Bi-directional sex change in a coral-dwelling goby

Bi-directional sex change has recently been reported among obligate coral-dwelling gobies of the genus Gobiodon. However, neither the functional role of this pattern of sex change nor the frequency of sex change in either direction in natural populations is known. We investigated the social structure and pattern of sex change of Gobiodon histrio at Lizard Island on the Great Barrier Reef. The social structure of G. histrio within coral colonies usually consisted of a single juvenile or a heterosexual adult pair. The size of adult social groups was not constrained by coral colony size. In contrast to expectations for pair-forming species, G.␣histrio was primarily a protogynous hermaphrodite. All immature G. histrio were females and sex change from female to male occurred readily when two mature females were placed in a coral colony. In addition, male G. histrio were able to change back to females when two mature males were placed in a coral. Sex change from female to male, however, occurred with over twice the frequency of sex change from male to female. Where two males were placed in a coral colony, heterosexual pairs were most frequently re-established by immigration of females from outside the treatment population. This pattern might be predicted if sex change from male to female is more expensive than sex change from female to male for G. histrio. Where sex change is expensive, movement may be favoured over sex change, particularly where coral densities are high and movement among corals incurs little mortality risk. Received: 10 November 1997 / Accepted after revision: 16 May 1998     

Parental investment, adult sex ratios, and sexual selection in a socially monogamous seabird

Although most birds are monogamous, theory predicts that greater female parental investment and female-biased adult sex ratios will lower the polygyny threshold. This should result in polygynous mating, unless obligate biparental care or the spatial and temporal distribution of fertilizable females constrains a male’s ability to take advantage of a lowered polygyny threshold. Here we present data on the extent of male sexually dimorphic plumage, adult sex ratios and breeding season synchrony in three populations of a socially monogamous seabird, the brown booby Sula leucogaster. For one of these populations, San Pedro Mártir Island, we also present data on differences in male and female parental investment, mortality and probability of pairing. The extent of plumage dimorphism varied among populations. Sex ratios were female biased in all populations. On San Pedro Mártir Island, parental investment was female biased, females failed more often than males to find a mate, but there was no polygyny. We suggest that on San Pedro Mártir: (1) a period of obligate biparental care coupled with a relatively synchronous breeding season constrained the ability of males to take advantage of a high environmental polygamy potential and (2) the resulting socially monogamous mating system, in combination with the female-biased adult sex ratio, caused females to be limited by the availability of males despite their greater parental investment. Received: 18 November 1999 / Accepted: 24 January 2000     

Male mating opportunities affect sex allocation in a protrandric-simultaneous hermaphroditic shrimp

Sex allocation theory predicts phenotypic adjustments by individuals in their investments into the male and female reproductive function in response to environmental conditions. I tested for phenotypically plastic shifts in sex allocation in a protandric simultaneous hermaphrodite, in which individuals mature and reproduce as males first, and later in life, as simultaneous hermaphrodites. I predicted that initially maturing males should adjust the timing of maturation as hermaphrodites according to male mating opportunities mediated by population size of hermaphrodites. In a first experiment, males maintained with only one hermaphrodite reduced the time they spent as males in comparison to males maintained with no conspecifics, presumably because total reproductive output is maximized by two individuals being simultaneous hermaphrodites when the mating system is a pair. Conversely, males maintained in groups with two or more hermaphrodites increased the time they spent as males in comparison to single males. This delay in maturation was not an effect of resource depletion with increasing shrimp density because the growth rate of males did not differ among most of the experimental treatments. One hypothesis to explain this social mediation of sex allocation is that the smaller males are more successful in mating as males than are the larger hermaphrodites: it will pay reproductively for males to delay maturation as hermaphrodites in large but not in small groups. In agreement with this notion, a second experiment demonstrated that smaller males were four times more successful than were larger hermaphrodites in inseminating shrimps reproducing as females. The informative cue that males may use to perceive different group sizes deserves further attention.     

