Humoral immune response in relation to senescence, sex and sexual ornamentation in the barn swallow (Hirundo rustica)

Infection with a non-lethal fungal parasite is associated with increased immune investment in the ant Myrmica scabrinodis

Social insects, such as ants, are preferred host organisms of pathogens and parasites because colonies are densely populated, and the number of potential hosts is high in the same place and time. Within a colony, individuals are exposed differentially to risks according to their function and age. Thus, older individuals forage and are therefore the most exposed to infection, predation, or physical stress, while young workers mostly stay inside the sheltered nest being less exposed. Immune investment is considered to be dependent on an individual's age and pathogen pressure. Long-term exposure to a parasite could affect the immune activity of individuals in an intriguing way that interferes with the age-dependent decline in immunocompetence. However, there are only few cases in which such interferences can be studied. The myrmecopathogenic fungus Rickia wasmannii, which infects entire colonies without killing the workers, is a suitable candidate for such studies. We investigated the general immunocompetence of Myrmica scabrinodis ant workers associated with non-lethal fungal infection by measuring the levels of active phenoloxidase (PO) and total PO (PPO) (reflecting the amount of both active and inactive forms of the enzyme) in two age classes. The level of PO proved to be higher in infected workers than in uninfected ones, while the level of PPO increased with age but was not affected by infection. Overall, these results indicate that a long-term infection could go hand in hand with increased immune activity of ant workers, conferring them higher level of protection.

Variations in immune parameters with age in a wild rodent population and links with survival

Recent findings suggest that immune functions do not unidirectionally deteriorate with age but that a potentially adaptive remodeling, where functions of the immune system get downregulated while others get upregulated with age could also occur. Scarce in wild populations, longitudinal studies are yet necessary to properly understand the patterns and consequences of age variations of the immune system in the wild. Meanwhile, it is challenging to understand if the observed variations in immune parameters with age are due to changes at the within‐individual level or to selective (dis)appearance of individuals with peculiar immune phenotypes. Thanks to a long‐term and longitudinal monitoring of a wild Alpine marmot population, we aimed to understand within‐ and between‐individual variation in the immune phenotype with age, in order to improve our knowledge about the occurrence and the evolutionary consequences of such age variations in the wild. To do so, we recorded the age‐specific leukocyte concentration and leukocyte profile in repeatedly sampled dominant individuals. We then tested whether the potential changes with age were attributable to within‐individual variations and/or selective (dis)appearance. Finally, we investigated if the leukocyte concentration and profiles were correlated to the probability of death at a given age. The leukocyte concentration was stable with age, but the relative number of lymphocytes decreased, while the relative number of neutrophils increased, over the course of an individual's life. Moreover, between individuals of the same age, individuals with fewer lymphocytes but more neutrophils were more likely to die. Therefore, selective disappearance seems to play a role in the age variations of the immune parameters in this population. Further investigations linking age variations in immune phenotype to individual fitness are needed to understand whether remodeling of the immune system with age could or could not be adaptive.

Total white blood cell counts but not HL ratio changes in the transition from post-juvenile molt to autumn migration in the first-year Eurasian blackcap (Sylvia atricapilla)

Theoretically, seasonal changes in immune functioning in animals are shaped by the trade-off between a probability of encountering pathogens and availability of resources. We used leukocyte profile (absolute and relative leukocyte counts) as a simple measure of immune system condition to study how it changes during the transition from postjuvenile molt to autumn migration in a free-living migratory songbird, the Eurasian blackcap (Sylvia atricapilla). We observed the higher white blood cells (WBC) and lymphocyte counts in molting birds compared to migrating individuals, but we did not find differences in heterophils and ratio of heterophils to lymphocytes (HL ratio). We suppose that the high number of WBC in molting blackcaps could reflect the heightened ability of their immune system to resists infections. The lower WBC counts in migrants compared to molting birds were mostly due to reduced lymphocyte numbers, thus representing in a downregulation of specific immunity. An absence of heterophil differences between molt and migration might indicate that various components of immunity can change relatively independently (or at different pace). Fat scores had no effect on WBC counts and HL ratio. Therefore, we found no strong evidence for a resource-immune functionality trade-off during transition from postjuvenile molt to autumn migration in immature Eurasian blackcap. This study is an important step in understanding how immune system in general and leukocyte profile in particular changes in transition between life-history stages in migratory songbirds.

