Variation In Reproductive Success Research Articles

Effect of age, breeding experience and senescence on corticosterone and prolactin levels in a long-lived seabird: The wandering albatross

Although several studies have described age-specific pattern of breeding performance, physiological and endocrine mechanisms underlying the variation in reproductive success in relation to age are poorly understood. We described baseline levels of: (1) corticosterone, which can trigger nest desertion when secreted at high levels, and (2) prolactin, a hormone known to trigger parental behaviour in incubating known-aged (7–39 years old) wandering albatrosses ( Diomedea exulans). In this long-lived species, reproductive performance increases with age and breeding experience before it stabilizes in middle-aged individuals and declines in senescent individuals. We found that breeding experience was a much better statistical predictor of hormone levels than age. Baseline corticosterone levels slightly increased with advancing experience and reached maximal levels about the sixth breeding attempt. Similarly, prolactin levels were positively correlated with breeding experience in males. No such relationship was found in females, but first-time breeding females had lower prolactin levels than experienced females. This parallel increases in hormonal levels with advancing experience could result from improvements of skills, development of an endocrine system adapted to energetic constraints of reproduction or may mirror a higher investment in reproduction. Corticosterone levels decreased in senescent birds, but such a decline was not observed for prolactin. Low corticosterone levels in senescent birds could be associated with a lower ability to secrete/sustain elevated corticosterone levels or with a decreased parental expenditure.

SITE-SPECIFIC SURVIVAL OF BLACK-HEADED GROSBEAKS AND SPOTTED TOWHEES AT FOUR SITES WITHIN THE SACRAMENTO VALLEY, CALIFORNIA

We estimated apparent annual survival and recapture probabilities for adult Black-headed Grosbeaks (Pheucticus melanocephalus) and Spotted Towhees (Pipilo maculatus) at four sites along the Sacramento River, California. To calculate our estimates, we used capture-recapture mist-net data collected over two time periods at four study sites: from 1993 to 1995 at Flynn, Ohm, and Sul Norte, and from 1995 to 2000 at Ohm and Phelan Island. Our primary objective was to determine whether there were site-specific differences in adult survival and recapture probabilities for each species. Such differences are rarely investigated, yet, if present, suggest site-specific differences in habitat quality, with important implications for source/sink dynamics. We found site-specific variation in Black-headed Grosbeak survival within both the 1993–1995 dataset (Flynn = 0.797 ± 0.496, Ohm = 0.158 ± 0.191, Sul Norte = 0.773 ± 0.131) and the 1995–2000 dataset (Ohm = 0.088 ± 0.090, Phelan Island = 0.664 ± 0.111). For Spotted Towhees (1993–1995 data), the most supported model assumed constant survival across sites (0.602 ± 0.240), but there was some support for site variation in survival, as well (Flynn = 0.653 ± 0.365, Ohm = 0.214 ± 0.253, Sul Norte = 0.632 ± 0.258). These results clearly suggest site variation for Black-headed Grosbeaks, and weak evidence of site variation for Spotted Towhees. For both species, the general pattern was low survival at Ohm, suggesting low-quality habitat there and/or reduced site fidelity. The magnitude of site-to-site variation in survival observed in the Black-headed Grosbeak, and suggested for Spotted Towhee, has strong implications for determining source versus sink population status. To determine source versus sink status, we conclude that investigators must not only take into account site variation in reproductive success, but also consider site-specific estimation of adult survival.

Evidence for isolation by time in the European eel (Anguilla anguilla L.)

