Cell-mediated immunosenescence in birds

Photo-Enhanced Oil Toxicity to Alcid Immune Function

Oil spills are devastating to seabirds, causing high levels of mortality and toxic physiological effects, especially to immune function. Sunlight exposure can further enhance the toxicity of oil to marine species by generating photodegradation products. Photo-enhanced oil toxicity to marine birds has not been studied. Therefore, the goal of the present study was to investigate the toxicity and photo-enhanced toxicity of oil to lymphocyte proliferation, macrophage phagocytosis, and reactive oxygen species production in three alcid species, common murres (Uria aalge), tufted puffins (Fratercula cirrhata), and horned puffins (Fratercula corniculata). Intrinsic factors (species, age, and sex) had a more significant effect on lymphocyte proliferation than exposure to oil or photoactivated oil. Macrophage phagocytosis was significantly reduced in oil and photoactivated oil treatments, whereas hydrogen peroxide production was significantly increased. Interestingly, nonphotoactivated oil stimulated significantly more hydrogen peroxide than photoactivated oil. The results suggest that alcid immune function could be variably influenced during an oil spill depending on the species, sex, and age of the bird as well as the season and level of sunlight exposure. Environ Toxicol Chem 2023;42:2701-2711. © 2023 SETAC.

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.

Immune parameters in two different laying hen strains during five production periods

During life, the number and function of immune cells change with potential consequences for immunocompetence of an organism. In laying hens, studies have primarily focused on early development of immune competence and only few have investigated systemic and lymphatic distribution of leukocyte subsets during adolescence and the egg-laying period. The present study determined the number of various leukocyte types in blood, spleen, and cecal tonsils of 10 Lohmann Brown-Classic and 10 Lohmann LSL-Classic hens per wk of life 9/10, 15/16, 23/24, 29/30, and 59/60, encompassing important production as well as developmental stages, by flow cytometry. Although immune traits differed between the 2 hen strains, identical patterns of age-related immunological changes were found. The numbers of all investigated lymphocyte types in the spleen as well as the numbers of blood γδ T cells increased from wk 9/10 to 15/16. This suggests an ongoing release of lymphocytes from primary lymphoid tissues and an influx of blood lymphocytes into the spleen due to novel pathogen encounters during adolescence. A strong decrease in the number of CTL and γδ T cells and an increase in innate immune cells within blood and spleen were found between wk of life 15/16 and 23/24, covering the transition phase to egg-laying activity. Numbers of peripheral and splenic lymphocytes remained low during the egg-laying period or even further decreased, for example blood CD4⁺ T cells and splenic γδ T cells. Functional assessments showed that in vitro IFN-γ production of mitogen-stimulated splenocytes was lower in wk 60. Taken together, egg-laying activity seems to alter the immune system toward a more pronounced humoral and innate immune response, with probable consequences for the immunocompetence and thus for productivity, health and welfare of the hens.

Immunocompetent birds choose larger breeding colonies

Optimal size of social groups may vary between individuals, depending on their phenotypic traits, such as dominance status, age or personality. Larger social groups often enhance transmission rates of pathogens and should be avoided by individuals with poor immune defences. In contrast, more immunocompetent individuals are expected to take advantage of larger group sizes (e.g. better protection, information transfer) with smaller extra costs from pathogen or parasite pressure. Here, we hypothesized that immunocompetence may be a key determinant of group size choice and tested this hypothesis in a colonial waterbird, the common tern Sterna hirundo. We used a unique experimental framework, where formation of breeding colonies of different sizes was induced under uniform environmental conditions. For this purpose, different-size patches of attractive nesting substrate (artificial floating rafts) were provided at a single site with limited availability of natural nesting habitat. Colony size was identified as the only significant predictor of both innate (natural antibody-mediated complement activation) and adaptive (immunoglobulin concentrations) immunological traits in the common terns, as more immunocompetent birds settled in larger experimental colonies. In contrast, we found no significant associations between colony size and genetic diversity of key pathogen-recognition receptors, toll-like receptors (TLRs) and the Major Histocompatibility Complex (MHC) or genome-wide heterozygosity. We conclude that settlement decisions may be flexible within individuals and, thus, are likely to be primarily determined by the current immunological status, rather than fixed immunogenetic traits. Our study sheds new light on the complex interface between immunity and sociality in animals.