Population dynamics, sex ratio and size at sex change in a protandric simultaneous hermaphrodite, the spiny shrimp Exhippolysmata oplophoroides

One of the main goals of sex allocation theory is understanding sex ratio evolution. However, theoretical studies predicting sex ratios in species with unusual sexual systems, such as protandric simultaneous (PS) hermaphroditism, are rare. In PS hermaphrodites, juveniles first develop into functional males that mature into simultaneous hermaphrodites later in life. Here, we report on the sex ratio (males/males + hermaphrodites) in the PS hermaphroditic shrimp Exhippolysmata oplophoroides. A 2-year study demonstrated that hermaphrodites dominated the population in two different bays. This skewed sex ratio may be explained by limited encounter rates among conspecifics. In agreement with this idea, the density of shrimps was extremely low (≤1 shrimp km⁻²) at the two study sites. Size at sex phase change and sex ratios remained relatively stable through time at the two bays. The stability of these parameters might be explained by the rather steady population structure of this species during the study period. A review of sex ratios in PS hermaphroditic shrimps (Lysmata and Exhippolysmata) revealed considerable variation; some species have male- and others hermaphrodite-skewed sex ratios. The conditions explaining inter- and intra-specific sex ratio variation in protandric simultaneous hermaphroditic species remain to be addressed.     

Breeding Periodicity for Male Sea Turtles,Operational Sex Ratios,and Implications in the Face of Climate Change

Abstract: Species that have temperature‐dependent sex determination (TSD) often produce highly skewed offspring sex ratios contrary to long‐standing theoretical predictions. This ecological enigma has provoked concern that climate change may induce the production of single‐sex generations and hence lead to population extirpation. All species of sea turtles exhibit TSD, many are already endangered, and most already produce sex ratios skewed to the sex produced at warmer temperatures (females). We tracked male loggerhead turtles (Caretta caretta) from Zakynthos, Greece, throughout the entire interval between successive breeding seasons and identified individuals on their breeding grounds, using photoidentification, to determine breeding periodicity and operational sex ratios. Males returned to breed at least twice as frequently as females. We estimated that the hatchling sex ratio of 70:30 female to male for this rookery will translate into an overall operational sex ratio (OSR) (i.e., ratio of total number of males vs females breeding each year) of close to 50:50 female to male. We followed three male turtles for between 10 and 12 months during which time they all traveled back to the breeding grounds. Flipper tagging revealed the proportion of females returning to nest after intervals of 1, 2, 3, and 4 years were 0.21, 0.38, 0.29, and 0.12, respectively (mean interval 2.3 years). A further nine male turtles were tracked for short periods to determine their departure date from the breeding grounds. These departure dates were combined with a photoidentification data set of 165 individuals identified on in‐water transect surveys at the start of the breeding season to develop a statistical model of the population dynamics. This model produced a maximum likelihood estimate that males visit the breeding site 2.6 times more often than females (95%CI 2.1, 3.1), which was consistent with the data from satellite tracking and flipper tagging. Increased frequency of male breeding will help ameliorate female‐biased hatchling sex ratios. Combined with the ability of males to fertilize the eggs of many females and for females to store sperm to fertilize many clutches, our results imply that effects of climate change on the viability of sea turtle populations are likely to be less acute than previously suspected.     

Male influence on sex allocation in the parasitoid wasp Nasonia vitripennis

Sex allocation is an important reproductive decision for parents. However, it is often assumed that females have substantial control over sex allocation decisions, and this is particularly true in haplodiploid insects, in which females apparently determine sex by deciding whether to fertilise an egg (and produce a diploid daughter) or not (and produce a haploid son). Mechanisms by which males may influence sex allocation are not so straightforward, and their potential influence on sex ratios has been somewhat neglected. Here, we test whether males influence offspring sex ratios in the parasitoid wasp Nasonia vitripennis. We show that some of the variation in observed sex ratios can be attributed to males when comparing the affect of male strain on sex ratio. We did not find among-male variation in sex ratio with a less powerful experiment using males from only one strain or an effect of male mating environment. Our data suggest that males can influence female sex ratios and contribute to the variation around the sex ratios optimal for females. However, the influence is not large, suggesting that females have more influence on sex allocation than do males. We conclude by considering whether male influences on sex ratio represent differences in male reproductive competence or deliberate attempts by males to increase their fitness by influencing daughter production.     

