Elevational trends in life histories: revising the pace‐of‐life framework

Life‐history traits in birds, such as lifespan, age at maturity, and rate of reproduction, vary across environments and in combinations imposed by trade‐offs and limitations of physiological mechanisms. A plethora of studies have described the diversity of traits and hypothesized selection pressures shaping components of the survival–reproduction trade‐off. Life‐history variation appears to fall along a slow–fast continuum, with slow pace characterized by higher investment in survival over reproduction and fast pace characterized by higher investment in reproduction over survival. The Pace‐of‐Life Syndrome (POLS) is a framework to describe the slow–fast axis of variation in life‐history traits and physiological traits. The POLS corresponds to latitudinal gradients, with tropical birds exhibiting a slow pace of life. We examined four possible ways that the traits of high‐elevation birds might correspond to the POLS continuum: (i) rapid pace, (ii) tropical slow pace, (iii) novel elevational pace, or (iv) constrained pace. Recent studies reveal that birds breeding at high elevations in temperate zones exhibit a combination of traits creating a unique elevational pace of life with a central trade‐off similar to a slow pace but physiological trade‐offs more similar to a fast pace. A paucity of studies prevents consideration of the possibility of a constrained pace of life. We propose extending the POLS framework to include trait variation of elevational clines to help to investigate complexity in global geographic patterns.     

Effects of a food supplementation experiment on reproductive investment and a post-mating sexually selected trait in magpies Pica pica

Food availability is an important factor affecting breeding success in birds. Food supplementation experiments in birds have in general focused on the effects on reproductive success in terms of female investment (laying date, clutch size, egg size), however, it is also known that the estimation of mate quality based on sexually selected signals influences female reproductive investment. In the particular case of magpies, females use nest size, a post-mating sexually selected signal, to assess male's likelihood to invest in reproduction, and accordingly adjust reproductive investment (clutch size). Then, the possible effects of food supplementation on female reproductive investment could be mediated by other variables related to parental quality, such as nest size in magpies. In the present study, we explore if higher food availability in a magpie territory affected both male sexually selected traits (i.e. nest size) and female reproductive investment (laying date, egg size, clutch size). We performed a food supplementation experiment in which we experimentally increased food availability in several magpie territories, keeping others as controls. In food-supplemented territories, males built significantly larger nests and females significantly increased egg size by 4.1% compared to control females. Results suggest that the continuous provisioning of protein rich food allowed magpie females to increase egg size. However, laying date and clutch size did not differ between control and food-supplemented magpie pairs. Food availability also affected the relationship between female reproductive investment and nest size. In control territories, females decreased their egg size in response to a larger nest, whereas a tendency for the opposite relationship was revealed in food-supplemented territories. We discuss the possibility that magpie females adopt different strategies for reproductive investment according to food availability.     

A review of urban impacts on avian life‐history evolution: Does city living lead to slower pace of life?

The concept of a pace‐of‐life syndrome describes inter‐ and intraspecific variation in several life‐history traits along a slow‐to‐fast pace‐of‐life continuum, with long lifespans, low reproductive and metabolic rates, and elevated somatic defences at the slow end of the continuum and the opposite traits at the fast end. Pace‐of‐life can vary in relation to local environmental conditions (e.g. latitude, altitude), and here we propose that this variation may also occur along an anthropogenically modified environmental gradient. Based on a body of literature supporting the idea that city birds have longer lifespans, we predict that urban birds have a slower pace‐of‐life compared to rural birds and thus invest more in self maintenance and less in annual reproduction. Our statistical meta‐analysis of two key traits related to pace‐of‐life, survival and breeding investment (clutch size), indicated that urban birds generally have higher survival, but smaller clutch sizes. The latter finding (smaller clutches in urban habitats) seemed to be mainly a characteristic of smaller passerines. We also reviewed urbanization studies on other traits that can be associated with pace‐of‐life and are related to either reproductive investment or self‐maintenance. Though sample sizes were generally too small to conduct formal meta‐analyses, published literature suggests that urban birds tend to produce lower‐quality sexual signals and invest more in offspring care. The latter finding is in agreement with the adult survival hypothesis, proposing that higher adult survival prospects favour investment in fewer offspring per year. According to our hypothesis, differences in age structure should arise between urban and rural populations, providing a novel alternative explanation for physiological differences and earlier breeding. We encourage more research investigating how telomere dynamics, immune defences, antioxidants and oxidative damage in different tissues vary along the urbanization gradient, and suggest that applying pace‐of‐life framework to studies of variation in physiological traits along the urbanization gradient might be the next direction to improve our understanding of urbanization as an evolutionary process.     