Assessing age, breeding stage, and mating activity as drivers of variation in the reproductive microbiome of female tree swallows

Sexually transmitted microbes are hypothesized to influence the evolution of reproductive strategies. Though frequently discussed in this context, our understanding of the reproductive microbiome is quite nascent. Indeed, testing this hypothesis first requires establishing a baseline understanding of the temporal dynamics of the reproductive microbiome and of how individual variation in reproductive behavior and age influence the assembly and maintenance of the reproductive microbiome as a whole. Here, we ask how mating activity, breeding stage, and age influence the reproductive microbiome. We use observational and experimental approaches to explain variation in the cloacal microbiome of free-living, female tree swallows (Tachycineta bicolor). Using microsatellite-based parentage analyses, we determined the number of sires per brood (a proxy for female mating activity). We experimentally increased female sexual activity by administering exogenous 17ß-estradiol. Lastly, we used bacterial 16S rRNA amplicon sequencing to characterize the cloacal microbiome. Neither the number of sires per brood nor the increased sexual activity of females significantly influenced female cloacal microbiome richness or community structure. Female age, however, was positively correlated with cloacal microbiome richness and influenced overall community structure. A hypothesis to explain these patterns is that the effect of sexual activity and the number of mates on variation in the cloacal microbiome manifests over an individual's lifetime. Additionally, we found that cloacal microbiome alpha diversity (Shannon Index, Faith's phylogenetic distance) decreased and community structure shifted between breeding stages. This is one of few studies to document within-individual changes and age-related differences in the cloacal microbiome across successive breeding stages. More broadly, our results contribute to our understanding of the role that host life history and behavior play in shaping the cloacal microbiomes of wild birds.

Masculinity and immune system efficacy in men

Masculinity-related morphological traits are supposed to be honest indicators of a man's biological quality. While some studies showed that sexually dimorphic traits are related to various aspects of biological condition such as general health, immunity or fertility, still little is known about the relationship between masculine traits and the effectiveness of innate and adaptive immunity in humans. The aim of this study was to see if masculine traits, which are dependent on androgen levels in foetal and pubertal stages of development, are related to the immune quality in healthy men. The immune quality was evaluated for 91 healthy men aged 19-36 years. Immunity measurements included innate and adaptive parameters. General health status, age, testosterone level, BMI, physical activity, and smoking were controlled. The shoulder-to-hip ratio (SHR), 2D:4D digit ratio and hand-grip strength (HGS) were used as markers of masculinization. The regressions showed that when controlling for confounds, masculinity-related traits were in general not related to innate and adaptive immunity. Only a weak association was observed for right 2D:4D ratio and T-lymphocyte counts (but it becomes non-significant after adjustment for multiple comparisons). Our results do not support the premise that masculinity is a cue for immunological quality in men. However, the positive association between right 2D:4D and T lymphocytes might suggest that further studies are needed to verify if androgen stimulation in prenatal development might be related to immunity in adulthood.

Foot web pentosidine does not covary strongly with age in four species of wild seabirds

Age is an important parameter for a variety of ecological applications, including population viability analyses, contaminants monitoring and targeting of individuals for conservation. While many organisms can be aged by annual rings, dentition and other techniques (i.e., fish otoliths, clam growth rings, mammal tooth wear), there are no minimally invasive biomarkers for accurately aging birds in the wild. For the past century, banding has been the only way to identify a bird of known age, which requires continuous effort on a large scale with possibly low return rates. Recent studies have identified pentosidine as a potential biomarker of chronological aging in several bird species. To test this idea in four species of long-lived seabirds, we collected skin biopsies from the foot webs of previously banded, known-age seabirds: black-legged kittiwakes (Rissa tridactyla; 0-19 y old), Atlantic puffins (Fratercula arctica; 5-26 y old), razorbills (Alca torda; 0-15 d old) and thick-billed murres (Uria lomvia; 0-35 y old). Foot web samples were specifically chosen because this was the least invasive site for substantial skin biopsy. Samples were analysed with high performance liquid chromatography to quantify pentosidine levels. Collagen levels were estimated through hydroxyproline assays to normalize pentosidine content across individuals. Kittiwakes displayed a weak correlation (r² = 0.20) between age and pentosidine/collagen. Puffins (adults only, r² = 0.02), razorbills (chicks only, r² = 0.08), and murres (adults, r² = 0.04) did not show any associations with age. We concluded that pentosidine content in the foot web does not appear to be a reliable method for aging seabirds in the wild. An absence of change in pentosidine in the foot web with age is further evidence that long-lived seabirds may maintain physiological performance into old age.

