Partitioning of resources: the evolutionary genetics of sexual conflict over resource acquisition and allocation

Fitness depends on both the resources that individuals acquire and the allocation of those resources to traits that influence survival and reproduction. Optimal resource allocation differs between females and males as a consequence of their fundamentally different reproductive strategies. However, because most traits have a common genetic basis between the sexes, conflicting selection between the sexes over resource allocation can constrain the evolution of optimal allocation within each sex, and generate trade‐offs for fitness between them (i.e. ‘sexual antagonism’ or ‘intralocus sexual conflict’). The theory of resource acquisition and allocation provides an influential framework for linking genetic variation in acquisition and allocation to empirical evidence of trade‐offs between distinct life‐history traits. However, these models have not considered the emergence of trade‐offs within the context of sexual dimorphism, where they are expected to be particularly common. Here, we extend acquisition–allocation theory and develop a quantitative genetic framework for predicting genetically based trade‐offs between life‐history traits within sexes and between female and male fitness. Our models demonstrate that empirically measurable evidence of sexually antagonistic fitness variation should depend upon three interacting factors that may vary between populations: (1) the genetic variances and between‐sex covariances for resource acquisition and allocation traits, (2) condition‐dependent expression of resource allocation traits and (3) sex differences in selection on the allocation of resource to different fitness components.     

Early‐life telomere length predicts lifespan and lifetime reproductive success in a wild bird

Poor conditions during early development can initiate trade‐offs that favour current survival at the expense of somatic maintenance and subsequently, future reproduction. However, the mechanisms that link early and late life‐history are largely unknown. Recently it has been suggested that telomeres, the nucleoprotein structures at the terminal end of chromosomes, could link early‐life conditions to lifespan and fitness. In wild purple‐crowned fairy‐wrens, we combined measurements of nestling telomere length (TL) with detailed life‐history data to investigate whether early‐life TL predicts fitness prospects. Our study differs from previous studies in the completeness of our fitness estimates in a highly philopatric population. The association between TL and survival was age‐dependent with early‐life TL having a positive effect on lifespan only among individuals that survived their first year. Early‐life TL was not associated with the probability or age of gaining a breeding position. Interestingly, early‐life TL was positively related to breeding duration, contribution to population growth and lifetime reproductive success because of their association with lifespan. Thus, early‐life TL, which reflects growth, accumulated early‐life stress and inherited TL, predicted fitness in birds that reached adulthood but not noticeably among fledglings. These findings suggest that a lack of investment in somatic maintenance during development particularly affects late life performance. This study demonstrates that factors in early‐life are related to fitness prospects through lifespan, and suggests that the study of telomeres may provide insight into the underlying physiological mechanisms linking early‐ and late‐life performance and trade‐offs across a lifetime.     

Age at mating and male quality influence female patterns of reproductive investment and survival

The trade‐off between the allocation of resources toward somatic maintenance or reproduction is one of the fundamentals of life history theory and predicts that females invest in offspring at the expense of their longevity or vice versa. Mate quality may also affect life history trade‐offs through mechanisms of sexual conflict; however, few studies have examined the interaction between mate quality and age at first mating in reproductive decisions. Using house crickets (Acheta domesticus), this study examines how survival and reproductive trade‐offs change based on females’ age at first reproduction and exposure to males of varying size. Females were exposed to either a large (presumably high‐quality) or small male at an early (young), middle (intermediate), or advanced (old) age, and longevity and reproductive investment were subsequently tracked. Females mated at a young age had the largest number of eggs but the shortest total lifespans while females mated at older ages produced fewer eggs but had longer total lifespans. The trade‐off between age at first mating and eggs laid appears to be mediated through higher egg‐laying rates and shorter postmating lifespans in females mated later in life. Exposure to small males resulted in shorter lifespans and higher egg‐laying rates for all females indicating that male manipulation of females, presumably through spermatophore contents, varies with male size in this species. Together, these data strongly support a trade‐off between age at first reproduction and lifespan and support the role of sexual conflict in shaping patterns of reproduction.     

