Environmental proxies of antigen exposure explain variation in immune investment better than indices of pace of life

Correlated evolution of oxidative physiology and MHC-based immunosurveillance in birds

Maintenance and activation of the immune system incur costs, not only in terms of substrates and energy but also via collateral oxidative damage to host cells or tissues during immune response. So far, associations between immune function and oxidative damage have been primarily investigated at intra-specific scales. Here, we hypothesized that pathogen-driven selection should favour the evolution of effective immunosurveillance mechanisms (e.g. major histocompatibility complex, MHC) and antioxidant defences to mitigate oxidative damage resulting from immune function. Using phylogenetically informed comparative approaches, we provided evidence for the correlated evolution of host oxidative physiology and MHC-based immunosurveillance in birds. Species selected for more robust MHC-based immunosurveillance (higher gene copy numbers and allele diversity) showed stronger antioxidant defences, although selection for MHC diversity still showed a positive evolutionary association with oxidative damage to lipids. Our results indicate that historical pathogen-driven selection for highly duplicated and diverse MHC could have promoted the evolution of efficient antioxidant mechanisms, but these evolutionary solutions may be insufficient to keep oxidative stress at bounds. Although the precise nature of mechanistic links between the MHC and oxidative stress remains unclear, our study suggests that a general evolutionary investment in immune function may require co-adaptations at the level of host oxidative metabolism.

Constitutive immune function is not associated with fuel stores in spring migrating passerine birds

Migratory birds may either upregulate their immune system during migration as they might encounter novel pathogens or downregulate their immune system as a consequence of trade-offs with the resource costs of migration. Support for the latter comes not least from a study that reports a positive correlation in autumn migrating birds between fuel stores and parameters of innate and acquired immune function, that is, energy-exhausted migrants appear to have lowered immune function. However, to our knowledge, no study has tested whether this pattern exists in spring migrating birds, which may face other trade-offs than autumn migrants. Here, we investigate if in spring there is a relationship between fuel stores and microbial-killing ability, a measure of innate immune function, and total immunoglobulin (IgY), a measure of acquired immune function, in four migrating bird species: chaffinches (Fringilla coelebs), dunnocks (Prunella modularis), song thrushes (Turdus philomelos) and northern wheatears (Oenanthe oenanthe). Our findings indicate no significant correlation between fuel stores and either microbial killing ability or IgY levels when considering all species collectively. When analysing species separately, we found a significant negative correlation between fuel stores and microbial-killing ability in chaffinches and a positive correlation between fuel stores and IgY levels in wheatears. In song thrushes, but not in any of the other species, there was a significant negative correlation between relative arrival date and microbial-killing ability and between arrival date and IgY levels. Sex did not affect immune function in any of the species. Our study suggests that the relationship between immune function and fuel stores may be different during spring migration compared to autumn migration. Differences in the speed of migration or pathogen pressure may result in different outcomes of the resource trade-off between investment in immune function and migration among the seasons.

Corticosterone and immune responses to dehydration in squamate reptiles

Many environments present some degree of seasonal water limitations; organisms that live in such environments must be adapted to survive periods without permanent water access. Often this involves the ability to tolerate dehydration, which can have adverse physiological effects and is typically considered a physiological stressor. While having many functions, the hormone corticosterone (CORT) is often released in response to stressors, yet increasing plasma CORT while dehydrated could be considered maladaptive, especially for species that experience predictable bouts of dehydration and have related coping mechanisms. Elevating CORT could reduce immunocompetence and have other negative physiological effects. Thus, such species likely have CORT and immune responses adapted to experiencing seasonal droughts. We evaluated how dehydration affects CORT and immune function in eight squamate species that naturally experience varied water limitation. We tested whether hydric state affected plasma CORT concentrations and aspects of immunocompetence (lysis, agglutination, bacterial killing ability and white blood cell counts) differently among species based on how seasonally water limited they are and whether this is constrained by phylogeny. The species represented four familial pairs, with one species of each pair inhabiting environments with frequent access to water and one naturally experiencing extended periods (>30 days) with no access to standing water. The effects of dehydration on CORT and immunity varied among species. Increases in CORT were generally not associated with reduced immunocompetence, indicating CORT and immunity might be decoupled in some species. Interspecies variations in responses to dehydration were more clearly grouped by phylogeny than by habitat type.

Seasonal changes in diet, immune function, and oxidative stress in three passerines inhabiting a Mediterranean climate

Oxidative status and immune function are energy-demanding traits closely linked to diet composition, particularly resource availability and nutritional value. In seasonal environments, nutrient availability and diet quality fluctuate, potentially influencing these traits. However, limited evidence exists regarding these dietary effects on immune function in seasonal environments. In this study, we employed stable isotope analysis to assess the impact of seasonal changes in niche width and trophic level (i.e., δ15N) on two immune variables (hemolysis and hemagglutination scores) and two oxidative status parameters (lipid peroxidation and total antioxidant capacity) in three passerine species: Zonotrichia capensis (omnivorous), Troglodytes aedon (insectivorous), and Spinus barbatus (granivorous). We found that hemolysis scores varied seasonally in Z. capensis, with higher values in winter compared to summer. Total antioxidant capacity (TAC) also increased during the winter in Z. capensis and S. barbatus. The isotopic niche width for Z. capensis and S. barbatus was smaller in winter than in summer, with the omnivorous species exhibiting a decrease in δ15N. Despite the seasonal shifts in ecological and physiological traits in Z. capensis, we identified no correlation between immune response and TAC with trophic level. In contrast, in the granivorous S. barbatus, the lower trophic level resulted in an increase in TAC without affecting immunity. Our findings revealed that dietary shifts do not uniformly impact oxidative status and immune function across bird species, highlighting species-specific responses to seasonal changes. This underscores the importance of integrating ecological and evolutionary perspectives when examining how diet shapes avian immunity and oxidative balance.

