Sex Ratio (sex + ratio)

Distribution by Scientific Domains
Distribution within Life Sciences

Kinds of Sex Ratio

  • adult sex ratio
  • birth sex ratio
  • brood sex ratio
  • colony sex ratio
  • equal sex ratio
  • female sex ratio
  • female-biased sex ratio
  • offspring sex ratio
  • operational sex ratio
  • population sex ratio
  • primary sex ratio
  • progeny sex ratio
  • secondary sex ratio
  • skewed sex ratio

  • Terms modified by Sex Ratio

  • sex ratio bias
  • sex ratio distortion
  • sex ratio variation

  • Selected Abstracts


    EVOLUTION, Issue 2 2008
    Max Reuter
    Theory predicts that males adapt to sperm competition by increasing their investment in testis mass to transfer larger ejaculates. Experimental and comparative data support this prediction. Nevertheless, the relative importance of sperm competition in testis size evolution remains elusive, because experiments vary only sperm competition whereas comparative approaches confound it with other variables, in particular male mating rate. We addressed the relative importance of sperm competition and male mating rate by taking an experimental evolution approach. We subjected populations of Drosophila melanogaster to sex ratios of 1:1, 4:1, and 10:1 (female:male). Female bias decreased sperm competition but increased male mating rate and sperm depletion. After 28 generations of evolution, males from the 10:1 treatment had larger testes than males from other treatments. Thus, testis size evolved in response to mating rate and sperm depletion, not sperm competition. Furthermore, our experiment demonstrated that drift associated with sex ratio distortion limits adaptation; testis size only evolved in populations in which the effect of sex ratio bias on the effective population size had been compensated by increasing the numerical size. We discuss these results with respect to reproductive evolution, genetic drift in natural and experimental populations, and consequences of natural sex ratio distortion. [source]


    EVOLUTION, Issue 8 2007
    Jon R. Linklater
    We assessed the extent to which traits related to ejaculate investment have evolved in lines of Drosophila melanogaster that had an evolutionary history of maintenance at biased sex ratios. Measures of ejaculate investment were made in males that had been maintained at male-biased (MB) and female-biased (FB) adult sex ratios, in which levels of sperm competition were high and low, respectively. Theory predicts that when the risk of sperm competition is high and mating opportunities are rare (as they are for males in the MB populations), males should increase investment in their few matings. We therefore predicted that males from the MB lines would (1) exhibit increased investment in their first mating opportunities and (2) deplete their ejaculates at a faster rate when mating multiply, in comparison to FB males. To investigate these predictions we measured the single mating productivity of males from three replicates each of MB and FB lines mated to five wild-type virgin females in succession. In contrast to the first prediction, there was no evidence for differences in productivity between MB and FB line males in their first matings. The second prediction was upheld: mates of MB and FB males suffered increasingly reduced productivity with successive matings, but the decline was significantly more pronounced for MB than for FB males. There was a significant reduction in the size of the accessory glands and testes of males from the MB and FB regimes after five successive matings. However, the accessory glands, but not testes, of MB males became depleted at a significantly faster rate than those of FB males. The results show that male reproductive traits evolved in response to the level of sperm competition and suggest that the ability to maintain fertility over successive matings is associated with the rate of ejaculate, and particularly accessory gland, depletion. [source]


    EVOLUTION, Issue 9 2002
    Therese Ann Markow
    Abstract., As commonly observed among closely related species within a variety of taxa, Drosophila species differ considerably in whether they exhibit sexual dimorphism in coloration or morphology. Those Drosophila species in which male external sexual characters are minimal or absent tend, instead, to have exaggerated ejaculate traits such as sperm gigantism or seminal nutrient donations. Underlying explanations for the interspecific differences in the presence of external morphological sexual dimorphism versus exaggerated ejaculate traits are addressed here by examining the opportunity for sexual selection on males to occur before versus after mating in 21 species of Drosophila. Female remating frequency, an important component of the operational sex ratio, differs widely among Drosophila species and appears to dictate whether the arena of sexual selection is prior to, as opposed to after, copulation. Infrequent female mating results in fewer mating opportunities for males and thus stronger competition for receptive females that favors the evolution of male characters that maximize mating success. On the other hand, rapid female remating results in overlapping ejaculates in the female reproductive tract, such that ejaculate traits which enhance fertilization success are favored. The strong association between female remating frequency in a given species and the presence of sexually selected external versus internal male characters indicates that the relationship be examined in other taxa as well. [source]


    J16 The beginning of the present century has been marked by a shift in attention from "excess" female mortality to discrimination in natality in explaining the "lowness" of the sex ratio or proportion of women in India's population. Such a shift in focus seemingly suggests that discrimination in intra-family allocation of resources has reduced substantially in India. In this context, an attempt has been made to decompose the observed lowness of the sex ratio in India vis-à-vis that of the stable population into that attributable to: (1) age structure difference, (2) excess female mortality, and (3) abnormalities in sex ratios at birth in India. Estimated contributions by each factor suggest that, as late as 2001, excess female mortality or the lowness of the relative survival advantage of women is the single most important determinant of "missing" women in India. The results also point to the importance of age structure difference, which accounts for a little more than 17% of the lowness of the sex ratio in India in 2001. [source]


