Investment In Current Reproduction Research Articles

Unravelling the relationships between life history, behaviour and condition under the pace-of-life syndromes hypothesis using long-term data from a wild bird

The hypothesis of pace-of-life syndromes (POLS) predicts relationships between traits including life history traits and risk-taking behaviour that can be mediated by the trade-off between current and future reproductive value. However, alternative causal mechanisms may also generate covariance among these traits without trade-offs. We investigated the relationships between survival to the next year, current reproductive investment and risk-taking behaviour (flight initiation distance) in male collared flycatchers, Ficedula albicollis, using long-term data. We used structural equation modelling (SEM) to uncover whether the associations among traits are mediated by a common latent factor that determines how individuals balance the trade-off between current and future reproductive value. As trade-offs could be concealed when there are differences in resource acquisition between individuals, we also included potential causes of these differences, body mass and body size, in the analysis. We found that risk-taking behaviour was positively related to reproductive investment and negatively to survival to the next year as could be predicted if investment into a risky behaviour is traded against future prospects. However, the most supported SEM model also suggested that survival to the next year was positively related to current reproductive investment, contrary to predictions of a hypothesis based on trade-off. These results remained qualitatively similar when controlling for body condition. In conclusion, we only could derive partial support for the POLS hypothesis. We suggest that aspects of individual quality, and not only trade-offs, should also be considered when interpreting the relationships between life history and behavioural traits. We investigated the association between two life history components (survival to the next year and current reproductive effort) and risk-taking behaviour, relying on long-term records from a passerine bird, to investigate the predictions of the pace-of-life syndrome (POLS) hypothesis. Using structural equation modelling, we found support for a causal model that implies that risk-taking negatively affects survival to the next year and that survival to the next year and current reproductive effort are strongly and positively associated. Controlling for the effect of body condition did not fundamentally change these relationships. We could not find conclusive evidence for the investigated traits being mediated by a common underlying factor, as generally predicted by the POLS hypothesis. However, the sign of the relationship between risk-taking behaviour and survival to the next year was as predicted by the POLS hypothesis.

Age-dependent variation in the terminal investment threshold in male crickets.

The terminal investment hypothesis proposes that decreased expectation of future reproduction (e.g., arising from a threat to survival) should precipitate increased investment in current reproduction. The level at which a cue of decreased survival is sufficient to trigger terminal investment (i.e., the terminal investment threshold) may vary according to other factors that influence expectation for future reproduction. We test whether the terminal investment threshold varies with age in male crickets, using heat-killed bacteria to simulate an immune-inducing infection. We measured calling effort (a behavior essential for mating) and hemolymph antimicrobial activity in young and old males across a gradient of increasing infection cue intensity. There was a significant interaction between the infection cue and age in their effect on calling effort, confirming the existence of a dynamic terminal investment threshold: young males reduced effort at all infection levels, whereas old males increased effort at the highest levels relative to naïve individuals. A lack of a corresponding decrease in antibacterial activity suggests that altered reproductive effort is not traded against investment in this component of immunity. Collectively, these results support the existence of a dynamic terminal investment threshold, perhaps accounting for some of the conflicting evidence in support of terminal investment.

No short-term physiological costs of offspring care in a cooperatively breeding bird.

The cost of reproduction results in a life-history trade-off where investment in current reproduction via costly parental care decreases subsequent fitness. Although this trade-off is thought to occur ubiquitously across animals, there is equivocal evidence that parental care behaviours are costly. A major challenge of studying the cost of parental care has been a lack of consensus over which physiological mechanisms underlie this trade-off. Here, we compare four traits believed to mediate the cost of parental care by examining whether glucocorticoids, oxidative stress, immune function or body condition represent a cost of performing offspring care and shape subsequent fitness. We use a 4year dataset collected in free-living cooperatively breeding superb starlings (Lamprotornis superbus), a species in which parental and alloparental care effort varies widely among individuals and across years. Our results showed that within-individual change in physiology was unrelated to investment in offspring care, and physiological state during chick rearing did not predict the likelihood that an individual would breed in subsequent seasons. Instead, individuals that had elevated baseline corticosterone during incubation performed more nest guarding, suggesting that this hormone may play a preparatory role for investing in offspring care. Together, our results indicate that superb starlings modify their investment in offspring care according to their physiological state during incubation, despite there being no evidence of a short-term physiological cost of parental or alloparental care. Thus, breeding cooperatively appears to provide individuals with the flexibility to adjust their investment in offspring care and overcome any potential costs of reproduction.

