Differences In Reproductive Investment Research Articles

The Fallacy of Year-Round Breeding in Polyphagous Tropical Fruit Flies (Diptera: Tephritidae): Evidence for a Seasonal Reproductive Arrestment in Bactrocera Species.

The genus Bactrocera (Diptera: Tephritidae) is endemic to the monsoonal rainforests of South-east Asia and the western Pacific where the larvae breed in ripe, fleshy fruits. While most Bactrocera remain rainforest restricted, species such as Bactrocera dorsalis, Bactrocera zonata and Bactrocera tryoni are internationally significant pests of horticulture, being both highly invasive and highly polyphagous. Almost universally in the literature it is assumed that Bactrocera breed continuously if temperature and hosts are not limiting. However, despite that, these flies show distinct seasonality. If discussed, seasonality is generally attributed to the fruiting of a particular breeding host (almost invariably mango or guava), but the question appears not to have been asked why flies do not breed at other times of the year despite other hosts being available. Focusing initially on B. tryoni, for which more literature is available, we demonstrate that the seasonality exhibited by that species is closely correlated with the seasons of its endemic rainforest environment as recognised by traditional Aboriginal owners. Evidence suggests the presence of a seasonal reproductive arrest which helps the fly survive the first two-thirds of the dry season, when ripe fruits are scarce, followed by a rapid increase in breeding at the end of the dry season as humidity and the availability of ripe fruit increases. This seasonal phenology continues to be expressed in human-modified landscapes and, while suppressed, it also partially expresses in long-term cultures. We subsequently demonstrate that B. dorsalis, across both its endemic and invasive ranges, shows a very similar seasonality although reversed in the northern hemisphere. While high variability in the timing of B. dorsalis population peaks is exhibited across sites, a four-month period when flies are rare in traps (Dec-Mar) is highly consistent, as is the fact that nearly all sites only have one, generally very sharp, population peak per year. While literature to support or deny a reproductive arrest in B. dorsalis is not available, available data is clear that continuous breeding does not occur in this species and that there are seasonal differences in reproductive investment. Throughout the paper we reinforce the point that our argument for a complex reproductive physiology in Bactrocera is based on inductive reasoning and requires specific, hypothesis-testing experiments to confirm or deny, but we do believe there is ample evidence to prioritise such research. If it is found that species in the genus undergo a true reproductive diapause then there are very significant implications for within-field management, market access, and biosecurity risk planning which are discussed. Arguably the most important of these is that insects in diapause have greater stress resistance and cold tolerance, which could explain how tropical Bactrocera species have managed to successfully invade cool temperate regions.

Sex biased effect of acute heat shock on the antioxidant system of non-native round goby Neogobius melanostomus.

Monitoring oxidative stress biomarkers has become a powerful and common tool to estimate organismal condition and response to endogenous and environmental factors. In the present study, we used round goby (Neogobius melanostomus) from non-native European populations, as a model species to test sex differences in oxidative stress biomarkers. Considering sex differences in reproductive investment, we hypothesized that males would display lower resistance to abiotic stress. Fish were exposed to a heat shock (temperature elevated by 10°C) for 1h, 6h, and 12h and catalase activity (CAT), reduced glutathione (GSH), total antioxidant capacity (TAC) and lipid peroxidation (LPO) were measured in liver and muscle tissues. Liver of males was significantly more responsive compared to liver of females in all tested parameters. GSH was found to be the most responsive to heat stress exposure in both sexes. The results supported our hypothesis that male reproductive investment (territoriality, courtship, and brood care) and likelihood of only a single spawning period in their lifetime influenced on higher sensitivity of their antioxidant defence. On the other hand, for females antioxidant defence is considered more important to survive the environmental changes and successfully reproduce in the next season. Our experiments exposed fish to acute thermal stress. Further research should determine the effects of exposure to chronic thermal stress to corroborate our understanding on sex differences in antioxidant defence in the round goby.

A shift towards the annual habit in selfing Arabidopsis lyrata.