    EVOLUTION, Issue 1 2010
    Gabriel D. G. Debout
    We investigated sex allocation in the Neotropical ant Allomerus octoarticulatus var. demerarae. Because Allomerus is a plant symbiont, we could make geographically extensive collections of complete colonies and of foundresses in saplings, allowing us to estimate not only population- and colony-level sex allocation but also colony resource levels and the relatednesses of competing ant foundresses. This species exhibits a strongly split sex ratio, with 80% of mature colonies producing ,90% of one sex or the other. Our genetic analyses (DNA microsatellites) reveal that Allomerus has a breeding system characterized by almost complete monogyny and a low frequency of polyandry. Contrary to theoretical explanations, we find no difference in worker relatedness asymmetries between female- and male-specialist colonies. Furthermore, no clear link was found between colony sex allocation and life history traits such as the number of mates per queen, or colony size, resource level, or fecundity. We also failed to find significant support for male production by workers, infection by Wolbachia, local resource competition, or local mate competition. We are left with the possibility that Allomerus exhibits split sex ratios because of the evolution of alternative biasing strategies in queens or workers, as recently proposed in the literature. [source]


    EVOLUTION, Issue 12 2005
    Ken R. Helms
    Abstract Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation. [source]


    EVOLUTION, Issue 5 2004
    Ken R. Helms
    Abstract A recent study by Fournier et al. (2003) provides important new information on sex allocation in the ant Pheidole pallidula, and proposes a new scenario for sex-ratio evolution in P. pallidula and similar species. However, Helms proposed to the authors that two important conclusions of the study were questionable because of potential problems with the analyses. Here we provide new data and a reanalysis that strengthens the conclusion that colony sex ratio is associated with breeding system (i.e., polygyny or monogyny). However, the proposal that colonies shift from monogyny to polygyny when they become larger and more productive is weakened because there is substantial overlap in productivity between monogynous and polygynous colonies. [source]

    Adaptive Offspring Sex Ratio Depends on Male Tail Length in the Guppy

    ETHOLOGY, Issue 11 2006
    Kenji Karino
    A biased sex ratio in a brood is considered to be an adaptive strategy under certain circumstances. For example, if the expected reproductive success of one sex is greater than that of the other, parents should produce more offspring of the former sex than the latter. A previous study has documented that in the guppy, Poecilia reticulata, the female offspring of males possessing proportionally longer tails exhibit smaller body sizes and show decreased reproductive outputs than those of males having shorter tails. On the other hand, the total lengths of the male offspring of the long-tailed males are larger because of their longer tails; consequently, they exhibit greater sexual attractiveness to females. Therefore, it has been hypothesized that this asymmetry in the expected reproductive success between the male and female offspring of long-tailed males may result in a biased sex ratio that is dependent on the tail lengths of their fathers. This hypothesis was tested in the present study. The results showed that the females that mated with long-tailed males produced more male offspring than those that mated with short-tailed males. Logistic regression analysis showed that the ratio of tail length to the standard length of the fathers is a determinant factor of the sex of their offspring. These results suggest that the manipulation of the offspring sex ratios by parents enhances the overall fitness of the offspring. [source]

    Operational Sex Ratio and Alternative Reproductive Behaviours in Chinese Bushcricket, Gampsocleis gratiosa

    ETHOLOGY, Issue 4 2006
    Yong Gao
    The effects of operational sex ratio (OSR) on male mating tactics in the Chinese bushcricket Gampsocleis gratiosa were investigated in male- and female-biased environments. We measured fresh and dry spermatophore contents and copulation duration, and counted sperm numbers of each copulation. The fresh weight of spermatophore and spermatophylax was positively correlated with male body weight. The males in a strongly male-biased environment produced significantly heavier fresh ampulla and more sperm per ejaculation, which were likely tactics for successful matings under the competition of rivals. The spermatophore might function as a structure to protect the fertilization potential of the ejaculate from rival males. [source]

    Effect of Dietary Administration of 17,-methyltestosterone on the Sex Ratio of Postlarval Freshwater Prawn, Macrobrachium rosenbergii, During the Nursery Stage of Culture

    Cortney L. Ohs
    First page of article [source]

    Sex Ratio of Some Long-Lived Dioecious Plants in a Sand Dune Area

    PLANT BIOLOGY, Issue 5 2004
    T. J. de Jong
    Abstract: In dioecious plants the fraction of males among flowering plants in the field (the secondary sex ratio) is the result of the fraction of males in the seeds (the primary sex ratio) and the subsequent survival and age at first reproduction of the two genders. It has been assumed that survival and age at first reproduction are the main determinants of biased secondary sex ratio but, especially for long-lived perennials, few data are available. We address this issue for natural populations of four long-lived perennials in a dune area. In Asparagus officinale and Bryonia dioica, the secondary sex ratio was unbiased. In Salix repens the secondary sex ratio was female-biased (0.337). Hippophae rhamnoides populations were male-biased; the average sex ratio of flowering plants was 0.658, while the fraction of males varied between 0.39 near the sea to 0.84 at the inland side of the dunes. The primary sex ratio was estimated by germinating seeds and growing plants under favourable conditions with minimal mortality. In S. repens the primary sex ratio in seeds was variable among mother plants and was, on average, female-biased (0.289). This is close to the secondary sex ratio, suggesting that the female bias already originates in the seed stage. In Hippophae rhamnoides the primary sex ratio was slightly male-biased (0.564). We argue that in this species, apart from the primary sex ratio, higher mortality and a later age at first reproduction for females contribute to the strong male bias among flowering plants in the field. [source]

