Natal legacies cause social and spatial marginalization during dispersal.

Early-life experiences can drive subsequent variation in social behaviours, but how differences among individuals emerge remains unknown. We combined experimental manipulations with GPS-tracking to investigate the pathways through which developmental conditions affect social network position during the early dispersal of wild red kites (Milvus milvus). Across 211 juveniles from 140 broods, last-hatched chicks-the least competitive-had the fewest number of peer encounters after fledging. However, when food supplemented, they had more encounters than all others. Using 4425 bird-days of GPS data, we revealed that this was driven by differential responses to competition, with less competitive individuals naturally spreading out into marginal areas, and clustering in central foraging areas when food supplemented. Our results suggest that early-life adversities can cause significant natal legacies on individual behaviour beyond independence, with potentially far-reaching consequences on the social and spatial structure of animal populations.

Winter Tracking Data Suggest that Migratory Seabirds Transport Per- and Polyfluoroalkyl Substances to Their Arctic Nesting Site.

Seabirds are often considered sentinel species of marine ecosystems, and their blood and eggs utilized to monitor local environmental contaminations. Most seabirds breeding in the Arctic are migratory and thus are exposed to geographically distinct sources of contamination throughout the year, including per- and polyfluoroalkyl substances (PFAS). Despite the abundance and high toxicity of PFAS, little is known about whether blood concentrations at breeding sites reliably reflect local contamination or exposure in distant wintering areas. We tested this by combining movement tracking data and PFAS analysis (nine compounds) from the blood of prelaying black-legged kittiwakes (Rissa tridactyla) nesting in Arctic Norway (Svalbard). PFAS burden before egg laying varied with the latitude of the wintering area and was negatively associated with time upon return of individuals at the Arctic nesting site. Kittiwakes (n = 64) wintering farther south carried lighter burdens of shorter-chain perfluoroalkyl carboxylates (PFCAs, C9-C12) and heavier burdens of longer chain PFCAs (C13-C14) and perfluorooctanesulfonic acid compared to those wintering farther north. Thus, blood concentrations prior to egg laying still reflected the uptake during the previous wintering stage, suggesting that migratory seabirds can act as biovectors of PFAS to Arctic nesting sites.

Spring arrival of the common cuckoo at breeding grounds is strongly determined by environmental conditions in tropical Africa.

Failure to adapt migration timing to changes in environmental conditions along migration routes and at breeding locations can result in mismatches across trophic levels, as occurs between the brood parasitic common cuckoo Cuculus canorus and its hosts. Using satellite tracking data from 87 male cuckoos across 11 years, we evaluate why the cuckoo has not advanced its arrival to the UK. Across years, breeding ground arrival was primarily determined by timing of departure from stopover in West Africa before northward crossing of the Sahara. Together with high population synchrony and low apparent endogenous control of this event, this suggests that a seasonal ecological constraint operating here limits overall variation in breeding grounds arrival, although this event was itself influenced by carry-over from timing of arrival into tropical Africa. Between-year variation within individuals was, in contrast, mostly determined by northward migration through Europe, probably due to weather conditions. We find evidence of increased mortality risk for (a) early birds following migration periods positively impacting breeding grounds arrival, and (b) late birds, possibly suffering energy limitation, after departure from the breeding grounds. These results help identify areas where demands of responding to global change can potentially be alleviated by improving stopover quality.

Distance doesn't matter: migration strategy in a seabird has no effect on survival or reproduction.

Migrating animals show remarkable diversity in migration strategies, even between individuals from the same population. Migrating longer distances is usually expected to be costlier in terms of time, energy expenditure and risks with potential repercussions for subsequent stages within the annual cycle. Such costs are expected to be balanced by increased survival, for example due to higher quality wintering areas or lower energy expenditure at lower latitudes. We compared reproductive parameters and apparent survival of lesser black-backed gulls (Larus fuscus) breeding in The Netherlands, whose winter range extends from the UK to West Africa, resulting in one-way migration distances that differ by more than 4500 km. Individuals migrating furthest arrived later in the colony than shorter distance migrants, but still laid in synchrony with the colony and consequently had a shorter pre-laying period. This shorter pre-laying period affected neither egg volumes nor hatching success. We found no relationship between migration distance and apparent survival probability, corresponding with previous research showing that annual energy expenditure and distance travelled throughout the year is similar across migration strategies. Combined, our results indicate an equal fitness payoff across migration strategies, suggesting there is no strong selective pressure acting on migration strategy within this population.