Hoopoe males experience intra-seasonal while females experience inter-seasonal reproductive costs.

Reproductive and survival costs due to reproductive investment are a central element for the evolution of life histories. Both intra- (reduction of reproductive performance of second brood due to investment in first brood) and inter-seasonal costs (reduction of reproductive performance or annual survival due to reproductive investment in preceding year) may appear in multiple breeding species. Knowledge about how trade-offs within and between seasons shape individual trajectories and influence fitness are crucial in life-history evolution, yet intra- and inter-seasonal reproductive costs are rarely analysed simultaneously. We investigated sex-specific differences in intra- and inter-seasonal reproductive and survival costs in response to previous reproductive effort in a monogamous, double-brooding bird, the hoopoe (Upupa epops), accounting for heterogeneity in individual and annual quality. Intra-seasonal reproductive costs were detected in males and inter-seasonal reproductive and survival costs were detected in females. In males, the probability of being a successful double breeder was negatively correlated with the number of hatchlings produced in the first brood. In females, the number of fledglings raised in the first brood was negatively correlated with the reproductive effort in the preceding season. Female annual survival was also negatively influenced by the number of broods produced in the previous reproductive season. Most of these reproductive costs were detected only in years with low productivity, suggesting that costs become evident when environmental conditions are harsh. Our results illustrate how different investment in current vs. future reproduction and survival shape different life-history strategies in males and females of a monogamous bird species.

A dynamic threshold model for terminal investment.

Although reproductive strategies can be influenced by a variety of intrinsic and extrinsic factors, life history theory provides a rigorous framework for explaining variation in reproductive effort. The terminal investment hypothesis proposes that a decreased expectation of future reproduction (as might arise from a mortality threat) should precipitate increased investment in current reproduction. Terminal investment has been widely studied, and a variety of intrinsic and extrinsic cues that elicit such a response have been identified across an array of taxa. Although terminal investment is often treated as a static strategy, the level at which a cue of decreased future reproduction is sufficient to trigger increased current reproductive effort (i.e., the terminal investment threshold) may depend on context, including the internal state of the organism or its current external environment, independent of the cue that triggers a shift in reproductive investment. Here, we review empirical studies that address the terminal investment hypothesis, exploring both the intrinsic and extrinsic factors that mediate its expression. Based on these studies, we propose a novel framework within which to view the strategy of terminal investment, incorporating factors that influence an individual's residual reproductive value beyond a terminal investment trigger - the dynamic terminal investment threshold.

Paternal investment with an uncertain future: effects of predator exposure on filial cannibalism and nesting behaviour

Owing to trade-offs between investment in current and future reproduction, factors that diminish a parent's survival prospects, such as predation threat, are expected to increase investment in existing young. Nevertheless, effects of predation risk on parental investment have only rarely been examined, and not at all within the context of filial cannibalism (parental consumption of their own offspring). We examined filial cannibalism and nest characteristics in a small fish with paternal egg care, the sand goby, Pomatoschistus minutus, both when exposed to a common piscivore, the perch, Perca fluviatilis, and in the absence of predators. We found that when males consumed only some of their eggs (partial filial cannibalism), the number of eaten eggs did not depend on predation threat, possibly indicating that partial clutch consumption is largely motivated by benefits to existing young. Total filial cannibalism (whole clutch consumption) was marginally less common under predator exposure, while its strongest predictor was small clutch size. This suggests that the return on parental investment has a greater influence on total filial cannibalism than the likelihood of future breeding. Regarding nest architecture, males that consumed their entire brood after exposure to a predator built larger nest entrances, possibly to facilitate predator evasion. Males that cared for at least part of their brood, however, maintained small nest entrances regardless of predation threat. Furthermore, more elaborate nests were not associated with greater egg consumption, suggesting that filial cannibalism is not employed to sustain nest building.

Butterflies Do Not Alter Conspecific Avoidance in Response to Variation in Density.