Life history traits of highly vagile marine species, such as adult reproductive success and larval dispersal, are strongly determined by oceanographic and climatic forces. Nevertheless, marine organisms may show restricted dispersal in time and space. Patterns of isolation by distance (IBD) have been repeatedly observed in marine species. If spawning time is a function of geographical location, temporal and spatial isolation, can easily be confounded or misinterpreted. In this study, we aimed at discriminating between various forces shaping the genetic composition of recruiting juveniles of the European eel (Anguilla anguilla L.). By controlling for geographical variation, we assessed temporal variation and tested for possible isolation by time (IBT) between recruitment waves within and between years. Using 12 polymorphic allozyme and six variable microsatellite loci, we show that genetic differentiation was low (F(ST) = 0.01-0.002) and significant between temporal samples. Regression analysis between genetic and temporal distance, was consistent with a subtle interannual pattern of IBT. Our data suggest that the population dynamics of the European eel may be governed by a double pattern of temporal variance in genetic composition: (i) a broad-scale IBT of spawning cohorts, possibly as a consequence of the large migration loop in anguillids and strong variance in annual adult reproductive contribution; and (ii) a smaller-scale variance in reproductive success (genetic patchiness) within cohorts among seasonally separated spawning groups, most likely originating from fluctuating oceanic and climatic forces. The consistency of both mechanisms remains to be verified with fine-scale analyses of both spawning/migrating aged adults and their offspring to confirm the stochastic/deterministic nature of the IBT pattern in eel.

Tactics for male reproductive success in plants: contrasting insights of sex allocation theory and pollen presentation theory

The basic tenet of sex allocation theory is that an organism's reproductive success, through either male or female function, can be represented as a sex-specific, monotonic, increasing function of the organism's investment of resources in that function. The shapes of these curves determine what patterns of resource allocation can be evolutionarily stable. Although SA theory has stimulated creative thinking about plant sexual tactics, quantifying the shapes of male and female gain curves has proven so difficult that other approaches must be considered. I contrast sex allocation theory to a different, emerging viewpoint, pollen presentation theory (PPT), which attempts to address variation in reproductive success by measuring and modeling the quantitative fates of pollen grains. Models suggest that RS through male function depends heavily on the packaging and gradual dispensing of pollen to pollinators, even with the amount of investment held constant. Many plants do deploy pollen gradually, through morphological and "behavioral" mechanisms that range from obvious to subtle. They may thereby influence many aspects of the evolution of sexual modes in plants, including transitions between dioecy and cosexuality. After reviewing the main implications of the models, I discuss recent work aimed at testing some key assumptions and predictions by functional and comparative studies in the genus Penstemon. Species of Penstemon conform to PPT predictions that bee-adapted flowers will restrict per-visit pollen availability more than hummingbird-adapted flowers.

Widespread gene flow and high genetic variability in populations of water volesArvicola terrestrisin patchy habitats

Theory predicts that the impact of gene flow on the genetic structure of populations in patchy habitats depends on its scale and the demographic attributes of demes (e.g. local colony sizes and timing of reproduction), but empirical evidence is scarce. We inferred the impact of gene flow on genetic structure among populations of water voles Arvicola terrestris that differed in average colony sizes, population turnover and degree of patchiness. Colonies typically consisted of few reproducing adults and several juveniles. Twelve polymorphic microsatellite DNA loci were examined. Levels of individual genetic variability in all areas were high (H(O) = 0.69-0.78). Assignments of juveniles to parents revealed frequent dispersal over long distances. The populations showed negative F(IS) values among juveniles, F(IS) values around zero among adults, high F(ST) values among colonies for juveniles, and moderate, often insignificant, F(ST) values for parents. We inferred that excess heterozygosity within colonies reflected the few individuals dispersing from a large area to form discrete breeding colonies. Thus pre-breeding dispersal followed by rapid reproduction results in a seasonal increase in differentiation due to local family groups. Genetic variation was as high in low-density populations in patchy habitats as in populations in continuous habitats used for comparison. In contrast to most theoretical predictions, we found that populations living in patchy habitats can maintain high levels of genetic variability when only a few adults contribute to breeding in each colony, when the variance of reproductive success among colonies is likely to be low, and when dispersal between colonies exceeds nearest-neighbour distances.