Changes in timing, duration, and symmetry of molt of Hawaiian forest birds

Food limitation greatly affects bird breeding performance, but the effect of nutritive stress on molt has barely been investigated outside of laboratory settings. Here we show changes in molting patterns for an entire native Hawaiian bird community at 1650-1900 m elevation on the Island of Hawaii between 1989-1999 and 2000-2006, associated with severe food limitation throughout the year beginning in 2000. Young birds and adults of all species took longer to complete their molt, including months never or rarely used during the 1989-1999 decade. These included the cold winter months and even the early months of the following breeding season. In addition, more adults of most species initiated their molt one to two months earlier, during the breeding season. Suspended molt, indicated by birds temporarily not molting primary flight feathers during the months of peak primary molt, increased in prevalence. Food limitation reached the point where individuals of all species had asymmetric molt, with different primary flight feathers molted on each wing. These multiple changes in molt, unprecedented in birds, had survival consequences. Adult birds captured during January to March, 2000-2004, had lower survival in four of five species with little effect of extended molt. Extended molt may be adaptive for a nutrient stressed bird to survive warm temperatures but not cool winter temperatures that may obliterate the energy savings. The changing molt of Hawaiian birds has many implications for conservation and for understanding life history aspects of molt of tropical birds.     

Functional linkages for the pace of life, life-history, and environment in birds

For vertebrates, body mass underlies much of the variation in metabolism, but among animals of the same body mass, metabolism varies six-fold. Understanding how natural selection can influence variation in metabolism remains a central focus of Physiological Ecologists. Life-history theory postulates that many physiological traits, such as metabolism, may be understood in terms of key maturational and reproductive characteristics over an organism's life-span. Although it is widely acknowledged that physiological processes serve as a foundation for life-history trade-offs, the physiological mechanisms that underlie the diversification of life-histories remain elusive. Data show that tropical birds have a reduced basal metabolism (BMR), field metabolic rate, and peak metabolic rate compared with temperate counterparts, results consistent with the idea that a low mortality, and therefore increased longevity, and low productivity is associated with low mass-specific metabolic rate. Mass-adjusted BMR of tropical and temperate birds was associated with survival rate, in accordance with the view that animals with a slow pace of life tend to have increased life spans. To understand the mechanisms responsible for a reduced rate of metabolism in tropical birds compared with temperate species, we summarized an unpublished study, based on data from the literature, on organ masses for both groups. Tropical birds had smaller hearts, kidneys, livers, and pectoral muscles than did temperate species of the same body size, but they had a relatively larger skeletal mass. Direct measurements of organ masses for tropical and temperate birds showed that the heart, kidneys, and lungs were significantly smaller in tropical birds, although sample sizes were small. Also from an ongoing study, we summarized results to date on connections between whole-organism metabolism in tropical and temperate birds and attributes of their dermal fibroblasts grown in cell culture. Cells derived from tropical birds had a slower rate of growth, consistent with the hypothesis that these cells have a slower metabolism. We found that dermal fibroblasts from tropical birds resisted chemical agents that induce oxidative and non-oxidative stress better than do cells from temperate species, consistent with the hypothesis that birds that live longer invest more in self-maintenance such as antioxidant properties of cells.     

Does food supplementation really enhance productivity of breeding birds?