An annual life history is often associated with the ability to self-fertilize. However, it is unknown whether the evolution of selfing commonly precedes the evolution of annuality, or vice versa. Using a 2-year common garden experiment, we asked if the evolution of selfing in the normally perennial Arabidopsis lyrata was accompanied by a shift towards the annual habit. Despite their very recent divergence from obligately outcrossing populations, selfing plants exhibited a 39% decrease in over-winter survival after the first year compared with outcrossing plants. Our data ruled out the most obvious underlying mechanism: differences in reproductive investment in the first year did not explain differences in survival. We conclude that transitions to selfing in perennial A. lyrata may be accompanied by a shift towards annuality, but drivers of the process require further investigation.

Flowering sex ratios and costs of reproduction in gynodioeciousOcotea oblonga(Lauraceae)

AbstractIn gynodioecious plant species, both female and hermaphrodite individuals produce fruit, but only hermaphrodites produce pollen. Such sex-specific differences in reproductive investment may contribute to dimorphism, but the magnitude and ecological effects are still unclear, especially for gynodioecious tropical trees where collecting flowers and determining the sex is complex. We documented flowering and fruiting over three years in a natural population of Ocotea oblonga (Lauraceae) trees in a tropical moist forest, Panama. We determined sex from freshly collected flowers, counted and measured fruit, and used long-term growth data for each individual. We confirmed that O. oblonga is gynodioecious. No tree switched sex or had flowers of both sexes. The population was hermaphrodite-biased. We found no ecological differences in reproductive investment (seed, fruit, or tree size, or growth rate) between the sexes, indicating that the sex differential in the cost of reproduction is much smaller in woody gynodioecious taxa than in dioecious taxa. Females produced more fruit than hermaphrodites, which may contribute to their persistence in the population. Accordingly, and contrary to most studies of temperate gynodioecious populations, our study of a tropical tree shows no differential cost of reproduction in a hermaphrodite-biased population. Consequently, other factors such as seed fertility or herbivory could drive the biased sex ratio in this population.

Behavioural, physiological and molecular changes in alloparental caregivers may be responsible for selection response for female reproductive investment in honey bees.

Reproductive investment is a central life history variable that influences all aspects of life. Hormones coordinate reproduction in multicellular organisms, but the mechanisms controlling the collective reproductive investment of social insects are largely unexplored. One important aspect of honey bee (Apis mellifera) reproductive investment consists of raising female-destined larvae into new queens by alloparental care of nurse bees in form of royal jelly provisioning. Artificial selection for commercial royal jelly production over 40years has increased this reproductive investment by an order of magnitude. In a cross-fostering experiment, we establish that this shift in social phenotype is caused by nurse bees. We find no evidence for changes in larval signalling. Instead, the antennae of the nurse bees of the selected stock are more responsive to brood pheromones than control bees. Correspondingly, the selected royal jelly bee nurses are more attracted to brood pheromones than unselected control nurses. Comparative proteomics of the antennae from the selected and unselected stocks indicate putative molecular mechanisms, primarily changes in chemosensation and energy metabolism. We report expression differences of several candidate genes that correlate with the differences in reproductive investment. The functional relevance of these genes is supported by demonstrating that the corresponding proteins can competitively bind one previously described and one newly discovered brood pheromone. Thus, we suggest several chemosensory genes, most prominently OBP16 and CSP4, as candidate mechanisms controlling queen rearing, a key reproductive investment, in honey bees. These findings reveal novel aspects of pheromonal communication in honey bees and explain how sensory changes affect communication and lead to a drastic shift in colony-level resource allocation to sexual reproduction. Thus, pheromonal and hormonal communication may play similar roles for reproductive investment in superorganisms and multicellular organisms, respectively.