No evidence for the immunocompetence handicap hypothesis in male humans

The observations that testosterone might be immunosuppressive, form the basis for the immunocompetence handicap hypothesis (ICHH). According to ICHH only high-quality individuals can maintain high levels of testosterone and afford the physiological cost of hormone-derived immunosuppression. The animal and human studies that attempted to support the ICHH by precisely defined impairment of immunity associated with high testosterone levels are inconclusive. Furthermore, human studies have used only selected immune functions and varying testosterone fractions. This is the first study examining the relationship between multiple innate and adaptive immunity and serum levels of free testosterone, total testosterone, DHT and DHEA in ninety-seven healthy men. Free testosterone and marginally DHT levels were positively correlated with the strength of the influenza post-vaccination response. Total testosterone and DHEA showed no immunomodulatory properties. Our findings did not support ICHH assumptions about immunosuppressive function of androgens. In the affluent society studied here, men with higher levels of free testosterone could afford to invest more in adaptive immunity. Since the hormone-immune relationship is complex and may depend on multiple factors, including access to food resources, androgens should be treated as immunomodulators rather than implicit immunosuppressants.

Capacity of blood plasma is higher in birds breeding in radioactively contaminated areas

Background

Environmental pollution in general, and radioactive contamination in particular, may deeply affect host-parasite relationships and their consequences for the evolution of organisms. The nuclear accident that occurred more than 30 years ago in Chernobyl resulted in significant changes in diversity and richness of microbial communities that could influence characteristics of animal-bacteria interactions, including host immune responses and competitive interference by bacteria. Given the high mortality rate of birds breeding in radioactively contaminated zones, those with stronger defences against infections should experience significant fitness advantages.

Methodology/Principal Findings

Here we characterized antimicrobial capacity of barn swallows (Hirundo rustica) from different Ukrainian populations (subject to a gradient of ionizing radiation) against 12 bacterial species. We also quantified constitutive innate immunity, which is the non-specific first barrier of protection of hosts against microbial parasites. We found a positive association between specific antimicrobial capacity of individual hosts and radiation levels in breeding habitats even after controlling for other confounding variables such as sex and age. However, no significant relationship was found between immunocompetence (non-specific response) and background radiation.

Conclusions/Significance

These results suggest that radiation selects for broad antimicrobial spectra of barn swallows, although not for all bacterial strains. We discuss these results in the framework of host-parasite evolution under extreme environmental conditions.

Does baseline innate immunity change with age? A multi-year study in great tits

Throughout their life animals progressively accumulate mostly detrimental changes in cells, tissues and their functions, causing a decrease in individual performance and ultimately an increased risk of death. The latter may be amplified if it also leads to a deterioration of the immune system which forms the most important protection against the permanent threat of pathogens and infectious diseases. Here, we investigated how four baseline innate immune parameters (natural antibodies, complement activity, concentrations of haptoglobin and concentrations of nitric oxide) changed with age in free-living great tits (Parus major). We applied both cross-sectional and longitudinal approaches as birds were sampled for up to three years of their lives. Three out of the four selected innate immune parameters were affected by age. However, the shape of the response curves differed strongly among the innate immune parameters. Natural antibody levels increased during early life until mid-age to decrease thereafter when birds aged. Complement activity was highest in young birds, while levels slightly decreased with increasing age. Haptoglobin levels on the other hand, showed a linear, but highly variable increase with age, while nitric oxide concentrations were unaffected by age. The observed differences among the four studied innate immune traits not only indicate the importance of considering several immune traits at the same time, but also highlight the complexity of innate immunity. Unraveling the functional significance of the observed changes in innate immunity is thus a challenging next step.

Host sex and age influence endoparasite burdens in the gray mouse lemur

Introduction

Immunosenescence (deteriorating immune function at old age) affects humans and laboratory animals, but little is known about immunosenescence in natural populations despite its potential importance for population and disease dynamics and individual fitness. Although life histories and immune system profiles often differ between the sexes, sex-specific effects of aging on health are rarely studied in the wild. Life history theory predicts that due to their shorter lifespan and higher investment into reproduction at the expense of immune defences, males might experience accelerated immunosenescence. We tested this hypothesis by examining sex-specific age trajectories of endoparasite burden (helminth prevalence and morphotype richness measured via fecal egg counts), an indicator of overall health, in wild gray mouse lemurs (Microcebus murinus). To account for potential interactions between seasonality and host sex or age we examined the predictors of parasite burdens separately for the dry and rainy season.