Food availability influences multiple stages of the breeding cycle of birds, and supplementary feeding has helped in its understanding. Most supplementation studies have reported advancements of laying, whilst others, albeit less numerous, have also demonstrated fitness benefits such as larger clutches, shorter incubation periods, and greater hatching success. Relatively few studies, however, have investigated the effects of supplementary feeding for protracted periods across multiple stages of the breeding cycle. These effects are important to understand since long-term food supplementation of birds is recommended in urban habitats and is used as a tool to increase reproductive output in endangered species. Here, we compare the breeding phenology and productivity of blue tits Cyanistes caeruleus and great tits Parus major breeding in food-supplemented and non-supplemented blocks in a broadleaf woodland in central England over three seasons (2006–2008). Supplementation was provided continuously from several weeks pre-laying until hatching, and had multiple significant effects. Most notably, supplementation reduced brood size significantly in both species, by half a chick or more at hatching (after controlling for year and hatching date). Reduced brood sizes in supplemented pairs were driven by significantly smaller clutches in both species and, in blue tits, significantly lower hatching success. These are novel and concerning findings of food supplementation. As expected, supplementary feeding advanced laying and shortened incubation periods significantly in both species. We discuss the striking parallels between our findings and patterns in blue and great tit reproduction in urban habitats, and conclude that supplementary feeding may not always enhance the breeding productivity of birds.     

Comparing food limitation among three stages of nesting: supplementation experiments with the burrowing owl

Food availability is an important limiting factor for avian reproduction. In altricial birds, food limitation is assumed to be more severe during the nestling stage than during laying or incubation, but this has yet to be adequately tested. Using food‐supplementation experiments over a 5‐year period, we determined the degree and timing of food limitation for burrowing owls (Athene cunicularia) breeding in Canada. Burrowing owls are an endangered species and food limitation during the nestling stage could influence reproductive performance of this species at the northern extent of their range. Supplemented pairs fledged on average 47% more owlets than unfed pairs, except during a year when natural food was not limiting (i.e., a prey irruption year). The difference in fledgling production resulted from high nestling mortality in unfed broods, with 96% of all nestling deaths being attributed to food shortage. Supplemental feeding during the nestling period also increased fledgling structural size. Pairs fed from the start of laying produced the same number of hatchlings as pairs that received no supplemental food before hatch. Furthermore, pairs supplemented from egg laying to fledging and pairs supplemented during the nestling period alone had the same patterns of nestling survival, equal numbers of fledglings, and similar fledgling mass and structural size. Our results provide empirical support for the hypothesis that the nestling period is the most food‐limited phase of the breeding cycle. The experimental design we introduce here could be used with other altricial species to examine how the timing of food limitation differs among birds with a variety of life‐history strategies. For burrowing owls, and other species with similar life histories, long‐term, large‐scale, and appropriately timed habitat management increasing prey abundance or availability is critical for conservation.     

The phenology of molting,breeding and their overlap in central Amazonian birds

The energetically challenging periods of molting and breeding are usually temporally separated in temperate birds, but can occur simultaneously in tropical birds, a condition known as molt–breeding overlap. Here, we document great variation in the timing and duration of molting and breeding, and in the extent of molt–breeding overlap, among 87 species of understory passerines in central Amazonia. We analyzed molt and breeding from 26 871 birds captured over a 30‐yr period near Manaus, Brazil. Although most species typically bred during the late dry season (about October through January), many thamnophilids apparently bred year‐round, whereas a few other species from a variety of families bred mainly during the wet season (about January through May). Of all breeding birds with an active brood patch, 12.7% were simultaneously molting. Molt–breeding overlap was more frequently observed among suboscines (13.3%), especially thamnophilids (23.0%), than oscines (6.4%). Some families had <5% molt–breeding overlap frequency, including Tyrannidae (4.4%), Tityridae (0.0%), Pipridae (1.5%), Turdidae (0.0%), and Thraupidae (0.0%), indicating that not all tropical species exhibit molt–breeding overlap. Among 31 well‐sampled species (n ≥15 brood patches), variation in molt–breeding overlap frequency was positively correlated with each species’ average duration of flight feather replacement (range 98–301 d). We also measured feather growth rates of individual birds in nine species; in five of these, slower‐growing feathers increased with an individual's probability of having molt–breeding overlap. Among furnariids, molt–breeding overlap occurred either at the beginning or end of the molt cycle, suggesting that physiological mechanisms typically separate molting from breeding. Thamnophilids showed a much different pattern; molt–breeding overlap occurred at any stage of feather replacement, apparently not regulated to be independent of breeding. These results reveal substantial life‐history variation among Amazonian birds. Future work to resolve the physiological regulation of molting and breeding in tropical birds will greatly contribute to understanding these patterns and their relevance to avian diversity.     