Gonad morphology, sexual development, and colony composition in the obligate coral-dwelling damselfish Dascyllus aruanus

Gonad morphology and colony composition support the existing supposition that the obligate coral-dwelling damselfish Dascyllus aruanus has a protogynous hermaphroditic sexual pattern. Adults had either an active ovary containing vitellogenic oocytes, an ovotestis, or a spermiated testis and were classified as adult female, hermaphrodite, or adult male, respectively. Among individuals having male function, the testis (or testis portion of the ovotestis) takes the form of an unrestricted spermatogonial lobular testis. Among hermaphrodites having an ovotestis, a small proportion of individuals had a gonad in which both the ovarian and testicular portions were inactive (inactive hermaphrodites), whereas the majority had a predominantly testicular ovotestis that contained spermatozoa (male-active hermaphrodites). The size range of individuals within gonadal classes indicates that all D. aruanus first develop an ovariform gonad. Some individuals then undergo ovarian maturation to become adult females while others develop testicular tissue to form an ovotestis and become male-active hermaphrodites. Subsequently, progressive loss of ovarian tissue results in the development of a secondary testis from an ovotestis with the retention of a residual, afunctional lumen among adult males. The wide size range of individuals having an ovotestis suggests that some hermaphrodites function as adult females before developing testicular tissue while other individuals do not pass through an adult female stage. If this is the case, D. aruanus exhibits a diandric protogynous hermaphroditic sexual pattern. The apparent prolonged retention of an ovotestis with both healthy oocytes and an ovarian-type lumen in a spermiated ovotestis, as well as a functional sex ratio of 1:1 for adult females:adult males plus male-active hermaphrodites also raises the possibility that D. aruanus may be capable of bidirectional sex change during the hermaphroditic stage. Such a capability would be highly adaptive for a species having limited mobility and unpredictable recruitment of new colony members resulting in unpredictable mating opportunities.     

A new method to determine in situ growth rates of decapod shrimp: a case study with brown shrimp Crangon crangon

Shrimps are economically and ecologically very important, yet a lack of ageing techniques and hence unknown growth rates often impairs analytical assessments and management. A new method for the determination of in situ growth rates of shrimps is presented, based on dry weight condition. Since this index oscillates from low values directly after moult to highest values prior to moult in constantly feeding shrimp, the lowest observed pre-moult condition followed by a moult was introduced as a reference value to separate growing and starving individuals in field data. Experiments with Crangon crangon confirmed that (1) post-moult condition varies in a narrow physiologically optimal range, regardless of recent growth increments, and (2) dry weight condition prior to moult is closely related to the subsequent length increment. The method was applied to estimate growth increments from in situ dry weight condition data of C. crangon. The new method can easily be applied to other related species, since the required data can be obtained from very simple short-term experiments.     

Evidence for seasonal mate limitation in populations of an intertidal amphipod, Corophium volutator (Pallas)