Reproductive Cycle, Size and Age at Sexual Maturity, and Sexual Dimorphism in the Stream-Breeding Frog Crossodactylus schmidti (Hylodidae)

A reproductive strategy includes physiological, morphological, behavioral traits dependent on exogenous and endogenous regulators, including size/age at maturity, temporal variation in gametogenic cycles, and size-fecundity relationships. Sexual differences in reproductive investment, territoriality and resource use may result in sexual dimorphism on body size and other morphological traits. These life-history components are key for understanding the selective forces underlying male and female reproductive strategies in a population. We investigated the reproductive biology of the stream-breeding frog Crossodactylus schmidti through the histological analysis of testes, ovaries, and bone samples. Mature males and females and free-swimming tadpoles occurred in all months, though the monthly proportion of mature females was variable. Free spermatozoa, sperm bundles, and germ cell cysts in several stages occurred in testes of all except two males, whilst ovaries of most females presented all follicle stages, indicating continuous gametogenesis in both sexes. There was no significant seasonal variation in snout–vent length (SVL), body mass (BM) or in the gonadosomatic index for neither sex, further indicating a continuous pattern of reproduction. Minimum SVL and age at sexual maturity was 22.14 mm and 2 a, respectively, in males, and 21.78 mm and 3 a, respectively, in females; both sexes reached up to six years of longevity. Gonad mass increased with BM in both sexes, though this effect was much stronger in females. After controlling for age, females were significantly larger than males in SVL, BM, and MW (mouth width): the larger female body size suggests selection for increased fecundity, whereas the larger MW possibly favors the consumption of large, energetically important prey for sustaining the continuous egg production. Finally, there was a positive correlation between female SVL and number of thumb spines, but the number of thumb spines did not differ between the sexes; we briefly discuss a possible role of thumb spines in the courtship of C. schmidti based on field observations.

Sex-specific plasticity of reproductive allocation in response to water depth in a clonal, dioecious macrophyte.

Sex-specific differences in reproductive investment contribute to sexual dimorphism in dioecious plants. Along environmental gradients, males and females may plastically adjust reproductive allocation differently because of contrasting reproductive costs. In dioecious macrophytes, variation in water depth is likely to influence reproductive allocation but has not been investigated in detail. Vallisneria spinulosa was grown in aquatic mesocosms at water depths of 50, 100 and 150 cm for 14 weeks. Plasticity in allocation was measured to investigate whether sexual dimorphism in reproductive allocation and vegetative growth changed in response to varying water depths. Females invested a higher fraction of resources to sexual reproduction than males across all water depths and decreased proportional allocation to sexual structures in shallow and deep water compared to intermediate water depth. In contrast, males maintained similar sexual allocation across all water depths. Females displayed larger vegetative size than males, despite greater sexual investment, but decreased vegetative biomass more than males in shallow or deep water. The sexes invested similarly in clonal propagation by tubers at all water depths, but a trade-off with sexual reproduction was only evident in females. Our results suggest that females of V. spinulosa have mechanisms to compensate for the costs of sexual reproduction in heterogeneous environments. Compared to males, females expressed greater plasticity in biomass allocated to sexual reproduction and vegetative growth in response to water depth variation. Environmental variation in underwater light availability probably caused the sex-specific allocation strategies found in V. spinulosa.

A common environment experiment reveals plastic and genetic contributions to the fast life‐history strategy of an invasive fish

AbstractInvasive species often exhibit a suite of life‐history traits that promote rapid population growth, including early age and small size at maturation, and high reproductive investment. The common expression of these “fast” life‐history traits in invasive populations could be the result of plastic and/or genetic responses to the non‐native environment, or in response to the process of range expansion. To determine the relative importance of plastic and genetic contributions to the expression of life‐history traits, we reared two native Canadian and two invasive Spanish populations of Pumpkinseed sunfish (Lepomis gibbosus) in a common environment in central Ontario, Canada. In the wild, European Pumpkinseed tend to exhibit faster juvenile growth rates, younger age and smaller size at maturity, and higher reproductive investment than native North American populations. When reared in a common environment, both native and invasive populations exhibited similar juvenile growth rates, and similar age and size at maturity, suggesting that the differences seen among wild populations are a plastic response to the warmer non‐native environment. However, reproductive investment was consistently higher in the Spanish populations regardless of rearing environment, suggesting a genetic difference in reproductive investment between native and invasive populations. Selection for greater reproductive investment in non‐native Pumpkinseed may have contributed to their widespread success in Europe.