Who rules over immunology? Sseasonal variation in body temperature,, steroid hormones, and immune variables in a tegu lizard

Multiple factors can influence the immune response of ectothermic vertebrates, including body temperature, gonadal steroids, and seasonality, in ways that are thought to reflect trade-offs between energetic investment in immunity vs. reproduction. Hibernating tegu lizards (Salvator merianae) are a unique model to investigate how immunocompetence might be influenced by different factors during their annual cycle. We assessed immunological measures (plasma bacterial killing ability, total and differential leukocyte count), plasma hormone levels (testosterone in males, estradiol and progesterone in females, and corticosterone in both sexes), body temperature, and body condition from adult tegus during each stage of their annual cycle: reproduction, post-reproduction/preparation for hibernation, and hibernation. Our hypothesis that immune traits present higher values during the reproductive phase, and a sharp decrease during hibernation, was partially supported. Immune variables did not change between life history stages, except for total number of leukocytes, which was higher at the beginning of the reproductive season (September) in both males and females. Average body temperature of the week prior to sampling was positively correlated with number of eosinophils, basophils, monocytes and azurophils, corroborating other studies showing that when animals maintain a high Tb, there is an increase in immune activity. Surprisingly, no clear relationship between immune traits and gonadal steroids or corticosterone levels was observed, even when including life history stage in the model. When gonadal hormones peaked in males and females, heterophil:lymphocyte ratio (which often elevates during physiological stress) also increased. Additionally, we did not observe any trade-off between reproduction and immunity traits, sex differences in immune traits or a correlation between body condition and immune response. Our results suggest that variation in patterns of immune response and correlations with body condition and hormone secretion across the year can depend upon the specific hormone and immune trait, and that experienced Tb is an important variable determining immune response in ectotherms.

Lack of statistical power as a major limitation in understanding MHC-mediated immunocompetence in wild vertebrate populations

Disentangling the sources of variation in developing an effective immune response against pathogens is of major interest to immunoecology and evolutionary biology. To date, the link between immunocompetence and genetic variation at the major histocompatibility complex (MHC) has received little attention in wild animals, despite the key role of MHC genes in activating the adaptive immune system. Although several studies point to a link between MHC and immunocompetence, negative findings have also been reported. Such disparate findings suggest that limited statistical power might be affecting studies on this topic, owing to insufficient sample sizes and/or a generally small effect of MHC on the immunocompetence of wild vertebrates. To clarify this issue, we investigated the link between MHC variation and seven immunocompetence proxies in a large sample of barn owls and estimated the effect sizes and statistical power of this and published studies on this topic. We found that MHC poorly explained variation in immunocompetence of barn owls, with small-to-moderate associations between MHC and immunocompetence in owls (effect size: .1 ≥ r ≤ .3) similar to other vertebrates studied to date. Such small-to-moderate effects were largely associated with insufficient power, which was only sufficient (>0.8) to detect moderate-to-large effect sizes (r ≥ .3). Thus, studies linking MHC variation with immunocompetence in wild populations are underpowered to detect MHC effects, which are likely to be of generally small magnitude. Larger sample sizes (>200) will be required to achieve sufficient power in future studies aiming to robustly test for a link between MHC variation and immunocompetence.

Immune parameters in different age classes of captive male Steller's eiders (Polysticta stelleri)

The immune system is important for host defense against antigens, but little is known about Steller's eider (Polysticta stelleri) immunology. This study compared hematological parameters, serum protein levels, lymphocyte proliferation, heat shock protein levels and oxidative damage in four different age classes of captive male Steller's eiders. The hatch year cohort had significantly higher total white blood cell and lymphocyte counts. The second year cohort had significantly higher albumin, alpha globulins and lymphocyte proliferation, and significantly lower beta globulin levels. The 9 year old males had a significantly higher IgY:IgY(ΔFc) ratio. The oldest eiders in the study, 14 + year old males, had significantly higher serum IgY, pre-albumin and glutathione reductase activity, and the lowest lymphocyte proliferation. This study provided a baseline of immune parameters in captive male Steller's eiders, and the results suggested the parameters were influenced by age-related changes.