High conspecific densities are associated with increased levels of intraspecific competition and a variety of negative effects on performance. However, changes in life history strategy could compensate for some of these effects. For instance, females in crowded conditions often have fewer total offspring, but they may invest more in each one. Such investment could include the production of larger offspring, more time spent engaging in parental care, or more choosy decisions about where offspring are placed. For animals that have a relatively immobile juvenile stage, the costs of competition can be particularly high. Females may be able to avoid such costs by investing more in individual reproductive decisions, rearing young or laying eggs in locations away from other females. We tested the hypothesis that conspecific density cues during juvenile and adult life stages lead to changes in life history strategy, including both reproduction and oviposition choices. We predicted that high-density cues during the larval and adult stages of female Pieris rapae butterflies lead to lower fecundity but higher conspecific avoidance during oviposition, compared to similar low-density cues. We used a 2×2 factorial design to examine the effects of low and high conspecific density during the larval and adult stages of butterflies on avoidance behavior and fecundity. We found that past information about conspecific density did not matter; all butterflies exhibited similar levels of fecundity and a low level of conspecific avoidance during oviposition regardless of their previous experience as larvae and adults. These results suggest that P. rapae females use a fixed, rather than flexible, conspecific avoidance strategy when making oviposition decisions, and past information about conspecific density has no effect on life history and current reproductive investment. We speculate that this may be partially because past conspecific density per se is not a reliable cue for predicting current density and levels of competition, and thus it does not affect the development of life history strategies in this system.

Age, timing, and a variable environment affect double brooding of a long-lived seabird

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 564:187-197 (2017) - DOI: https://doi.org/10.3354/meps11988 Age, timing, and a variable environment affect double brooding of a long-lived seabird Michael E. Johns1,*, Pete Warzybok2, Russell W. Bradley2, Jaime Jahncke2, Mark Lindberg1, Greg A. Breed1 1Department of Biology and Wildlife and Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA 2Point Blue Conservation Science, 3820 Cypress Drive, Suite 11, Petaluma, CA 94954, USA *Corresponding author: mejohns3@alaska.edu ABSTRACT: Differences in reproductive output for marine vertebrates are driven by a combination of environmental, physiological, and behavioral cues. Individuals use these cues when balancing the tradeoff between current reproductive investment and future survival, particularly when resources vary on spatial and temporal scales. A common strategy for maximizing fecundity in birds is to produce 2 broods in a single season, a behavior known as double brooding. Cassin’s auklets Ptychoramphus aleuticus are among the relatively few seabirds that use this strategy; however, the proportion of breeding pairs that attempt double brooding is highly variable among years. We investigated the source of this variation using long-term monitoring data from Southeast Farallon Island off central California, USA. Double-brooding rates ranged from 0 to 90% over a 26 yr period, with an overall rate of 32% (95% CI ± 4.16%). Parameter estimates from generalized linear mixed models indicate that older females, earlier breeding initiation dates, stronger upwelling, and an interaction between age and upwelling strength increased the likelihood of double brooding in this population. Using a within-subject centering technique, the effects of age included both a within- and between-individual improvement in the likelihood of double brooding with age. Our findings indicate that females of higher quality and reproductive experience drive double brooding in this population and that these individuals are especially able to adopt a more flexible breeding regime in years characterized by high marine productivity. KEY WORDS: Age-based demography · California Current · Cassin’s auklet · Reproductive strategy · Seabird · Upwelling · Double brooding · Variable environment Full text in pdf format PreviousNextCite this article as: Johns ME, Warzybok P, Bradley RW, Jahncke J, Lindberg M, Breed GA (2017) Age, timing, and a variable environment affect double brooding of a long-lived seabird. Mar Ecol Prog Ser 564:187-197. https://doi.org/10.3354/meps11988 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 564. Online publication date: February 03, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Is hissing behaviour of incubating great tits related to reproductive investment in the wild?