Spatial and seasonal variability in reproductive investment of Baltic sprat

Fecundity of marine fish species is highly variable, but trade-offs between fecundity and egg quality have rarely been observed at the individual level. We investigated spatial differences in reproductive investment of individual European sprat Sprattus sprattus (Linnaeus 1758) females by determining batch fecundity, condition indices (somatic condition index and gonadosomatic index) as well as oocyte dry weight, protein content, lipid content, spawning batch energy content, and fatty acid composition. Sampling was conducted in five different spawning areas within the Baltic Sea between March and May 2012. Spawning sprat from the Kiel Bight were in a better nutritional condition compared to sprat from the Arkona Basin, Bornholm Basin, and the Gdansk Deep. These females were also producing up to twice as many oocytes, and invested significantly more energy in reproduction, than their counterparts sampled in the eastern part of the Baltic Sea. Still, oocytes produced by Kiel Bight sprat contained significantly lower fractions of the essential fatty acids 20:4 (n-6) and 22:6 (n-3). A seasonal trade-off between oocyte weight/lipid content and fecundity was found for Baltic sprat, albeit the gross energy invested into spawning remained constant. Observed spatial and seasonal differences in sprat reproductive investment may be linked to hydrographic conditions and food availability and will impact the survival probability of yolk-sac and first feeding larvae. These findings indicate that Baltic sprat is able to adapt its reproductive tactics to the highly variable pelagic habitat of the Baltic Sea.

Fine scale spatio-temporal life history shifts in an invasive species at its expansion front

The dynamic shifts in life history traits by non-native populations at an invasion front may be driven by a combination of phenotypic plasticity and micro-evolutionary processes, allowing for appropriate responses to changes in biotic and abiotic factors associated with range expansion. The temporal and spatial scale at which these shifts occur, however, is largely unexplored. We examined the age-specific life-history responses of female round goby (Neogobius melanostomus) across a fine spatial scale by comparing traits of individuals at the expanding front of an invasive pathway with those of individuals living at the previous year’s front. Females in the new front were younger, exhibited higher body condition index and fecundity, and greater reproductive allocation than those living at the previous year’s front. No difference in back-calculated length at age or in seasonal growth increment was found between occupants of the old and new fronts. Age-specific differences in reproductive investment were observed; age-1 females from the new front exhibited greater reproductive effort but similar reproductive allocation, whereas age-2 females showed greater reproductive allocation in the new front, but similar reproductive effort. These age-specific differences may be related to differences in population density and other biotic factors encountered during ontogeny, or to micro-evolutionary processes. Our results demonstrate that fine scale changes in energy allocation towards reproduction through increases in reproductive allocation and fecundity occur at the invasion front, highlight how quickly those shifts can occur at the expansion front, and suggest that such changes in newly colonised areas facilitate range expansion.

Masked Expression of Life-History Traits in a Highly Variable Environment

Differing life-history strategies may act as a constraint on reproductive expression that ultimately limits the ability of individual species to respond to changes in the magnitude or frequency of environmental variation, and potentially underlies the variation often inherent in phenotypic and evolved responses to anthropogenic change. Alternatively, if there are environmental cues that predict reproductive potential, differential expression of life-history strategies may represent differences in the adaptive capacity to optimize current reproductive value given variation in environmental conditions. We compared several aspects of walleye Sander vitreus spawning ecology at two reservoirs that differ in environmental variability (i.e., annual water-level fluctuation) to identify the capacity of phenotypic expression and the corresponding association with age. Despite significant differences in female body and liver masses between reservoirs that differ in environmental variability, we found no difference in reproductive investment measured by egg size and fecundity. Walleye in a highly variable environment appear to exhibit reproductive traits more typical of a short-lived life-history strategy, which may be resultant from the interaction of environmental and anthropogenic pressures. This finding emphasizes the need to identify the degree to which life-history expression represents physiological constraints versus ecological optimization, particularly as anthropogenic change continues to alter environmental conditions.

The cost of reproduction: differential resource specialization in female and male California sea otters.