Short- and long-term consequences of heat exposure on mitochondrial metabolism in zebra finches (Taeniopygia castanotis).

Understanding the consequences of heat exposure on mitochondrial function is crucial as mitochondria lie at the core of metabolic processes, also affecting population dynamics. In adults, mitochondrial metabolism varies with temperature but can also depend on thermal conditions experienced during development. We exposed zebra finches to two alternative heat treatments during early development: "constant", maintained birds at ambient 35 °C from parental pair formation to fledglings' independence, while "periodic" heated broods at 40 °C, 6 h daily at nestling stage. Two years later, we acclimated birds from both experiments at 25 °C for 21 days, before exposing them to artificial heat (40 °C, 5 h daily for 10 days). After both conditions, we measured red blood cells' mitochondrial metabolism using a high-resolution respirometer. We found significantly decreased mitochondrial metabolism for Routine, Oxidative Phosphorylation (OxPhos) and Electron Transport System maximum capacity (ETS) after the heat treatments. In addition, the birds exposed to "constant" heat in early life showed lower oxygen consumption at the Proton Leak (Leak) stage after the heat treatment as adults. Females showed higher mitochondrial respiration for Routine, ETS and Leak independent of the treatments, while this pattern was reversed for OxPhos coupling efficiency (OxCE). Our results show that short-term acclimation involved reduced mitochondrial respiration, and that the reaction of adult birds to heat depends on the intensity, pattern and duration of temperature conditions experienced at early-life stages. Our study provides insight into the complexity underlying variation in mitochondrial metabolism and raises questions on the adaptive value of long-lasting physiological adjustments triggered by the early-life thermal environment.

The real-time and carry-over effects of injustice on performance and service quality in a ridesharing driver scenario.

The nature of gig work and its growth have important implications for organizational justice theory. Aspects of gig work, including the transactional compensation arrangement, strict algorithmic rating system, and power asymmetry between drivers and customers, have implications for understanding how dimensions of distributive, informational, and interpersonal injustice manifest and impact job performance in the gig context. An understanding of this topic can inform justice theory more broadly and help explain inconsistent findings in the literature. Here, we report the results of two studies examining the unique effects of these respective dimensions of injustice on emotions and, ultimately, the driving performance and service quality in a ridesharing service context. In Study 1, we modeled the passenger-driver interaction of the ridesharing context using a driving simulator in a laboratory setting to differentiate the real-time and carry-over effects of specific dimensions of injustice. The results from 99 participants showed that perceptions of interpersonal injustice increased anger and unhappiness during the ride, in turn impairing driving and service performance. Antecedent-focused emotion regulation strategies (ERS) reduced felt unhappiness. Moreover, unexpectedly, perceived distributive injustice as caused by the customer rating had opposite (direct versus indirect) effects on service performance in the subsequent ride. Study 2 was an online simulation vignette scenario with 294 participants. The results replicated the findings of Study 1 and revealed two moderators of the unexpected distributive justice-performance relationship. Supplementary Information: The online version contains supplementary material available at 10.1007/s12144-022-04215-3.

Differences in the temporal scale of reproductive investment across the slow-fast continuum in a passerine.

Life-history strategies differ with respect to investment in current versus 'future' reproduction, but when is this future? Under the novel 'temporality in reproductive investment hypothesis', we postulate variation should exist in the time frame over which reproductive costs are paid. Slow-paced individuals should pay reproductive costs over short (e.g. inter-annual) time scales to prevent reproductive costs accumulating, whereas fast-paced individuals should allow costs to accumulate (i.e. senescence). Using Fourier transforms, we quantify adjustments in clutch size with age, across four populations of blue tits (Cyanistes caeruleus). Fast populations had more prevalent and stronger long-term changes in reproductive investment, whereas slower populations had more prevalent short-term adjustments. Inter-annual environmental variation partly accounted for short-, but not long-term changes in reproductive investment. Our study reveals individuals differ in when they pay the cost of reproduction and that failure to partition this variation across different temporal scales and environments could underestimate reproductive trade-offs.