Anti-predator behaviour of breeding animals is a complex trait that depends on current reproductive investment as well as individual differences in risk-taking propensities. In response to nest predation, many bird species produce specific sounds, such as the hissing calls in incubating great tits (Parus major), that may provoke an acoustic startle response in the predator. However, it is still unclear whether the propensity of incubating females to produce hissing calls towards nest predators depends on the reproductive investment. With our 3-year study, we show that response type (females that do not hiss versus females giving hissing calls) to a potential nest predator, the woodpecker, is a repeatable trait. We found no differences in the studied reproductive traits between hissing and non-hissing birds. Interestingly, among hissing birds, fast-responding females started egg-laying earlier than slow-responding ones. Among non-hissing birds, heavier birds initiated clutches earlier. We also revealed that hissing birds breed in areas with decreased nest-box occupancy, suggesting either that they potentially select different areas to breed or that territory size is larger as a result of hissing birds being more aggressive. These findings demonstrate that response type is not related to the early reproductive value of the brood across distinct behavioural groups. However, our results do suggest that non-hissing and hissing females differ in terms of individual quality or dominance or personality related aspects.

Trade‐offs among locomotor performance, reproduction and immunity in lizards

Summary Life‐history theory predicts that investment of acquired energetic resources to a particular trait denies those same resources from being allocated to a different trait, resulting in life‐history trade‐offs. Dynamic, whole‐organism performance traits, including locomotor capacity, are key to fitness and fit within this framework. Such performance traits are typically energetically expensive, but are seldom integrated into life‐history studies. We manipulated diet and allocation of resources to performance, via exercise training, to examine trade‐offs among endurance capacity, growth, immune function and current reproductive investment. Captive green anole lizards were assigned to one of four treatment combinations across two factors (diet restricted or not and endurance trained or not) over the course of 9 weeks. Our results show that both diet restriction and training dramatically suppressed reproduction and immune function, but there were opposing effects of diet restriction and training on growth. Elevated corticosterone from training was associated with suppression of immunity, and decreased fat stores from diet restriction were associated with suppressed reproduction in both sexes. Our results suggest that locomotor performance is an important part of energy allocation decisions and thus a key component of life‐history trade‐offs.

Terminal investment in the gustatory appeal of nuptial food gifts in crickets.

Investment in current versus future reproduction represents a prominent trade-off in life-history theory and is likely dependent on an individual's life expectancy. The terminal investment hypothesis posits that a reduction in residual reproductive value (i.e. potential for future offspring) will result in increased investment in current reproduction. We tested the hypothesis that male decorated crickets (Gryllodes sigillatus), when cued to their impending mortality, should increase their reproductive effort by altering the composition of their nuptial food gifts (i.e. spermatophylaxes) to increase their gustatory appeal to females. Using a repeated-measures design, we analysed the amino acid composition of spermatophylaxes derived from males both before and after injection of either a saline control or a solution of heat-killed bacteria. The latter, although nonpathogenic, represents an immune challenge that may signal an impending survival threat. One principal component explaining amino acid variation in spermatophylaxes, characterized by a high loading to histidine, was significantly lower in immune-challenged versus control males. The relevance of this difference for the gustatory appeal of gifts to females was assessed by mapping spermatophylax composition onto a fitness surface derived in an earlier study identifying the amino acid composition of spermatophylaxes preferred by females. We found that immune-challenged males maintained the level of attractiveness of their gifts post-treatment, whereas control males produced significantly less attractive gifts post-injection. These results are consistent with the hypothesis that cues of a survival-threatening infection stimulate terminal investment in male decorated crickets with respect to the gustatory appeal of their nuptial food gifts.

Male Snakes Allocate Time and Energy according to Individual Energetic Status: Body Condition, Steroid Hormones, and Reproductive Behavior in Timber Rattlesnakes, Crotalus horridus.

Life-history theory predicts that organisms will hedge current reproductive investment against potential costs in terms of survivorship and future fecundity. However, little is known regarding the endocrine mechanisms underlying bet-hedging strategies in free-ranging male vertebrates. We examined the relationships among individual energetic status, steroid hormones, mate search, and reproductive behavior in free-ranging male timber rattlesnakes. Snakes were monitored over four active seasons in order to test two hypotheses: (1) males adjust the amount of time and energy allocated toward reproduction according to the level of individual energy stores, and (2) observed condition-dependent reproductive allocation is associated with circulating concentrations of steroid hormones (testosterone and corticosterone) thought to regulate reproductive behaviors in vertebrates. A positive relationship between body condition and testosterone was observed in both the field and the laboratory. Male mate search effort was positively correlated with both body condition and testosterone. Body condition and testosterone concentrations were negatively related to time allocated toward foraging during the breeding season. A strong effect of year was observed in the analysis of testosterone and search effort, suggesting that multiple environmental factors impact hormone production and reproductive investment. Corticosterone was not related to any measured variable. Therefore, our results did not indicate a clear role of corticosterone in mediating observed relationships between energetic status and behavior. Observed relationships are consistent with the hypothesis that males allocate time and energy toward reproduction according to individual energetic status and that testosterone plays a role in mediating the trade-off between current reproductive investment and residual reproductive value.