Evolutionary trade-off between innate and acquired immune defences in birds

BACKGROUND

The development, maintenance, and use of immune defences are costly. Therefore, animals face trade-offs in terms of resource allocation within their immune system and between their immune system and other physiological processes. To maximize fitness, evolution may favour investment in one immunological defence or subsystem over another in a way that matches a species broader life history strategy. Here, we used phylogenetically-informed comparative analyses to test for relationships between two immunological components. Natural antibodies and complement were used as proxies for the innate branch; structural complexity of the major histocompatibility complex (MHC) region was used for the acquired branch.

RESULTS

We found a negative association between the levels of natural antibodies (i.e., haemagglutination titre) and the total MHC gene copy number across the avian phylogeny, both at the species and family level. The family-level analysis indicated that this association was apparent for both MHC-I and MHC-II, when copy numbers within these two MHC regions were analysed separately. The association remained significant after controlling for basic life history components and for ecological traits commonly linked to pathogen exposure.

CONCLUSION

Our results provide the first phylogenetically robust evidence for an evolutionary trade-off within the avian immune system, with a more developed acquired immune system (i.e., more complex MHC architecture) in more derived bird lineages (e.g., passerines) being accompanied by an apparent downregulation of the innate immune system.

Cloacal microbiota are biogeographically structured in larks from desert, tropical and temperate areas

BACKGROUND

In contrast with macroorganisms, that show well-documented biogeographical patterns in distribution associated with local adaptation of physiology, behavior and life history, strong biogeographical patterns have not been found for microorganisms, raising questions about what determines their biogeography. Thus far, large-scale biogeographical studies have focused on free-living microbes, paying little attention to host-associated microbes, which play essential roles in physiology, behavior and life history of their hosts. Investigating cloacal gut microbiota of closely-related, ecologically similar free-living songbird species (Alaudidae, larks) inhabiting desert, temperate and tropical regions, we explored influences of geographical location and host species on α-diversity, co-occurrence of amplicon sequence variants (ASVs) and genera, differentially abundant and dominant bacterial taxa, and community composition. We also investigated how geographical distance explained differences in gut microbial community composition among larks.

RESULTS

Geographic location did not explain variation in richness and Shannon diversity of cloacal microbiota in larks. Out of 3798 ASVs and 799 bacterial genera identified, 17 ASVs (< 0.5%) and 43 genera (5%) were shared by larks from all locations. Desert larks held fewer unique ASVs (25%) than temperate zone (31%) and tropical larks (34%). Five out of 33 detected bacterial phyla dominated lark cloacal gut microbiomes. In tropical larks three bacterial classes were overrepresented. Highlighting the distinctiveness of desert lark microbiota, the relative abundances of 52 ASVs differed among locations, which classified within three dominant and 11 low-abundance phyla. Clear and significant phylogenetic clustering in cloacal microbiota community composition (unweighted UniFrac) showed segregation with geography and host species, where microbiota of desert larks were distinct from those of tropical and temperate regions. Geographic distance was nonlinearly associated with pairwise unweighted UniFrac distances.

CONCLUSIONS

We conclude that host-associated microbiota are geographically structured in a group of widespread but closely-related bird species, following large-scale macro-ecological patterns and contrasting with previous findings for free-living microbes. Future work should further explore if and to what extent geographic variation in host-associated microbiota can be explained as result of co-evolution between gut microbes and host adaptive traits, and if and how acquisition from the environmental pool of bacteria contributes to explaining host-associated communities.

Plastic plumage colouration in response to experimental humidity supports Gloger's rule

Knowing how animals adapt their phenotype to local temperature and humidity is key to understanding not only ecogeographical rules, but also how species will manage climate change, as current models predict changes in global patterns of temperature and precipitation. In endotherms, colour adaptations in response to climate have been under investigated, and their acclimatization-the individual capacity to reversibly adjust phenotype in response to different environments-is unknown. Geographic trends can provide clues about abiotic variables involved in colouration, as postulated by Gloger's rule, which predicts darker individuals in warm and humid regions. We tested whether house sparrows (Passer domesticus) can adjust colouration when faced with varying humidity conditions. We exposed birds to either a dry (humidity 45%) or a wet environment (70%) six months before their moult, and measured colouration in newly developed feathers in five parts of the body (bib, crown, crown stripe, belly and rump). As predicted by Gloger's rule, birds in wet conditions developed darker (bib and belly) and larger (bib) melanised plumage patches, than birds in dry conditions. Our result provides the first unequivocal evidence that the ability of individual birds to adjust their colouration may be a potential adaptation to climatic changes in endotherms.

