Subsequent Reproductive Success Research Articles

Food availability during migratory stopover affects testis growth and reproductive behaviour in a migratory passerine

Long-distance migratory passerines initiate testicular recrudescence during spring migration to meet the demands of timely reproduction upon immediate arrival on the breeding grounds. The degree of testicular development is known to depend on environmental factors like stopover habitat quality; reproductive performance may be strongly impacted by testicular maturation upon arrival on the breeding grounds. We investigated the effect of stopover food availability on subsequent reproductive performance in garden warblers ( Sylvia borin). Spring migration was simulated by repeated food deprivation and re-feeding to imitate the alternation of flight and stopover periods. During the two final stopover periods, males were either kept under ad libitum food (ad libitum males) or under limited food conditions (limited males). After simulated arrival in the breeding area, manipulation of previous stopover food availability resulted in significantly slower testicular recrudescence ( p < 0.001) and decreased plasma testosterone ( p < 0.01) in limited males compared to ad libitum males. Body mass change was not significantly different between the two groups ( p = 0.38). Limited males also exhibited reduced performance in reproductive behaviours employed in territorial and sexual contexts. Limited males had a longer ‘freezing’ interval ( p < 0.05) and decreased activity ( p < 0.01) when challenged with a live male decoy. In direct confrontation between limited and ad libitum males in the presence of a female, limited males exhibited significantly fewer behavioural traits in sexual context, i.e. directed to the female ( p < 0.001). Therefore, we suggest that conditions encountered during previous migratory stopover may affect subsequent annual reproductive success by influencing key reproductive behaviours.

Reproductive success and individual variation in feeding frequency of House Sparrows (Passer domesticus)

Parental care is assumed to be closely associated with individual differences in reproductive success. We investigated how feeding frequencies varied among parents and how this affected the subsequent reproductive success in insular populations of House Sparrows Passer domesticus in northern Norway. Female parents fed their offspring more than male parents did, and the feeding rates were positively related to the feeding rates of the partner. A positive relationship between feeding rates and bill depth was present in females. In males, the feeding rates were negatively related to total badge size and positively related to visible badge size, after the effect of other variables had been taken into account. A non-linear convex relationship between feeding frequency and hatch day was present in males, which could reflect either the seasonal change in weather conditions or the seasonal variation in food availability. For both sexes, feeding frequencies increased with increasing brood size, but at the same time the average feeding rate per nestling decreased with increased brood sizes. Finally, our results indicate that the amount of parental investment, measured as feeding rates during the nestling stage, may have a positive long-term influence both on the number of fledglings that recruit as well as the probability that fledglings survive until recruitment.

Impacts of Endothall Applications on Largemouth Bass Spawning Behavior and Reproductive Success

AbstractAquatic herbicides are commonly used to control excessive and nuisance levels of submersed aquatic vegetation (SAV). At times, these applications tend to be more effective at the onset of fish spawning in spring; however, they can cause conflicts between anglers and lake management personnel assigned to control excessive SAV. We evaluated the direct application of an aquatic herbicide (Aquathol K [dipotassium salt of endothall]) on the nest behavior and fidelity of largemouth bass Micropterus salmoides and measured the subsequent reproductive success of these fish. Endothall was applied to achieve a shoreline concentration of 5 mg/L of water volume in 0.1‐ha ponds (3 mg/L whole‐pond concentration). It was applied directly on nests in three ponds in March 2005, 2006, and 2007 when largemouth bass initiated spawning; spawning activity was monitored over 14–15 d. Using similar techniques, we applied water to another three ponds where largemouth bass were nesting. For all 3 years, endothall did not affect nest guarding by largemouth bass, and nest fidelity was similar to that for ponds treated with water. Endothall concentrations declined about sevenfold over 21 d but persisted in the water during spawning. Electrofishing of young largemouth bass 2–3 months after spawning showed that their relative abundance and size were similar among the ponds treated with endothall and the control ponds. Similarly, the density and biomass of age‐0 largemouth bass did not vary between treatments for fish killed with rotenone in late August each year. Managers can use these results to address angler concerns about endothall application during largemouth bass spawning and thereby lessen conflicts over efforts to control excessive SAV.