Reproduction is not costly in terms of oxidative stress.

One of the core assumptions of life-history theory is the negative trade-off between current and future reproduction. Investment in current reproduction is expected to decrease future reproductive success or survival, but the physiological mechanisms underlying these costs are still obscure. To test for a role of oxidative stress, we measured oxidative damage to lipids and proteins in liver, heart, kidneys and muscles, as well as the level of antioxidants (total glutathione and catalase), in breeding and non-breeding bank voles. We used females from lines selected for high aerobic metabolism and non-selected control lines and manipulated their reproductive investment by decreasing or increasing litter size. Unlike in most previous studies, the females reared four consecutive litters (the maximum possible during a breeding season). Contrary to predictions, oxidative damage in reproducing females was decreased or not changed, and did not differ between the selected and control lines. Oxidative damage to lipids and proteins in the liver was lower in females that weaned enlarged litters than in non-breeding ones, and was intermediate in those with reduced litters. Oxidative damage to proteins in the heart also tended to be lower in breeding females than in non-breeding ones. A negative relationship between the level of oxidative damage and activity of catalase in kidneys indicated a protective action of antioxidants. In conclusion, our study falsified the hypothesis that oxidative stress is a part of the proximate physiological mechanism underlying the fundamental life-history trade-off between current and future reproduction.

Does predation risk affect mating behavior? An experimental test in dumpling squid (Euprymna tasmanica).

IntroductionOne of the most important trade-offs for many animals is that between survival and reproduction. This is particularly apparent when mating increases the risk of predation, either by increasing conspicuousness, reducing mobility or inhibiting an individual's ability to detect predators. Individuals may mitigate the risk of predation by altering their reproductive behavior (e.g. increasing anti-predator responses to reduce conspicuousness). The degree to which individuals modulate their reproductive behavior in relation to predation risk is difficult to predict because both the optimal investment in current and future reproduction (due to life-history strategies) and level of predation risk may differ between the sexes and among species. Here, we investigate the effect of increased predation risk on the reproductive behavior of dumpling squid (Euprymna tasmanica).ResultsFemales, but not males, showed a substantial increase in the number of inks (an anti-predator behavior) before mating commenced in the presence of a predator (sand flathead Platycephalus bassensis). However, predation risk did not affect copulation duration, the likelihood of mating, female anti-predator behavior during or after mating or male anti-predator behavior at any time.ConclusionsInking is a common anti-predator defense in cephalopods, thought to act like a smokescreen, decoy or distraction. Female dumpling squid are probably using this form of defense in response to the increase in predation risk prior to mating. Conversely, males were undeterred by the increase in predation risk. A lack of change in these variables may occur if the benefit of completing mating outweighs the risk of predation. Prioritizing current reproduction, even under predation risk, can occur when the chance of future reproduction is low, there is substantial energetic investment into mating, or the potential fitness payoffs of mating are high.

To breed or not: a novel approach to estimate breeding propensity and potential trade‐offs in an Arctic‐nesting species