Early and Late Migrating Avian Individuals Differ in Constitutive Immune Function and Blood Parasite Infections – But Patterns Depend on the Migratory Strategy

Billions of birds migrate every year. To conduct a successful migration, birds undergo a multitude of physiological adaptions. One such adaptation includes adjustments of immune function, however, little is known about intraspecies (between-individual) and interspecies (between-species) variation in immune modulations during migration. Here, we explore if early and late migrating individuals differ in their immune function, and if such patterns differ among species with short- vs. long-distance migration strategies. We quantified three parameters of baseline (constitutive) innate immune function and one parameter of baseline (constitutive) acquired immune function in 417 individuals of 10 species caught during autumn migration at Falsterbo (Sweden). Early and late migrating individuals differed in lysis and total immunoglobulins (IgY), but the patterns show different directions in long-distance migrants (LDMs) (wintering in Africa) as compared to short-distance migrants (SDMs) (wintering within Europe). Specifically, early migrating LDMs had lower lysis but higher immunoglobulin levels than late migrating individuals. In short distance migrants, there was no difference in lysis between early and late migrating individuals, but immunoglobulin levels were higher in late migrating individuals. We found no correlation between timing of migration and haptoglobin, but LDMs had lower levels of haptoglobin than SDMs. We also found that the prevalence of haemosporidian blood parasite infections decreased in LDMs, but increased in SDMs, as the autumn progressed. Taken together, our study suggests that the investment into immune function depends on the migratory strategy (short- vs. long-distance migrants), and that early and late migrating individuals of a migration strategy might invest differently in baseline immune function, potentially driven by differences in the trade-offs with timing and speed of migration. Our study highlights the potential adaptations of immune function that could help explain trade-offs with other physiological systems, and behavioural responses during migration.

Immunosenescence in the wild? A longitudinal study in a long-lived seabird

Longitudinal studies of various vertebrate populations have demonstrated senescent declines in reproductive performance and survival probability to be almost ubiquitous. Longitudinal studies of potential underlying proximate mechanisms, however, are still scarce. Due to its critical function in the maintenance of health and viability, the immune system is among the potential (mediators of) proximate mechanisms that could underlie senescence. Here, we studied three innate immune parameters-haemagglutination titre, haemolysis titre and haptoglobin concentration-in a population of common terns (Sterna hirundo) known to undergo actuarial senescence. We repeatedly sampled birds of known sex and age across 11 years and used random regression models to (a) quantify how immune parameters vary among individuals and (b) describe within-individual age-specific changes in, and potential trade-offs between, immune parameters. Our models revealed no differences between males and females in haemagglutination titre and haptoglobin concentration, and very low among-individual variation in these parameters in general. Within individuals, haemagglutination titre increased with age, while haptoglobin concentration did not change. We found no indication for selective (dis)appearance in relation to haemagglutination titre or haptoglobin concentration, nor for the existence of a trade-off between them. Haemolysis was absent in the majority (76%) of samples. Common terns do not exhibit clear senescence in haemagglutination titre and haptoglobin concentration and show very little among-individual variation in these parameters in general. This may be explained by canalisation of the immune parameters or by the colonial breeding behaviour of our study species, but more longitudinal studies are needed to facilitate investigation of links between species' characteristics and immunosenescence in wild animals.

Immune function differs among tropical environments but is not downregulated during reproduction in three year-round breeding equatorial lark populations

Seasonal variation in immune function can be attributed to life history trade-offs, and to variation in environmental conditions. However, because phenological stages and environmental conditions co-vary in temperate and arctic zones, their separate contributions have not been determined. We compared immune function and body mass of incubating (female only), chick-feeding (female and male), and non-breeding (female and male) red-capped larks Calandrella cinerea breeding year-round in three tropical equatorial (Kenya) environments with distinct climates. We measured four immune indices: haptoglobin, nitric oxide, agglutination, and lysis. To confirm that variation in immune function between breeding (i.e., incubating or chick-feeding) and non-breeding was not confounded by environmental conditions, we tested if rainfall, average minimum temperature (T_min), and average maximum temperature (T_max) differed during sampling times among the three breeding statuses per location. T_min and T_max differed between chick-feeding and non-breeding, suggesting that birds utilized environmental conditions differently in different locations for reproduction. Immune indices did not differ between incubating, chick-feeding and non-breeding birds in all three locations. There were two exceptions: nitric oxide was higher during incubation in cool and wet South Kinangop, and it was higher during chick-feeding in the cool and dry North Kinangop compared to non-breeding birds in these locations. For nitric oxide, agglutination, and lysis, we found among-location differences within breeding stage. In equatorial tropical birds, variation in immune function seems to be better explained by among-location climate-induced environmental conditions than by breeding status. Our findings raise questions about how within-location environmental variation relates to and affects immune function.

Parental morph combination does not influence innate immune function in nestlings of a colour-polymorphic African raptor

Conditions experienced during early life can have long-term individual consequences by influencing dispersal, survival, recruitment and productivity. Resource allocation during development can have strong carry-over effects onto these key parameters and is directly determined by the quality of parental care. In the black sparrowhawk (Accipiter melanoleucus), a colour-polymorphic raptor, parental morphs influence nestling somatic growth and survival, with pairs consisting of different colour morphs ('mixed-morph pairs') producing offspring with lower body mass indices, but higher local apparent survival rates. Resource allocation theory could explain this relationship, with nestlings of mixed-morph pairs trading off a more effective innate immune system against somatic growth. We quantified several innate immune parameters of nestlings (hemagglutination, hemolysis, bacteria-killing capacity and haptoglobin concentration) and triggered an immune response by injecting lipopolysaccharides. Although we found that nestlings with lower body mass index had higher local survival rates, we found no support for the proposed hypothesis: neither baseline immune function nor the induced immune response of nestlings was associated with parental morph combination. Our results suggest that these immune parameters are unlikely to be involved in providing a selective advantage for the different colour morphs' offspring, and thus innate immunity does not appear to be traded off against a greater allocation of resources to somatic growth. Alternative hypotheses explaining the mechanism of a low nestling body mass index leading to subsequent higher local survival could be related to the post-fledgling dependency period or differences in dispersal patterns for the offspring from different morph combinations.