Sperm Quantity Explains Age‐Related Variation in Fertilization Success in the Hide Beetle

AbstractAge‐related variation in sperm quality and quantity may have a dramatic impact on female fecundity and fertilization success. Despite this, the relationships between these parameters are rarely investigated. Moreover, studies exploring age‐related variation in male mating success generally fail to consider the entire lifespan of an individual male; instead they restrict analyses to a small number of defined age classes. As a consequence they are unable to assess the impact of early and late life history components on male reproductive success. In this study, we explore these questions using the hide beetle, Dermestes maculatus, a model species for investigating age‐related male mating success. We first employed a longitudinal design to explore whether patterns of sperm transfer and subsequent reproductive success varied over the reproductive lives of a cohort of males. Secondly, we investigated age‐related variation in sperm viability, a surrogate measure of sperm quality. Finally, we combined these data and assessed whether the observed patterns of sperm transfer were correlated with fertilization success. We found that the quantity of sperm varied with male age: the amount of sperm transferred to a female increased with male age until 9 wk and then started to decline. Similarly, female fecundity and fertilization success were related to male age: females mating with males when they were relatively young (1 wk) or relatively old (13 wk) suffered reductions in fecundity and fertilization. Our data suggest that fertilization success is driven at least in part by the quantity of sperm transferred during mating.

EFFECTS OF SPRING BODY CONDITION AND AGE ON REPRODUCTION IN MALLARDS (ANAS PLATYRHYNCHOS)

We explored predictive models relating body condition and age to nesting propensity, timing of nest initiation, clutch size of first nests, aggregated nest survival, hatching success, and timing of hatch in Mallards (Anas platyrhynchos). Nesting propensity had a positive linear relationship with body condition, and second-year (SY) females had a lower probability of nesting than after-second-year (ASY) females (84% and 94%, respectively, at average body condition). Females in better body condition initiated nesting ∼15 days earlier than those in poor condition, and SY females nested ∼4 days later than ASY females at average body condition. Clutch size of first nests exhibited a curvilinear decline with body condition, such that large clutches were characteristic of females in the best condition that nested early. Nest survival and hatching probability were unaffected by body condition. Younger females had somewhat lower nest survival (11%) than older females (14%), which, in combination with renesting effort, resulted in an age difference in hatching probability (22% and 33%, respectively). Females in better condition hatched nests ∼15 days earlier than those in poor condition, and SY females hatched nests ∼4 days later, on average, than ASY females. Our results indicate that factors influencing the body condition of female Mallards arriving on breeding areas could influence subsequent reproductive investment and success. Condition effects are primarily through the mechanisms of nesting propensity, clutch size, and timing of nest initiation and hatch. Additionally, we provide evidence that Mallards in their first breeding season have lower reproductive potential than older females.

Offspring are lighter at birth and smaller in adulthood when born after a brother versus a sister in humans

In mammals, including humans, it is more costly to produce sons than it is to produce daughters, with maternal survival and subsequent reproductive success diminished more by producing male over female offspring. It is therefore predicted that offspring who are produced by mothers who have previously produced sons versus daughters will be compromised by the relatively high cost their mother incurred in the previous reproductive episode. Such effects are potentially important because characters that determine offspring survival and fecundity ultimately contribute to maternal fitness. Using questionnaire-based data from a contemporary human population, I show that birthweight (irrespective of their sex) is lower in individuals born after an elder brother than in those born after an elder sister. In addition, I show that both men and women who were born after a male versus a female sibling have reduced adulthood height, a known correlate of reproductive success in both sexes. The results suggest that producing sons may have a negative effect on the fitness of subsequent offspring, which has implications for calculations of maternal fitness and for optimal sex allocation.

Diets of Lesser Scaup during Spring Migration throughout the Upper-Midwest are Consistent with the Spring Condition Hypothesis

Abstract The spring condition hypothesis (SCH) states that the current decline of the North American scaup population (Lesser [Aythya affinis] and Greater Scaup [A. marila] combined) is due to a decline in quality or availability of scaup foods on wintering, spring migration, or breeding areas that has caused a reduction in female body condition and subsequent reproductive success. Our previous research indicated that forage quality in diets of Lesser Scaup (hereafter scaup) at two sites in Northwestern Minnesota was lower in springs 2000-2001 than that reported for springs 1986-1988, consistent with the SCH. Accordingly, we further tested the SCH at a landscape scale, by comparing amounts of amphipods in diets (index of forage quality) of scaup (N = 263) collected in springs 2003-2005 from seven eco-physiographic regions in Iowa, Minnesota, and North Dakota in relation to data from Northwestern Minnesota during springs 2000-2001. We found that aggregate percentages of Gammarus lacustris and Hyalella azte...