Breeding propensity, i.e., the probability that a mature female attempts to breed in a given year, is a critical demographic parameter in long‐lived species. Life‐history theory predicts that this trait should be affected by reproductive trade‐offs so that the probability of future reproduction should depend on the current reproductive investment. However, breeding propensity is one of the most difficult parameters to estimate because nonbreeders are often absent from the breeding area, thereby requiring the inclusion of unobservable states in the analysis. We developed a new methodological approach by integrating a robust design sampling scheme within the multi‐event capture–recapture framework. Our new model accounted for uncertainty in state assignation while allowing for departure of individuals between secondary sampling occasions. We applied this model to a long‐term data set of female Greater Snow Geese (Chen caerulescens atlantica) to estimate breeding propensity and to investigate potential reproductive costs. We combined resightings during the nesting stage and recapture at the end of the breeding season to estimate breeding propensity and nesting success, and added recoveries to improve survival probability estimates. We found that both breeding propensity and nesting success depended upon breeding status in the previous year, though not survival. Successful breeders had a lower breeding propensity than failed breeders in the following year, but a higher nesting success. Individuals absent from the breeding colony had a low breeding propensity, but a high nesting success the following year. Our results suggest a cost of reproduction on breeding propensity in the next year, but once females decide to breed, nesting success is likely driven by individual quality. An added benefit of our model is that, unlike previous models with unobservable states, all parameters were identifiable when survival and breeding probabilities were fully state dependent. Our new multi‐event framework is a flexible tool that can be applied to a large range of species to estimate breeding propensity and to investigate reproductive trade‐offs.

Context‐dependent stress responses and their connections to fitness in a landscape of fear

AbstractThe acute glucocorticoid stress response is presumed to facilitate escape from life‐threatening situations such as predation and thus it is assumed to be linked to fitness. However, the fitness effects of glucocorticoid reactivity remain controversial, as these effects may be context‐dependent. Individuals differing in their emphasis on current versus future reproduction may differ in their risk‐taking under threat of predation; this variation in risk‐taking may be mediated by variations in stress reactivity. We set out to test whether predation risk (island‐ and year‐specific proportion of depredated nests) modified the relationships between stress responsiveness and current reproductive investment (clutch weight) and between stress responsiveness and reproductive success (viable proportion of the clutch) in the long‐lived female eider Somateria mollissima. This study system shows large spatial and annual fluctuations in predation risk, indexed by the annual island‐specific proportion of depredated nests. The capture stress‐related corticosterone output was attenuated with increasing clutch weight under low predation pressure but elevated under severe predation pressure, and females in well‐concealed nests had lower stress responsiveness. The viable proportion of the clutch decreased with increasing corticosterone reactivity under low to moderate predation pressure, but slightly increased under severe predation pressure. The acute stress response may thus mediate adaptive plasticity; dampened stress reactivity may ensure successful reproduction under low predation threat or in nest sites reducing detection by visual predators, whereas preparing for potential attacks may be favoured under elevated predation risk.

Litter size manipulation in laboratory mice: an example of how proteomic analysis can uncover new mechanisms underlying the cost of reproduction.

BackgroundLife history theories predict that investment in current reproduction comes at a cost for future reproduction and survival. Oxidative stress is one of the best documented mechanisms underlying costs of reproduction to date. However, other, yet to be described molecular mechanisms that play a short term role during reproduction may explain the negative relationships underlying the cost of reproduction. To identify such new mechanisms, we used a global proteomic determination of liver protein profiles in laboratory adult female mice whose litter size had been either reduced or enlarged after birth. This litter size manipulation was expected to affect females by either raising or decreasing their current reproductive effort. At the same time, global parameters and levels of oxidative stress were also measured in all females.ResultsBased on plasma analyses, females with enlarged litters exhibited increased levels of oxidative stress at the date of weaning compared to females with reduced litters, while no significant difference was found between both the latter groups and control females. None of the liver proteins related to oxidative balance were significantly affected by the experimental treatment. In contrast, the liver protein profiles of females with enlarged and reduced litters suggested that calcium metabolism and cell growth regulation were negatively affected by changes in the number of pup reared.ConclusionsPlasma oxidative stress levels in reproductive mice revealed that the degree of investment in reproduction can actually incur a cost in terms of plasmatic oxidative stress, their initial investment in reproduction being close to maximum and remaining at a same level when the energy demand of lactation is reduced. Liver proteomic profiles in reproductive females show that hepatic oxidative stress is unlikely to be involved in the cost of reproduction. Reproductive females rather exhibited liver protein profiles similar to those previously described in laboratory ageing mice, thus suggesting that hepatic cell pro-ageing processes may be involved in the cost of reproduction. Overall, our data illustrate how a proteomic approach can unravel new mechanisms sustaining life-history trade-offs, and reproduction costs in particular.