Aging-associated dysregulation of homeostatic immune response termination (and not initiation)

Immunosenescence is a state of unbalanced immune responsiveness, characterized by a diverse repertoire of seemingly discreet and paradoxical alterations in all aspects of immunity arising in an aging‐associated manner. We asked whether aging‐associated alterations in the ability of apoptotic cells to elicit immunomodulatory responses (innate apoptotic immunity; IAI) or in IAI responses themselves might underlie the confounding aging‐associated anomalies of immunosenescence. We explored this question by examining, as a function of animal age, responsiveness of murine macrophages on the single cell level. We monitored the expression of pro‐ and anti‐inflammatory cytokines cytofluorimetrically in response to pro‐inflammatory Toll‐like receptor (TLR) stimulation and anti‐inflammatory treatment with apoptotic cells. While we found no alterations with age in the potency of apoptotic cells or in the initiation and magnitude of IAI responses, we did identify a cell‐intrinsic deficiency in anti‐inflammatory IAI response termination linked with age and preceding manifestations of immunosenescence. Further, we found that an aging‐associated deficiency in response termination also is evident following TLR stimulation. These surprising observations reveal that a loss of homeostatic immune control with animal age results from the dysregulation of response termination (as distinct from response initiation) and is exerted on the level of transcription. We suggest that, with advancing age, cells become locked into relatively longer‐lived response states. Aging‐associated immune dysfunctions may reflect a diminution in the cellular nimbleness of immune responsiveness.

ECOLOGICAL DETERMINANTS OF AVIAN INFLUENZA VIRUS, WEST NILE VIRUS, AND AVIAN PARAMYXOVIRUS INFECTION AND ANTIBODY STATUS IN BLUE-WINGED TEAL (ANAS DISCORS) IN THE CANADIAN PRAIRIES

The Canadian prairies are one of the most important breeding and staging areas for migratory waterfowl in North America. Hundreds of thousands of waterfowl of numerous species from multiple flyways converge in and disperse from this region annually; therefore this region may be a key area for potential intra- and interspecific spread of infectious pathogens among migratory waterfowl in the Americas. Using Blue-winged Teal (Anas discors, BWTE), which have the most extensive migratory range among waterfowl species, we investigated ecologic risk factors for infection and antibody status to avian influenza virus (AIV), West Nile virus (WNV), and avian paramyxovirus-1 (APMV-1) in the three prairie provinces (Alberta, Saskatchewan, and Manitoba) prior to fall migration. We used generalized linear models to examine infection or evidence of exposure in relation to host (age, sex, body condition, exposure to other infections), spatiotemporal (year, province), population-level (local population densities of BWTE, total waterfowl densities), and environmental (local pond densities) factors. The probability of AIV infection in BWTE was associated with host factors (e.g., age and antibody status), population-level factors (e.g., local BWTE population density), and year. An interaction between age and AIV antibody status showed that hatch year birds with antibodies to AIV were more likely to be infected, suggesting an antibody response to an active infection. Infection with AIV was positively associated with local BWTE density, supporting the hypothesis of density-dependent transmission. The presence of antibodies to WNV and APMV-1 was positively associated with age and varied among years. Furthermore, the probability of being WNV antibody positive was positively associated with pond density rather than host population density, likely because ponds provide suitable breeding habitat for mosquitoes, the primary vectors for transmission. Our findings highlight the importance of spatiotemporal, environmental, and host factors at the individual and population levels, all of which may influence dynamics of these and other viruses in wild waterfowl populations.

The influence of year, laying date, egg fertility and incubation, individual hen, hen age and mass and clutch size on maternal immunoglobulin Y concentration in captive Steller's and spectacled eider egg yolk

Steller's eiders and spectacled eiders are sea duck species whose populations have declined significantly and infectious diseases could influence offspring survival. Therefore, the maternal transfer of immunoglobulin Y (IgY) into yolk was investigated in captive Steller's and spectacled eiders during the 2007-2013 breeding seasons. This project had two objectives: establish baseline IgY levels in Steller's and spectacled eider yolk under controlled captive conditions and evaluate the effect of year, laying date, egg fertility, egg incubation duration, individual hen, hen age and mass, and laying order to determine which variables influenced IgY levels. Average IgY concentrations were 0.03-0.48 mg ml(-1) in Steller's eider yolk and 0.10-0.51 mg ml(-1) in spectacled eider yolk. The year and individual hen influenced IgY concentration in Steller's and spectacled eider yolk. The laying date was negatively correlated with egg IgY levels for most Steller's eider hens, but laying order was positively correlated with egg IgY concentration for spectacled eiders.