Current and time-lagged effects of climate on innate immunity in two sympatric snake species

Changing environments result in alterations at all levels of biological organization, from genetics to physiology to demography. The increasing frequency of droughts worldwide is associated with higher temperatures and reduced precipitation that can impact population persistence via effects on individual immune function and survival.We examined the effects of annual climate variation on immunity in two sympatric species of garter snakes from four populations in California over a seven-year period that included the record-breaking drought.We examined three indices of innate immunity: bactericidal competence (BC), natural antibodies (NABs), and complement-mediated lysis (CL).Precipitation was the only climatic variable explaining variation in immune function: spring precipitation of the current year was positively correlated to Thamnophis sirtalis BC and NABs, whereas spring precipitation of the previous year was positively correlated to T. elegans BC and NABs. This suggests that T. elegans experiences a physiological time-lag in response to reduced precipitation, which may reflect lack of capital for investment in immunity in the year following a dry year.In general, our findings demonstrate compelling evidence that climate can influence wild populations through effects on physiological processes, suggesting that physiological indices such as these may offer valuable opportunities for monitoring the effects of climate.

Jackdaw nestlings rapidly increase innate immune function during the nestling phase but no evidence for a trade-off with growth

Although animals are born with a protective immune system, even the innate immune system is under development from birth to adulthood and this development may be affected by sex and growth. However, most knowledge comes from captive animals or long-lived slow growing species. Moreover, little is known about how innate immune function, the important first line of defence, develops during early life in fast-growing animals such as free-living passerines. We studied development of innate baseline immune function in nestlings of free-living jackdaws Corvus monedula. We measured four immune parameters (hemolysis, hemagglutination, bacterial-killing capacity, haptoglobin concentration) and structural body size (body mass, wing length, tarsus length) at day 12 and day 29 post-hatching. We found that three out of four immune parameters (hemolysis, hemagglutination, bacterial-killing capacity) substantially increased with nestling age and had roughly reached adult levels shortly prior to fledging. We found little differences in immune development between males and females despite them differing in structural development. We also found no evidence that the nestlings traded off immune development with growth. That nestlings rapidly increase innate baseline immune function during early life and similarly in males and females indicates the importance of a well-functioning immune system already during the nestling phase.

Microbial environment shapes immune function and cloacal microbiota dynamics in zebra finches Taeniopygia guttata

Background

The relevance of the host microbiota to host ecology and evolution is well acknowledged. However, the effect of the microbial environment on host immune function and host microbiota dynamics is understudied in terrestrial vertebrates. Using a novel experimental approach centered on the manipulation of the microbial environment of zebra finches Taeniopygia guttata, we carried out a study to investigate effects of the host’s microbial environment on: 1) constitutive immune function, 2) the resilience of the host cloacal microbiota; and 3) the degree to which immune function and host microbiota covary in microbial environments that differ in diversity.

Results

We explored immune indices (hemagglutination, hemolysis, IgY levels and haptoglobin concentration) and host-associated microbiota (diversity and composition) in birds exposed to two experimental microbial environments differing in microbial diversity. According to our expectations, exposure to experimental microbial environments led to differences related to specific antibodies: IgY levels were elevated in the high diversity treatment, whereas we found no effects for the other immune indices. Furthermore, according to predictions, we found significantly increased richness of dominant OTUs for cloacal microbiota of birds of the high diversity compared with the low diversity group. In addition, cloacal microbiota of individual females approached their baseline state sooner in the low diversity environment than females in the high diversity environment. This result supported a direct phenotypically plastic response of host microbiota, and suggests that its resilience depends on environmental microbial diversity. Finally, immune indices and cloacal microbiota composition tend to covary within treatment groups, while at the same time, individuals exhibited consistent differences of immune indices and microbiota characteristics.

Conclusion

We show that microbes in the surroundings of terrestrial vertebrates can influence immune function and host-associated microbiota dynamics over relatively short time scales. We suggest that covariation between immune indices and cloacal microbiota, in addition to large and consistent differences among individuals, provides potential for evolutionary adaptation. Ultimately, our study highlights that linking environmental and host microbiotas may help unravelling immunological variation within and potentially among species, and together these efforts will advance the integration of microbial ecology and ecological immunology.

Geographic variation in baseline innate immune function does not follow variation in aridity along a tropical environmental gradient

Geographic variation in aridity determines environmental productivity patterns, including large-scale variability in pathogens, vectors and associated diseases. If disease risk decreases with increasing aridity and is matched by immune defense, we predict a decrease in innate immune function along a gradient of increasing aridity from the cool-wet forest to the hot-dry Sahel, from south to north in Nigeria. We sampled blood and measured five innate immune indices from 286 Common Bulbuls Pycnonotus barbatus between 6 and 13°N. We sampled in the dry season; we resampled the first location (Jos) also as the last sample location to test temporal change in immune function. Immune indices did not decrease with aridity. One immune index, nitric oxide concentration showed a weak quadratic pattern. In Jos, ovotransferrin concentration, haemagglutination and haemolysis titres increased 12 weeks into the dry season, contrary to expectations that immune indices should decrease with increased dryness. In this tropical system, innate immune function does not decrease with increasing aridity but temporal factors within a location may influence immune function more strongly than spatial variation in aridity, suggesting that immune variation does not follow a simple environmental productivity pattern. Consequently, caution should probably be exercised in predicting effects of climate variability on immune function or disease risk.