    Equal Sex Ratios of a Marine Green Alga, Bryopsis plumosa

    Tatsuya Togashi
    Abstract By finding some important culture conditions as below, we succeeded in experimentally controlling the whole life history of a dioecious marine green alga, Bryopsis plumosa (Hudson) C. Agardh. In this study, we focused on the primary and secondary sex ratios (i.e. at inception and maturity) using these culture techniques. Gametogenesis was induced by culturing haploid gametophytes with Provasoli's enriched seawater (PES) medium under a 14:10 h light : dark cycle at 14 °C. Formed zygotes grew into diploid sporophytes, which were cultured for 3 months with PES medium under a 14:10 h light : nbsp;dark cycle at 18 °C. Then they were transferred into Schreiber medium and cultured under a 10:14 h light : dark cycle at 22 °C. Within 1 week, zoosporogenesis was observed. Zoospores were released within a couple of days. Each zoospore soon germinated and grew into a unisexual gametophyte. The primary sex ratio was examined in gametophytes that originated from a single sporophyte. The secondary sex ratio was studied in the field. Both were estimated as 1:1. Synchronized meiotic cell divisions might occur during zoosporogenesis dividing each sex-determining factor evenly among zoospores. Given the equal sex ratio at maturity, there seems to be no environmental factor that differentially affects the survival of male or female gametophytes in nature. [source]

    Fertility and Distorted Sex Ratios in a Rural Chinese County: Culture, State, and Policy

    Rachel Murphy
    This article explores how gender bias in population policies interacts with local culture to reinforce distortions in sex ratios among infants and young children in rural China. It argues that population policies introduce new sources of inequality into local culture while, conversely, gender inequalities embedded in local culture influence formal population policy and practice. Applying an institutional approach to the study of an agricultural county in Jiangxi province, southeast China, the analysis identifies four ways in which an interplay between gender bias in policy and culture produces gendered fertility outcomes: (1) the creation of gendered official categories such as "daughter-only households"; (2) a male bias embedded in local government; (3) the use of local gender norms in state pedagogy; and (4) the reworking or subverting of official norms in ways that reinforce gender inequalities in local reproductive culture. The article concludes that despite indications of contestation of village patriarchy, discrimination against daughters is likely to persist. [source]

    Ultrasonography and Sex Ratios in China

    Hongbin LI
    J13; J16; O10 This paper directly measures the causal effects of sex-selective abortions on the sex ratio at birth by exploiting the exogenous county-level variation in the availability of B-ultrasound machines. Using data from the 1990 Census of Fujian Province and local records on the introduction time of B-ultrasound machines, we find that the availability of B-ultrasound machines increases the sex ratio at birth by 0.025 in rural areas and 0.117 in urban areas. The rise of sex ratio is especially significant for second births in rural areas when the first birth is a girl. [source]

    Dietary exposure to low doses of bisphenol A: Effects on reproduction and development in two generations of C57BL/6J mice

    Kenichi Kobayashi
    Abstract The present study was conducted to examine the effects of low-dose exposure to bisphenol A on reproduction and development in two generations of mice. Pregnant female C57BL/6J mice (F0) were fed a diet containing low doses of bisphenol A (0, 0.33, 3.3, or 33 ppm) from gestational day 6 through postnatal day 22, and the weanlings (F1 and F2) from each F0 and F1 dam group, respectively, were also fed these same concentrations of bisphenol A ad libitum until sacrifice. There were no treatment-related changes in body weight, body weight gain, food consumption, gestation length, or the number of live births on postnatal day 1 in F0 dams between the control group and bisphenol A groups. Sex ratio and viability were similar in all F1 pups. No treatment-related changes were observed in body weight, food consumption, developmental parameters, anogenital distance, or weight of any of the organs (liver, kidney, heart, spleen, thymus, testis, ovary, or uterus) in F1 and F2 adults in either sex. The epididymis weight was slightly higher with 0.33 and 3.3 ppm in F1 males, but this slight increase was neither dose dependent nor seen across generations. There were no treatment-related effects of bisphenol A on cauda epididymal sperm count or sperm motility in F1 or F2 males. These findings indicate that dietary exposure to bisphenol A between 0.33 and 33 ppm does not adversely affect reproduction or development as assessed in two generations of mice. [source]

    Sex ratio of siblings of children diagnosed with autism spectrum disorder

    No abstract is available for this article. [source]

    Distribution, zoogeography and biology of the Murchison River hardyhead (Craterocephalus cuneiceps Whitley, 1944), an atherinid endemic to the Indian Ocean (Pilbara) Drainage Division of Western Australia