Mating systems and male reproductive success in Nile tilapia ( Oreochromis niloticus) in breeding hapas: A microsatellite analysis

Fry production in Nile tilapia, Oreochromis niloticus is typically achieved by mass spawning of males and females stocked in large hapas suspended in ponds. In such spawning aggregations, territorial behavior and reproductive competition among males may lead to a large variance in reproductive success among individual males. We analyzed parentage and quantified male reproductive success of O. niloticus under commercial hatchery conditions in two breeding hapas each stocked with 12 males and 25 females. Eleven microsatellite markers were used to estimate the reproductive success of individual males and females. In total, 760 offspring from 76 spawnings were included in the paternity analysis. Mating patterns in Nile tilapia ranged from single pair to promiscuous mating. Multiple paternity was detected in 70% of the broods, with up to 4 males fertilizing a single clutch. Multiple maternity was also detected in over 30% of the clutches analyzed. There was a very high variance in male reproductive success, with one third of males siring more than 70% of the offspring. Male condition factor had a significant effect on reproductive success with larger males siring a large proportion of offspring. The high reproductive variance resulted in a N e of around 16 for hapas A and B, and a low N e/ N ratio, 0.43 and 0.45 for hapas A and B, respectively. The rate of inbreeding, Δ F, for each hapa, was estimated to be around 3.0% per generation, which is about twice the inbreeding expected in an idealized population of the same census size. When designing fish breeding programmes based on mass spawning, the higher inbreeding and lower effective population size should be taken into consideration.

A bias correction for estimates of effective population size based on linkage disequilibrium at unlinked gene loci*

Analysis of linkage disequilibrium (\(\hat{r}^2\)=mean squared correlation of allele frequencies at different gene loci) provides a means of estimating effective population size (N e) from a single sample, but this method has seen much less use than the temporal method (which requires at least two samples). It is shown that for realistic numbers of loci and alleles, the linkage disequilibrium method can provide precision comparable to that of the temporal method. However, computer simulations show that estimates of N e based on \(\hat{r}^2\) for unlinked, diallelic gene loci are sharply biased downwards (\(\hat{N}_{\rm e}/N <0.1\) in some cases) if sample size (S) is less than true N e. The bias is shown to arise from inaccuracies in published formula for \(E(\hat{r}^2)\) when S and/or N e are small. Empirically derived modifications to \(E(\hat{r}^2)\) for two mating systems (random mating and lifetime monogamy) effectively eliminate the bias (residual bias in \(\hat{N}_{\rm e}<5\)% in most cases). The modified method also performs well in estimating N e in non-ideal populations with skewed sex ratio or non-random variance in reproductive success. Recent population declines are not likely to seriously affect \(\hat{N}_{\rm e}\), but if N has recently increased from a bottleneck \(\hat{N}_{\rm e}\) can be biased downwards for a few generations. These results should facilitate application of the disequilibrium method for estimating contemporary N e in natural populations. However, a comprehensive assessment of performance of \(\hat{r}^2\) with highly polymorphic markers such as microsatellites is needed.

Rearing conditions determine offspring survival independent of egg quality: a cross‐foster experiment with Oystercatchers Haematopus ostralegus

Variation in rearing conditions, due either to parental or to environmental quality, can result in offspring of different quality (e.g. body condition, immune function). However, evidence is accumulating that egg size and composition can also affect offspring quality. In Oystercatchers Haematopus ostralegus, high‐quality rearing conditions result in a higher quantity as well as quality of offspring. This is thought to be caused by increased parental food provisioning to the chicks in high‐quality environments. However, variation in egg quality between rearing conditions could also affect the quantity and quality of offspring. Determining the mechanism and ontogeny of quality differences is important in unravelling the causes of variation in reproductive success. To disentangle the effects of egg quality, and quality of the rearing conditions, on the future survival of offspring, we cross‐fostered complete clutches between nests. When reared under conditions of similar environmental quality, chicks originating from eggs laid in low‐quality environments survived as well as chicks originating from eggs laid in a high‐quality environment. However, chicks reared in high‐quality environments survived twice as long as chicks reared in low‐quality environments, independent of the environmental quality in which the eggs were laid. This suggests that variation in the future survival of offspring is primarily caused by differences in environmental and/or parental quality, with no clear effect of egg quality (size).