When relative allocation depends on total resource acquisition: implication for the analysis of trade‐offs

A central tenet of evolutionary biology states that life‐history traits are linked via trade‐offs, as classically exemplified by the van Noordwijk and de Jong model. This model, however, assumes that the relative resource allocation to a biological function varies independently of the total resource acquisition. Based on current empirical evidence, we first explored the dependency between the total resource acquisition and the relative resource allocation to reproduction and showed that such dependency is the rule rather than the exception. We then derived the expression of the covariance between traits when the assumption of independence is relaxed and used simulations to quantify the importance of such dependency on the detection of trade‐offs between current reproduction and future survival. We found that the dependency between the total energy acquisition and the relative allocation to reproduction can influence the probability to detect trade‐offs between survival and reproduction. As a general rule, a negative dependency between the total energy acquisition and the relative allocation to reproduction should lead to a higher probability of detecting a trade‐off in species with a fast pace of life, whereas a positive dependency should lead to a higher probability of detecting a trade‐off in species with a slow pace of life. In addition to confirming the importance of resource variation to reveal trade‐offs, our finding demonstrates that the covariance between resource allocation and resource acquisition is generally not null and also plays a fundamental role in the detection of trade‐offs.     

What shall I do now? State-dependent variations of life-history traits with aging in Wandering Albatrosses

Allocation decisions depend on an organism's condition which can change with age. Two opposite changes in life‐history traits are predicted in the presence of senescence: either an increase in breeding performance in late age associated with terminal investment or a decrease due to either life‐history trade‐offs between current breeding and future survival or decreased efficiency at old age. Age variation in several life‐history traits has been detected in a number of species, and demographic performances of individuals in a given year are influenced by their reproductive state the previous year. Few studies have, however, examined state‐dependent variation in life‐history traits with aging, and they focused mainly on a dichotomy of successful versus failed breeding and non‐breeding birds. Using a 50‐year dataset on the long‐lived quasi‐biennial breeding wandering albatross, we investigated variations in life‐history traits with aging according to a gradient of states corresponding to potential costs of reproduction the previous year (in ascending order): non‐breeding birds staying at sea or present at breeding grounds, breeding birds that failed early, late or were successful. We used multistate models to study survival and decompose reproduction into four components (probabilities of return, breeding, hatching, and fledging), while accounting for imperfect detection. Our results suggest the possible existence of two strategies in the population: strict biennial breeders that exhibited almost no reproductive senescence and quasi‐biennial breeders that showed an increased breeding frequency with a strong and moderate senescence on hatching and fledging probabilities, respectively. The patterns observed on survival were contrary to our predictions, suggesting an influence of individual quality rather than trade‐offs between reproduction and survival at late ages. This work represents a step further into understanding the evolutionary ecology of senescence and its relationship with costs of reproduction at the population level. It paves the way for individual‐based studies that could show the importance of intra‐population heterogeneity in those processes.     

Trade‐offs between life‐history traits at range‐edge and central locations

The allocation of resources to different life‐history traits should represent the best compromise in fitness investment for organisms in their local environment. When resources are limiting, the investment in a specific trait must carry a cost that is expressed in trade‐offs with other traits. In this study, the relative investment in the fitness‐related traits, growth, reproduction and defence were compared at central and range‐edge locations, using the seaweed Ascophyllum nodosum as a model system. Individual growth rates were similar at both sites, whereas edge populations showed a higher relative investment in reproduction (demonstrated by a higher reproductive allocation and extended reproductive periods) when compared to central populations that invested more in defence. These results show the capability of A. nodosum to differentially allocate resources for different traits under different habitat conditions, suggesting that reproduction and defence have different fitness values under the specific living conditions experienced at edge and central locations. However, ongoing climate change may threaten edge populations by increasing the selective pressure on specific traits, forcing these populations to lower the investment in other traits that are also potentially important for population fitness.     