The relative influence of cross-seasonal and local weather effects on the breeding success of a migratory songbird.

In seasonal environments, fluctuating early-season weather conditions and short breeding windows limit reproductive opportunities such that breeding earlier or later than the optimum may be particularly costly. Given the risk of early-season energy limitations, time- and energy-based carry-over effects stemming from environmental conditions across the annual cycle may have pronounced consequences for breeding phenology and fitness. Generally, when and where environmental conditions are most influential are poorly understood, limiting our ability to predict the future of climate-sensitive populations. For an alpine-breeding, migratory population of horned lark Eremophila alpestris in northern British Columbia, Canada (54.8°N), we assessed how weather conditions across the annual cycle influenced clutch initiation date and offspring development. We also addressed how cross-seasonal effects on breeding parameters combine to influence reproductive fitness. With 12 years of breeding data and 3 years of migration data, we used a sliding window approach to identify points during the annual cycle when weather events most influenced breeding phenology and offspring development. Consequences for breeding success were assessed using nest survival simulations. Average clutch initiation date varied up to 11 days among years but did not advance from 2003 to 2019. Warmer temperatures at stopover and breeding sites advanced clutch initiation, but winter conditions had no effect. Sub-zero stopover temperatures carried over to prolong offspring development independent of clutch initiation date, potentially indicating energy-based carry-over effects acting on parental investment. Nest survival decreased with both later clutch initiation and prolonged offspring development such that females nesting earlier and fledging offspring at a younger age were up to 45% more likely to reproduce successfully. We demonstrate that stronger carry-over effects originated from environmental conditions closer to the breeding site in time and space, as well as the potential for energy-based mechanisms to link pre-breeding conditions to reproductive fitness. We also highlight the importance of extended stopovers for songbirds breeding in seasonal environments, particularly given that climatic conditions are becoming increasingly decoupled across stages of the annual cycle. Understanding the cross-seasonal mechanisms shaping breeding decisions in stochastic environments allows for more accurate predictions of population-level responses to climate change.

Les variations saisonnières de l'environnement, notamment due aux conditions climatiques changeantes en début de saison ainsi que la réduction de la période propice à l'accouplement contraint fortement les possibilités de reproduction. Dans ces conditions, s'accoupler avant ou après le moment optimal peut s'avérer particulièrement coûteux. Les effets de report (temporels et énergétiques) causés par ces variations environnementales peuvent avoir des conséquences notables sur la phénologie de la reproduction, et ultimement sur la valeur sélective des individus. Où et quand les effets des conditions environnementales sont le plus critiques reste encore méconnu, limitant notre capacité à prédire le futur des populations sensibles aux variations climatiques. Ici, nous avons évalué comment des conditions climatiques au cours de l'année influencent la date de ponte et le développement des oisillons dans une population migratrice d'Alouette hausse-col Eremophila alpestris se reproduisant en milieux alpin. Nous tirons profit de données issues de 12 ans de suivit de la reproduction et de 3 ans de suivit migratoire, et utilisons une approche dite de 'sliding window' pour identifier les moments du cycle annuel pour lequel le climat a eu le plus d'influence sur la phénologie de la reproduction et le développement des oisillons. La date moyenne de ponte s'avère variable d'une année à l'autre (certaine différence allant jusqu'à 11 jours), mais ne se décale pas sur la période de 2003 à 2019. Nos résultats montrent que des conditions climatiques plus chaudes lors des haltes migratoires ainsi que sur les sites de reproductions rendent la date de ponte plus précoces. Des températures négatives lors des haltes migratoires aussi ont pour conséquences un temps de développement des oisillons plus long, et ceci indépendamment de la date de ponte. Cela suggère des effets de report, notamment énergétique, affectant l'investissement des parents. Nos résultats montrent que la survie au nid diminue lorsque la date de ponte est plus tardive ou que le temps de développement des oisillons est rallongé. De cette manière, les femelles commençants la nidification plus tôt et pour qui les oisillons quittent le nid plus tôt ont 45% plus de chance de se reproduire avec succès. Nous démontrons que des conditions environnementales proche du site de reproduction (que ce soit dans le temps ou dans l'espace) cause un fort effet de report, et suggérons un possible mécanisme reliant les conditions climatiques pré-reproductives au succès reproducteur. De plus, nous mettons en lumière l'importance des haltes migratoires prolongées pour la reproduction des passereaux en environnement saisonnier, particulièrement du fait que les conditions climatiques sont de plus en plus découplées au cours des les étapes du cycle annuel. Une meilleure compréhension des mécanismes inter-saisonniers influençant les décisions de reproduction en environnement stochastique permettrait de mieux prédire les réponses des populations aux changements climatiques.