Distylic Hottonia palustris shows high reproductive success in small populations despite low genetic variability

Hottonia palustris L. (Primulaceae) is characterized by a heteromorphic incompatibility system. The strategy of distylic ramets is believed to promote outcrossing, to maintain overall genetic diversity and to prevent inbreeding depression. In spite of this distyly, an extremely low amount of allozymic diversity was observed in 545 individual ramets from 14 populations in Flanders (Belgium). A possible explanation for such low genetic variation is discussed in relation to the vegetative propagating abilities and the ecological niche width of the species. In contrast to the uniformity in allozymes as well as to the feature of single morph populations, there was a high variability in reproductive success between populations such as the number of seeds per ramet (425–2633), the number of flowers per ramet (9–36) and the mean weight of seeds (0.03–0.17 mg). Small populations and even those consisting of only one style morph may show a high reproductive success. As a whole, H. palustris showed a negative relationship of reproductive success with the surface area of its populations.

Phenotypic and genetic consequences of size selection at the larval stage in the Pacific oyster ( Crassostrea gigas)

The life histories of oysters in the genus Crassostrea, like those of most marine bivalves, are typified by high fecundity and low survival in nature. Rearing conditions in hatcheries however ensure optimized density, diet, and temperature. Hatcheries are becoming increasingly important for the production of juveniles in aquaculture, and their culture practices often include culling of slow growing larvae to reduce and synchronize the time taken to reach settlement. Because previous studies have found substantial genetic variation for early life developmental traits in Crassostrea gigas, these culling practices are likely to cause highly different selective pressures in hatcheries from those in the natural environment. We studied the phenotypic and genetic impact of such culling practices in a factorial cross between 10 males and 3 females subjected to progressive culling of the smallest 50% of larvae, compared with a non-culled control. Measurements were made on larval growth, survival, time taken to attain pediveliger stage and settlement success. Culling had a larger effect on the variance of these larval traits than on their means. The larvae in culled cultures were approximately 10% larger than those in controls, whereas the coefficient of variation was reduced by 30–40%. Culling also reduced the mean time to settlement by 12% and its variance by 55%. Using a multiplexed set of microsatellite markers to trace parentage, we also estimated the variance in reproductive success in a controlled experiment to quantify the consequences of intensive hatchery rearing practices. We also focused on changes in effective population size and genetic structure over time (and developmental stages). Our results show a loss of genetic diversity following removal of the smallest larvae by culling, as well as temporally varying genetic structure of the larval population. This supports the existence of genetic variability in early life developmental traits in C. gigas. Culling in hatcheries, like size-related selective pressures in the wild, are likely to have a significant genetic impact, through their effects on the timing of settlement.

Genetic patchiness among recruits in the European eel Anguilla anguilla

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 307:209-217 (2006) - doi:10.3354/meps307209 Genetic patchiness among recruits in the European eel Anguilla anguilla J. M. Pujolar1,2,*, G. E. Maes1, F. A. M. Volckaert1 1Laboratory of Aquatic Ecology, Katholieke Universiteit Leuven, Ch. de Bériotstraat 32, 3000 Leuven, Belgium2Present address: Department of Ecology and Evolutionary Biology, Earth & Marine Sciences Building, University of California, Santa Cruz, California 95064, USA *Email: pujolar@biology.ucsc.edu ABSTRACT: Heterogeneity in genetic composition among recruits of marine species is mostly due to a large variance in reproductive success mediated by oceanographic processes. Temporal genetic variation in a population of the European eel was quantified over 2 time scales among glass eel (1) inter-annual samples (cohorts), and (2) intra-annual samples within cohorts (‘arrival waves’). A total of 789 glass eels comprising 11 different arrival waves were collected at Den Oever in The Netherlands over the period 2001 to 2003. All samples were screened for genetic variation using 10 allozyme and 6 microsatellite loci. The main result from this study is the highly significant genetic differentiation among arrival waves, despite the low FST values (FST = 0.0036). Heterogeneity in genetic composition was observed both among cohorts and among samples within cohorts. Genetic differentiation partitioned within cohorts was more than 10-fold the differences among cohorts. Genetic heterogeneity is likely to result from a large variance in the contribution of individuals to each cohort determined by genetic drift. Although natural selection and gene flow could also play a role in the observed genetic pattern, we suggest that large variances in reproductive success are a contributing factor to the recruit differentiation. If only a subset of the adults contribute to the new recruits, effective population size in European eel might be much lower than the census size. A low effective population size combined with fluctuating oceanic conditions might have contributed to the current dramatic decline in abundance of European eel. KEY WORDS: Allozymes · Arrival waves · European eel · Genetic patchiness · Microsatellites Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 307. Online publication date: January 24, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Catastrophic Reproductive Failure, Terrestrial Survival, and Persistence of the Marbled Salamander