    M. G. Allen
    Abstract , The Murchison River hardyhead (Craterocephalus cuneiceps) is endemic to the extremely arid Indian Ocean (Pilbara) Drainage Division of Western Australia, where it is found in the Greenough, Hutt, Murchison, Wooramel, Gascoyne and DeGrey rivers, but is absent from numerous rivers within its range. The most likely explanation for the disjunct contemporary distribution is that C. cuneiceps has simply never inhabited the rivers from which it is conspicuously absent (e.g. Ashburton and Fortescue). Biogeographical, geological and palaeoclimatic evidence is presented to support this hypothesis. In the Murchison River, breeding was extremely protracted with recruitment occurring throughout the year. The largest female and male specimens captured were 96 mm total length (TL; 7.73 g) and 86 mm TL (5.57 g), respectively. Sex ratio was 1.09 females:1 male. Batch fecundity ranged from 46 to 454 (mean 167.5 ± 25.7 SE). Estimates for the length at which 50 and 95% of females first spawned were 36.4 and 44.3 mm TL, respectively. Craterocephalus cuneiceps is essentially a detritivore, but also feeds on aquatic invertebrates. Rainfall in the Murchison River catchment is unpredictable and pH, salinity and temperature are variable. A specialised diet, small size and young age at maturity and protracted spawning period, coupled with serial spawning and high fecundity, allows the numerical dominance of this species in competitive, harsh, arid and unpredictable desert environments. Resumen 1. Craterocephalus cuneiceps es una especie endémica de las cuencas del Océano Indico (i.e., Pilbara) de Australia Occidental. Se encuentra en los ríos Greenough, Hutt, Murchison, Wooramel, Gascoyne y DeGrey pero está ausente en numerosos ríos dentro de su área de distribución. La explicación más probable para esta distribución separada en la actualidad es que C. cuneiceps no ha habitado nunca los ríos en los que está ausente tales como los ríos Ashburton y Fortescue. Presentamos evidencia bio-geográfica, geológica y paleo-climática para soportar esta hipótesis. 2. En el río Murchison, la reproducción es extremadamente prolongada con reclutamiento a lo largo de todo el año. Los mayores machos y hembras capturados alcanzaron 96 mm LT (7.73 g) y 86 mm LT (5.57 g), respectivamente. La proporción de sexos fue 1.09 hembras: 1 macho. La fecundidad varió entre 46 y 454 (media 167.5 ± 25.7 SE) y la longitudes a la que el 50 y el 95% de las hembras se reproducen por primera vez alcanzaron 36.4 y 44.3 mm LT, respectivamente. 3. C. cuneiceps es esencialmente detritívoro pero también se alimenta de invertebrados acuáticos. La lluvia sobre la cuenca del río Murchison es impredecible y el pH, la salinidad y la temperatura son variables. Una dieta especializada, pequeño tamaño, una edad joven en la madurez, y un período reproductivo prolongado, ademos de una freza seriada y alta fecundidad, permiten la dominancia numérica de la especie en ambientes competitivos, duros, áridos e impredecibles. [source]

    Sex ratio, reproductive mode and genetic diversity in Triops cancriformis

    FRESHWATER BIOLOGY, Issue 7 2009
    Summary 1. Aquatic invertebrates display a wide array of alternative reproductive modes from apomixis to hermaphroditism and cyclical parthenogenesis. These have important effects on genetic diversity and population structure. Populations of the ,living fossil'Triops cancriformis display a range of sex ratios, and various reproductive modes are thought to underlie this variation. Using sex ratio information and histological analyses European populations have been inferred to be gonochoric (with separate males and females), selfing hermaphroditic and androdioecious, a rare reproductive mode in which selfing hermaphrodites coexist with variable proportions of males. In addition, some populations have been described as meiotic parthenogens. 2. Here we use population genetic analysis using microsatellite loci in populations with a range of sex ratios including a gonochoric population, and marker segregation patterns in heterozygote individuals reared in isolation, to clarify the reproductive mode in this species. 3. Our data show that populations in general have very low levels of genetic diversity. Non-gonochoric populations show lower genetic diversity, more heterozygote deficiencies, higher inbreeding coefficients and stronger linkage disequilibria than the gonochoric population. The maintenance of some heterozygosity in populations is consistent with some male influence in T. cancriformis populations, as would be expected from an androdioecious reproductive system. Results of marker segregation in eggs produced in isolation from non-gonochoric populations indicate that meiosis occurs and are consistent with two reproductive modes: selfing hermaphroditism and a type of ameiotic parthenogenesis. 4. Overall, our data indicate that androdioecy and selfing hermaphroditism are the most likely reproductive modes of non-gonochoric European Triops populations. Triops populations are strongly structured, suggesting high genetic drift and low levels of gene flow. [source]

    Sex ratio and maturity indicate the local dispersal and mortality of adult stoneflies