Fine-scale spatial age segregation in the limited foraging area of an inshore seabird species, the little penguin

Competition for food resources can result in spatial and dietary segregation among individuals from the same species. Few studies have looked at such segregations with the combined effect of sex and age in species with short foraging ranges. In this study we examined the 3D spatial use of the environment in a species with a limited foraging area. We equipped 26 little penguins (Eudyptula minor) of known age, sex, and breeding output with GPS (location) and accelerometer (body acceleration and dive depth) loggers. We obtained dietary niche information from the isotopic analysis of blood tissue. We controlled for confounding factors of foraging trip length and food availability by sampling adults at guard stage when parents usually make one-day trips. We observed a spatial segregation between old (>11 years old) and middle-aged penguins (between 5 and 11 years old) in the foraging area. Old penguins foraged closer to the shore, in shallower water. Despite observing age-specific spatial segregation, we found no differences in the diving effort and foraging efficiency between age classes and sexes. Birds appeared to target similar prey types, but showed age-specific variation in their isotopic niche width. We hypothesize that this age-specific segregation was primarily determined by a "cohort effect" that would lead individuals sharing a common life history (i.e. having fledged and dispersed around the same age) to forage preferentially together or to share similar foraging limitations.

Extrinsic- and intrinsic-dependent variation in component communities and patterns of aggregations in helminth parasites of great cormorant (Phalacrocorax carbo) from N.E. Poland

In the present study, we investigate the effect extrinsic (habitat and season) and intrinsic (host’s age and sex) factors on the richness, diversity, and structure of parasite component communities and aggregation patterns in the helminth fauna of the great cormorant Phalacrocorax carbo from northeastern Poland. The helminth fauna of cormorants from the brackish water habitat was far richer (30 species) than in those from freshwater lakes (18 species) and strongly depend on season and age of the host. The values of diversity index strongly varied in relation to habitat and host age with clear seasonal differences in the value of diversity index, i.e., its value increased over time in adults from the brackish water habitat and decreased in those from the freshwater lakes. The number of helminths in adult and immature birds varied, depending on the season and habitat: in the brackish water habitat, the overall percentage of helminths was higher in spring than in summer, while in the freshwater habitat a higher proportion of helminths was recorded in summer. During spring, in the brackish water habitat, we observed a higher level of aggregation (for all groups of helminths) than in autumn. The opposite pattern was found in the freshwater habitat. However, this regularity was typical of adult birds only. In immature hosts, the level of aggregation was not predictable and varied among the higher taxa. Our study clearly showed that processes determinate diversity, structure, richness, and patterns of aggregation in helminth assemblages of avian hosts are multi-origin and highly complex.

Eco-endo-immunology across avian life history stages

Broadly distributed songbirds, particularly those that migrate, encounter a wide range of pathogens. Both pathogen exposure and energy available for immune responses are expected to be affected by environmental variation in climate, habitat quality, and social interactions as well as hormonal mechanisms. Comparisons of Aves in the field have begun to build the framework for understanding how such environmental variation interacts with disease environments as reflected in endocrine and immune responses. In this review, the roles of hormones and immune function across the various life history stages that make up the avian annual cycle are considered with an emphasis on free-living songbirds and the various hormones known to mediate the innate and acquired immune systems including melatonin, prolactin, growth hormone, and several neuroendocrine hormones. Finally, hormone-immune interactions are considered within the framework of disease ecology.

Age-related differences in the cloacal microbiota of a wild bird species

Background

Gastrointestinal bacteria play a central role in the health of animals. The bacteria that individuals acquire as they age may therefore have profound consequences for their future fitness. However, changes in microbial community structure with host age remain poorly understood. We characterised the cloacal bacteria assemblages of chicks and adults in a natural population of black-legged kittiwakes (Rissa tridactyla), using molecular methods.

Results

We show that the kittiwake cloaca hosts a diverse assemblage of bacteria. A greater number of total bacterial OTUs (operational taxonomic units) were identified in chicks than adults, and chicks appeared to host a greater number of OTUs that were only isolated from single individuals. In contrast, the number of bacteria identified per individual was higher in adults than chicks, while older chicks hosted more OTUs than younger chicks. Finally, chicks and adults shared only seven OTUs, resulting in pronounced differences in microbial assemblages. This result is surprising given that adults regurgitate food to chicks and share the same nesting environment.