Intraspecific variation in behavior and diet can have important consequences for population and ecosystem dynamics. Here, we examine how differences in reproductive investment and spatial ecology influence individual diet specialization in male and female southern sea otters (Enhydra lutris nereis). We hypothesize that greater reproductive constraints and smaller home ranges of females lead to more pronounced intraspecific competition and increased specialization. We integrate stable carbon (δ(13)C) and nitrogen (δ(15)N)isotope analysis of sea otter vibrissae with long-term observational studies of five subpopulations in California. We define individual diet specialization as low ratios of within-individual variation (WIC) to total population niche width (TNW). We compare isotopic and observational based metrics of WIC/TNW for males and females to data on population densities, and movement patterns using both general linear and linear mixed-effects models. Consistent with our hypothesis, increasing population density is associated with increased individual diet specialization by females but not by males. Additionally, we find the amount of coastline in a sea otter's home range positively related with individual dietary variability, with increased range span resulting in weaker specialization for both males and females. We attribute our results to sex-based differences in movement, with females needing to specialize in their small ranges to maximize energy gain, and posit that the paradigm of individual prey specialization in sea otters with increased intraspecific competition may be a pattern driven largely by females. Our work highlights a potentially broader role of sex in the mechanistic pressures promoting and maintaining dietspecialization.

The consequences of demand‐driven seed provisioning for sexual differences in reproductive investment in Thalictrum occidentale (Ranunculaceae)

Summary Many iteroparous angiosperms may benefit from flexible annual resource allocation in response to variable reproductive opportunities induced by external conditions. If maximal reproductive investment is fixed, lack of reproductive sinks would cause resource redistribution to other sinks. Alternatively, reproductive investment may vary depending on the demand of reproductive sinks, changing source–sink relations. In particular, differential responses by males and females to the demands of flower and seed production may cause sexual dimorphism. We assess the occurrence of demand‐driven seed allocation by females and its implications for sexual differences in reproductive investment, including the dynamics of mass and carbon allocation and the physiological cost of reproduction, for a dioecious, perennial herb, Thalictrum occidentale. We specifically quantified allocation responses to partial defoliation, which reduced current resource supply, and partial flower/fruit removal, which reduced the aggregate demand of reproductive sinks. During flowering, males preferentially invested carbon and mass in flowers at the expense of vegetative organs, whereas females allocated less mass to flowers and invested more in new rhizome production for future performance than males. In contrast, during early fruiting, both sexes had new rhizomes of similar size and a doubling of reproductive mass by females after flowering resulted in similar total reproductive investment for both sexes. Manipulation of the source–sink balance did not influence carbon allocation, except that partial fruit removal increased new rhizome mass compared with intact plants. Females with many fertilized ovules invested proportionally more in seed number and mass per seed than females with few fertilized ovules, indicating both demand‐driven seed maturation and its elimination of seed size‐number trade‐off. Furthermore, males consistently exhibited size‐dependent flower production, whereas females exhibited size‐dependent flower production only if they had not reproduced during the previous year. Synthesis. This study demonstrates that sexual differences in policies of reproductive investment and the timing of the physiological costs of reproduction impose contrasting allocation schedules. Males invested in reproduction proportionally with their size, whereas females invested flexibly in seeds in response to the demand of developing embryos. Thus, the contrasting certainty and timing of reproductive resource requirements between the sexes contribute to sexual dimorphism.

Sex-biased immunity is driven by relative differences in reproductive investment.

Sex differences in immunity are often observed, with males generally having a weaker immune system than females. However, recent data in a sex-role-reversed species in which females compete to mate with males suggest that sexually competitive females have a weaker immune response. These findings support the hypothesis that sexual dimorphism in immunity has evolved in response to sex-specific fitness returns of investment in traits such as parental investment and longevity, but the scarcity of data in sex-reversed species prevents us from drawing general conclusions. Using an insect species in which males make a large but variable parental investment in their offspring, we use two indicators of immunocompetence to test the hypothesis that sex-biased immunity is determined by differences in parental investment. We found that when the value of paternal investment was experimentally increased, male immune investment became relatively greater than that of females. Thus, in this system, in which the direction of sexual competition is plastic, the direction of sex-biased immunity is also plastic and appears to track relative parental investment.

Chemical and physical defense traits in two sexual forms of Opuntia robusta in Central Eastern Mexico.