Avian Health across the Landscape: Nestling Immunity Covaries with Changing Landcover

The landscape composition of an organism's home range or territory should influence aspects of its condition, including measures of immune function. Changes in immunocompetence arising from variation in landcover may provide important links between habitat changes and patterns of disease spread. To establish a baseline understanding for whether immune measures covary with changes in landcover, we examined associations between immunological parameters and landcover composition for adults and nestlings of five shrubland bird species. Specifically, we examined the bacteria-killing ability (BKA) of the blood plasma and profiles of the five avian leukocytes as our measures of immune function, and assessed the proportion of area around each bird's nest that was composed of the four major landcover types in the Midwestern USA: row crop agriculture, developed, forest, and grass/shrub. We performed landcover assessments at 100 and 1000 m radius buffers to identify whether associations between habitat and immune function differed at the two spatial scales. As part of this work, we examined age and species-related immunological variation, as well as associations among the immune parameters. There was little evidence linking variation in immune function to landcover composition for the adults at either spatial scale, but there were numerous associations for nestlings, and these were stronger at the 1000 than 100 m spatial scale. The proportion of grass/shrub around the nest had the largest impact on immune function, although the effect varied by immune parameter and species. BKA and basophils were inversely associated with grass/shrub for all species, whereas lymphocytes were positively associated with grass/shrub for all species. We also documented species-level differences among adults and nestlings for BKA and all leukocytes except monocytes. As expected, we found that nestlings had reduced levels of BKA, lymphocytes, monocytes, and elevated heterophils compared with adults (except for field sparrow-Spizella pusilla-nestlings, which had higher lymphocytes). Basophils generally did not differ by age class, and eosinophils exhibited species-specific patterns, in which they were higher for nestling American robins (Turdus migratorius) and gray catbirds (Dumetella carolinensis) compared with adults, but lower in the other nestlings. Heterophils and lymphocytes were inversely associated for all species and age classes, and basophil levels were positively associated with BKA across species and age classes. Together, these findings bolster our understanding of age and species-specific variation in immune function, and provide evidence that immune measures can covary with changes in landcover.

Macroimmunology: The drivers and consequences of spatial patterns in wildlife immune defence

The prevalence and intensity of parasites in wild hosts varies across space and is a key determinant of infection risk in humans, domestic animals and threatened wildlife. Because the immune system serves as the primary barrier to infection, replication and transmission following exposure, we here consider the environmental drivers of immunity. Spatial variation in parasite pressure, abiotic and biotic conditions, and anthropogenic factors can all shape immunity across spatial scales. Identifying the most important spatial drivers of immunity could help pre-empt infectious disease risks, especially in the context of how large-scale factors such as urbanization affect defence by changing environmental conditions. We provide a synthesis of how to apply macroecological approaches to the study of ecoimmunology (i.e. macroimmunology). We first review spatial factors that could generate spatial variation in defence, highlighting the need for large-scale studies that can differentiate competing environmental predictors of immunity and detailing contexts where this approach might be favoured over small-scale experimental studies. We next conduct a systematic review of the literature to assess the frequency of spatial studies and to classify them according to taxa, immune measures, spatial replication and extent, and statistical methods. We review 210 ecoimmunology studies sampling multiple host populations. We show that whereas spatial approaches are relatively common, spatial replication is generally low and unlikely to provide sufficient environmental variation or power to differentiate competing spatial hypotheses. We also highlight statistical biases in macroimmunology, in that few studies characterize and account for spatial dependence statistically, potentially affecting inferences for the relationships between environmental conditions and immune defence. We use these findings to describe tools from geostatistics and spatial modelling that can improve inference about the associations between environmental and immunological variation. In particular, we emphasize exploratory tools that can guide spatial sampling and highlight the need for greater use of mixed-effects models that account for spatial variability while also allowing researchers to account for both individual- and habitat-level covariates. We finally discuss future research priorities for macroimmunology, including focusing on latitudinal gradients, range expansions and urbanization as being especially amenable to large-scale spatial approaches. Methodologically, we highlight critical opportunities posed by assessing spatial variation in host tolerance, using metagenomics to quantify spatial variation in parasite pressure, coupling large-scale field studies with small-scale field experiments and longitudinal approaches, and applying statistical tools from macroecology and meta-analysis to identify generalizable spatial patterns. Such work will facilitate scaling ecoimmunology from individual- to habitat-level insights about the drivers of immune defence and help predict where environmental change may most alter infectious disease risk.

Not just fuel: energy stores are correlated with immune function and oxidative damage in a long-distance migrant

In many animals, catabolic and anabolic periods are temporally separated. Migratory birds alternate energy expenditure during flight with energy accumulation during stopover. The size of the energy stores at stopover affects the decision to resume migration and thus the temporal organization of migration. We now provide data suggesting that it is not only the size of the energy stores per se that may influence migration scheduling, but also the physiological consequences of flying. In two subspecies of the northern wheatear Oenanthe oenanthe, a long-distance migrant, estimated energy stores at a stopover during autumn migration were positively related with both constitutive innate and acquired immune function, and negatively related with oxidative damage to lipids. In other words, migrants' physiological condition was associated with their energetic condition. Although time spent at stopover before sampling may have contributed to this relationship, our results suggest that migrants have to trade-off the depletion of energy stores during flight with incurring physiological costs. This will affect migrants' decisions when to start and when to terminate a migratory flight. The physiological costs associated with the depletion of energy stores may also help explaining why migrants often arrive at and depart from stopover sites with larger energy stores than expected. We propose that studies on the role of energy stores as drivers of the temporal organization of (avian) migration need to consider physiological condition, such as immunological and oxidative states.