Conclusions

Our findings suggest that chick gastrointestinal tracts are colonised by many transient species and that bacterial assemblages gradually transition to a more stable adult state. Phenotypic differences between chicks and adults may lead to these strong differences in bacterial communities. These data provide the framework for future studies targeting the causes and consequences of variation in bacterial assemblages in wild birds.

Interrelations among immune defense indexes reflect major components of the immune system in a free-living vertebrate

Understanding the relationships among immune components in free-living animals is a challenge in ecoimmunology, and it is important not only for selecting the immune assays to be used but also for more knowledgeable interpretation of results. In this study, we investigated the relationships among six immune defense indexes commonly used by ecoimmunologists and measured simultaneously in individual free-living tree swallows. Three main axes of variation in immune function were identified using a principal components analysis, representing variation in T-cell, B-cell, and innate immunity. Measures within each axis tended to be positively correlated among individuals, while measures in different axes were uncorrelated. A trade-off between T-cell function and B-cell function became apparent only when variation among individuals in body condition, age, and general quality was taken into account. Interestingly, the level of natural antibodies, a component of innate immunity, showed the strongest association with components of acquired B-cell function, possibly reflecting a common underlying genetic mechanism, as has been documented in poultry. Our results indicate that despite the complexity of the immune system, important insights can be gained by using the currently available assays but in a more comprehensive approach than has generally been used in the field of ecoimmunology.

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.

State-dependent physiological maintenance in a long-lived ectotherm, the painted turtle (Chrysemys picta)

Energy allocation among somatic maintenance, reproduction and growth varies not only among species, but among individuals according to states such as age, sex and season. Little research has been conducted on the somatic (physiological) maintenance of long-lived organisms, particularly ectotherms such as reptiles. In this study, we examined sex differences and age- and season-related variation in immune function and DNA repair efficiency in a long-lived reptile, the painted turtle (Chrysemys picta). Immune components tended to be depressed during hibernation, in winter, compared with autumn or spring. Increased heterophil count during hibernation provided the only support for winter immunoenhancement. In juvenile and adult turtles, we found little evidence for senescence in physiological maintenance, consistent with predictions for long-lived organisms. Among immune components, swelling in response to phytohemagglutinin (PHA) and control injection increased with age, whereas basophil count decreased with age. Hatchling turtles had reduced basophil counts and natural antibodies, indicative of an immature immune system, but demonstrated higher DNA repair efficiency than older turtles. Reproductively mature turtles had reduced lymphocytes compared with juvenile turtles in the spring, presumably driven by a trade-off between maintenance and reproduction. Sex had little influence on physiological maintenance. These results suggest that components of physiological maintenance are modulated differentially according to individual state and highlight the need for more research on the multiple components of physiological maintenance in animals of variable states.

Variation in the seasonal patterns of innate and adaptive immunity in the red-eared slider (Trachemys scripta)

The primary function of the immune system is to protect the organism from invading pathogens. In vertebrates, this has resulted in a multifaceted system comprised of both innate and adaptive components. The immune system of all jawed vertebrates is complex, but unlike the endothermic vertebrates, relatively little is known about the functioning of the ectothermic vertebrate immune system, especially the reptilian system. Because turtles are long-lived ectotherms, factors such as temperature and age may affect their immune response, but comprehensive studies are lacking. We investigated variation in immune responses of adult male and female red-eared sliders (Trachemys scripta) across the entire active season. We characterized seasonal variation in innate, cell-mediated and humoral components via bactericidal capacity of plasma, delayed-type hypersensitivity and total immunoglobulin levels, respectively. Results indicate that all immune measures varied significantly across the active season, but each measure had a different pattern of variation. Interestingly, temperature alone does not explain the observed seasonal variation. Immune measures did not vary between males and females, but immunoglobulin levels did vary with age. This study demonstrates the highly dynamic nature of the reptilian immune system, and provides information on how biotic and abiotic factors influence the immune system of a long-lived ectotherm.