Results

Contrary to the prediction of immunosenescence, parasite prevalence and morphotype richness decreased at old age in the dry season, indicating acquired immunity by older animals. This pattern was primarily caused by within-individual decline in parasite loads rather than the earlier mortality of highly parasitized individuals. With the exception of an increasing cestode prevalence in males from yearlings to prime age in the rainy season, no evidence was found of male-biased ageing in parasite resistance. Besides this sex*age interaction, host age was uncorrelated with rainy season parasite loads. Seasonality did not affect the overall parasite loads but seasonal patterns were found in the predictors of parasite prevalence and morphotype richness.

Conclusions

These results provide rare information about the age-related patterns of health in a wild vertebrate population and suggest improvement rather than senescence in the ability to resist helminth infections at old age. Overall, males appear not to suffer from earlier immunosenescence relative to females. This may partially reflect the earlier mortality of males, which can render senescence difficult to detect. While helminth infections are not strongly associated with survival in wild gray mouse lemurs, parasite load may, however, reflect overall good phenotypic quality of long-lived individuals, and is a potential correlate of fitness.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-015-0118-9) contains supplementary material, which is available to authorized users.

The prudent parent meets old age: a high stress response in very old seabirds supports the terminal restraint hypothesis

The reproductive success of wild animals usually increases with age before declining at the end of life, but the proximate mechanisms underlying those patterns remain elusive. Young animals are expected to invest less in current reproduction due to high prospects for future reproduction (the "restraint" hypothesis). The oldest animals may also show restraint when conditions are sub-optimal where even a small increase in reproductive investment may lead to death ("terminal restraint"). Alternatively, reproduction may be constrained by lack of experience and senescence (the "constraint" hypothesis). In two species of breeding seabirds, behavioural (time to return the offspring, calmness during restraint) and physiological (metabolism, glucose and corticosterone) parameters responded similarly to stress with advancing age, implying a generalized stress response. Across those parameters, birds were "shy" (high stress response) when young or old, and "bold" (low stress response) when middle-aged. Specifically, free corticosterone, the principal avian glucocorticoid responsible for directing energy away from reproduction and towards immediate survival following stress, was highest in both young and very old stressed birds. All age groups had a similar adrenal capacity to produce corticosterone, implying that middle-aged birds were showing restraint. Because the stress response, was highest at ages when the probability of current reproduction was lowest rather than at ages when the probability of future reproduction was highest we concluded that birds restrained reproductive investment based on current conditions rather than potential future opportunities. In particular, old birds showed terminal restraint when stressed. Hormonal cues promoted investment in adult survival over reproductive output at both the start and end of life consistent with the restraint hypothesis.

Reproductive consequences of host age in a desert flea

We tested for the effect of age of a rodent host (Meriones crassus) on reproductive performance of fleas in terms of number and quality of offspring and predicted that fleas would perform better on juvenile and old than on subadult and adult hosts. The number of flea offspring was evaluated via egg and new imago production, while their quality was estimated via duration of development, resistance to starvation and body size. Although fleas produced more eggs when they exploited adults than when they exploited juvenile, subadult and old hosts, significantly more new imago emerged from fleas fed on juvenile and old hosts than on subadult and adult hosts. Fleas performed better when they fed on juvenile and/or old hosts than on subadult and adult hosts in 2 of 3 measures of offspring quality (duration of development and body size). Nevertheless, when offspring quality was estimated via resistance to starvation of a new imago, fleas demonstrated good performance in young (juvenile and subadult) hosts, while they performed poorly in old hosts. Thus, general reproductive performance of fleas was better when they exploited young and old hosts than when they exploited median age cohorts. However, the effect of host age on flea reproductive performance was manifested somewhat differently between (a) male and female hosts and (b) male and female flea offspring.

Evaluation of lymphocyte levels in a random sample of 218 elderly individuals from São Paulo city

Background

Age-associated changes in the immune system cause decreased protection after vaccination and increased rates of infections and tumor development.