Potential rates of reproduction (PRR) differ between the sexes of many animal species. Adult sex ratios together with PRR are expected to determine the operational sex ratio (OSR) defined as the ratio of fertilizable females to sexually active males at any given time. OSR is expected to determine the degree to which one sex competes for another—the limiting sex. We explored the potential for mate limitation in an intertidal amphipod, Corophium volutator (Pallas). Males have higher PRR than females, but males may be limiting because of extreme female-biased sex ratios observed in this species. Consistent with this idea, late season females were less likely to be ovigerous and had smaller size-specific clutches, both of which were associated with seasonal declines in availability of males of reproductive size. Seasonal changes in ovigery could not be explained by seasonal changes across sites in other factors (e.g., female body size or phenology of breeding). Smaller females were less likely to become ovigerous later in the season at three of four sites. Seasonal reductions in clutch size also occurred among small females expected to be reproducing for their first time. In complimentary laboratory experiments, reduced likelihood of ovigery and reduced fecundity occurred when the number of receptive females was increased relative to availability of a reproductively active male. Our results suggest male mate limitation can occur seasonally in this species and that male limitation is regionally widespread and may affect recruitment.     

Growth,survivorship, life-span,and sex change in the hermaphroditic shrimp<Emphasis Type="Italic"> Lysmata wurdemanni</Emphasis> (Decapoda: Caridea: Hippolytidae)

Lysmata wurdemanni (Gibbes) is a protandric simultaneous hermaphrodite. All individuals first mature as a male-phase individual (MP) and then later change to a female-phase individual (FP) that spawns and broods embryos but can also mate as a male. A Gulf of Mexico population was sampled monthly for 1 year and bimonthly the next. Estimates of basic population parameters were obtained from cohort analysis to reveal possible factors explaining the unusual sexual biology of L. wurdemanni as well as the broad variation in the size (age) of change from MP to FP. Growth rates of individuals from cohorts varied from 4–7 mm carapace length year ^-1. Growth of small MPs in the laboratory was somewhat faster but concordant with growth rates estimated from field samples. The period from recruitment to >50% sex change in cohorts varied from 3 months to 1 year. In the laboratory, the size and interval to sex change was similar to that of the most rapidly changing cohort observed. Survivorship of cohorts was high until later in life; life-span was estimated to be 12–18 months. Rates of sex change were highest from late winter through spring, in time for the spring–summer breeding season. The size and age of sex change in cohorts were related to the season of recruitment. MPs recruited from late winter to mid-spring rapidly changed to FPs at a relatively small size. A majority of MPs recruited in the summer and autumn did not change to FPs until the following late winter to spring, and they did so at a larger size. Rates of sex change were not correlated with the sexual composition of the population. We conclude that seasonal factors related to female breeding greatly influence sex change in L. wurdemanni. We found no evidence to support demographically influenced and socially mediated environmental sex determination, which has been suggested for L. wurdemanni and other sex-changing caridean shrimps.     

Do protandric pandalid shrimp have environmental sex determination?

Based on demographic analysis of four different temporally enclosed demes of Pandalus borealis in Gullmarsfjorden on the Swedish west coast and published information, I discuss the notion of environmental sex determination in protandric pandalid shrimps as suggested by sex allocation theory. Demographic results showed that age structure and mortality rates varied substantially between four studied periods without noticeable effects on the age of sex change in the fjord populations. The majority of shrimps changed sex at an approximate age of 3 years and became females at an age of 3 to 4 years, however, low proportions (4 to 7%) of 2nd year females (transitional at ca. 1 year) and 3rd year females (5 to 12%) (transitional at ca. 2 years) were present each year. Low proportions of primary females were also found in two of the temporally enclosed demes, in 1985 (9%) and in 1987 (5%). These results do not indicate that yearly variations in age structure, mortality, or frequency of older breeders in the breeding population affect the age of sex change in temporally enclosed fjord demes of this protandric species. An alternative to assuming that the protandric mode of reproduction in pandalid shrimp is combined with environmental sex determination is suggested. Received: 24 September 1996 / Accepted: 6 November 1996     

Female reproductive strategies in bar-headed geese (Anser indicus): Why are geese monogamous?