Sexually dimorphic plants provide an excellent opportunity for examining the differences in the extent of their defense against herbivores because they exhibit sex-related differences in reproductive investment. Such differences enable comparison of the sex with high reproduction expenses with the sex that expends less. The more costly sex is usually also better defended against herbivores. Generally, females are considered more valuable than hermaphrodites in terms of fitness; however, hermaphrodites are more valuable if they can produce seed by autonomous selfing, provided that the inbreeding depression is low and pollen is limited. We studied a gynodioecious population of Opuntia robusta from Central-Eastern Mexico, which has been reported to be trioecious, dioecious, or hermaphrodite, and addressed the following questions: 1) Is the hermaphrodite's reproductive output higher than the female's, and are hermaphrodites thus better defended? 2) Are plant tissues differentially defended? 3) Do trade-offs exist among different physical defense traits? and 4) among physical and chemical defense traits? We found that 1) hermaphrodites had a higher seed output and more spines per areola than females and that their spines contained less moisture. Non-reproductive hermaphrodite cladodes contained more total phenolic compounds (TPCs) than female ones. In addition, 2) hermaphrodite reproductive cladodes bore more spines than female cladodes, and 3) and 4) we found a negative relationship between spine number per areola and areola number per cladode and a positive relationship between spine number per areola per plant and TPC concentration per plant. Non-reproductive hermaphrodite cladodes contained a higher concentration of TPCs than female cladodes, and parental cladodes contained fewer TPCs than both reproductive and empty cladodes.

No evidence for sex biases in milk macronutrients, energy, or breastfeeding frequency in a sample of filipino mothers

Maternal reproductive investment includes both the energetic costs of gestation and lactation. For most humans, the metabolic costs of lactation will exceed those of gestation. Mothers must balance reproductive investment in any single offspring against future reproductive potential. Among mammals broadly, mothers may differentially invest in offspring based on sex and maternal condition provided such differences investment influence future offspring reproductive success. For humans, there has been considerable debate if there are physiological differences in maternal investment by offspring sex. Two recent studies have suggested that milk composition differs by infant sex, with male infants receiving milk containing higher fat and energy; prior human studies have not reported sex-based differences in milk composition. This study investigates offspring sex-based differences in milk macronutrients, milk energy, and nursing frequency (per 24 h) in a sample of 103 Filipino mothers nursing infants less than 18 months of age. We found no differences in milk composition by infant sex. There were no significant differences in milk composition of mothers nursing first-born versus later-born sons or daughters or between high- and low-income mothers nursing daughters or sons. Nursing frequency also showed no significant differences by offspring sex, sex by birth order, or sex by maternal economic status. In the Cebu sample, there is no support for sex-based differences in reproductive investment during lactation as indexed by milk composition or nursing frequency. Further investigation in other populations is necessary to evaluate the potential for sex-based differences in milk composition among humans.

Plant Phenology and Absence of Sex-Biased Gall Attack on Three Species of Baccharis

BackgroundDioecy represents a source of variation in plant quality to herbivores due to sexual differences in intensity and timing of resource allocation to growth, defense and reproduction. Male plants have higher growth rates and should be more susceptible to herbivores than females, due to a lower investment in defense and reproduction.Methodology/Principal FindingsWe compared resource investment to growth and reproduction and its consequences to herbivore attack on three Baccharis species along one year (B. dracunculifolia, B. ramosissima, and B. concinna). Phenological patterns presented by the three species of Baccharis were quite different over time, but the number of fourth-level shoots and plant growth rate did not differ between sexes in any studied species. Intersexual difference in reproductive investment was only observed for B. concinna, with female individuals supporting higher inflorescence density than male individuals throughout the year. Gall abundance on the three Baccharis species was not influenced by plant sex. However, all plant traits evaluated here positively influenced the gall abundance on B. concinna, whereas only the number of fourth-level shoots positively influenced gall abundance on B. ramosissima and B. dracunculifolia.Conclusions/SignificanceThe absence of differential reproductive allocation may have contributed to similar growth and shoot production between the sexes, with bottom-up effects resulting in gender similarities in gall abundance patterns. The number of fourth-level shoots, an indicator of meristem availability to herbivores, was the most important driver of the abundance of the galling insects regardless of host plant gender or species. Albeit the absence of intersexual variation in insect gall abundance is uncommon in the literature, the detailed study of the exceptions may bring more light to understand the mechanisms and processes behind such trend.