Life‐history trade‐offs promote the evolution of dioecy

Most dioecious plants are perennial and subject to trade‐offs between sexual reproduction and vegetative performance. However, these broader life‐history trade‐offs have not usually been incorporated into theoretical analyses of the evolution of separate sexes. One such analysis has indicated that hermaphroditism is favoured over unisexuality when female and male sex functions involve the allocation of nonoverlapping types of resources to each sex function (e.g. allocations of carbon to female function vs. allocations of nitrogen to male function). However, some dioecious plants appear to conform to this pattern of resource allocation, with different resource types allocated to female vs. male sex functions. Using an evolutionarily stable strategy approach, we show that life‐history trade‐offs between sexual reproduction and vegetative performance enable the evolution of unisexual phenotypes even when there are no direct resource‐based trade‐offs between female and male sex functions. This result might help explain the preponderance of perennial life histories among dioecious plants and why many dioecious plants with annual life histories have indeterminate growth with ongoing trade‐offs between sexual reproduction and vegetative growth.     

STRONG SELECTION GENOME‐WIDE ENHANCES FITNESS TRADE‐OFFS ACROSS ENVIRONMENTS AND EPISODES OF SELECTION

Fitness trade‐offs across episodes of selection and environments influence life‐history evolution and adaptive population divergence. Documenting these trade‐offs remains challenging as selection can vary in magnitude and direction through time and space. Here, we evaluate fitness trade‐offs at the levels of the whole organism and the quantitative trait locus (QTL) in a multiyear field study of Boechera stricta (Brassicaceae), a genetically tractable mustard native to the Rocky Mountains. Reciprocal local adaptation was pronounced for viability, but not for reproductive components of fitness. Instead, local genomes had a fecundity advantage only in the high latitude garden. By estimating realized selection coefficients from individual‐level data on viability and reproductive success and permuting the data to infer significance, we examined the genetic basis of fitness trade‐offs. This analytical approach (Conditional Neutrality‐Antagonistic Pleiotropy, CNAP) identified genetic trade‐offs at a flowering phenology QTL (costs of adaptation) and revealed genetic trade‐offs across fitness components (costs of reproduction). These patterns would not have emerged from traditional ANOVA‐based QTL mapping. Our analytical framework can be applied to other systems to investigate fitness trade‐offs. This task is becoming increasingly important as climate change may alter fitness landscapes, potentially disrupting fitness trade‐offs that took many generations to evolve.     

Experimental evolution reveals differences between phenotypic and evolutionary responses to population density

Group living can select for increased immunity, given the heightened risk of parasite transmission. Yet, it also may select for increased male reproductive investment, given the elevated risk of female multiple mating. Trade‐offs between immunity and reproduction are well documented. Phenotypically, population density mediates both reproductive investment and immune function in the Indian meal moth, Plodia interpunctella. However, the evolutionary response of populations to these traits is unknown. We created two replicated populations of P. interpunctella, reared and mated for 14 generations under high or low population densities. These population densities cause plastic responses in immunity and reproduction: at higher numbers, both sexes invest more in one index of immunity [phenoloxidase (PO) activity] and males invest more in sperm. Interestingly, our data revealed divergence in PO and reproduction in a different direction to previously reported phenotypic responses. Males evolving at low population densities transferred more sperm, and both males and females displayed higher PO than individuals at high population densities. These positively correlated responses to selection suggest no apparent evolutionary trade‐off between immunity and reproduction. We speculate that the reduced PO activity and sperm investment when evolving under high population density may be due to the reduced population fitness predicted under increased sexual conflict and/or to trade‐offs between pre‐ and post‐copulatory traits.     