Variable temperature regimes and wetland salinity reduce performance of juvenile wood frogs.

On a changing planet, amphibians must respond to weather events shifting in frequency and magnitude, and to how those temperature and precipitation changes interact with other anthropogenic disturbances that modify amphibian habitat. To understand how drastic changes in environmental conditions affect wood frog tadpoles, we tested five temperature manipulations, including Ambient (water temperatures tracking daily air temperatures), Elevated (+ 3 °C above ambient), Nightly (removal of nightly lows), Spike (+ 6 °C above ambient every third week), and Flux (alternating ambient and + 3 °C weekly) crossed with Low Salt (specific conductivity: 109-207 µS-cm) and High Salt (1900-2000 µS-cm). We replicated each of the ten resulting treatments four times. High-salinity conditions produced larger metamorphs than low-salinity conditions. Tadpole survival was reduced only by the Spike treatment (P = 0.017). Elevated temperatures did not shorten larval periods; time to metamorphosis did not differ among temperature treatments (P = 0.328). We retained 135 recently metamorphosed frogs in outdoor terrestrial enclosures for 10 months to investigate larval environment carryover effects. Juvenile frogs grew larger in low-density terrestrial enclosures than high density (P = 0.015) and frogs from Ambient Low Salt larval conditions grew and survived better than frogs from manipulated larval conditions. Frogs from High Salt larval conditions had lower survival than frogs from Low Salt conditions. Our results suggest that anthropogenic disturbances to larval environmental conditions can affect both larval and post-metamorphic individuals, with detrimental carryover effects of high-salinity larval conditions not emerging until the juvenile life stage.

Phenotypic plasticity as a cause and consequence of population dynamics.

Predicting complex species-environment interactions is crucial for guiding conservation and mitigation strategies in a dynamically changing world. Phenotypic plasticity is a mechanism of trait variation that determines how individuals and populations adapt to changing and novel environments. For individuals, the effects of phenotypic plasticity can be quantified by measuring environment-trait relationships, but it is often difficult to predict how phenotypic plasticity affects populations. The assumption that environment-trait relationships validated for individuals indicate how populations respond to environmental change is commonly made without sufficient justification. Here we derive a novel general mathematical framework linking trait variation due to phenotypic plasticity to population dynamics. Applying the framework to the classical example of Nicholson's blowflies, we show how seemingly sensible predictions made from environment-trait relationships do not generalise to population responses. As a consequence, trait-based analyses that do not incorporate population feedbacks risk mischaracterising the effect of environmental change on populations.

Birds of different morphs use slightly different strategies to achieve similar reproductive performance following heatwave exposure.