Carotenoids modulate the trade-off between egg production and resistance to oxidative stress in zebra finches

The allocation of resources to reproduction and survival is a central question of studies of life history evolution. Usually, increased allocation to current reproduction is paid in terms of reduced future reproduction and/or decreased survival. However, the proximal mechanisms underlying the cost of reproduction are poorly understood. Recently, it has been shown that increased susceptibility to oxidative stress might be one of such proximate links between reproduction and self-maintenance. Organisms possess a range of antioxidant defenses, including endogenously produced molecules (e.g., enzymes) and compounds ingested with food (e.g., carotenoids). If reproductive effort increases the production of reactive oxygen species, the availability of antioxidant defenses may partly or fully counteract the free-radical damages. One could, therefore, expect that the trade-off between reproduction and oxidative stress is modulated by the availability of antioxidant defenses. We tested this hypothesis in zebra finches. We manipulated reproductive effort by either allowing or preventing pairs to breed. Within each breeding or non-breeding group, the availability of antioxidant compounds was manipulated by supplementing or not supplementing the drinking water with carotenoids. We found that although birds in the breeding and non-breeding groups did not differ in their resistance to oxidative stress (the breakdown of red blood cells submitted to a controlled free-radical attack), one aspect of breeding effort (i.e., the number of eggs laid by birds in both breeding and non-breeding groups) was negatively correlated with resistance to oxidative stress only in birds that did not benefit from a carotenoid-supplemented diet. This result therefore suggests that carotenoid availability can modulate the trade-off between reproduction and resistance to oxidative stress.     

Limitation of reproductive success by food availability and litter size in the bank vole,Clethrionomys glareolus

Food limitation has been suggested as one of the most important factors affecting life history evolution in terrestrial vertebrates. However, this inference is based mainly on evidence from birds, and reproductive trade-offs may differ among groups with different forms of parental care. To study whether the costs of enlarged litters (decreased mass of offspring) would appear when food is not limiting, we performed outdoor enclosure experiments in which we manipulated simultaneously the litter size (control versus litter plus two pups) and food availability (control versus food-supplemented) of female bank voles, Clethrionomys glareolus. The weaning success of females increased significantly in response to supplementary food. When females were provided with extra food, no differences were observed in body masses of weanlings of control and enlarged litters. Further, food-supplemented females grew to larger sizes during nursing than unsupplemented females. Our experiment suggests that energetic requirements during nursing constrain the number of offspring that can be raised successfully in a particular breeding attempt. The results also indicate that unlimited food resources may increase future reproductive potential of females because they can use more energy for somatic growth.     

Testosterone and year‐round territoriality in tropical and non‐tropical songbirds

Past studies have suggested a fundamental difference in testosterone concentrations between tropical and northern latitude male birds, with the convention being that males in the tropics express much lower levels of testosterone. However, recent comparative studies have shown that tropical males with a short and synchronous breeding season (i.e. a breeding season typical of northern species) express maximum testosterone levels similar to those of northern latitude birds. Here, we ask the converse: do northern latitude songbirds that express a defining life‐history characteristic typical of the tropics, i.e. year‐round territoriality, have an annual testosterone profile similar to that of tropical songbirds? For the few year‐round territorial species for which data are available, we found that seasonal testosterone profiles and seasonal maxima in plasma testosterone were similar between males of tropical and non‐tropical species. For example, males of both groups expressed seasonal maxima during the period when females were fertile, and testosterone levels at this time were similar. In contrast, this and other studies show that species with seasonal territories typically express maximum testosterone levels earlier in the breeding cycle, when territories are first being established. Taken together, we suggest that specific life‐history traits may play a more important role in determining testosterone profiles of tropical and non‐tropical birds than breeding latitude and encourage further studies to allow for more formal comparisons.     