Effects of environmental change on wildlife health

Environmental change has negatively affected most biological systems on our planet and is becoming of increasing concern for the well-being and survival of many species. At an organism level, effects encompass not only endocrine disruptions, sex-ratio changes and decreased reproductive parameters, but also include teratogenic and genotoxic effects, immunosuppression and other immune-system impairments that can lead directly to disease or increase the risk of acquiring disease. Living organisms will strive to maintain health by recognizing and resolving abnormal situations, such as the presence of invading microorganisms or harmful peptides, abnormal cell replication and deleterious mutations. However, fast-paced environmental changes may pose additional pressure on immunocompetence and health maintenance, which may seriously impact population viability and persistence. Here, we outline the importance of a functional immune system for survival and examine the effects that exposure to a rapidly changing environment might exert on immunocompetence. We then address the various levels at which anthropogenic environmental change might affect wildlife health and identify potential deficits in reproductive parameters that might arise owing to new immune challenges in the context of a rapidly changing environment. Throughout the paper, a series of examples and case studies are used to illustrate the impact of environmental change on wildlife health.

Dynamics of PHA-induced immune response and plasma carotenoids in birds: should we have a closer look?

Allocation trade-offs of limited resources are thought to ensure the honesty of sexual signals and are often studied using controlled immune challenges. One such trade-off between immunity and ornaments is that involving carotenoids. Phytohemagglutinin (PHA)-induced immune response is a widely used immune challenge, yet more details on the underlying physiological mechanisms and potential costs are needed. We investigated the temporal dynamics of PHA-induced immune response and associated changes in blood carotenoids, body mass and a carotenoid-based coloured signal. We found variation in individual response patterns to PHA after peak swelling was reached, with birds showing either a rapid or a slow subsequent decrease in swelling, suggesting variation in the duration of the immune response and/or inflammation. Body mass did not affect immune response. Plasma carotenoids followed a transient decrease closely matching the dynamics of the swelling. The peak of the immune response was negatively related to initial plasma carotenoid levels and positively correlated to the relative decrease in plasma carotenoids. Individual variation in duration of the swelling could be partly explained by plasma carotenoids; high initial carotenoid levels were associated with a slower decrease of the swelling. These contradictory effects of carotenoids suggest a complex role in the immune response. Bill colour was positively correlated to initial plasma carotenoid concentration but it did not predict or change as a consequence of immune response to PHA. Bill colour thus reflects medium- or long-term quality rather than immediate quality. Taking into account the dynamics of the immune response and that of associated physiological parameters would thus yield new insights into our interpretation of variation in PHA response.

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.

Ontogeny of innate and adaptive immune defense components in free-living tree swallows, Tachycineta bicolor

Little is known about the development of immune function in wild animals. We investigated the ontogeny of immune defense in a free-living bird, the tree swallow. We assessed total and differential leukocyte counts, natural antibodies, complement activity, in vivo skin swelling response, and in vitro lymphocyte proliferation and compared the levels of development between nestlings and young adults. We also assessed whether body condition explained variation in these immune components. We found some support for the prediction that innate defenses, which do not need to generate a broad repertoire of specific receptors, would reach adult levels earlier than adaptive defenses. In contrast, we found limited support for the prediction that adaptive defenses, which are thought to be more costly to develop, would be more related to body condition than innate defenses. We discuss our findings in the context of other studies on the ontogeny of immune function.

Testosterone-mediated trade-offs in the old age: a new approach to the immunocompetence handicap and carotenoid-based sexual signalling

The immunocompetence handicap hypothesis proposes that testosterone mediates a trade-off between sexual signalling and immunocompetence in males. Such a trade-off could favour the reliability of sexual signals on the basis that testosterone required for signal expression also promotes immunosuppression. However, the immunosuppressive activity of testosterone has not been convincingly demonstrated. We propose that the optimal solution to the testosterone-mediated trade-off should change with age, explaining ambiguous results in the past. Testosterone and ageing would promote two simultaneous immunosuppressive challenges unaffordable for low-quality males. Oxidative stress, as intimately related to ageing and immunosenescence, could contribute to enhance signal reliability. In this context, traits coloured by carotenoids (yellow-red traits) could play a crucial role due to the immunostimulatory and antioxidant properties of these pigments. Here, old and middle-aged male red-legged partridges were treated with testosterone or manipulated as controls. In the presence of high-testosterone levels, middle-aged males increased both circulating carotenoid levels and colour expression, whereas their cell-mediated immunity was not significantly altered. However, in old males, neither circulating carotenoids nor sexual signalling increased when treated with testosterone, but immunosuppression was detected. The link between testosterone and carotenoids could favour the reliability of sexual signals throughout the life.