Seasonal differences in baseline innate immune function are better explained by environment than annual cycle stage in a year-round breeding tropical songbird

Seasonal variation in innate immunity is often attributed to either temporal environmental variation or to life-history trade-offs that arise from specific annual cycle stages but decoupling them is difficult in natural populations. Here, we effectively decouple seasonal environmental variation from annual cycle stage effects by exploiting cross-seasonal breeding and moult in the tropical Common Bulbul Pycnonotus barbatus. We test how annual cycle stage interacts with a key seasonal environmental variable, rainfall, to determine immunity at population and individual level. If immune challenge varies with precipitation, we might expect immune function to be higher in the wet season due to increased environmental productivity. If breeding or moult imposes resource constraints on birds, depending on or independent of precipitation, we might expect lower immune indices during breeding or moult. We sampled blood from 818 birds in four annual cycle stage categories: breeding, moult, simultaneous breeding and moulting, or neither. We quantified indices of innate immunity (haptoglobin, nitric oxide (NOx ) and ovotransferrin concentrations, and haemagglutination and haemolysis titres) over two annual cycles of wet and dry seasons. Environment (but not annual cycle stage or interactions between both) explained variation in all immune indices, except NOx . NOx concentration differed between annual cycle stages but not between seasons. However, within the wet season, haptoglobin, NOx , ovotransferrin and haemolysis differed significantly between breeding and non-breeding females. Aside from some recorded inconsistencies, population level results were largely similar to results within individuals that were measured repeatedly. Unexpectedly, most immune indices were higher in the dry season and during breeding. Higher immune indices may be explained if fewer or poorer quality resources force birds to increase social contact, thereby exposing individuals to novel antigens and increased infection risk, independently of environmental productivity. Breeding birds may also show higher immunity if less immune-competent and/or infected females omit breeding. We conclude that seasonal environmental variation impacts immunity more directly in natural animal populations than via resource trade-offs. In addition, immune indices were more often variable within than among individuals, but some indices are characteristic of individuals, and so may offer selective advantages if heritable.

Different incubation patterns affect selective antimicrobial properties of the egg interior: experimental evidence from eggs of precocial and altricial birds

Avian eggs contend with omnipresent microorganisms entering the egg interior, where they affect embryo viability and hatchling phenotype. The incubation behaviour and deposition of egg white antimicrobial proteins (AMPs) vary highly across the avian altricial-precocial spectrum. Experimental evidence of how these alterations in avian reproductive strategies affect the antimicrobial properties of the precocial and altricial egg interior is lacking, however. Here, we tested the egg white antimicrobial activity in eggs of two representative model species, from each end of the avian altricial-precocial spectrum, against potentially pathogenic and beneficial probiotic microorganisms. Eggs were experimentally treated to mimic un-incubated eggs in the nest, partial incubation during the egg-laying period, the onset of full incubation and the increased deposition of two main egg white AMPs, lysozyme and ovotransferrin. We moreover assessed to what extent egg antimicrobial components, egg white pH and AMP concentrations varied as a result of different incubation patterns. Fully incubated precocial and altricial eggs decreased their antimicrobial activity against a potentially pathogenic microorganism, whereas partial incubation significantly enhanced the persistence of a beneficial probiotic microorganism in precocial eggs. These effects were most probably conditioned by temperature-dependent alterations in egg white pH and AMP concentrations. While lysozyme concentration and pH decreased in fully incubated precocial but not altricial eggs, egg white ovotransferrin increased along with the intensity of incubation in both precocial and altricial eggs. This study is the first to experimentally demonstrate that different incubation patterns may have selective antimicrobial potential mediated by species-specific effects on antimicrobial components in the egg white.

Short-Term Climate Variation Drives Baseline Innate Immune Function and Stress in a Tropical Bird: A Reactive Scope Perspective

Investment in immune function can be costly, and life-history theory predicts trade-offs between immune function and other physiological demands. Environmental heterogeneity may constrain or change the optimal strategy and thereby alter baseline immune function (possibly mediated by stress responses). We tested several hypotheses relating variation in climatic, ecological, and social environments to chronic stress and levels of baseline innate immunity in a wild, cooperatively breeding bird, the purple-crowned fairy-wren (Malurus coronatus coronatus). From samples collected biannually over 5 yr, we quantified three indexes of constitutive innate immune function (haptoglobin/PIT54, natural antibodies, complement activity) and one index of chronic stress (heterophil-lymphocyte ratio; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>n</mml:mi><mml:mo>=</mml:mo><mml:mn>513</mml:mn><mml:mtext>-</mml:mtext><mml:mn>647</mml:mn></mml:mrow></mml:math> ). Using an information-theoretic and multimodel inference statistical approach, we found that habitat quality and social group size did not affect any immune index, despite hypothesized links to resource abundance and parasite pressure. Rather, short-term variation in temperature and rainfall was related to immune function, while overall differences between seasons were small or absent, despite substantial seasonal variation in climate. Contrary to our expectation, we found no evidence that physiological stress mediated any effects of short-term climatic variables on immune indexes, and alternative mechanisms may be involved. Our results may be interpreted from the perspective of reactive scope models, whereby predictive homeostasis maintains standing immune function relative to long-term demands, while short-term environmental change, being less predictable, has a greater influence on baseline immune function.