Age at the onset of senescence in birds and mammals is predicted by early-life performance

Life-history theory predicts that traits involved in maturity, reproduction and survival correlate along a fast–slow continuum of life histories. Evolutionary theories and empirical results indicate that senescence-related traits vary along this continuum, with slow species senescing later and at a slower pace than fast species. Because senescence patterns are typically difficult to estimate from studies in the wild, here we propose to predict the associated trait values in the frame of life-history theory. From a comparative analysis based on 81 free-ranging populations of 72 species of birds and mammals, we find that a nonlinear combination of fecundity, age at first reproduction and survival over the immature stage can account for ca two-thirds of the variance in the age at the onset of actuarial senescence. Our life-history model performs better than a model predicting the onset based on generation time, and it only includes life-history traits during early life as explanatory variables, i.e. parameters that are both theoretically expected to shape senescence and are measurable within relatively short studies. We discuss the good-fit of our life-history model to the available data in the light of current evolutionary theories of senescence. We further use it to evaluate whether studies that provided no evidence for senescence lasted long enough to include the onset of senescence.     

Insights into life history theory: a brood size manipulation on a southern hemisphere species,Tachycineta leucorrhoa,reveals a fast pace of life

Life history traits exhibit substantial geographical variation associated with the pace of life. Species with a slow pace are expected to invest more in their future/residual reproductive value and are more common at tropical latitudes, whereas species from high latitudes, with a faster pace, are expected to prioritize the current reproductive effort. Most evidence supporting this pattern comes from studies conducted in tropical and north temperate species; very little is known about patterns in southern South American species. Here, we describe the life history of a southern swallow Tachycineta leucorrhoa and use an experimental approach to test their breeding strategy over four breeding seasons. We manipulated brood size for 105 nests of white‐rumped swallows to measure whether costs of reproduction were borne by adults or nestlings as alternative selection strategies towards maintaining residual or current reproductive value. Adults increased their feeding effort in enlarged broods, at least enough to maintain nestlings’ development/growth. In addition, adults decreased the number of visits to the nest (without having a negative effect on nestlings) in reduced broods. We did not detect differences in fledging success among treatments, suggesting there were no differences in nestlings’ survival. However, enlarged broods more frequently incurred in complete nest failure, suggesting only some adults were able to cope with increased costs of reproduction. We conclude this species is characterized by a fast pace of life similar to their northern congeners and less like its tropical ones. This is one of the first studies to use an experimental approach to test a life history hypothesis of pace of life using data from a southern South American species. We encourage researches to include southern species when evaluating latitudinal variations as we still do not have enough evidence to assume all southern subtropical species are indeed similar to tropical ones.     

The scheduling of molt in migratory birds

Summary We model the yearly cycle of small migratory birds to explain the variation in scheduling of complete molt, in particular why some birds molt immediately after breeding on the breeding grounds (summer molt) whereas others migrate to their wintering grounds before molt is initiated (winter molt). We employ the method of dynamic programming, because of its suitability for modelling life history traits. Feather quality and latitude entered the model as state variables and were assumed to affect survival rate and reproductive success. Migration and molt were assumed to be associated with increased mortality risks. By changing the parameters in the model we were able to generate most existing molt patterns, including summer and winter molt, biannual (summer and winter) molt, and molt migration. Our model suggests that the scheduling of molt is basically a result of a trade-off between having a high feather quality during breeding versus during the non-breeding period. A high impact of feather quality on survival rate in combination with low costs of molt resulted in biannual molt. Winter molt became more likely as the survival rateper se increased. A low seasonal amplitude in survival rate is a prerequisite for the occurrence of molt migration. Molt duration, migration costs and reproductive successper se were found to have no impact on the timing of molt. We also investigated the effect of benefits from prior occupancy at breeding and winter grounds.     