EVOLUTION OF TRADE-OFFS BETWEEN SEXUAL AND ASEXUAL PHASES AND THE ROLE OF REPRODUCTIVE PLASTICITY IN THE GENETIC ARCHITECTURE OF APHID LIFE HISTORIES

Life‐history theory postulates that evolution is constrained by trade‐offs (i.e., negative genetic correlations) among traits that contribute to fitness. However, in organisms with complex life cycles, trade‐offs may drastically differ between phases, putatively leading to different evolutionary trajectories. Here, we tested this possibility by examining changes in life‐history traits in an aphid species that alternates asexual and sexual reproduction in its life cycle. The quantitative genetics of reproductive and dispersal traits was studied in 23 lineages (genotypes) of the bird cherry‐oat aphid Rhopalosiphum padi, during both the sexual and asexual phases, which were induced experimentally under specific environmental conditions. We found large and significant heritabilities (broad‐sense) for all traits and several negative genetic correlations between traits (trade‐offs), which are related to reproduction (i.e., numbers of the various sexual or asexual morphs) or dispersal (i.e., numbers of winged or wingless morphs). These results suggest that R. padi exhibits lineage specialization both in reproductive and dispersal strategies. In addition, we found important differences in the structure of genetic variance–covariance matrices ( G ) between phases. These differences were due to two large, negative genetic correlations detected during the asexual phase only: (1) between fecundity and age at maturity and (2) between the production of wingless and winged parthenogenetic females. We propose that this differential expression in genetic architecture results from a reallocation scheme during the asexual phase, when sexual morphs are not produced. We also found significant G × E interaction and nonsignificant genetic correlations across phases, indicating that genotypes could respond independently to selection in each phase. Our results reveal a rather unique situation in which the same population and even the same genotypes express different genetic (co)variation under different environmental conditions, driven by optimal resource allocation criteria.     

Sexual signals reflect telomere dynamics in a wild bird

Telomere dynamics in natural populations have been linked to survival, reproduction, and energetic investment. Given their putative role in mediating life‐history trade‐offs, telomeres are also a likely candidate for maintaining honesty in sexually selected signals; few studies to date, however, have demonstrated a correlation between sexual signals and telomere dynamics. Here, we show that plumage coloration in male common yellowthroats (Geothlypis trichas) is correlated with both relative telomere length and with the rate of telomere loss between years. Elevated antioxidant capacity is also associated with reduced telomere loss, but only among older males. Previous work in this population has demonstrated that males with brighter plumage are in better condition, have higher reproductive success, and are more likely to survive over winter. Thus, the signal attribute associated with mate choice in this system also conveys reliable information about telomere dynamics. At present, it is unclear whether telomere maintenance plays a causal role in maintaining signal honesty or whether the correlation arises due to underlying variation in individual resources or genotypes. We suggest that subsequent work should consider the possibility that fundamental trade‐offs between signal investment and cell‐level processes that influence aging and reproductive senescence may provide a foundation for understanding the maintenance of sexual signal honesty.     

Effects of variation in resource acquisition during different stages of the life cycle on life‐history traits and trade‐offs in a burying beetle

Individual variation in resource acquisition should have consequences for life‐history traits and trade‐offs between them because such variation determines how many resources can be allocated to different life‐history functions, such as growth, survival and reproduction. Since resource acquisition can vary across an individual's life cycle, the consequences for life‐history traits and trade‐offs may depend on when during the life cycle resources are limited. We tested for differential and/or interactive effects of variation in resource acquisition in the burying beetle Nicrophorus vespilloides. We designed an experiment in which individuals acquired high or low amounts of resources across three stages of the life cycle: larval development, prior to breeding and the onset of breeding in a fully crossed design. Resource acquisition during larval development and prior to breeding affected egg size and offspring survival, respectively. Meanwhile, resource acquisition at the onset of breeding affected size and number of both eggs and offspring. In addition, there were interactive effects between resource acquisition at different stages on egg size and offspring survival. However, only when females acquired few resources at the onset of breeding was there evidence for a trade‐off between offspring size and number. Our results demonstrate that individual variation in resource acquisition during different stages of the life cycle has important consequences for life‐history traits but limited effects on trade‐offs. This suggests that in species that acquire a fixed‐sized resource at the onset of breeding, the size of this resource has larger effects on life‐history trade‐offs than resources acquired at earlier stages.     