    FRESHWATER BIOLOGY, Issue 8 2006
    Summary 1.,Despite a recent upsurge in interest, there remains remarkably little information about the dispersal and survival of the adults of most stream-dwelling insects, although this is a basic requirement for understanding their long-term population dynamics. 2.,Using Malaise traps for a whole annual flight period, we captured adult stoneflies (Leuctra nigra) along transects perpendicular to three upland Welsh streams. We assessed spatial and temporal patterns in sex ratio to infer local dispersal and, using maturity as an age marker, estimated the mortality of adult females. 3.,Nearly all adult stoneflies (99%) were taken in the period 26 April,23 July, and the onset of the male and female flight periods was the same. Most males (90%) had been caught by late June. Females were classified as immature (without ripe eggs) or mature, and 90% of immature females had been caught by mid-late June (depending on catchment). As immature females declined in the catch, mature females increased, 10% having been caught by late-May to early June and 90% by early to mid-July. The median catches of immature and mature females were separated by 32 days in all three catchments. 4.,There was a female bias in the sex ratio overall, which increased as time passed and was attributable partially to the greater longevity of females. Late in the flight period, however, female bias was also greater near the stream channel implying a return of mature females (but not males) from the riparian vegetation, presumably to oviposit. 5.,The number of mature females was less than the number of immatures in two of the three channels. Over all three catchments, the overall mortality of females over the 32 days taken to mature was estimated at 29%, equivalent to an exponential daily rate of 1.1% day,1. The apparently negative mortality rate in one catchment (i.e. more matures than immatures being caught) could be due to an influx of adult females from elsewhere along the channel to oviposit. 6.,Natural markers of age and population structure, such as sex ratio and female maturity, thus enabled us to detect a return of females to the stream to oviposit, after prior limited dispersal into the riparian zone, and to infer longitudinal movements in search of suitable sites. We were also able to estimate mortality in the field. Such natural markers seem to have been underexploited in the study of adult aquatic insects. [source]

    Offspring sex ratio skew in the sexually monomorphic house martin Delichon urbicum

    Monika Zieli
    Sex ratio at conception may be under selection pressure, given that male and female offspring differ in the cost of production or generate different fitness returns under specific conditions. We studied adjustments in the primary, secondary and tertiary sex ratio in house martin Delichon urbicum, which is a sexually monomorphic, socially monogamous, colonial bird. Males of this species engage in extra-pair copulations with heavy males acquiring the highest fertilization success. We analyzed variation in the sex ratio in relation to clutch size and parental characteristics including body condition, wing length, as well as length and pigmentation of the white rump patch during three breeding seasons. The only variable which significantly explained the variation in the sex ratio was wing length of the social father and mother. The proportion of sons among offspring was positively correlated to wing length of the social father and negatively correlated to mother wing length. Social father wing length positively correlated with mean brood body mass at fledging, which may suggest that females that mated with long-winged males produced sons, which acquired the highest total fertilization success. Consequently, our results indicate that house martin females may adaptively adjust offspring sex composition at egg laying in relation to the characteristics of their social mate. [source]

    Testing the abundant-centre hypothesis using intertidal porcelain crabs along the Chilean coast: linking abundance and life-history variation

    Marcelo M. Rivadeneira
    Abstract Aim, The abundant-centre hypothesis (ACH) is based on the assumption that physiological constraints limit populations at the edges of their distributional range, yet the geographical variation of physiological performance or life-history traits has rarely been examined. Here we examine the applicability of the ACH in a marine system by testing whether physiological predictions are reflected in large-scale variations of life-history traits. Location, The Chilean coast (18°,42° S), encompassing more than 2500 km along the Pacific coast of South America. Methods, Five porcelain crab species (Petrolisthes granulosus, Petrolisthes laevigatus, Petrolisthes tuberculatus, Petrolisthes violaceus and Allopetrolisthes angulosus) were sampled on intertidal boulder beaches at 13 sampling sites. For each species and site we evaluated: (1) relative abundance (density), (2) maximum size, (3) size at maturity, (4) sex ratio, (5) proportion of ovigerous females, and (6) presence of recruits. The shape of the spatial distribution of each trait was evaluated statistically against the prediction of four hypothetical models (normal, ramped-south, ramped-north and abundant-edge). Results, The relative abundance and life-history traits showed different spatial patterns among species. Relative abundance (across sites) was fitted by a normal model in only two species. No model fitted the spatial variation in body size and size at first maturity, which showed a slight but monotonic poleward increase in all species. Sex ratio showed a prominent hump-shaped pattern, with females prevailing in the centre of the ranges and males dominating towards the range boundaries; this pattern was statistically significant in three of the five studied species. The proportion of ovigerous females showed no clear latitudinal trends, and mature individuals were observed across most of the geographical range of the species. However, recruits tended to be absent towards the southern (poleward) boundaries of the distribution. Main conclusions, The ACH does not apply to all species equally. The link between abundance and life-history traits is complex and variable among the porcelain crab species studied. Overall, the observed patterns were consistent with the idea that equatorward boundaries might be controlled by physiological restrictions mainly affecting adult survival, whereas poleward boundaries might be shaped by limitations in reproductive output and larval survival. Our results underline the importance of incorporating ecological, physiological and life-history studies in future tests of the ACH. [source]

    Density dependence and population dynamics of black rhinos (Diceros bicornis michaeli) in Kenya's rhino sanctuaries