Wide variation in reproductive success is common among amphibians that breed in seasonal ponds, but persistence of adults can buffer against these fluctuations, particularly for long-lived species. We hypothesized that the frequent episodes of catastrophic failure of the marbled salamander (Ambystoma opacum) enhance the importance of high terrestrial survival. At Rainbow Bay in South Carolina reproductive success was poor (< 1 metamorph/breeding female) in nearly half of the 22 years that the species bred. Complete failure occurred in 6 of 22 years. To study catastrophic failure, we adapted an age-structured, individual-based model with density-dependent growth and survival of larvae. The model was based on extensive data from local field studies and experiments. With consistently good survival in the pond stages, the simulated population required survival probabilities in the upland stages (juveniles and adults) near 0.5/year to persist and near 0.8/year to achieve the increases observed. Catastrophic failure, occurring randomly with probability 0.5/year created additional fluctuations in the population, raised the thresholds of survival required for persistence, and caused extinction under conditions that were otherwise favorable. The marbled salamander at Rainbow Bay is not at great risk of extinction because of catastrophic failure, but the risk increases dramatically if life span is decreased or frequency of failure is increased. Any reduction in terrestrial survival will have deleterious consequences by reducing the breeding populations at equilibrium, even if it does not jeopardize persistence. Our model provides assessments of risk that can be applied to poorly studied species with similar life histories, such as the endangered flatwoods salamander (A. cingulatum).

Optimal Clone Number for Seed Orchards with Tested Clones

Abstract The optimal number of clones in seed orchards is discussed. A model is constructed to maximize a goodness criterion (“benefit”) for seed orchards. This criterion is a function of: 1) the number of tested genotypes available for selection and planted in seed orchard; 2) the contribution to pollination from: a) the ramet itself; b) the closest neighbors; c) the rest of the orchard and sources outside the orchard (contamination); 3) variation among genotypes for fertility; 4) frequency of selfing; 5) production of selfed genotypes; 6) gene diversity (= status number); 7) influence of contamination; 8) genetic variation among candidates; 9) correlation between selection criterion (e.g. height in progeny test) and value for forestry (e.g. production in forests from the orchard); and 10) the number of clones harvested. Numeric values of the entries are discussed, and values were chosen to be relevant for scenarios with Swedish conifers (focusing on Scots pine) and for loblolly pine. Benefit was maximized considering the number of clones. The optimum was 16 clones for the Swedish scenario, while less for the loblolly pine scenario. The optimum was rather broad, thus it is not essential to deploy the exact optimum, and an approximate optimum will do. A sensitivity analyses was performed to evaluate the importance of the likely uncertainty and variation in different entries. Quantification of the benefit of gene diversity is important. Other significant considerations are the genetic variance in the goal character and the ability to predict it, as well as the impact of selfing and the variation in reproductive success between clones. Twenty clones is suggested as a thumb rule for Swedish conifers.

Bateman's Principle and Simultaneous Hermaphrodites: A Paradox

Bateman's principle states that reproductive success is limited a) in females by the resources available for egg production; and b) in males, only by access to females and/or eggs. The principle has been used to generate predictions for two aspects of hermaphroditism; a) the advantage of hermaphroditism and b) sexual conflict. Comparing these predictions to the empirical data offers tests of Bateman's principle. Charnov's prediction that hermaphroditism would occur under circumstances where Bateman's principle does not apply is found to be largely correct. However, the prediction as to the association of hermaphroditism and low fixed costs is inconsistent with the data. Alternative explanations that predict that hermaphroditism is a strategy for reducing variance in reproductive success may better explain the data. Probability theory demonstrates that where two strategies have equal mean fitness, which must be the case for male and female function, the strategy with the lower variance in reproductive success must have higher fitness (Gillespie's principle). Bateman's principle predicts that this will be the female role in hermaphrodites. However, Charnov, assuming Bateman's principle, predicted that sexual conflict stemming from a preference for the male role would be important in hermaphrodite mating systems, creating a paradox. Many hermaphrodite mating systems are based on conditional reciprocity with a preferred sexual role indicating sexual conflict. The data demonstrate that the preferred role varies among taxa, contrary to the predictions of Bateman's principle. It has been suggested that Bateman's principle can explain cases in which the female role is preferred (sperm-trading) as involving energy rather than gamete trading. However, energetic considerations suggest that energy trading would only be adaptive if Bateman's principle does not apply, paradoxically. The gamete trading model, based on the prediction that the role that offers control of fertilization will be preferred, is more consistent with the data. Application of Bateman's principle to hermaphrodites leads to contradictory predictions and does not offer the basis for a coherent theory of sexual selection, as Bateman proposed.