Evidence for baseline glucocorticoids as mediators of reproductive investment in a wild bird

Determining the mechanisms that mediate investment decisions between current and future reproductive attempts is still a key goal of life-history studies. Since baseline levels of stress hormones (glucocorticoids – GCs) act as predictive and labile regulators of daily energetic balance in vertebrates they remain excellent candidates for mediating investment decisions both within and across reproductive attempts. Using free-living female European starlings (Sturnus vulgaris) we experimentally reduced investment in current reproduction (number of offspring raised in the first brood) to examine whether baseline corticosterone (CORT) acted as a hormonal mediator preparing individuals for a predictable increase in future investment (number of offspring raised in the second brood). Although treatment and control birds raised the same total amount of offspring across two broods, the experimental birds increased reproductive investment in second broods to compensate for the reduced investment in the first brood. Data on both mean and intra-individual changes in baseline CORT support the idea that an increase in baseline CORT between the incubation stages in treatment birds strongly predicted this increase in investment. Importantly, we measured the increase in baseline CORT during late incubation prior to the increase in energetic demand associated with increased reproductive investment in offspring, indicating that flexible within-individual changes in baseline GCs can act as a labile mechanism preparing individuals for predictable increases in reproductive investment. As such, our experimental results indicate that elevated baseline GCs can prepare individuals for investment in energetically expensive life-history stages, rather than simply being elevated as a consequence of increased effort or demand. This suggests that short-term preparative increases in baseline GCs benefit individuals by successfully allowing them to maximize fitness under varying environmental conditions.

Females better face senescence in the wandering albatross

Sex differences in lifespan and aging are widespread among animals. Since investment in current reproduction can have consequences on other life-history traits, the sex with the highest cost of breeding is expected to suffer from an earlier and/or stronger senescence. This has been demonstrated in polygynous species that are highly dimorphic. However in monogamous species where parental investment is similar between sexes, sex-specific differences in aging patterns of life-history traits are expected to be attenuated. Here, we examined sex and age influences on demographic traits in a very long-lived and sexually dimorphic monogamous species, the wandering albatross (Diomedea exulans). We modelled within the same model framework sex-dependent variations in aging for an array of five life-history traits: adult survival, probability of returning to the breeding colony, probability of breeding and two measures of breeding success (hatching and fledging). We show that life-history traits presented contrasted aging patterns according to sex whereas traits were all similar at young ages. Both sexes exhibited actuarial and reproductive senescence, but, as the decrease in breeding success remained similar for males and females, the survival and breeding probabilities of males were significantly more affected than females. We discuss our results in the light of the costs associated to reproduction, age-related pairing and a biased operational sex-ratio in the population leading to a pool of non-breeders of potentially lower quality and therefore more subject to death or breeding abstention. For a monogamous species with similar parental roles, the patterns observed were surprising and when placed in a gradient of observed age/sex-related variations in life-history traits, wandering albatrosses were intermediate between highly dimorphic polygynous and most monogamous species.

Physiological costs and carry-over effects of avian interspecific brood parasitism influence reproductive tradeoffs

Although models of co-evolution between brood parasites and their hosts primarily focus upon the cost to hosts in the current reproductive bout, the impact of brood parasitism may carry over to future reproductive attempts by altering resource allocation. Glucocorticoid stress hormones help mediate resource allocation to reproduction, yet they have rarely been examined in brood parasitic systems. Here we determined if shifts in parental care and corticosterone had carry-over effects on future reproductive effort in the rufous-and-white wren (Thryophilus rufalbus), a host of the Central American striped cuckoo (Tapera naevia). We found that parasitized parents had significantly higher stress-induced, but not baseline, corticosterone than natural parents during the fledgling stage, which was associated with changes in parental care. The high investment in current reproduction while parasitized may be due to the value of fledged chicks in tropical systems. This maladaptive response by parasitized parents was associated with delayed re-nesting and a reduced likelihood of nesting in the subsequent breeding season. Although a reduction in future reproductive effort can result from a combination of factors, this work suggests that fitness costs of brood parasitism are mediated by changes in corticosterone and parental care behavior that carry over into subsequent breeding seasons.