Methods

Lymphocyte percentages were compared by gender and age to establish differences between subtypes. Three mL blood samples were obtained from 218 randomly selected individuals (60-101 years old) who live in São Paulo city. Blood was lysed with Tris phosphate buffer and stained for 30 minutes with monoclonal antibodies (CD3PerCP, CD4FITC, CD8Pe, CD19Pe) for analysis by flow cytometry. Statistical analysis was by ANOVA.

Results

The percentage of CD4⁺ T cells (p-value = 0.005) and the CD4/CD8 ratio (p-value = 0.010) were lower in men, whereas the percentage of CD8⁺ T cells was lower (p-value = 0.002) in women; the percentage of B cells (CD19⁺ ) was similar between groups. Individuals grouped by gender and age range and compared showed a drop in CD4⁺ cells in 75 to 79-year-old men (female: 46.1% ± 8.1% and male: 38.8% ± 10.5%; p-value = 0.023). Also, the 80 to 84-year-old group of men had a higher percentage of CD8⁺ (female: 20.8% ± 8.2%, and male: 27.2% ± 8.2%; p-value = 0.032). Low percentages of B cells were detected in men in the 75 to 79-year-old (p-value = 0.003), 85 to 89-year-old (p-value = 0.020) and older than 90 year old (p-value = 0.002) age ranges.

Conclusion

Elderly men present with more changes in lymphocyte subsets compared to elderly women. These findings could demonstrate impairment in the immune response since the lower CD4⁺ in men would provide less help to B cells (also lower in men) in terms of antibody production. In addition, the increase in CD8⁺ cells in this group could represent chronic inflammation observed during the aging process.

Does age matter in song bird vocal interactions? Results from interactive playback experiments

The song of oscines provides an extensively studied model of age-dependent behaviour changes. Male and female receivers might use song characteristics to obtain information about the age of a signaller, which is often related to its quality. Whereas most of the age-dependent song changes have been studied in solo singing, the role of age in vocal interactions is less well understood. We addressed this issue in a playback study with common nightingales (Luscinia megarhynchos). Previous studies showed that male nightingales had smaller repertoires in their first year than older males and males adjusted their repertoire towards the most common songs in the breeding population. We now compared vocal interaction patterns in a playback study in 12 one year old and 12 older nightingales (cross-sectional approach). Five of these males were tested both in their first and second breeding season (longitudinal approach). Song duration and latency to respond did not differ between males of different ages in either approach. In the cross-sectional approach, one year old nightingales matched song types twice as often as did older birds. Similarly, in the longitudinal approach all except one bird reduced the number of song type matches in their second season. Individuals tended to overlap songs at higher rates in their second breeding season than in their first. The higher levels of song type matches in the first year and song overlapping by birds in their second year suggest that these are communicative strategies to establish relationships with competing males and/or choosy females.

Consequences of immune system aging in nature: a study of immunosenescence costs in free-living Tree Swallows

Immunosenescence, the aging of the immune system, is well documented in humans and laboratory models and is known to increase infection risk, morbidity, and mortality among the old. Immunosenescence patterns have recently been unveiled in various free-living populations, but their consequences in the wild have not been explored. We investigated the consequences of immunosenescence in free-living Tree Swallows Tachycineta bicolor through a field experiment simulating a bacterial infection (challenge with lipopolysaccharide, LPS) in females of different ages during the nestling rearing period. We assessed behavioral and physiological responses of females, as well as growth and quality of their offspring, to determine the costs associated with the simulated infection. Results of the experiment differed between the two years of study. In the first year, old females challenged with LPS lost more body mass and reduced their nest visitation rates more, and their offspring tended to grow slower compared to similarly challenged younger females. In contrast, in the second year, old females did not appear to suffer larger costs than younger ones. Interestingly, immunosenescence was only detected during the first year of the study, suggesting that it is the dysregulated immune function characteristic of immunosenescent individuals rather than age per se that can lead to higher costs of immune defense in old individuals. These results provide the first evidence of costs of immunosenescence in free-living animals and support the hypothesis that old, immunosenescent individuals pay higher costs than younger ones when faced with a challenge to their immune system. Our results also suggest that these costs are mediated by an exaggerated sickness behavior, as seen in laboratory models, and can be modulated by ecological factors such as weather conditions and food availability.