The Distribution of Male and Female Reproductive Success in a Broadcast Spawning Marine Invertebrate

Many studies have addressed sexual selection in animals, but few data are available on animals that release eggs and sperm into the environment for external fertilization. Although this reproductive mode represents the ancestral condition and is still a very common reproductive strategy, it is underrepresented in empirical studies and theoretical treatments. Here I present data on the pattern of reproductive success in male and female sea urchins. The results suggest that the strength of sexual selection and the differences between the sexes in the intensity of sexual selection depend on mate density. In general, despite the high degree of multiple paternity, the variance in reproductive success appears to be lower in males and higher in females than it is in polygamous species with internal fertilization. These results may provide insight into the patterns of effective population size in marine invertebrates and also more generally the evolutionary transition from sexual monomorphism to polymorphism in adult traits.

Bateman's Principle in Cooperatively Breeding Vertebrates: The Effects of Non-breeding Alloparents on Variability in Female and Male Reproductive Success

The sex-specific slopes of Bateman's gradients have important implications for understanding animal mating systems, including patterns of sexual selection and reproductive competition. Intersexual differences in the fitness benefits derived from mating with multiple partners are expected to yield distinct patterns of reproductive success for males and females, with variance in direct fitness predicted to be greater among males. These analyses assume that typically all adults are reproductive and that failure to produce offspring is non-adaptive. Among some species of cooperatively breeding birds and mammals, however, non-breeding adult alloparents are common and may comprise the majority of individuals in social groups. The presence of a large number of non-breeding adults, particularly when coupled with greater social suppression of reproduction among females, may alter the relative variance in direct fitness between the sexes, thereby generating an apparent contradiction to Bateman's Paradigm. To explore quantitatively the effects of non-breeding alloparents on variance in reproductive success, we used genetic estimates of parentage and reproductive success drawn from the literature to calculate the relative variability in direct fitness for females and males in alloparental and "other" societies of birds and mammals. Our analyses indicate that in mammals and, to a lesser extent, in birds, variability in direct fitness is greater among females in species characterized by the presence of non-breeding alloparents. These data suggest that social interactions, including social suppression of reproduction, are powerful determinants of individual direct fitness that may modify sex-specific patterns of reproductive variance from those described by Bateman.

Sex ratio and male sexual characters in a population of blue tits, Parus caeruleus

Sex allocation theory proposes that parents should bias the sex ratio of their offspring if the reproductive value of one sex is greater than that of the other. In the monogamous blue tit (Parus caeruleus), males have a greater variance in reproductive success than females, and high-quality males have higher reproductive success than high-quality females due to extrapair paternity. Consequently, females mating with attractive males are expected to produce broods biased toward sons, as sons benefit more than daughters from inheriting their father’s characteristics. Song and plumage color in birds are secondary sexual characters indicating male quality and involved in female choice. We used these male sexual traits in blue tits to investigate adaptive sex ratio manipulation by females. We did not find any relationship between male color ornamentation and brood sex ratio, contrary to previous studies. On the other hand, the length of the strophe bout (i.e., the mean number of strophes per strophe bout) of fathers was positively related with the proportion of sons in their broods. The length of the strophe bout is supposed to reflect male quality in terms of neuromuscular performance. We further showed that sons produced in experimentally enlarged broods had shorter strophe bouts than sons raised in reduced broods. These results are consistent with the hypothesis that females adjust the sex ratio of their broods in response to the phenotype of their mate. Key words: dawn chorus, male song, Parus caeruleus, plumage color, sex ratio. [Behav Ecol]