Previous Experience and Reproductive Investment of Female Columbian Ground Squirrels

Reproductive success of females in iteroparous species usually increases early in life, then remains constant or declines. The restraint hypothesis suggests that primiparous females limit their 1st reproduction in order to avoid future costs to survival, reproduction, or both, whereas constraint indicates that primiparous females are limited in their 1st reproductive attempt either because they lack sufficient resources (measured by body mass) or lack experience at weaning a litter. To evaluate these alternatives, we examined age- and experience-related patterns of reproduction in female Columbian ground squirrels (Spermophilus columbianus) with data from a long-term study and by experimentally altering age at 1st reproduction. In the results of the long-term study, experienced females had significantly higher reproductive investment and reproductive success (number of offspring surviving to yearling age) compared to inexperienced females. Annual survival of females was not affected by their level of previous weaning experience or current reproductive investment. Additionally, subsequent reproductive success was not impaired by earlier reproduction. In the experimental results, experienced females had significantly higher reproductive investments compared to inexperienced females. Constraint in terms of previous weaning experience best explained the differences in reproductive investment and reproductive success between primiparous and multiparous females.

Heterogeneity in individual quality overrides costs of reproduction in female reindeer

Reproductive allocation at one age is predicted to reduce the probability of surviving to the next year or to lead to a decrease in future reproduction. This prediction assumes that reproduction involves fitness costs. However, few empirical studies have assessed whether such costs may vary with the age at primiparity or might be overridden by heterogeneities in individual quality. We used data from 35 years' monitoring of individually marked semi-domestic reindeer females to investigate fitness costs of reproduction. Using multi-state statistical models, we compared age-specific survival and reproduction among four reproductive states (never reproduced, experienced non-breeders, reproduced but did not wean offspring, and reproduced and weaned offspring) and among contrasted age at primiparity. We assessed whether reproductive costs occurred, resulting in a trade-off between current reproduction and future reproduction or survival, and whether early maturation was costly or rather reflected differences in individual quality of survival and reproduction capabilities. We did not find any evidence for fitness costs of reproduction in female reindeer. We found no cost of gestation and lactation in terms of future reproduction and survival. Conversely, successful breeders had higher survival and subsequent reproductive success than experienced non-breeders and unsuccessful breeders, independently of the age at primiparity. Moreover, it was beneficial to mature earlier, especially for females that successfully weaned their first offspring. Successful females at early primiparity remained successful throughout their life, clearly supporting the existence of marked among-female differences in quality. The weaning success peaked for multiparous females and was lower for first-time breeders, indicating a positive effect of experience on reproductive performance. Our findings emphasize an overwhelming importance of individual quality and experience to account for observed variation in survival and reproductive patterns of female reindeer that override trade-offs between current reproduction and future performance, at least in the absence of harsh winters.

Effects of plant abundance on reproductive success in the biennial Sabatia angularis (Gentianaceae): spatial scale matters

Summary Small and low‐density populations often suffer significant reductions in reproduction, as exemplified by studies on rare and threatened species. While this phenomenon is less studied in common species, if general, it should not be restricted to those in peril. We addressed this assertion by examining the effects of plant abundance, measured at population‐level (population size, density) and local‐level (local neighbourhood size, LNS) spatial scales, on fruit set, seed set and subsequent reproductive success (RS) across 19 natural populations of the widespread biennial Sabatia angularis (Gentianaceae). We also examined stigmatic pollen loads across a subset of populations to address whether changes in reproduction were related to reduced pollination. Effects of plant abundance on reproduction were evident at both spatial scales. At the population level, population size – but not density – affected fruit set significantly. This effect was non‐linear. Fruit set in populations larger than 15 individuals was relatively constant, but dropped abruptly in populations below that size. This relationship did not prevail between population size and RS. Instead, RS declined significantly with population density. At the local level, increases in LNS between 1‐m and 4‐m radii from study plants increased fruit set significantly. In contrast, increases within 1 m significantly reduced seed set. Both of these effects prevailed at different spatial scales for RS; increases in 1‐m LNS reduced RS, but increases beyond that distance and within 4 m increased RS. Although non‐significant, trends in the data are consistent with facilitative interactions for pollination influencing local‐level effects on fruit set. However, negative interactions, presumably because of competition for resources, are more important for determining seed set at local scales. Both interactions influence RS, but the effect of competition appears to dominate at the population level. Synthesis. This study demonstrates that although significant reductions in fruit set in small S. angularis populations imply that common species are not impervious to reproductive disadvantages, small populations need not experience reduced RS. Furthermore, low‐density populations actually enjoy increased RS. As such, this study highlights the complexity of ecological interactions affecting reproduction and the importance of incorporating multiple spatial scales when examining population dynamics.