Assessing the reproductive consequences of mate retention and pair bond duration in Thorn-tailed Rayadito (Aphrastura spinicauda), a short-lived,socially monogamous neotropical bird

The adaptive value of mate retention has been studied in several socially monogamous birds but evidence of reproductive benefits for short-lived species is inconclusive. Most studies come from northern latitudes but more research on tropical birds is needed, as these species typically show higher survival rates and longer pair bonds than those from temperate regions. We gathered data on the reproductive biology of a subtropical, isolated population of Thorn-tailed Rayadito Aphrastura spinicaudaduring 2008–2017 to evaluate the reproductive consequences of mate retention. We examined data from 243 breeding attempts made by 159 breeding pairs. We found that ~30% of all breeding pairs bred together during at least two consecutive years, and some were mated for 6 years. The main cause of pair dissolution was mate loss, not divorce. Mixed-effects models provided moderate evidence for positive effects of mate retention and successive remating on reproductive success. Newly formed pairs laid eggs later and had slightly smaller clutches than remated pairs. Furthermore, clutch size seemed to increase with successive remating. Overall, our results suggest that newly formed pairs are less efficient in reproduction and that minor yearly reproductive benefits of mate retention might accumulate for birds that are able to breed with the same partner over many years. Because breeding habitat is limited in our study population, Thorn-tailed Rayaditos could benefit from remating if the number of individuals that can breed exceeds the number of available breeding positions. Profitable long-term pair bonds might be more frequent in tropical birds and therefore more studies are needed to assess the prevalence of remating-mediated effects on reproduction in relatively short-lived monogamous species breeding in tropical regions.     

Food Supplementation Fails to Reveal a Trade-Off between Incubation and Self-Maintenance in Female House Wrens

Incubating birds must allocate their time and energy between maintaining egg temperature and obtaining enough food to meet their own metabolic demands. We tested the hypothesis that female house wrens (Troglodytes aedon) face a trade-off between incubation and self-maintenance by providing females with supplemental food during incubation. We predicted that food supplementation would increase the amount of time females devoted to incubating their eggs, lower their baseline plasma corticosterone levels (a measure of chronic stress), and increase their body mass, haematocrit (a measure of anaemia), and reproductive success relative to control females. As predicted, food-supplemented females spent a greater proportion of time incubating their eggs than control females. Contrary to expectation, however, there was no evidence that food supplementation significantly influenced female baseline plasma corticosterone levels, body mass, haematocrit, or reproductive success. However, females with high levels of corticosterone at the beginning of incubation were more likely to abandon their nesting attempt after capture than females with low levels. Corticosterone significantly increased between the early incubation and early nestling stages of the breeding cycle in all females. These results suggest that although food supplementation results in a modest increase in incubation effort, it does not lead to significantly lower levels of chronic stress as reflected in lower baseline corticosterone levels. We conclude that female house wrens that begin the incubation period with low levels of plasma corticosterone can easily meet their own nutritional needs while incubating their eggs, and that any trade-off between incubation and self-feeding does not influence female reproductive success under the conditions at the time of our study.     

Using molt cycles to categorize the age of tropical birds: an integrative new system

ABSTRACT Accurately differentiating age classes is essential for the long‐term monitoring of resident New World tropical bird species. Molt and plumage criteria have long been used to accurately age temperate birds, but application of temperate age‐classification models to the Neotropics has been hindered because annual life‐cycle events of tropical birds do not always correspond with temperate age‐classification nomenclature. However, recent studies have shown that similar molt and plumage criteria can be used to categorize tropical birds into age classes. We propose a categorical age‐classification system for tropical birds based on identification of molt cycles and their inserted plumages. This approach allows determination of the age ranges (in months) of birds throughout plumage succession. Although our proposed cycle‐based system is an improvement over temperate calendar‐based models, we believe that combining both systems provides the most accurate means of categorizing age and preserving age‐related data. Our proposed cycle‐based age‐classification system can be used for all birds, including temperate species, and provides a framework for investigating molt and population dynamics that could ultimately influence management decisions.     