Breeding Structure of Steelhead Inferred from Patterns of Genetic Relatedness among Nests

AbstractTen polymorphic microsatellite loci were used to infer kinship among steelhead Oncorhynchus mykiss sampled from nest sites in the Hamma Hamma River, Washington. Fertilized eggs were removed from 8 of 11 redds (i.e., individual nests within redds) surveyed in 1998, tagged with unique identifiers, and reared until adulthood as part of a conservation program. On average, individuals sampled from the same redd were more closely related than individuals sampled from different redds, and a tree generated from the pairwise relatedness estimates shows clear clustering of individuals by redd. Despite this general pattern, there was substantial overlap in the pairwise relatedness estimates within and among most redds. Of the eight redds, seven contained more than four alleles at a minimum of one locus and five contained more than six alleles, indicating that most redds were the result of matings by more than a single pair of parents. In total, the eight redds were created by a minimum of 21 parents, which we inferred to consist of at least 5 females and 16 males based on the location and timing of redd construction. The male‐biased sex ratio is probably due to matings by either male resident trout or precocial steelhead parr with female steelhead. The results of this study indicate that although multiple matings by both sexes are common in this population, the variance in reproductive success is much higher for males than females.

Egg size in relation to sex of embryo, brood sex ratios and laying sequence in northern lapwings (Vanellus vanellus)

AbstractThis study focuses on possible sources of egg size variation in the northern lapwing Vanellus vanellus, a polygynous shorebird with more variable male than female reproductive success. From the hypothesis that female birds allocate more resources into male than female eggs to improve their sons' abilities to compete for mates, male eggs were predicted to be larger than female eggs, and the proportion of sons was predicted to increase with mean egg volume of clutches. However, no size differences between male and female eggs were found. Corresponding with earlier reports in the species, egg size was positively correlated with female body condition, but no relationship between offspring sex ratios and intra‐clutch mean egg volumes was detected. These results suggest that a higher variation in reproductive success among males than among females is not sufficient for sexually dimorphic eggs to evolve in birds. Moreover, previous studies have suggested that shorebird eggs are subject to stabilizing selection for equal size within clutches. In contrast, this study demonstrates a consistent variation in egg size, with a significant increase in egg breadth between first and second eggs, and decreases in both breadth and volume between the third and the final fourth egg. The volume of last‐laid eggs was on average 2.3% smaller than the mean size of the preceding three eggs. Among the shorebirds studied so far, this consistent pattern of intra‐clutch egg size variation is unique to lapwings.

Corticosteroids as potential mechanism regulating variability in reproductive success in monogamous oldfield mice ( Peromyscus polionotus)

Male and female mammals undergo profound hormonal changes during pregnancy, some of which are sufficiently dramatic to influence offspring survival. In order to understand the proximate mechanisms regulating the variability in reproductive success within and between individuals, we monitored changes in fecal corticosteroid concentrations over the reproductive cycle in male and female oldfield mice ( Peromyscus polionotus subgriseus) to test whether corticosteroid concentrations during pregnancy were associated with offspring survival. In females that successfully raised litters to weaning, fecal corticosteroid concentrations were low until mid-gestation and increased significantly towards term; in females that did not raise their pups to weaning, fecal corticosteroid concentrations were significantly higher at mid-gestation, and remained high until late gestation. The difference in fecal corticosteroid concentrations at mid-gestation between successful and unsuccessful females can be explained by the fact that successful females were lactating. Lactation has been associated with a down-regulation of the hypothalamic–pituitary–adrenal (HPA) axis and, accordingly, a decrease in plasma corticosterone (CORT) in several species, including humans. Males that successfully raised their litters had low fecal corticosteroid concentrations throughout their partner's pregnancy. Unsuccessful males, however, had significantly higher fecal corticosteroid concentrations at term than males that raised their pups to weaning. While these preliminary data require further investigation, we suggest that pre-partum fecal corticosteroid concentrations in males were responsible for the variability in reproductive success.