Growing old with the immune system: a study of immunosenescence in the zebra finch (Taeniopygia guttata)

Immunosenescence has not received much attention in birds and the few existing studies indicate that the occurrence of immunosenescence and/or its extent may differ between species. In addition, not much information is available on the immunosenescence patterns of different immune parameters assessed simultaneously in both sexes within a single species. The present study reports the results on immunosenescence in innate immunity and both cellular and humoral acquired immunity of both sexes in a captive population of zebra finch (Taeniopygia guttata) using three age groups (approximately 0.2, 2.5 and 5.1 years). Both male and female finches showed an inverse U-shaped pattern in cellular immune function with age, quantified by a PHA response. Males showed stronger responses than females at all ages. In contrast, an increase with age in humoral immunity, quantified through total plasma immunoglobulin Y levels, was found in both sexes. However, our measurements of innate immunity measured through the bacteria-killing ability against Escherichia coli gave inconclusive results. Still, we conclude that both cellular and humoral acquired immunity are susceptible to immunosenescence, and that the sexes differ in cellular immunity.

The aging immune system and its relationship to the development of chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory disease of the lungs that usually manifests late in life. Physiologic and immunologic changes that occur in COPD often mimic changes seen in the aging lung. This has led some to characterize COPD as an "accelerated aging phenotype." At the molecular level, COPD and aging share common mechanisms and are associated with significant dysregulation of the immune systems. Aging and COPD are characterized by increases in proinflammatory cytokines such as interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha, which are implicated in aging-related inflammatory diseases and correlate with degree of obstruction in COPD. There is an age-dependent decline in naïve T cells with oligoclonal expansion of CD8(+) CD28(null) T cells from chronic antigenic stimulation. The increase in CD8(+) CD28 (null) T regulatory cells inhibits antigen-specific CD4(+) T cell responses, leading to a decline in adaptive immune response. To compensate for the decline in the adaptive immune function there is a paradoxical up-regulation of innate immune system resulting in a proinflammatory state. The dysregulated adaptive immune system with activated innate immune responses seen with aging results in recruitment and retention of neutrophils, macrophages, and CD4(+) and CD8(+) T cells in the lungs of smokers with COPD. Once the inflammation is triggered, there is a self-perpetuating cascade of inflammation and lung parenchymal damage. This review will focus on how the aging immune system may contribute to COPD development later in life in susceptible individuals.

Ageing in a variable habitat: environmental stress affects senescence in parasite resistance in St Kilda Soay sheep

Despite widespread empirical evidence for a general deterioration in the majority of traits with advancing age, it is unclear whether the progress of senescence is chronologically determined, or whether factors such as environmental conditions experienced over the lifespan are more important. We explored the relative importance of ‘chronological’ and ‘environmental’ measures of age to changes in parasite resistance across the lifespan of free-living Soay sheep. Our results show that individuals experience an increase in parasite burden, as indicated by gastrointestinal helminth faecal egg count (FEC) with chronological age. However, chronological age fails to fully explain changes in FEC because a measure of environmental age, cumulative environmental stress, predicts an additional increase in FEC once chronological age has been accounted for. Additionally, we show that in females age-specific changes are dependent upon the environmental conditions experienced across individuals' life histories: increases in FEC with age were greatest among individuals that had experienced the highest degree of stress. Our results illustrate that chronological age alone may not always correspond to biological age, particularly in variable environments. In these circumstances, measures of age that capture the cumulative stresses experienced by an individual may be useful for understanding the process of senescence.

Immune function varies with reproductive stage and context in female and male tree lizards, Urosaurus ornatus

Competition among physiological processes for limited resources often results in trade-offs. Key among these processes is reproduction and immune function, and optimizing both appears to be difficult. To test the hypothesis that the resource demands of reproduction compromise immune function, we measured rates of wound healing, an integrated measure of innate immunity, across different reproductive stages in female and male tree lizards (Urosaurus ornatus) in both the field and the laboratory. The hypothesis predicted that immune function would be lowest when resource demands of reproduction are highest, i.e., vitellogenic females and reproductive males. In the field, vitellogenic females had significantly slower healing rate than females in other reproductive stages. However, in the laboratory, vitellogenic females had a healing rate similar to that of other females. Conversely, males showed suppression of healing in the laboratory but not in the field during the reproductive season. The results of this study support a trade-off between reproduction and immune function, and suggest that reproduction is given priority. However, the results also indicate that this trade-off is not fixed in the reproductive process and that it may instead be dependent on the context, sex or resource balance of the individual.