Methodological challenges to examining the causes of variation in stock reproductive potential

Understanding the causes of variation in stock reproductive potential (SRP) is challenging due to the difficulty in assessing the relative contribution of environmental, demographic and genotypic influences. This review explores how experimental and comparative field studies have been used to disentangle sources of variation in maturity, fecundity and the timing of spawning. By comparing reproductive parameters and corresponding environmental conditions among stocks of a species it is clear that demography, energetic state and temperature are important factors affecting variation in SRP. Common garden and environment experiments have confirmed that there is also a substantial genetic component to regional differences in reproductive investment and timing of spawning. Environmental manipulation experiments have helped to elucidate the proximate mechanisms underlying many reproductive processes. Together these different sources of information have provided a foundation for the development of statistical and individual based modelling approaches that help explain variation in SRP. In the near future, genomic investigations may provide a direct means to account for genetic influences on reproductive variation. Given that the age and size structure of many fish stocks has become truncated through fishing, a greater focus on the contribution of reproductive life span to SRP is also needed.

No Evidence for a Trade-Off between Reproductive Investment and Immunity in a Rodent

Life history theory assumes there are trade-offs between competing functions such as reproduction and immunity. Although well studied in birds, studies of the trade-offs between reproduction and immunity in small mammals are scarce. Here we examined whether reduced immunity is a consequence of reproductive effort in lactating Brandt's voles (Lasiopodomys brandtii). Specifically, we tested the effects of lactation on immune function (Experiment I). The results showed that food intake and resting metabolic rate (RMR) were higher in lactating voles (6≤ litter size ≤8) than that in non-reproductive voles. Contrary to our expectation, lactating voles also had higher levels of serum total Immunoglobulin G (IgG) and anti-keyhole limpet hemocyanin (KLH) IgG and no change in phytohemagglutinin (PHA) response and anti-KLH Immunoglobulin M (IgM) compared with non-reproductive voles, suggesting improved rather than reduced immune function. To further test the effect of differences in reproductive investment on immunity, we compared the responses between natural large (n≥8) and small litter size (n≤6) (Experiment II) and manipulated large (11–13) and small litter size (2–3) (Experiment III). During peak lactation, acquired immunity (PHA response, anti-KLH IgG and anti-KLH IgM) was not significantly different between voles raising large or small litters in both experiments, despite the measured difference in reproductive investment (greater litter size, litter mass, RMR and food intake in the voles raising larger litters). Total IgG was higher in voles with natural large litter size than those with natural small litter size, but decreased in the enlarged litter size group compared with control and reduced group. Our results showed that immune function is not suppressed to compensate the high energy demands during lactation in Brandt's voles and contrasting the situation in birds, is unlikely to be an important aspect mediating the trade-off between reproduction and survival.

Multiple Fitness Benefits of Polyandry in a Cephalopod

BackgroundSex differences in reproductive investment play a crucial role in sexual conflict. One intriguing aspect of sexual conflict is the evolution of female multiple mating (polyandry), particularly in systems where females receive no obvious direct benefits from males, and where mating is highly costly. Here, theory predicts that polyandrous females can increase their reproductive success by taking advantage of the genetic benefits of mating with multiple males. Cephalopods provide a model system for addressing this question, as all species mate multiply. Here we examine differences in reproductive success between monandrous, multiply mated (to the same male) and polyandrous female dumpling squid (Euprymna tasmanica).Methodology/Principal FindingsWe mated females in the laboratory with two different males (polyandrous; controlling for mating order), or with a single male (monandrous). To control for mating frequency, we mated monandrous females either once (monandrous 1), or with the same male twice (monandrous 2), and measured reproductive success for each of the three treatments (polyandrous, monandrous 1, monandrous 2). Females mated to two different males produced eggs faster and had larger hatchlings relative to egg mass than females mated once with a single male.Conclusions/SignificanceThe benefits of polyandry demonstrated here are the first, to our knowledge, in any cephalopod. These benefits may outweigh the significant costs associated with mating and help to explain how multiple mating has evolved (or is maintained) in this group.