Demographic consequences of greater clonal than sexual reproduction in Dicentra canadensis

Clonality is a widespread life history trait in flowering plants that may be essential for population persistence, especially in environments where sexual reproduction is unpredictable. Frequent clonal reproduction, however, could hinder sexual reproduction by spatially aggregating ramets that compete with seedlings and reduce inter‐genet pollination. Nevertheless, the role of clonality in relation to variable sexual reproduction in population dynamics is often overlooked. We combined population matrix models and pollination experiments to compare the demographic contributions of clonal and sexual reproduction in three Dicentra canadensis populations, one in a well‐forested landscape and two in isolated forest remnants. We constructed stage‐based transition matrices from 3 years of census data to evaluate annual population growth rates, λ. We used loop analysis to evaluate the relative contribution of different reproductive pathways to λ. Despite strong temporal and spatial variation in seed set, populations generally showed stable growth rates. Although we detected some pollen limitation of seed set, manipulative pollination treatments did not affect population growth rates. Clonal reproduction contributed significantly more than sexual reproduction to population growth in the forest remnants. Only at the well‐forested site did sexual reproduction contribute as much as clonal reproduction to population growth. Flowering plants were more likely to transition to a smaller size class with reduced reproductive potential in the following year than similarly sized nonflowering plants, suggesting energy trade‐offs between sexual and clonal reproduction at the individual level. Seed production had negligible effects on growth and tuber production of individual plants. Our results demonstrate that clonal reproduction is vital for population persistence in a system where sexual reproduction is unpredictable. The bias toward clonality may be driven by low fitness returns for resource investment in sexual reproduction at the individual level. However, chronic failure in sexual reproduction may exacerbate the imbalance between sexual and clonal reproduction and eventually lead to irreversible loss of sex in the population.     

Early reproductive investment,senescence and lifetime reproductive success in female Asian elephants

The evolutionary theory of senescence posits that as the probability of extrinsic mortality increases with age, selection should favour early‐life over late‐life reproduction. Studies on natural vertebrate populations show early reproduction may impair later‐life performance, but the consequences for lifetime fitness have rarely been determined, and little is known of whether similar patterns apply to mammals which typically live for several decades. We used a longitudinal dataset on Asian elephants (Elephas maximus) to investigate associations between early‐life reproduction and female age‐specific survival, fecundity and offspring survival to independence, as well as lifetime breeding success (lifetime number of calves produced). Females showed low fecundity following sexual maturity, followed by a rapid increase to a peak at age 19 and a subsequent decline. High early life reproductive output (before the peak of performance) was positively associated with subsequent age‐specific fecundity and offspring survival, but significantly impaired a female's own later‐life survival. Despite the negative effects of early reproduction on late‐life survival, early reproduction is under positive selection through a positive association with lifetime breeding success. Our results suggest a trade‐off between early reproduction and later survival which is maintained by strong selection for high early fecundity, and thus support the prediction from life history theory that high investment in reproductive success in early life is favoured by selection through lifetime fitness despite costs to later‐life survival. That maternal survival in elephants depends on previous reproductive investment also has implications for the success of (semi‐)captive breeding programmes of this endangered species.     

Recovery and immune priming modulate the evolutionary trajectory of infection‐induced reproductive strategies

In response to parasite exposure, organisms from a variety of taxa undergo a shift in reproductive investment that may trade off with other life‐history traits including survival and immunity. By suppressing reproduction in favour of somatic and immunological maintenance, hosts can enhance the probability of survival and recovery from infection. By plastically enhancing reproduction through terminal investment, on the other hand, hosts under the threat of disease‐induced mortality could enhance their lifetime reproductive fitness through reproduction rather than survival. However, we know little about the evolution of these strategies, particularly when hosts can recover and even bequeath protection to their offspring. In this study, we develop a stochastic agent‐based model that competes somatic maintenance and terminal investment strategies as they trade off differentially with lifespan, parasite resistance, recovery and transgenerational immune priming. Our results suggest that a trade‐off between reproduction and recovery can drive directional selection for either terminal investment or somatic maintenance, depending on the cost of reproduction to lifespan. However, some conditions, such as low virulence with a high cost of reproduction to lifespan, can favour diversifying selection for the coexistence of both strategies. The introduction of transgenerational priming into the model favours terminal investment when all strategies are equally likely to produce primed offspring, but favours somatic maintenance if it confers even a slight priming advantage over terminal investment. Our results suggest that both immune priming and recovery may modulate the evolution of reproductive shift diversity and magnitude upon exposure to parasites.     