Effects of food abundance, age, and flea infestation on the body condition and immunological variables of a rodent host, and their consequences for flea survival

Temporal variation in body condition and immunological variables of animals that harbor parasites may explain patterns of variation in infestation, as well as parasite impact on the host. We emulated such variability in Sundevall's jirds by manipulating food availability and flea infestation in juveniles and adults and examining how these changes affect survival of fleas on their hosts. Body condition of food-restricted jirds deteriorated, but there was no change in their immunological variables. Adult jirds were in better body condition and had higher immunocompetence than juveniles, however there were no significant effects of flea infestation on any of the variables examined. The main effects of flea infestation were a decrease in the response to phytohaemagglutinin injection, and an increase in the negative effects of food restriction on body mass. Flea survival was higher on juveniles, but fleas did not respond to temporal variability in body condition and immunocompetence of the jirds. We concluded that changes in body condition and immune responses due to growth or variability in food abundance are more important than changes caused by the fleas themselves. Flea infestation is more detrimental to jirds when they are not able to compensate for mass loss through increased food consumption.

Neuroendocrine and immune characteristics of aging in avian species

Avian species show a remarkable diversity in lifespan. The differing lifespan patterns are found across a number of birds, in spite of higher body temperature and apparent increased metabolic rate. These characteristics make study of age-related changes of great interest, especially for understanding the biology of aging associated with surprisingly long lifespan in some birds. Our studies have focused on a short-lived avian model, the Japanese quail in order to describe reproductive aging and the neuroendocrine characteristics leading to reproductive senescence. Biomarkers of aging used in mammalian species include telomere length, oxidative damage, and selected metabolic indicators. These markers provide confirming evidence that the long-lived birds appear to age more slowly. A corollary area of interest is that of immune function and aging. Immune responses have been studied in selected wild birds and there has been a range of studies that have considered the effects of stress in wild and domestic species. Our laboratory studies have specifically tested response to immune challenge relative to aging in the quail model and these studies indicate that there is an age-related change in the qualitative aspects of the response. However, there are also intriguing differences in the ability of the aging quail to respond that differ from mammalian data. Finally, another approach to understanding aging is to attempt to develop or test strategies that may extend lifespan and presumably health. One area of great interest has been to consider the effect of calorie restriction, which is a treatment shown to extend lifespan in a variety of species. This approach is routinely used in domestic poultry as a means for extending reproductive function and enhancing health. Our data indicate that moderate calorie restriction has beneficial effects, and that physiological and endocrine responses reflect these benefits.

Differential resource allocation in deer mice exposed to sin nombre virus

The resource allocation hypothesis predicts that reproductive activity suppresses immunocompetence; however, this has never been tested in an endemic disease system with free-ranging mammals. We tested the resource allocation hypothesis in wild deer mice (Peromyscus maniculatus) with natural exposure to Sin Nombre Virus (SNV). Immunocompetence was estimated from the extent of swelling elicited after deer mice were injected with phytohemagglutinin (PHA); swelling is positively correlated with immunocompetence. After livetrapping deer mice, we determined their reproductive state and SNV infection status. Males were more likely to be seropositive for SNV than females (37% vs. 25%) and exhibited 10% less swelling after PHA injection. The swelling response of females differed with both infection status and reproductive condition. There was also a significant infection status by reproductive condition interaction: non-reproductive, seropositive females experienced the least amount of swelling, whereas females in all other categories experienced significantly greater swelling. The swelling response of males differed with both SNV infection status and reproductive condition, but there was no significant infection status by reproductive condition interaction. Seronegative males elicited greater swelling than seropositive males regardless of reproductive status. In contrast to the resource allocation hypothesis, these results do not indicate that reproductive activity suppresses immunocompetence of deer mice but rather suggest that chronic SNV infection reduces immunocompetence. Sex-based differences in swelling indicate that SNV modulates the immune system of female deer mice differently than it does that of males, particularly during reproduction. We propose that differences in resource allocation between males and females could result from inherent sex-based differences in parental investment.

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.