    Benson Okita-Ouma
    Abstract Density-dependent feedback mechanisms provide insights into the population dynamics and interactions of large herbivores with their ecosystem. Sex ratio also has particularly important implications for growth rates of many large mammal populations through its influence on reproductive potential. Therefore, the interrelationships between density-dependent factors, comprising density, sex ratio and underlying growth rates (r) were examined for the Eastern black rhino (Diceros bicornis michaeli) living in three rhino sanctuaries in Kenya using four population models. The exponential and logistic models gave similar results and the former were accepted because they better portrayed the actual situation on the ground. Sex ratios in all sanctuary populations were positively correlated with r but interpreted with realization of other factors also affecting r. We caution that the results of population models should be interpreted alongside ground-truthed observations. We recommend that future translocation strategies should take into account sex and age structures of the donor population, while future studies of density dependence should take into account both biotic and abiotic factors. Résumé Des mécanismes de feedback de densité-dépendance chez les grands herbivores donnent un aperçu de la dynamique des populations et de l'interaction avec l'écosystème. Les sex-ratios ont aussi d'importantes implications pour la dynamique des populations de nombreux grands mammifères, spécialement par leur influence sur le potentiel reproducteur. On a étudié ces relations croisées entre les facteurs densité-dépendants du rhino noir de l'Est Diceros bicornis michaeli, le sex-ratio et le taux de croissance (r) sous-jacent dans trois sanctuaires de rhinos du Kenya en utilisant quatre modèles de population. Les modèles exponentiel et logistique donnaient des résultats similaires, les résultats du premier étant acceptés parce qu'ils représentaient la situation actuelle sur le terrain. Les sex-ratios de toutes les populations étaient positivement liés àr mais interprétés en réalisant que d'autres facteurs affectent aussi r. Nous attirons l'attention sur le fait que les résultats de la modélisation des populations doivent être interprétés tout en les confirmant par des observations sur le terrain; nous recommandons des stratégies de translocations qui prélèvent des individus dans les diverses structures de sexe et d'âge de la population d'origine; et nous suggérons que de futures études de densité-dépendance tiennent compte de facteurs biotiques et abiotiques. [source]

    Breeding patterns of Arvicanthis neumanni in central Tanzania

    A. W. Massawe
    Abstract The breeding pattern of the grass rat, Arvicanthis neumanni, was investigated in central Tanzania in 2002/2003. Nine hundred and forty-seven animals were captured in three age groups: juveniles, sub adults and adults. These age groups were present in the population for an extended duration, but there was an increase in the number of sexually active individuals and juveniles 2,3 months after the onset of the rains (December 2002,May 2003). A peak in reproductive activity occurred between January and April. Litter size ranged between 5.58 ± 0.42 and 6.1 ± 0.26 in two study sites. There were no significant differences in the number of embryos implanted in the right and left horns of the uterus of pregnant females (t22 = 0, P > 0.05 and t36 = 1.68, P > 0.05, respectively). Sex ratio of A. neumanni was not skewed to either males or females. Breeding was seasonal and seemed to be associated with seasonal variations in primary productivity, which relates to rainfall patterns. Résumé Le schéma de reproduction d'Arvicanthis neumanni a étéétudié dans le centre de la Tanzanie en 2002/2003. On a capturé 947 animaux appartenant à trois groupes d'âge : juvéniles, sub-adultes et adultes. Ces groupes d'âge étaient présents dans la population pendant une longue période, mais il y avait une augmentation du nombre d'individus sexuellement actifs et de juvéniles deux à trois mois après le début des pluies (décembre 2002-mai 2003). Il y eut un pic d'activité sexuelle entre janvier et avril. La taille des portées allait de 5.58 ± 0.42 à 6.1 ± 0.26 dans deux sites étudiés. Il n'y avait pas de différence significative du nombre d'embryons implantés dans les cornes droite et gauche de l'utérus des femelles (t22 = 0, P > 0.05 et t36 = 1.68, P > 0.05 respectivement). Le sex-ratio d'A. neumanni ne favorise ni les mâles, ni les femelles. La reproduction est saisonnière et semble associée aux variations saisonnières de la productivité primaire, liées, elles, aux chutes de pluie. [source]

    Effects of exploitation on reproductive capacity of blackspot snapper, Lutjanus fulviflamma (Pisces: Lutjanidae) in Mafia Island, Tanzania