Phenotypic sorting in morphology and reproductive investment among sociable weaver colonies

Colony sizes in birds can vary by orders of magnitude within species, and many studies have shown that selection pressures differ dramatically among small and large colonies. Does such selection result in phenotypic sorting at the level of individuals? This study describes inter-colony differences in morphology and reproductive investment in a population of a highly colonial, communal and sedentary African passerine bird, the sociable weaver Philetairus socius. Relative colony sizes were fairly stable over a 10-year period. Adults differed among colonies in terms of bill morphology, condition, body size and degree of ectoparasite infestation, and the last two declined consistently with colony size. In larger colonies, smaller eggs were laid, and nestlings were more parasite-infested, showed weaker cell-mediated immune responses, and experienced higher levels of brood reduction and snake predation. Taken together with another study showing that adult survival is higher in larger colonies, these results suggest that patterns of age-specific mortality are consistently related to colony size in the sociable weaver. Based on these observations I suggest two hypotheses that might account for the observed phenotypic sorting, involving colony size-dependent patterns in (1) density-dependent competition for food and (2) adaptive life-history adjustment.

Immunosenescence in some but not all immune components in a free-living vertebrate, the tree swallow

A wide diversity of free-living organisms show increases in mortality rates and/or decreases in reproductive success with advancing age. However, the physiological mechanisms underlying these demographic patterns of senescence are poorly understood. Immunosenescence, the age-related deterioration of immune function, is well documented in humans and laboratory models, and often leads to increased morbidity and mortality due to disease. However, we know very little about immunosenescence in free-living organisms. Here, we studied immunosenescence in a free-living population of tree swallows, Tachycineta bicolor, assessing three components of the immune system and using both in vivo and in vitro immunological tests. Immune function in tree swallow females showed a complex pattern with age; acquired T-cell mediated immunity declined with age, but neither acquired nor innate humoral immunity did. In vitro lymphocyte proliferation stimulated by T-cell mitogens decreased with age, suggesting that reduced T-cell function might be one mechanism underlying the immunosenescence pattern of in vivo cell-mediated response recently described for this same population. Our results provide the most thorough description of immunosenescence patterns and mechanisms in a free-living vertebrate population to date. Future research should focus on the ecological implications of immunosenescence and the potential causes of variation in patterns among species.

Immunosenescence and age-related susceptibility to influenza virus in Japanese quail

We evaluated juvenile, pubescent, reproductive adult, and aged Japanese quail (Coturnix coturnix japonica) to determine if there were age-related differences in immune function with the hypothesis that aged birds would have weaker immune responses. Immune responses were measured using phytohemagglutinin (PHA) skin test, antibody response to foreign red blood cells and exposure to an H9N2 influenza virus. Adult birds consistently had stronger immune responses than young and aged birds. Aged quail had skin responses 38% lower than adults. Pubescent birds' mean anti-red blood cell response was four-fold lower than adult birds. Adults had greater increase in total anti-viral antibody between primary and secondary infections than all other groups. Our data demonstrate an age-related difference in immune function in Japanese quail that has similarities to age-related immunity in humans; younger and older animals had weaker immune responses compared to young adults.

The design of complex sexual traits in male barn swallows: associations between signal attributes

Variation in the expression of sexually selected traits among individuals is widely investigated on the premise that these traits evolved to signal male quality. Significant repeatabilities of sexual signals and their associations with condition, mating success, survivorship and age may be the signatures of sexual selection. However, little is known about the relationship between these sexual attributes. Here we studied 28 acoustic and visual traits in the barn swallow, Hirundo rustica, that may potentially function in sexual selection. Based on effect sizes calculated at the between-individual level, we assessed the relationship between repeatability, condition-dependence, attractiveness, age-dependence and viability indicator value of sexual traits using sexual signals as the units of analyses. Those traits that showed high within-year repeatability also showed high between-year repeatability, indicating that between-individual variation is consistent within and among seasons. In addition, age-dependence of traits, probably causing between-year variation, was negatively related to between-year repeatability. Condition-dependence was negatively correlated with effect sizes for the extent to which traits predicted viability. Therefore, traits that are positively related to immediate condition are those that are negatively related to survival, which may be the signature of a trade-off between current and future reproductive success ultimately reflecting signal reliability. No other significant relationship was found between trait attributes. We conclude that multiple sexual signals reflect different aspects of male quality in the barn swallow.

