, increased density in breeding groups may affect mortality due to male aggression. In nature, adult sex ratios are negatively correlated with population density. The reproductive capacity of the population is not limited by a shortage of adult males, despite the low adult sex ratio.     

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.     

Local adjustment of sex ratio in the spider Pityohyphantes phrygianus

The sex ratio in five populations of the sheetweb spider Pityohyphantes phrygianus in southwest Sweden was investigated in July and September, the spiders being juveniles and subadults, respectively. In July, no heterogeneity between the five populations was found and the pooled samples revealed a sex ratio of 36% males and 64% females. A significant heterogeneity in sex ratio was observed in the September samples; the proportion of males varied between 23% and 40%. In several cases, male mean weight differed significantly between the five populations, comparing the July and September samples separately. This suggests that the prey availability varied considerably between the sites. In September, dead spiders were collected in two sites. This mortality was probably caused by starvation and the mortality rates differed significantly between the two sites. Indirect evidence suggested that the local sex ratio was affected by the prey availability, males being at a relative disadvantage in poor sites. Local environmental conditions may govern the adult sex ratio in this spider by causing differential mortality and/or migration.     

Fledgling sex ratios in relation to brood size in size-dimorphic altricial birds

In six species of dimorphic raptors (females larger than males)and one passerine (males larger than females), the sex ratioat fledging varied systematically with brood size at fledging.In all species the strongest bias toward the smaller sex wasestablished in the largest as well as the smallest broods; amore even distribution of males and females was observed inbroods of intermediate size. We explored a specific differentialmortality explanation for this sex ratio variation. Our hypothesispostulates that variation in mortality is caused by differencesin food demand between broods of the same size, due to theirsex composition. Data from the marsh harrier Circus aeruginosuson gender-related food demand and overall nestling mortalitywere used to predict the frequency of surviving males and femalesat fledging, assuming an even sex ratio at hatching and randommortality with respect to both sexes within broods. The modelquantitatively fits the marsh harrier data well, especiallyin broods originating from large dutches. Although we anticipatethat other mechanisms are also involved, the results supportthe hypothesis of sex-ratio-dependent mortality, differentialbetween broods, as the process generating the observed brood-sizedependence of fledgling sex ratios in sexually dimorphic birds.     

Reversal of evolutionary downsizing caused by selective harvest of large fish

Evolutionary responses to the long-term exploitation of individuals from a population may include reduced growth rate, age at maturation, body size and productivity. Theoretical models suggest that these genetic changes may be slow or impossible to reverse but rigorous empirical evidence is lacking. Here, we provide the first empirical demonstration of a genetically based reversal of fishing-induced evolution. We subjected six populations of silverside fish (Menidia menidia) to three forms of size-selective fishing for five generations, thereby generating twofold differences among populations in mean weight and yield (biomass) at harvest. This was followed by an additional five generations during which size-selective harvest was halted. We found that evolutionary changes were reversible. Populations evolving smaller body size when subjected to size-selective fishing displayed a slow but significant increase in size when fishing ceased. Neither phenotypic variance in size nor juvenile survival was reduced by the initial period of selective fishing, suggesting that sufficient genetic variation remained to allow recovery. By linear extrapolation, we predict full recovery in about 12 generations, although the rate of recovery may taper off near convergence. The recovery rate in any given wild population will also depend on other agents of selection determined by the specifics of life history and environment. By contrast, populations that in the first five generations evolved larger size and yield showed little evidence of reversal. These results show that populations have an intrinsic capacity to recover genetically from harmful evolutionary changes caused by fishing, even without extrinsic factors that reverse the selection gradient. However, harvested species typically have generation times of 3–7 years, so recovery may take decades. Hence, the need to account for evolution in managing fisheries remains.