Summary In a semi-captive flock of Anser indicus with a surplus of females, permanent harem groups consisting of one male with one to five females, lasting for up to several years, were regularly observed. Polygynous groups contain one paired female to which the male is most attentive and secondary females which follow the paired male and are tolerated by the pair. Average annual reproductive success was lowest in lone females (0.02 young fledged per year), higher in secondary females (0.23 young) and highest in paired females (0.56 young per year). Differences seemed due to different degrees of male assistance. Secondary females could not be shown to be competitors of paired females in annual reproductive success. Lone females became secondary females mainly after an age of 3 years, i.e. when their chances to pair had dropped significantly. Females were more likely to become secondary instead of paired females in years when the adult sex ratio was more heavily female biased. As sex ratios in wild geese are usually around 1:1 or even biased towards males, females will not usually need to resort to the suboptimal secondary-female strategy. Hence, geese usually live in monogamous pairs instead of harem groups.     

Is <Emphasis Type="Italic">Hippolyte williamsi</Emphasis> gonochoric or hermaphroditic? A multi-approach study and a review of sexual systems in <Emphasis Type="Italic">Hippolyte</Emphasis> shrimps

Sexual systems vary considerably among caridean shrimps and while most species are gonochoric, others are described as sequential protandric hermaphrodites or simultaneous hermaphrodites with an early male phase. At present, there is confusion about the sexual system exhibited by several species mostly because those studies attempting to reveal their sexual system draw inferences solely from the distribution of the sexes across size classes. Here we investigated the sexual system of the shrimp Hippolyte williamsi from Chile to determine if the species is protandric or gonochoric with sexual dimorphism (males smaller than females). Morphological identification and size frequency distributions indicated that the population comprised small males, small immature females, and large mature females, which was confirmed by dissections. No transitional individuals were found. Males maintained in the laboratory molted 1–8 times, and many grew up to reach sizes observed in only a small fraction of males in the field. No indication of sex change was recorded. Our results indicate that H. williamsi is a sexually dimorphic gonochoric species and emphasizes the importance of using several kinds of evidence (size measurements, growth experiments, morphological dissections, and histological studies) to reveal the sexual system of Hippolyte species. Whether the observed size dimorphism between males and females in many species of Hippolyte is expression of contrasting sexual and natural selection, and whether divergent sexual fitness functions can contribute to the evolution of hermaphroditism remains to be revealed in future studies.     

Effect of female molt stage and sex ratio on courtship behavior of the blue crab Callinectes sapidus

In many species, males and females actively participate in courtship, and the outcome of pre-mating interactions influences the mating success of both sexes. Female blue crabs, Callinectes sapidus, mate soon after their final molt to maturity; thus female molt stage dictates the timing of mating. In a field experiment, we manipulated female molt stage and sex ratio to test their effects on the courtship behavior of both sexes, if female behavior influences the behavior and pairing success of males, and if male courtship influences male pairing-success. Early-molt-stage females avoided males during courtship, whereas late-molt-stage females sought out males. As a result, males had to pursue and capture early-molt-stage females whereas males displayed to late-molt-stage females and more easily physically controlled them. Males sometimes abandoned late-molt-stage females, but this occurred more often when females were abundant. The rate at which females avoided males was positively correlated with that of males abandoning females, and males that were unsuccessful at pairing met with higher rates of female resistance than successful males, suggesting that female behavior influences male pairing-success. Unlike unsuccessful males, successful males more often made the transition between display and maintaining physical control of the female. At high male sex ratios, males initiated courtship more readily; thus both sexual competition and female behavior influence male courtship in this species. Received: 7 July 1996 / Accepted: 10 January 1998     

No evidence for offspring sex-ratio adjustment to social or environmental conditions in cooperatively breeding purple-crowned fairy-wrens