Can conditions experienced during migration limit the population levels of birds?

Populations of migratory birds are usually considered to be limited by conditions in breeding or wintering areas, but some might be limited by conditions encountered on migration. This could occur at stopover sites where competition for restricted food supplies can reduce subsequent survival or breeding success, or during the flights themselves, when adverse weather can occasionally kill large numbers of individuals. Competition for food could act in a density-dependent manner and help to regulate populations, whereas weather effects are more likely to act in a density-independent manner. The evidence for these views is explored in this paper. When preparing for migration, birds must normally obtain more food per day than usual, in order to accumulate the body reserves that fuel their flights. Birds often concentrate in large numbers at particular stopover sites, where food can become scarce, thus affecting migratory performance. Rates of weight gain, departure weights, and stopover durations often correlate with food supplies at stopover sites, sometimes influencing the subsequent survival and reproductive success of individuals, which can in turn affect subsequent breeding numbers. Many studies have provided evidence for interference and depletion competition at stopover sites, relatively few for migration conditions influencing the subsequent breeding or survival of individuals, and even fewer for effects on subsequent breeding numbers. Migrants in flight occasionally suffer substantial mortality in storms, especially over water, sometimes involving many thousands of birds at a time. Other mass mortalities have resulted from atypical ‘winter-like’ weather, occurring soon after the arrival of summer migrants in their breeding areas or just before their departure in autumn. Again, many thousands of birds at a time have been killed in such incidents, causing reductions of 30–90% in local breeding densities. In some bird species, migration-related events can at times have substantial effects on the year-to-year changes in breeding population levels. Nonetheless, the difficulties involved in investigating migrating birds at different points on their migration routes have so far limited the number of studies on the influence of events during migration periods on population levels.

Early experience and parent-of-origin-specific effects influence female reproductive success in mice

Recent studies on mammals investigating parent-of-origin-specific effects such as genomic imprinting and maternal effects have demonstrated their impact on short-term measures of fitness, for example offspring growth. However, the long-term fitness consequences of parent-of-origin-specific effects and their role outside the immediate mother-offspring interaction remain largely unexplored. Here, we show that female mice mated to males that inherited the same set of paternal and maternal genes as themselves have a higher reproductive success than females mated to males of reciprocal genotype. Furthermore, we demonstrate that the early social environment experienced by an individual influences its reproductive success. Females raised with unrelated siblings in a mixed litter had a subsequent lower reproductive success than those that were fostered together with all their biological siblings in unmixed litters. Our results highlight the important influence of parent-of-origin-specific effects and conditions in early development on long-term reproductive success in mammals and suggest that parent-of-origin-specific effects may provide the underlying mechanism for beneficial coadaptation between genotypes, for example, in mate choice.

Cotyledon damage at the seedling stage affects growth and flowering potential in mature plants

Seedling herbivory is an important selective filter influencing patterns of plant community composition. Nevertheless, while many of the mechanisms governing seedling selection by herbivores are well established, the effects of tissue loss at the seedling stage on subsequent plant development are poorly understood. Here we examined how the removal of 50 or approximately 100% of cotyledon area from 7-d-old chalk grassland seedlings affected subsequent plant growth and flowering over a 100-d period. Cotyledon damage had a significant effect on growth during the establishment phase for six of the nine species. For two species, significant effects on plant growth were manifest in 100-d-old plants. Of the five species that flowered, three developed fewer inflorescences or flowered later as a consequence of cotyledon damage suffered as a seedling. Our results show that, in addition to the direct effect of herbivory on seedling mortality, more subtle sublethal effects may also influence plant establishment. Reduced growth as a result of cotyledon damage may have implications for plant competition during the establishment phase, and on subsequent reproductive success at maturity.