Responses to extreme climatic events may differ between individuals of distinct morphs which differ in life-history strategies, resulting in climate change 'winners' and 'losers' within species. We examined the reproductive performance and carry-over effects on offspring of black- and red-headed Gouldian finches Erythrura gouldiae after exposure to simulated heatwaves of moderate or severe intensity. We expected black-headed pairs' reproductive performance to decline after the severe heatwave because only the condition of black-headed females deteriorates during such a heatwave. Supporting the fact that Gouldian finches of different morphs use alternative reproductive strategies, we found that black-headed females initiated egg-laying a month earlier than red-headed females after experiencing a severe heatwave. We also found that this severe heatwave resulted in shorter spermatozoa in males irrespective of their morph. Despite these effects associated with heatwave intensity, the overall reproductive performance of both morphs was not affected by this factor, which was possibly due to an increased nestling provisioning rate by parents after exposure to the severe heatwave. However, offspring still bore the cost of parental exposure to the severe heatwave, as they showed a reduced condition (lower plasma antioxidant capacity and transient lower breathing rate) and higher oxidative damage (at least in fledglings with black-headed parents). These results suggest that inter-morph phenotypic variability in the Gouldian finch does not result in clear differences in reproductive performance following heatwave exposure, despite basal phenotypic differences between morphs. Whether animals using alternative reproductive strategies are, in the end, differently affected by climate changes will likely depend on the capacity of their offspring to recover from altered developmental conditions.

Larval habitats impose trait-dependent limits on the direction and rate of adult evolution in dragonflies.

Natural selection on juveniles is often invoked as a constraint on adult evolution, but it remains unclear when such restrictions will have their greatest impact. Selection on juveniles could, for example, mainly limit the evolution of adult traits that mostly develop prior to maturity. Alternatively, selection on juveniles might primarily constrain the evolution of adult traits that experience weak or context-dependent selection in the adult stage. Using a comparative study of dragonflies, I tested these hypotheses by examining how a species' larval habitat was related to the evolution of two adult traits that differ in development and exposure to selection: adult size and male ornamentation. Whereas adult size is fixed at metamorphosis and experiences consistent positive selection in the adult stage, ornaments develop throughout adulthood and provide context-dependent fitness benefits. My results show that species that develop in less stable larval habitats have smaller adult sizes and slower rates of adult size evolution. However, these risky larval habitats do not limit ornament expression or rates of ornament evolution. Selection on juveniles may therefore primarily affect the evolution of adult traits that mostly develop prior to maturity.

Costs of reproduction and migration are paid in later return to the colony, not in physical condition, in a long-lived seabird.

Life history theory suggests a trade-off between costly activities such as breeding and migration and somatic self-maintenance. However, how the short-term cost of parental effort is expressed in species with a slow pace-of-life is not well understood. Also, investigating carry-over effects of migration is most meaningful when comparing migratory strategies within the same population, but this has rarely been done. We explore this hypothesis in a long-lived, pelagic seabird, the Cory's Shearwater, Calonectris borealis, where males display partial migration. By manipulating reproductive effort and taking advantage of the natural variation in migratory strategy, we investigate whether early reproductive failure and migratory strategy had implications on the physical condition of males on return to the colony the following year. We experimentally induced breeding failure from mid-incubation, tracked the over-winter movements of these males and of males that invested in parental effort, and assessed innate immunity, stress, and residual body mass the following year. Early breeding failure resulted in earlier return to the colony among all males, associated with greater probability of reproductive success. Residents had a lower tail feather fault bar intensity, an indicator of stress during the non-breeding period, compared to migrants. Reproductive effort and migratory strategy had no impact on physiological condition otherwise. Our results provide evidence that in species with a slow-pace of life, such as the Cory's Shearwater, somatic maintenance is prioritised, with the costs of reproduction and migration paid in delayed arrival date.

Assessing seasonal demographic covariation to understand environmental-change impacts on a hibernating mammal.

Natural populations are exposed to seasonal variation in environmental factors that simultaneously affect several demographic rates (survival, development and reproduction). The resulting covariation in these rates determines population dynamics, but accounting for its numerous biotic and abiotic drivers is a significant challenge. Here, we use a factor-analytic approach to capture partially unobserved drivers of seasonal population dynamics. We use 40 years of individual-based demography from yellow-bellied marmots (Marmota flaviventer) to fit and project population models that account for seasonal demographic covariation using a latent variable. We show that this latent variable, by producing positive covariation among winter demographic rates, depicts a measure of environmental quality. Simultaneously, negative responses of winter survival and reproductive-status change to declining environmental quality result in a higher risk of population quasi-extinction, regardless of summer demography where recruitment takes place. We demonstrate how complex environmental processes can be summarized to understand population persistence in seasonal environments.