When fitness returns or production costs vary between male and female offspring, selection is expected to favor females that adjust offspring sex ratio accordingly. However, to what extent vertebrates can do so is the subject of ongoing debate. Here, we explore primary sex ratios in 125 broods of cooperatively breeding purple-crowned fairy-wrens Malurus coronatus. We expected that females might adjust offspring sex ratio because this passerine species experiences considerable variation in social and environmental conditions. (1) However, although helpers substantially increase parental fitness, females (particularly in pairs and small groups) did not overproduce philopatric males (helper-repayment hypothesis). (2) Sex-ratio adjustment based on competition among individuals (helper-competition hypothesis) did not conceal helper-repayment effects or drive sex allocation on its own: while high-quality territories can accommodate more birds, brood sex ratios were independent of territory quality, alone or in interaction with group size. (3) Additionally, males are larger than females and are possibly more costly to produce (costly sex hypothesis), and (4) female offspring may benefit more from long-term effects of favorable conditions early in life (Trivers–Willard hypothesis). Nonetheless, large seasonal variation in food abundance was not associated with a consistent skew in primary sex ratios. Thus, overall, our results did not support the main hypotheses of adaptive sex-ratio adjustment in M. coronatus. We discuss that long-term differential costs and benefits may be insufficient to drive evolution of primary sex-ratio manipulation by M. coronatus females. More investigation is therefore needed to determine the general required sex differences in long-term fitness returns for mechanisms of primary sex-ratio manipulation to evolve.     

Variation in size at sex-change among natural populations of the protandrous hermaphrodite, <Emphasis Type="Italic">Crepidula fornicata</Emphasis> (Gastropoda,Calyptraeidae)

Protandrous hermaphrodites are predicted to change sex from male to female when relative reproductive fitness of females surpasses that of males. How size at sex transition varies with population, mating group and individual parameters was investigated for five populations of the protandrous hermaphrodite slipper snail, Crepidula fornicata. The populations varied for density, size distribution, average mating group size and sex ratio. Size at sex-change was correlated with the population sex ratio. Comparisons of multiple hypotheses revealed that variables predicting the sex of a snail vary among positions in the mating group. The variables included body size, the relative size of the snail sitting atop the focal snail and population density. Our data support the conclusions that size at sex-change (and by inference, the size at which one sex has relatively greater fitness) is not fixed for these hermaphrodites and that individual size, social conditions and population differences all influence variation in relative fitness.     

Not all ctenophores are hermaphrodites. Studies on the systematics,distribution, sexuality and development of two species of Ocyropsis

The lobate ctenophores Ocyropsis maculata and O. crystallina are not simultaneous hermaphrodites, based on morphological, histological and experimental evidence. Sex ratios in populations, sex ratios of size classes within populations, and average sizes of males and females support the hypothesis that both species are dioecious, rather than sequential hermaphrodites. We have divided each species into two subspecies, based on morphology and geographic distribution. Preliminary evidence suggests that the subspecies also differ in reproductive behavior. One subspecies, O. crystallina guttata, spawns on a daily cycle in the laboratory, and spawning becomes more synchronous when males and females are placed together. Species of Ocyropsis, all of which are oceanic in distribution, are probably descended from a common ancestor that was a simultaneous hermaphrodite. That this group of oceanic ctenophores evolved dioecy directly contradicts the assertion that there is a selective advantage to hermaphroditism in environments where the probability of finding a mate is reduced.     

Bi-directional sex change: testing the growth-rate advantage model

Bi-directional sex change in coral-dwelling fishes (genera Gobiodon and Paragobiodon) has been attributed to a growth-rate advantage for females during the non-breeding season. This model predicts that the smallest individual in a newly formed pair should always be female. To determine if a growth-rate advantage exists for female Gobiodon histrio, I monitored the growth of males and females in natural pairs during the breeding and non-breeding season. I then used a manipulative field experiment to test four predictions of the growth-rate advantage model: (1) the larger individual should change sex to male in new pairs containing two females; (2) the smaller individual should change sex to female in new pairs containing two males; (3) neither individual should change sex in heterosexual pairs where the male is larger than the female; and (4) both individuals should change sex in heterosexual pairs where the female is larger than the male. A growth-rate advantage was detected for female G. histrio during the non-breeding season; however, only the first three of the predicted outcomes were observed in the manipulative experiment. Sex change did not occur in heterosexual pairs where the female was larger than the male. Furthermore, growth did not differ between sex-changed and non-sex-changed fish; therefore, the absence of sex change in these pairs is not due to a growth cost to sex change. I propose that the risk of moving among spatially isolated habitat patches and the low probability of finding a mate have been more important than sex-specific differences in growth rates to the evolution of bi-directional sex change in coral-dwelling gobies.     