Avian immunosenescence

Immunosenescence, the aging of the immune system, is well studied in humans. Mammalian immune systems become less capable of fighting pathogens and individuals become more susceptible to infection and cancer in their elder years. Little is currently published on avian immunosenescence even though avian immune function has been well characterized and birds have been critical models in the study of immunology. The value of birds in the study of aging has been well established. Evidence demonstrates a decline in cell-mediated and (or) humoral immune function with age in four species: the barn swallow, collared flycatcher, ruff and Japanese quail. These studies suggest that birds may experience age-related changes in immune function similar to humans. Therefore, avian aging models should be evaluated for changes in immune function and comparisons made between short- and long-lived birds. Specifically, data on age-related changes in innate immunity are lacking for birds and should be characterized. These data will strengthen the use of avian models for the study of aging.

Epidemiology and risk factors analysis of elaphostrongylosis in red deer (Cervus elaphus) from Spain

We studied the distribution and faecal shedding pattern of the first-stage larvae (L1) of Elaphostrongylus cervi (Nematoda: Protostrongylidae) in the red deer (Cervus elaphus) across Spain, where excretion was widespread. We evaluated the effects of individual, population and environmental factors on E. cervi L1 counts in 18 free-ranging red deer populations in South Central Spain. In this area, prevalence was 71.42+/-2.14% (n = 448) and mean intensity (n = 320) was 74.50+/-10.35. Aggregation of deer at water-holes was positively associated with E. cervi L1 prevalence, possibly due to spatial and temporal odds of infected gastropods, red deer and infective E. cervi L1 larvae being encountered. Prevalence increased with age, and there was also a trend towards males having higher intensities than females. A slightly decreasing age-intensity profile was identified for females, which may suggest a role of acquired immunity.

Serum immunoglobulin G levels are positively related to reproductive performance in a long-lived seabird, the common tern (Sterna hirundo)

The evolution of longevity requires a low risk of mortality from extrinsic factors, relative to intrinsic factors, so that individuals that differentially invest in physiological self-maintenance and minimize their annual reproductive costs will maximize lifetime fitness through a prolonged reproductive lifespan. The trade-off between reproductive effort and self-maintenance, as measured by immune function, has been well documented in short-lived birds, but is difficult to demonstrate in long-lived birds. To assess self-maintenance in a long-lived seabird, we measured serum protein levels, including immunoglobulin G (IgG = IgY), in 30 breeding pairs of common terns (Sterna hirundo) and their first-hatched (A) chicks. Most parents were of known age from banding as hatchlings; our sample was selected to contrast young breeders (6-9 years) with very old birds (17-23 years). Body-mass of the parents declined by 5% during the chick-rearing period, while serum protein levels were stable. Serum IgG levels were higher in parents of offspring with faster growth rates, while IgG levels were lower in parents whose broods were reduced by starvation. A-chicks in broods of two had higher IgG levels than singleton chicks. Albumin levels were not related to reproductive performance. Thus, despite adequate statistical power, we could find no evidence for a trade-off between reproduction and self-maintenance in common terns, even in old age. The results are consistent with life-history predictions for long-lived vertebrates, in which selection favors sustained self-maintenance across the reproductive lifespan. The positive relationships between IgG levels and reproductive performance indicate that IgG can be used as an index of parental "quality."

Declining Immunity with Age in the Wild

Despite their higher metabolic rates and lifetime energy expenditures, birds generally outlive similar-sized mammals even in the wild, often reaching maturity and aging considerably more slowly. Wild populations of many bird species have been monitored for years using banding-and-recapture methods, allowing field ornithologists to document age-related declines in survival and reproductive success. Although elderly birds rarely reach advanced stages of senescence in nature, many show other signs of physiological deterioration. In this Perspective, we review recent reports of aging-related changes in the immune response of two small European songbirds, the barn swallow and the collared flycatcher. Researchers in both studies challenged birds' humoral immune response by administering antigen to free-ranging adults during the breeding season. Older barn swallows--particularly breeding females--showed lower antibody responses (both primary and secondary) to vaccination with Newcastle disease virus, an avian pathogen. In flycatchers, older females raised lower antibody titers than younger breeders did in response to an injection of sheep red blood cells, a nonpathogenic antigen, and produced offspring with lower average body masses. Although the relevance of such measures of "immunosenescence" to actual fitness, reproductive success, and mortality is still unclear, studies of wild vertebrate populations may ultimately provide an important link between laboratory research and our understanding of the natural history and evolution of basic mechanisms of aging.