The jury is still out regarding the generality of adaptive 'transgenerational' effects.

A recent meta-analysis concluded, 'transgenerational effects are widespread, strong and persistent'. We identify biases in the literature search, data and analyses, questioning that conclusion. Re-analyses indicate few studies actually tested transgenerational effects - making it challenging to disentangle condition-transfer from anticipatory parental effects, and providing little insight into the underlying mechanisms.

Personality-specific carry-over effects on breeding.

Carry-over effects describe the phenomenon whereby an animal's previous conditions influence its subsequent performance. Carry-over effects are unlikely to affect individuals uniformly, but the factors modulating their strength are poorly known. Variation in the strength of carry-over effects may reflect individual differences in pace-of-life: slow-paced, shyly behaved individuals are thought to favour an allocation to self-maintenance over current reproduction, compared to their fast-paced, boldly behaved conspecifics (the pace-of-life syndrome hypothesis). Therefore, detectable carry-over effects on breeding should be weaker in bolder individuals, as they should maintain an allocation to reproduction irrespective of previous conditions, while shy individuals should experience stronger carry-over effects. We tested this prediction in black-legged kittiwakes breeding in Svalbard. Using miniature biologging devices, we measured non-breeding activity of kittiwakes and monitored their subsequent breeding performance. We report a number of negative carry-over effects of non-breeding activity on breeding, which were generally stronger in shyer individuals: more active winters were followed by later breeding phenology and poorer breeding performance in shy birds, but these effects were weaker or undetected in bolder individuals. Our study quantifies individual variability in the strength of carry-over effects on breeding and provides a mechanism explaining widespread differences in individual reproductive success.

Survival varies seasonally in a migratory bird: Linkages between breeding and non-breeding periods.

Migratory species form an important component of biodiversity; they link ecosystems across the globe, but are increasingly threatened by global environmental change. Understanding and mitigating threats requires knowledge of how demographic processes operate throughout the annual cycle, but this can be difficult to achieve when breeding and non-breeding grounds are widely separated. Our goal is to quantify the importance of variability in survival during the breeding and non-breeding seasons in determining variation in annual survival using a single population and, more broadly, the extent to which annual survival across species reflects variation in probability of surviving the migratory period. We use a 25-year dataset in which individuals of a long-distance migratory bird, the alpine swift Tachymarptis melba, were captured towards the beginning and end of each breeding season to estimate age- and season-specific survival probabilities and incorporate explicit estimation of the correlations in survival between age-classes and seasons. Monthly survival was higher during the breeding period than during the rest of the year and strongly affected by conditions in the breeding season; effects that remained apparent in the following non-breeding season, but not subsequently. Recruitment of juveniles was dependent on the timing of breeding, being higher if egg-laying commenced before the median date, and substantially lower if not. Across migratory bird species, variation in annual survival largely reflects variation in the probability of surviving the migratory period. Using a double-capture approach, even within a single season, provides valuable insights into the demography of migratory species, which will help understand the extent and impacts of the threats they face in a changing world.

Sea crossing as a major determinant for the evolution of migratory strategies in soaring birds.

IN FOCUS: Santos, C. D., Silva, J. P., Muñoz, A.-R., Onrubia, A., & Wikelski, M. (2020). The gateway to Africa: What determines sea crossing performance of a migratory soaring bird at the strait of Gibraltar? Journal of Animal Ecology, 89, 1317-1328. Migrating birds undertake long journeys which pose several challenges. Water bodies are the most demanding ecological barriers for soaring birds, due to the increase in energy consumption and mortality risk. Through high-resolution GPS, Santos et al. (2020), analysed how the flight performance of 73 black kites crossing the Strait of Gibraltar was affected by external (e.g. weather conditions) and internal factors (individual experience). Kites waited for weaker crosswinds to start the crossing to minimize energy consumption, drift and altitude loss. Moreover, adults were quicker and lost less altitude than juveniles. These processes are likely to occur in all soaring species and have consequences also at a much wider spatial scale. In the Mediterranean region, species- and population-specific migration strategies appear to be influenced by interactions between species' morphology and the distribution of the land masses they traverse.