Sex ratio and sex change in wild and laboratory populations ofTyposyllis prolifera (Polychaeta)

Sex determination and sex differentiation in three local Istrian (Yugoslavia) populations of the polychaeteTyposyllis prolifera were studied both in the field and the laboratory. Significant differences in sex habits were revealed among the populations, the biological significance of which is unknown. (1) Sex ratios (% ) in natural populations from the Adriatic Sea amounted to 50% (Rovinj), 62% (Pore) and 77% (Pula). In the Pula-(but not in the Rovinj-)population, a correlation was found between population density on individualHalopteris scoparia thalli (the favorite habitat ofT. prolifera) and the respective sex ratio: The mean male protion was only a little more than 50% in densely populated thalli, and increased to more than 80% in sparsely populated thalli. Laboratory studies provided the information necessary to explain these field findings. (2) Laboratoryraised progenies showed an overall 1:1 primary (first sexual phase) sex ratio, independent of the local origin of the material and of whether worms were raised singly or under conditions of social contact. Nevertheless, the studied populations may differ with respect to the genetic mechanisms of sex determination: In the Pore- and Pulapopulation individual progenies often departed significantly from the 1:1 sex ratio, whereas individual progenies in the Rovinj- population never did so. (3) The populations proved to be partially protogynous. Sex differentiation in that half of the individuals which differentiated into males (primary males) was absolutely stable during sequential sexual phases. Worms which differentiated into females, however, often changed sex at an earlier or later stage of their life cycle (secondary males). Sex change was irreversible. As to the degree of lability of female differentiation under conditions of isolation, the findings suggest a genetic divergence among both populations and individuals of the same population. Female differentiation in the Rovinj-population was virtually stable, but Pula-females underwent rapid and complete masculinization. Pore-females were between these extremes. (4) In addition to genetic factors, exogenous conditions affected the incidence and time of sex change (Pula-population): Under conditions of social contact, sex change was delayed or suppressed as compared with isolated individuals. The degree of delay or inhibition was independent of the sex of the social partners, yet increased (to the point of saturation) with population density.     

Male limitation of female reproductive success in a pipefish: effects of body-size differences

Summary In the pipefish Syngnathus typhle, a species with exclusive male parental care, males limit female reproductive success because of their limited brood pouch space and long pregnancy. Sexual size dimorphism is absent in these 1-year-old animals but increases with age so that older females are larger than similarly aged males. Because fecundity is related to size in both sexes and increases more rapidly with body size in females than in males, the difference in growth increases female fecundity more, relative to male fecundity, as the fish get older. We therefore predicted that male limitation of female reproductive success is even more severe when all age classes are considered. To measure a female's maximum reproductive rate, she was provided with three males. Small 1-year-old females produced as many eggs, or produced eggs at the same rate, as a male of similar size could care for. Small females filled on average 1.06 males within the time span of one male pregnancy and actually produced on average 10 eggs fewer than needed to fill a similarly sized male. Large 2-year-old females, in contrast, produced on average a surplus of 149 eggs and filled 2.7 similarly sized males within the course of one pregnancy. The difference between females of the two size classes was highly significant. Males prefer to mate with larger females if given a choice. In nature sex ratios are equal, and males limit female reproductive success in the whole population. Therefore, small females are more severely constrained by mate availability than are larger females because males choose to mate with larger females. Offprint requests to: A Berglund