Immune function and blood parasite infections impact stopover ecology in passerine birds

Stopovers play a crucial role for the success of migrating animals and are key to optimal migration theory. Variation in refuelling rates, stopover duration and departure decisions among individuals has been related to several external factors. The physiological mechanisms shaping stopover ecology are, however, less well understood. Here, we explore how immune function and blood parasite infections relate to several aspects of stopover behaviour in autumn migrating short- and long-distance migrating songbirds. We blood sampled individuals of six species and used an automated radio-telemetry system in the stopover area to subsequently quantify stopover duration, 'bush-level' activity patterns (~ 0.1-30 m), landscape movements (~ 30-6000 m), departure direction and departure time. We show that complement activity, the acute phase protein haptoglobin and blood parasite infections were related to prolonged stopover duration. Complement activity (i.e., lysis) and total immunoglobulins were negatively correlated with bush-level activity patterns. The differences partly depended on whether birds were long-distance or short-distance migrants. Birds infected with avian malaria-like parasites showed longer landscape movements during the stopover than uninfected individuals, and birds with double blood parasite infections departed more than 2.5 h later after sunset/sunrise suggesting shorter flight bouts. We conclude that variation in baseline immune function and blood parasite infection status affects stopover ecology and helps explain individual variation in stopover behaviour. These differences affect overall migration speed, and thus can have significant impact on migration success and induce carry-over effects on other annual-cycle stages. Immune function and blood parasites should, therefore, be considered as important factors when applying optimal migration theory.

A mimicked bacterial infection prolongs stopover duration in songbirds-but more pronounced in short- than long-distance migrants

Migration usually consists of intermittent travel and stopovers, the latter being crucially important for individuals to recover and refuel to successfully complete migration. Quantifying how sickness behaviours influence stopovers is crucial for our understanding of migration ecology and how diseases spread. However, little is known about infections in songbirds, which constitute the majority of avian migrants. We experimentally immune-challenged autumn migrating passerines (both short- and long-distance migrating species) with a simulated bacterial infection. Using an automated radiotelemetry system in the stopover area, we subsequently quantified stopover duration, "bush-level" activity patterns (0.1-30 m) and landscape movements (30-6,000 m). We show that compared to controls, immune-challenged birds prolonged their stopover duration by on average 1.2 days in long-distance and 2.9 days in short-distance migrants, respectively (100%-126% longer than controls, respectively). During the prolonged stopover, the immune-challenged birds kept a high "bush-level" activity (which was unexpected) but reduced their local movements, independent of migration strategy. Baseline immune function, but not blood parasite infections prior to the immune challenge, had a prolonging effect on stopover duration, particularly in long-distance migrants. We conclude that a mimicked bacterial infection does not cause lethargy, per se, but restricts landscape movements and prolongs stopover duration, and that this behavioural response also depends on the status of baseline immune function and migration strategy. This adds a new level to the understanding of how acute inflammation affect migration behaviour and hence the ecology and evolution of migration. Accounting for these effects of bacterial infections will also enable us to fine-tune and apply optimal migration theory. Finally, it will help us predicting how migrating animals may respond to increased pathogen pressure caused by global change.

A genetics-based approach confirms immune associations with life history across multiple populations of an aquatic vertebrate (Gasterosteus aculeatus)

Understanding how wild immune variation covaries with other traits can reveal how costs and trade‐offs shape immune evolution in the wild. Divergent life history strategies may increase or alleviate immune costs, helping shape immune variation in a consistent, testable way. Contrasting hypotheses suggest that shorter life histories may alleviate costs by offsetting them against increased mortality, or increase the effect of costs if immune responses are traded off against development or reproduction. We investigated the evolutionary relationship between life history and immune responses within an island radiation of three‐spined stickleback, with discrete populations of varying life histories and parasitism. We sampled two short‐lived, two long‐lived and an anadromous population using qPCR to quantify current immune profile and RAD‐seq data to study the distribution of immune variants within our assay genes and across the genome. Short‐lived populations exhibited significantly increased expression of all assay genes, which was accompanied by a strong association with population‐level variation in local alleles and divergence in a gene that may be involved in complement pathways. In addition, divergence around the eda gene in anadromous fish is likely associated with increased inflammation. A wider analysis of 15 populations across the island revealed that immune genes across the genome show evidence of having diverged alongside life history strategies. Parasitism and reproductive investment were also important sources of variation for expression, highlighting the caution required when assaying immune responses in the wild. These results provide strong, gene‐based support for current hypotheses linking life history and immune variation across multiple populations of a vertebrate model.