    A. T. Kamukuru
    Abstract There is paucity of information on the effects of exploitation on reproductive characteristics of blackspot snapper, Lutjanus fulviflamma (Forsskål 1775) in Tanzanian coastal waters. We compared size at first sexual maturity (LM50), sex ratio, fecundity, and breeding season of L. fulviflamma in least fished Mafia Island Marine Park (MIMP) and intensively fished areas (IFA) between May 1999 and April 2001. Fish in MIMP matured at significantly smaller size (female: LM50 = 206.3 mm; male: LM50 = 195.5 mm) than in IFA (female: LM50 = 216.7 mm; male: LM50 =212.1 mm) total body length. Sex ratio was balanced at 1.03 : 1 (female : male) in MIMP, but it was skewed 0.9 : 1 (female : male) in IFA. Size-related differences in sex ratio were observed with males predominating in the smaller sizes and females in the larger sizes. Total fecundity of fish in MIMP was determined at 45,200,430,200 oocytes in females of between 207 and 293 mm total length. Lutjanus fulviflamma in MIMP has a prolonged spawning season lasting from September to March peaking in December. None of the fish from IFA were in breeding state, suggesting recruitment overfishing is an added matter of concern for the long-term sustainability of the fishery at the current exploitation level. Résumé Il existe très peu d'informations sur les effets de l'exploitation sur les caractéristiques de la reproduction de Lutjanus fulviflamma (Forsskål 1775) dans les eaux côtières de Tanzanie. Nous avons comparé la taille au début de la maturité sexuelle (LM50), le sex-ratio, la fécondité et la saison de reproduction de L. fulviflamma dans le Parc Marin de l'Ile de Mafia (MIMP) où la pêche est moins intense et dans des eaux intensément exploitées (IFA) entre mai 1999 et avril 2001. Les poissons du MIMP deviennent mâtures tout en ayant une longueur totale significativement plus petite (femelles: LM50 = 206,3 mm; mâles: LM50 = 195,5 mm) que dans les IFA (femelles: LM50 = 216,7 mm; mâles: LM50 = 212,1 mm). Le sex-ratio est équilibréà 1,03/1 (femelles/mâles) dans le MIMP, mais il est de 0,9/1 (femelles/mâles) dans les IFA. On a observé des différences du sex-ratio liées à la taille, avec les mâles qui prédominent dans les plus petites tailles et les femelles dans les plus grandes. On a déterminé la fécondité totale dans le MIMP, entre 45.200,430.000 ovocytes à l'intérieur des femelles comprises entre 207 et 293 mm de longueur totale. Dans le MIMP, Lutjanus fulviflamma a une saison de frai prolongée, allant de septembre à mars avec un pic en décembre. Aucun des poissons des IFA n'était en période de reproduction, ce qui signifie une sur-pêche du potentiel reproducteur, ce qui est un motif d'inquiétude pour la durabilitéà long terme de la pêche à son niveau actuel. [source]

    Demography of lions in relation to prey and habitat in the Maasai Mara National Reserve, Kenya

    J. O. Ogutu
    Abstract We studied lion demography in the Maasai Mara National Reserve between September 1990 and April 1992, with a special emphasis on the spatial and seasonal variation in demographic characteristics. Lion density (0.2,0.4 lions km,2) and pride size (range 8,48) were high because of a high resident prey biomass (10 335 kg km,2) augmented by migrant prey to 26 092 kg km,2 in the dry season. Overall, their sex ratio was almost at parity and varied neither spatially nor seasonally. Sex ratio was even among subadults but skewed toward males and females among cubs and adults, respectively. This implies an increasing differential mortality of males with age through subadulthood. The age ratio varied seasonally because of a birth peak in March,June and an influx of subadults into the reserve during July,August, coincident with increases in migrant prey. The birth peak was apparently preceded by another peak in mating activity falling between November and May. Further research should investigate the precise causes of the biased cub sex ratio, low lion density in the Mara Triangle and the higher ratio of subadults in Musiara than in the Mara Triangle or Sekenani. Résumé Nous avons étudié la démographie des lions dans la Réserve Nationale de Masai Mara entre septembre 1990 et avril 1992, en insistant particulièrement sur les variations spatiales et saisonnières des caractéristiques démographiques. La densité des lions (0.2,;0.4 lions/km2) et la taille des troupes (de 8 à 48) étaient élevées en raison de la forte biomasse des proies résidentes (10 335 kg/km2), portée à 26 092 kg/km2 en saison sèche par les proies migratrices. En général, le sex-ratio était à peu près égal et ne variait ni selon les saisons, ni selon les endroits. Les sex-ratio était égal parmi les sub-adultes, mais il déviait en faveur des mâles et des femelles chez les petits et les adultes, respectivement. Ceci implique une mortalité de plus en plus différentielle des mâles à l'âge sub-adulte. L'âge-ratio variait avec les saisons en raison d'un pic de natalité de mars à juin et d'une arrivée de sub-adultes dans la réserve en juillet-août, coïncidant avec l'augmentation des proies migratrices. Le pic des naissances était apparemment précédé par un autre pic des activités d'accouplement, entre novembre et mai. De nouvelles recherches devraient étudier les raisons précises du sex-ratio biaisé chez les lionceaux, de la faible densité des lions dans le Mara Triangle et du taux de sub-adultes plus élevéà Musiara que dans le Mara Triangle ou à Sekenani. [source]

    Prevalence of Pervasive Developmental Disorders in Two Canadian Provinces

    Hélène Ouellette-Kuntz
    Abstract, Although it is generally accepted that the proportion of children diagnosed with pervasive developmental disorders (PDDs) has increased in the past two decades, there is no consensus on the prevalence of these conditions. The accompanying large rise in demand for services, together with uncertainty regarding the extent to which the observed increases are due to a true change in risk, has made PDDs a major public health concern. As few data exist on the prevalence of PDDs in Canada, the aim of this study was to estimate the prevalence of diagnosed PDDs in two Canadian provinces (Manitoba and Prince Edward Island (PEI)) and compare characteristics of diagnosed cases between the two regions. To obtain the estimates, children under the age of 15 years with a PDD diagnosis who lived in either province in 2002 were identified by workers at Children's Special Services, a provincial government program that supports children with special needs in Manitoba, and by the PEI provincial early intervention coordinator (Department of Social Services and Seniors) and special education autism coordinator (Department of Education). The findings show that the prevalence among children 1,14 years of age was 28.4 per 10,000 (95% confidence interval: 26.1,30.8) in Manitoba and 35.2 per 10,000 (95% confidence interval: 28.2,43.4) in PEI. In Manitoba, children of aboriginal identity with PDDs (8.3%) were significantly underrepresented compared with the general population of aboriginal children living off native reserves (15.6%). Sex ratio, sibling risk, and age at initial diagnosis were similar in the two provinces. These findings can serve as a baseline from which to monitor the prevalence of these conditions over time, providing valuable data for researchers, planners, and service providers. [source]