Lactation and resource limitation affect stress responses,thyroid hormones,immune function,and antioxidant capacity of sea otters (Enhydra lutris)

Lactation is the most energetically demanding stage of reproduction in female mammals. Increased energetic allocation toward current reproduction may result in fitness costs, although the mechanisms underlying these trade‐offs are not well understood. Trade‐offs during lactation may include reduced energetic allocation to cellular maintenance, immune response, and survival and may be influenced by resource limitation. As the smallest marine mammal, sea otters (Enhydra lutris) have the highest mass‐specific metabolic rate necessitating substantial energetic requirements for survival. To provide the increased energy needed for lactation, female sea otters significantly increase foraging effort, especially during late‐lactation. Caloric insufficiency during lactation is reflected in the high numbers of maternal deaths due to End‐Lactation Syndrome in the California subpopulation. We investigated the effects of lactation and resource limitation on maternal stress responses, metabolic regulation, immune function, and antioxidant capacity in two subspecies of wild sea otters (northern: E. l. nereis and southern: E. l. kenyoni) within the California, Washington, and Alaska subpopulations. Lactation and resource limitation were associated with reduced glucocorticoid responses to acute capture stress. Corticosterone release was lower in lactating otters. Cortisol release was lower under resource limitation and suppression during lactation was only evident under resource limitation. Lactation and resource limitation were associated with alterations in thyroid hormones. Immune responses and total antioxidant capacity were not reduced by lactation or resource limitation. Southern sea otters exhibited higher concentrations of antioxidants, immunoglobulins, and thyroid hormones than northern sea otters. These data provide evidence for allocation trade‐offs during reproduction and in response to nutrient limitation but suggest self‐maintenance of immune function and antioxidant defenses despite energetic constraints. Income‐breeding strategists may be especially vulnerable to the consequences of stress and modulation of thyroid function when food resources are insufficient to support successful reproduction and may come at a cost to survival, and thereby influence population trends.     

Food availability as a major driver in the evolution of life‐history strategies of sibling species

Life‐history theory predicts trade‐offs between reproductive and survival traits such that different strategies or environmental constraints may yield comparable lifetime reproductive success among conspecifics. Food availability is one of the most important environmental factors shaping developmental processes. It notably affects key life‐history components such as reproduction and survival prospect. We investigated whether food resource availability could also operate as an ultimate driver of life‐history strategy variation between species. During 13 years, we marked and recaptured young and adult sibling mouse‐eared bats (Myotis myotis and Myotis blythii) at sympatric colonial sites. We tested whether distinct, species‐specific trophic niches and food availability patterns may drive interspecific differences in key life‐history components such as age at first reproduction and survival. We took advantage of a quasi‐experimental setting in which prey availability for the two species varies between years (pulse vs. nonpulse resource years), modeling mark‐recapture data for demographic comparisons. Prey availability dictated both adult survival and age at first reproduction. The bat species facing a more abundant and predictable food supply early in the season started its reproductive life earlier and showed a lower adult survival probability than the species subjected to more limited and less predictable food supply, while lifetime reproductive success was comparable in both species. The observed life‐history trade‐off indicates that temporal patterns in food availability can drive evolutionary divergence in life‐history strategies among sympatric sibling species.     