Experimentally increased brood size accelerates actuarial senescence and increases subsequent reproductive effort in a wild bird population.

The assumption that reproductive effort decreases somatic state, accelerating ageing, is central to our understanding of life-history variation. Maximal reproductive effort early in life is predicted to be maladaptive by accelerating ageing disproportionally, decreasing fitness. Optimality theory predicts that reproductive effort is restrained early in life to balance the fitness contribution of reproduction against the survival cost induced by the reproductive effort. When adaptive, the level of reproductive restraint is predicted to be inversely linked to the remaining life expectancy, potentially resulting in a terminal effort in the last period of reproduction. Experimental tests of the reproductive restraint hypothesis require manipulation of somatic state and subsequent investigation of reproductive effort and residual life span. To our knowledge the available evidence remains inconclusive, and hence reproductive restraint remains to be demonstrated. We modulated somatic state through a lifelong brood size manipulation in wild jackdaws and measured its consequences for age-dependent mortality and reproductive success. The assumption that lifelong increased brood size reduced somatic state was supported: Birds rearing enlarged broods showed subsequent increased rate of actuarial senescence, resulting in reduced residual life span. The treatment induced a reproductive response in later seasons: Egg volume and nestling survival were higher in subsequent seasons in the increased versus reduced broods' treatment group. We detected these increases in egg volume and nestling survival despite the expectation that in the absence of a change in reproductive effort, the reduced somatic state indicated by the increased mortality rate would result in lower reproductive output. This leads us to conclude that the higher reproductive success we observed was the result of higher reproductive effort. Our findings show that reproductive effort negatively covaries with remaining life expectancy, supporting optimality theory and confirming reproductive restraint as a key factor underpinning life-history variation.

Metabolic costs of altered growth trajectories across life transitions in amphibians.

Climate change is causing increases in temperature and in the frequency of extreme weather events. Under this scenario, organisms should maintain or develop strategies to cope with environmental fluctuations, such as the capacity to modify growth trajectories. However, altering growth can have negative consequences for organisms' fitness. Here, we investigated the metabolic alterations induced by compensatory growth during the larval development of the common frog (Rana temporaria), quantifying changes in oxidative stress, corticosterone levels and telomere length. We induced compensatory growth responses by exposing frog embryos to cold conditions (i.e. a 'false spring' scenario), which cause a delay in hatching. Once hatched, we reared larvae at two different photoperiods (24:0, representing the natural photoperiod of larvae, and 18:6) to test also for the interactive effects of light on growth responses. Larvae experiencing delayed hatching showed fast compensatory responses and reached larger size at metamorphosis. Larvae shortened their developmental period in response to delayed hatching. Non-permanent light conditions resulted in relaxed growth compared with larvae reared under permanent light conditions, which grew at their natural photoperiod and closer to their maximal rates. Growth responses altered the redox status and corticosterone levels of larvae. These physiological changes were developmental stage-dependent and mainly affected by photoperiod conditions. At catch-up, larvae reared at 18:6 light:dark cycles showed higher antioxidant activities and glucocorticoid secretion. On the contrary, larvae reared at 24:0 developed at higher rates without altering their oxidative status, likely an adaptation to grow under very restricting seasonal conditions at early life. At metamorphosis, compensatory responses induced higher cellular antioxidant activities probably caused by enhanced metabolism. Telomere length remained unaltered by experimental treatments but apparently tended to elongate across larval ontogeny, which would be a first evidence of telomere lengthening across metamorphosis. Under the forecasted increase in extreme climatic events, adjusting growth and developmental rates to the dynamics of environmental fluctuations may be essential for survival, but it can carry metabolic costs and affect later performance. Understanding the implications of such costs will be essential to properly estimate the impact of climate change on wild animals.