Manipulation of Floral Symmetry Does Not Affect Seed Production in Impatiens pallida

The outcomes of past research concerning the role of floral symmetry in plant‐pollinator interactions are mixed. Field manipulations of plants, as well as experiments using artificial arrays, have found evidence for and against a direct relationship between the degree of floral symmetry and subsequent reproductive success. In this study, we test the hypothesis that deviations from perfect floral symmetry in Impatiens pallida (Balsaminaceae) result in direct fitness costs. In two field seasons, we manipulated experimentally floral symmetry in separate populations of plants and measured seed set. We did not address any possible effects of floral asymmetry on male function. We found no significant differences in seed number per fruit among control, symmetrically cut (30% on both lobes), asymmetrically cut (60% on left lobe), and severely mutilated (100% on left lobe) flowers in the field. Post hoc power analyses show that a prohibitively large sample size would be required to detect the observed differences in seed set among treatments. Moreover, the magnitude of the observed differences among treatments is biologically irrelevant. We conclude that the degree of floral symmetry does not affect female reproductive success in this system.

Effects of early experience on subsequent parental behaviour and reproductive success in oldfield mice, Peromyscus polionotus

Studies of a variety of taxa have almost universally shown an increase in survivorship between the first cohort of young reared by a female (or both parents) and subsequent litters. Here, we examine the effect of early experience with sibling pups during the subadult period on later maternal behaviour and reproductive success in oldfield mice. ‘Experienced’ females remained with their mothers through the rearing of a litter of siblings. ‘Inexperienced’ females were removed from their parents at 20 days of age and had no prior exposure to pups before they reproduced. Twenty-four experienced females and 24 inexperienced females (each a full sibling of an experienced female) were paired with noninbred, inexperienced males through the birth and rearing of three litters. Litter survival improved significantly with experience and increasing parity. Pairs with experienced females showed improved nest-building behaviour and averaged significantly more surviving pups than did pairs with inexperienced females. Inexperienced mates of experienced females showed improved nest building, as well, consistent with the results of previous studies showing that paternal care tends to match that of mates. Maternal body weight did not appear to influence reproductive performance. These findings support the contention that individuals may accrue significant fitness benefits by delaying their own reproduction and gaining experience.

PUBERTAL DEVELOPMENT AS AN INDICATOR OF, AND CRITICAL PERIOD FOR, ENDOCRINE DISRUPTION

ISEE-277 Abstract: Pubertal development is a complex process involving the central nervous system, gonads and other endocrine organs. The first step in this process occurs in utero and extends into infancy. The surge in steroid hormones that occurs in infancy is sometimes referred to as “baby puberty” and may represent a critical window for the normal development of the endocrine system. Exposure to endocrine disrupting chemicals during this critical period may cause advanced or delayed puberty as well as later reproductive dysfunction. Likewise, the period of pubertal development itself is a critical period of gonadal maturation and preparation for reproduction. Exposures during this window may also have adverse consequences for reproduction, reproductive cancers and other endocrine-sensitive endpoints. We will review the endocrine events that occur during these developmental stages. A cohort of individuals exposed to polybrominated biphenyls (PBBs) as the result of an industrial accident provides a unique opportunity to observe the effects of exposure during these critical windows. We will present data on pubertal development and subsequent reproductive success among girls exposed in utero and through breastfeeding. Girls exposed in utero and through breastfeeding had their first menstrual period an average of one year earlier than unexposed girls (Blanck et al, 2000). We are now following these girls through the reproductive years and will present preliminary data on their reproductive success. Michigan Dept of Community Health, NIEHS, EPA.

Relationship between PCB accumulation and reproductive output in conditioned oysters Crassostrea virginica fed a contaminated algal diet

Because of their resistance to environmental degradation, polychlorinated biphenyls (PCBs) are among the most widespread environmental contaminants. PCBs have high bioaccumulation potential and may affect a number of biological/physiological processes including disruption of the endocrine system function, lipid metabolism and reproduction. The objective of this study was to test whether conditioning sexually immature oysters with PCB-contaminated algal diets affects their subsequent reproductive success. Sexually immature oysters were conditioned in individual containers and fed daily with 0.7 g algal paste containing 0, 0.35 or 3.5 μg PCBs for up to 76 days. The impact of sediment load on PCB accumulation in oysters was also tested by exposure of a subset of oysters to clay particles. Oysters in different treatments were sampled 56 days after conditioning with PCB-contaminated algal diets to determine uptake and distribution of PCBs in gonad, digestive gland, mantle, gill and muscle, and the presence of gametes. Tissues from oysters exposed to PCBs alone for 56 days were also analyzed for lipid and fatty acid composition. Following 61 and 76 days of PCB exposure, remaining oysters from all treatments were induced to spawn via thermal stimulation. Non-spawned oysters were stripped to determine if sexual products were present. Oysters exposed to PCBs alone and PCBs plus clay particles showed similar trends in PCB accumulation, but concentrations were generally lower in the latter. PCB accumulation in oysters increased with an increase in algal-associated PCB concentrations, varied with organ types and was correlated with lipid content. The highest PCB concentration was in the gonad and the lowest in gill and muscle. PCB-153, -138/158, -118, -90/101 and -149 were the dominant congeners in all tissue compartments, except the muscle where PCB-28/31 was the dominant congener pair. PCB exposure appeared to impair both lipid metabolism and reproductive success. Although PCB exposure produced only slight changes in the lipid class composition in the oysters, decreases in phospholipids were observed in gonad, muscle and mantle of oysters exposed to 3.5 μg PCBs daily for 56 days. After 56 days of conditioning with PCB-sorbed algal paste, no well-developed mature eggs were observed in any of the oysters examined for the presence of sexual products. No significant difference was noted in reproductive success (production of spawned females and males) between sediment-treated and non-treated groups after 76 days of PCB exposure compared to controls, PCB-exposed oysters produced fewer spawned females, but no dose-dependent relationship was observed.