RETRACTED ARTICLE: Latitude affects degree of advancement in laying by birds in response to food supplementation: a meta-analysis

Food supplementation experiments have provided considerable information about the importance of resource availability in timing reproduction. Supplemented birds usually advance breeding over non-supplemented controls. Initial observations suggested that degree of advancement in studies conducted at higher latitudes was less than in those at lower latitudes. We hypothesized that birds at high latitudes are less responsive to the "supplementary" cue of food. We tested this hypothesis using a literature-based meta-analysis of 36 papers which, because several papers presented separate data sets from different years, yielded 56 "studies." We used step-wise regression to determine whether latitude, elevation, the duration of supplementation, and the migratory status of the species predicted the degree to which mean clutch initiation dates of food supplemented birds differed from non-supplemented controls (i.e., effect size = [Formula: see text]). Consistent with our predictions, there was a significant inverse relationship between effect size and latitude: elevation, migratory status, and duration of treatment contributed little to the model. Because the response of animals' reproductive systems to environmental information is mediated by the neuroendocrine system, we discuss two models: (1) the adaptive specialization hypothesis in which higher latitude species that experience a relatively short breeding season have evolved a reliance on photic cues while exhibiting reduced sensitivity to non-photic cues; and (2) the conditional plasticity hypothesis in which an individual might show a marked response to non-photic information if it lived at low latitudes, but be largely driven by photic cues, endogenous rhythms, or both to the relative exclusion of non-photic information if it lived at higher latitudes.     

Cellular Metabolic Rate Is Influenced by Life-History Traits in Tropical and Temperate Birds

In general, tropical birds have a “slow pace of life,” lower rates of whole-animal metabolism and higher survival rates, than temperate species. A fundamental challenge facing physiological ecologists is the understanding of how variation in life-history at the whole-organism level might be linked to cellular function. Because tropical birds have lower rates of whole-animal metabolism, we hypothesized that cells from tropical species would also have lower rates of cellular metabolism than cells from temperate species of similar body size and common phylogenetic history. We cultured primary dermal fibroblasts from 17 tropical and 17 temperate phylogenetically-paired species of birds in a common nutritive and thermal environment and then examined basal, uncoupled, and non-mitochondrial cellular O₂ consumption (OCR), proton leak, and anaerobic glycolysis (extracellular acidification rates [ECAR]), using an XF24 Seahorse Analyzer. We found that multiple measures of metabolism in cells from tropical birds were significantly lower than their temperate counterparts. Basal and uncoupled cellular metabolism were 29% and 35% lower in cells from tropical birds, respectively, a decrease closely aligned with differences in whole-animal metabolism between tropical and temperate birds. Proton leak was significantly lower in cells from tropical birds compared with cells from temperate birds. Our results offer compelling evidence that whole-animal metabolism is linked to cellular respiration as a function of an animal’s life-history evolution. These findings are consistent with the idea that natural selection has uniquely fashioned cells of long-lived tropical bird species to have lower rates of metabolism than cells from shorter-lived temperate species.     

Kittiwakes strategically reduce investment in replacement clutches

Many life-history traits are expressed interactively in life, but to a varying extent on different occasions. Changes in trait expression can be accounted for by differences in the quality of the environment ('environmental constraint' hypothesis) or by strategic adjustments, if the relative contribution of the trait to fitness varies with time ('strategic allocation' hypothesis). In birds, egg production is lower in replacement clutches than in first clutches, but it is unknown whether this reduction results from an environmental constraint (e.g. food being less available at the time when the replacement clutch is produced) or from a strategic allocation of resources between the two breeding attempts. To distinguish between these two hypotheses, we performed an experiment with black-legged kittiwakes (Rissa tridactyla). Pairs were either food-supplemented or not before the first clutch was laid onwards and we induced them to produce a replacement clutch by removing eggs once when the first clutch was complete. As predicted by the 'strategic allocation' hypothesis, egg production of food-supplemented and non-food-supplemented birds decreased between first and replacement clutches. This suggests that kittiwakes strategically reduce investment in egg production for their replacement clutches compared to first clutches.