Predicting optimal transmission investment in malaria parasites

In vertebrate hosts, malaria parasites face a tradeoff between replicating and the production of transmission stages that can be passed onto mosquitoes. This tradeoff is analogous to growth‐reproduction tradeoffs in multicellular organisms. We use a mathematical model tailored to the life cycle and dynamics of malaria parasites to identify allocation strategies that maximize cumulative transmission potential to mosquitoes. We show that plastic strategies can substantially outperform fixed allocation because parasites can achieve greater fitness by investing in proliferation early and delaying the production of transmission stages. Parasites should further benefit from restraining transmission investment later in infection, because such a strategy can help maintain parasite numbers in the face of resource depletion. Early allocation decisions are predicted to have the greatest impact on parasite fitness. If the immune response saturates as parasite numbers increase, parasites should benefit from even longer delays prior to transmission investment. The presence of a competing strain selects for consistently lower levels of transmission investment and dramatically increased exploitation of the red blood cell resource. While we provide a detailed analysis of tradeoffs pertaining to malaria life history, our approach for identifying optimal plastic allocation strategies may be broadly applicable.     

Testing the reproductive and somatic trade‐off in female Columbian ground squirrels

Energetic trade‐offs in resource allocation form the basis of life‐history theory, which predicts that reproductive allocation in a given season should negatively affect future reproduction or individual survival. We examined how allocation of resources differed between successful and unsuccessful breeding female Columbian ground squirrels to discern any effects of resource allocation on reproductive and somatic efforts. We compared the survival rates, subsequent reprodction, and mass gain of successful breeders (females that successfully weaned young) and unsuccessful breeders (females that failed to give birth or wean young) and investigated “carryover” effects to the next year. Starting capital was an important factor influencing whether successful reproduction was initiated or not, as females with the lowest spring emergence masses did not give birth to a litter in that year. Females that were successful and unsuccessful at breeding in one year, however, were equally likely to be successful breeders in the next year and at very similar litter sizes. Although successful and unsuccessful breeding females showed no difference in over winter survival, females that failed to wean a litter gained additional mass during the season when they failed. The next year, those females had increased energy “capital” in the spring, leading to larger litter sizes. Columbian ground squirrels appear to act as income breeders that also rely on stored capital to increase their propensity for future reproduction. Failed breeders in one year “prepare” for future reproduction by accumulating additional mass, which is “carried over” to the subsequent reproductive season.     

Stress promotes changes in resource allocation to growth and reproduction in a simultaneous hermaphrodite with indeterminate growth

In iteroparous animals, investment in growth is compromised by investment in reproduction, especially in species with indeterminate growth. Life‐history theory predicts that growth should be favoured over reproduction, assuming size‐related fecundity or survival. Hence, increase body condition represents an increase in reproductive potential. Simultaneous hermaphrodites should adjust their resource allocation to each sex function in response to current conditions but, recently, it has been suggested that, in hermaphrodites, gender allocation should be considered as a three‐way trade‐off, including the investment in somatic growth. Due to the higher costs involved, the female function is affected to a greater extent by environmentally stressful conditions rather than the male function. To examine this, we induced stress in the hermaphroditic earthworm Eisenia fetida (Savigny, 1826) and looked for changes in resource allocation in nonreproductive and reproductive individuals. Experimental stress was induced by using tweezers to elicit contractile escape movements. We predicted that stressed earthworms would preferentially allocate resources to growth. In nonreproductive individuals, however, stress had a negative effect on growth, although weight recovery was rapid once manipulation ceased, indicating the importance of body condition, as well as the existence of mechanisms of compensatory growth for growth trajectories in this earthworm species. The response of reproductive individuals was consistent with our expectation: (1) stressed worms maintained their growth rate at the expense of current reproduction and (2) stressed earthworms laid 25% fewer cocoons, which were 30% lighter than cocoons laid by control earthworms. The present results suggest that E. fetida regulates its reproductive effort and that future reproduction has more impact on its fitness than current reproduction. The trade‐off between current and future reproduction should be taken into consideration in models of sex allocation in simultaneous hermaphrodites. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 91 , 593–600.