Early developmental conditions and reproductive success in humans: downstream effects of prenatal famine, birthweight, and timing of birth.

Growth, survival, and breeding success of individuals in populations of wild mammals are influenced by the climatic and nutritional conditions that individuals experience during their early development. Recent findings have shown that early conditions also have consequences for subsequent survival and reproductive performance in humans. Environmental conditions which affect early development of individuals, such as the quality and quantity of nutrition received in utero and infancy, predict the onset of many chronic diseases in adulthood, affect longevity and may also influence a range of measures of reproductive performance in both food-limited and contemporary Western human populations. These associations are proposed to result from foetal programming, where a stimulus or insult during a critical period early in life may permanently affect body structure, physiology, and metabolism. Here I review studies showing how birthweight, season of birth, or exposure to prenatal starvation affect different aspects of an individual's subsequent reproductive success in humans and the growth, survival, and reproductive performance of the offspring produced. I show that early maternal and environmental conditions can have a large impact on human reproductive strategies and fitness that can span across generations.

The influence of density on post‐weaning growth in roe deer Capreolus capreolus fawns

AbstractIn temperate ungulates, the body weight of juveniles at the onset of winter is a crucial determinant of survival and can also influence subsequent reproductive success. However, growth may be retarded post‐weaning, during winter, as a result of resource restriction and/or the demands of thermoregulation in harsh climatic conditions. Post‐weaning growth rates of juveniles were compared in relation to varying density in two populations of roe deer Capreolus capreolus (Chizé, Dourdan) monitored for 10 and 15 years, respectively. Body growth of fawns continued over the post‐weaning stage of the juvenile period (October–March) at the rate of 0.017 kg/day at Chizé and 0.014 kg/day at Dourdan. Deer density had no influence on this post‐weaning growth rate of juveniles in their first winter. However, deer born in years of high density weighed less at a given date than those born in years of low density because of their smaller body size at the onset of winter, indicating density‐dependent rates of growth before weaning. At Chizé, the sexes grew at the same rate, but sexual dimorphism was apparent as males weighed about 1 kg (8%) more than females at a given date. At Dourdan, no significant sexual dimorphism was detected, although females tended to be heavier than males at a given date. We conclude that density influences juvenile body weight (through its effect on birth weight and/or post‐natal growth rate) before weaning in this species and, despite continued growth after weaning, during winter, roe deer whose early growth is limited through interspecific competition cannot compensate for this early restriction.

Sex ratio in relation to timing of breeding, and laying sequence in a dimorphic seabird

When the cost of rearing sons and daughters differs and the subsequent survival and reproductive success of one sex is more dependent than the other, on the amount of parental investment, adult females tend to produce more chicks of the more dependent sex if the females are in good condition themselves. One method of varying the total investment in each sex is through modifying the sex ratio of offspring produced. This study shows that in broods of European ShagsPhalacrocorax aristotelis, the sex ratio varied with laying date. Presumably in this species, the lifetime reproductive success of males is more dependent on the level of parental investment. Early breeders are in better condition, the brood sex ratio of early broods was male biased (0.63), while that of late broods was female biased (0.36). The overall difference in sex ratio found between early and late nests could be attributed to manipulation of sex in the first laid egg. In early broods, 77% of the first hatched chicks were male but only 30% of the first hatched chicks in late broods were male. The sex combination of the first two chicks in a brood significantly affected growth as measured by asymptotic mass.