    Sustained, Natural Spawning of Southern Flounder Paralichthys lethostigma Under an Extended Photothermal Regime

    Wade O. Watanabe
    Hormone-induced spawning of southern flounder Paralichthys lethostigma has produced substantial numbers of viable eggs, but wide variations in fertilization and hatch rates have been reported. Recently, sustained natural spawning of southern flounder broodstock, without hormone induction, has been achieved in our laboratory. Adults (average weight = 1.12 kg; N= 25), including 6 captured as juveniles in 1993 and 19 captured as adults during September 1998, were stocked in two 4.8-m3 controlled-environment tanks in October 1998 and held under natural photothermal conditions until January 1999, when an artificial winter photo-period of 10 L:14 D was initiated and then maintained through April 1999. Sex ratio was approximately 13 females:8 males:7 unknown. Natural spawning was observed in early December 1998 and increased in frequency to a peak in March 1999, before declining in late April. Water temperature ranged from 13.9 to 24.5 C during the spawning period. Natural spawnings over 142 d produced a total of 18.3 × 106 eggs, with a mean fertilization rate of 28.0% (range = 0,100%), yielding 4.94 × 106 fertilized eggs. The mean percentage of eggs that remained buoyant in full-strength seawater (34 ppt) was 41.3% (0,98%), while hatching rate of buoyant eggs was 37.3% (0,99%) and survival of yolksac larvae to the first-feeding stage was 30.2% (0,100%). Gonadal biopsies in late April identified six females from both tanks as probable spawners. A preliminary comparison suggests that natural spawning produced much larger numbers of viable eggs per female, with higher egg quality (i.e., fertilization and hatching success) than hormone-induced spawning. In contrast to natural spawning, hormone-induced strip-spawning enabled timing of spawnings to be more precisely controlled. These results suggest that a combination of both natural and hormone-induced spawning of photothermally conditioned fish will help produce the large numbers of eggs required to support commercial production. [source]

    Familial monozygotic twinning: A report of seven pedigrees,

    Geoffrey Machin
    Abstract Seven families contained 19 MZ twin pairs (2.7 pairs/family), diagnosed by low-stringency variable number tandem repeats in DNA from placental tissue or blood. Chorion status was known in 10 pairs, 6 dichorionic, 4 monochorionic. Sex ratio was equal. Autosomal dominant inheritance is apparent. © 2009 Wiley-Liss, Inc. [source]

    Growth and reproductive performance of triploid and diploid blacklip abalone, Haliotis rubra (Leach, 1814)

    Wenshan Liu
    Abstract Growth and reproduction of triploid and diploid blacklip abalone Haliotis rubra (Leach, 1814) were compared in a 30-month study. Triploidy was induced by inhibition of the second polar body formation using 6-dimethylaminopurine (6-DMAP) or cytochalasin B (CB). There were no significant differences in growth and survivorship between triploid and diploid abalone. However, triploid abalone had a more elongated shell and greater foot muscles than diploid abalone. A slightly curvilinear growth in shell length was conformed to all treatments. While diploid abalone had reached sexual maturity and spawned during the study, gonadal development and gamete maturation were abnormal in triploids. Female triploids lacked an apparent gonad at the macroscopic level but microscopic examination revealed that they had a thin layer of oogonia development. In contrast, male triploids were able to form similar-sized gonads to diploids during most of the reproductive period, but with brown-yellow discolouration and stalled gametogenesis at spermatocyte formation. Sex ratio of triploid abalone did not deviate from 1:1. With the onset of sexual maturation, growth and gonadal maturation occurred concurrently in diploid abalone, and there was no indication that growth of (diploid) abalone was reduced. [source]

    Sex ratio, mating behaviour and Wolbachia infections in a sheetweb spider

    The solitary sheetweb spider Pityohyphantes phrygianus has a female-biased primary sex ratio. Earlier studies have indicated that female post-copulatory position may influence the degree of bias, but the adaptive significance of the departure from a Fisherian 1 : 1 sex ratio is not known. In this study we show that there is a significant relationship between abdominal position and offspring sex ratio in undisturbed females. Moreover, female mating behaviour (and thus sex ratio) is correlated with female size, but this relationship may shift from negative to positive depending on the presence of the maternally acquired endosymbiotic bacterium Wolbachia in both sexes. A survey of wild-caught individuals shows that the incidence of Wolbachia infection in natural populations is high. This indicates that Wolbachia may be a significant factor influencing female post-copulatory behaviour and sex ratio in natural populations. We discuss various adaptive reasons for sex-ratio bias and behavioural plasticity in females. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 181,186. [source]