Understanding immune function as a pace of life trait requires environmental context

This article provides a brief historical perspective on the integration of physiology into the concept of the pace of life of birds, evaluates the fit of immune function into this framework, and asks what it will take to fruitfully understand immune functioning of birds in pace of life studies in the future. In the late 1970s, physiology started to seriously enter avian life history ecology, with energy as the main currency of interest, inspired by David Lack's work in the preceding decades emphasizing how food availability explained life history variation. In an effort to understand the trade-off between survival and reproduction, and specifically the mortality costs associated with hard work, in the 1980s and 1990s, other physiological phenomena entered the realm of animal ecologists, including endocrinology, oxidative stress, and immunology. Reviewing studies thus far to evaluate the role of immune function in a life history context and particularly to address the questions whether immune function (1) consistently varies with life history variation among free-living bird species and (2) mediates life history trade-offs in experiments with free-living bird species; I conclude that, unlike energy metabolism, the immune system does not closely covary with life history among species nor mediates the classical trade-offs within individuals. Instead, I propose that understanding the tremendous immunological variation uncovered among free-living birds over the past 25 years requires a paradigm shift. The paradigm should shift from viewing immune function as a costly trait involved in life history trade-offs to explicitly including the benefits of the immune system and placing it firmly in an environmental and ecological context. A first step forward will be to quantify the immunobiotic pressures presented by diverse environmental circumstances that both shape and challenge the immune system of free-living animals. Current developments in the fields of infectious wildlife diseases and host-microbe interactions provide promising steps in this direction.

Investigating Relationships between Reproduction, Immune Defenses, and Cortisol in Dall Sheep

Life-history theory is fundamental to understanding how animals allocate resources among survival, development, and reproduction, and among traits within these categories. Immediate trade-offs occur within a short span of time and, therefore, are more easily detected. Trade-offs, however, can also manifest across stages of the life cycle, a phenomenon known as carryover effects. We investigated trade-offs on both time scales in two populations of Dall sheep (Ovis dalli dalli) in Southcentral Alaska. Specifically, we (i) tested for glucocorticoid-mediated carryover effects from the breeding season on reproductive success and immune defenses during parturition and (ii) tested for trade-offs between immune defenses and reproduction within a season. We observed no relationship between cortisol during mating and pregnancy success; however, we found marginal support for a negative relationship between maternal cortisol and neonate birth weights. Low birth weights, resulting from high maternal cortisol, may result in low survival or low fecundity for the neonate later in life, which could result in overall population decline. We observed a negative relationship between pregnancy and bacterial killing ability, although we observed no relationship between pregnancy and haptoglobin. Study site affected bactericidal capacity and the inflammatory response, indicating the influence of external factors on immune responses, although we could not test hypotheses about the cause of those differences. This study helps advance our understanding of the plasticity and complexity of the immune system and provides insights into the how individual differences in physiology may mediate differences in fitness.

Indices of immune function used by ecologists are mostly unaffected by repeated freeze-thaw cycles and methodological deviations

BACKGROUND

Over the past couple of decades, measuring immunological parameters has become widespread in studies of ecology and evolution. A combination of different immunological indices is useful for quantifying different parts of the immune system and comprehensively assessing immune function. Running multiple immune assays usually requires samples to be repeatedly thawed and re-frozen. There is some evidence that repeated freezing and thawing can affect assay results, but this has never been comprehensively studied in some common ecological immunology assays. We tested the effect of multiple (1, 2, 3, 4, 5, 10) freeze-thaw cycles on the results of four commonly used immunological assays: haemolysis-haemagglutination titres, haptoglobin concentration, bacterial killing capacity and total immunoglobulins (IgY). We tested five different bird species from four different bird orders (Passeriformes, Columbiformes, Charadriiformes and Galliformes), and we included both captive and free-living individuals. In addition, we tested for haptoglobin concentrations and the haemolysis-haemagglutination assay if re-analysing samples 1 year apart led to different results. For the haemolysis-haemagglutination assay we also tested two different sources of rabbit blood, and we compared untreated microtitre plates with plates that were "blocked" to prevent nonspecific interactions between the plate and assay reagents.

RESULTS

Repeated freezing and thawing of plasma had no effect on lysis titres, haptoglobin concentrations, bacterial killing capacity, or total immunoglobulin levels. Agglutination titres were unaffected by up to five cycles but were lower after ten freeze-thaw cycles. For the haemolysis-haemagglutination assay and haptoglobin concentrations, re-analysing samples 1 year apart yielded highly correlated data. For the haemolysis-haemagglutination assay, the source of rabbit blood did not influence the results, and the untreated vs. blocked plates differed slightly overall, but at the individual level assay results were highly correlated. Using different rabbit blood sources or different types of microtitre plates yielded highly correlated data.

CONCLUSIONS

Our data suggest that repeated freeze-thaw cycles do not impair assay results to the point of influencing ecological or evolutionary conclusions. Plasma samples can be safely stored in one tube and thawed repeatedly for different assays. Nevertheless, we recommend consistent treatment of samples in terms of freeze-thaw cycles or other laboratory treatments to minimize the potential for introducing a systematic bias.

Geographical and temporal variation in environmental conditions affects nestling growth but not immune function in a year-round breeding equatorial lark

Background

Variation in growth and immune function within and among populations is often associated with specific environmental conditions. We compared growth and immune function in nestlings of year-round breeding equatorial Red-capped Lark Calandrella cinerea from South Kinangop, North Kinangop and Kedong (Kenya), three locations that are geographically close but climatically distinct. In addition, we studied growth and immune function of lark nestlings as a function of year-round variation in breeding intensity and rain within one location. We monitored mass, wing, and tarsus at hatching (day 1) and at 4, 7, and 10 days post-hatch, and we quantified four indices of immune function (haptoglobin, agglutination, lysis and nitric oxide) using blood samples collected on day 10.

Results

Nestling body mass and size at hatching, which presumably reflect the resources that females allocated to their eggs, were lowest in the most arid location, Kedong. Contrary to our predictions, nestlings in Kedong grew faster than nestlings in the two other cooler and wetter locations of South and North Kinangop. During periods of peak reproduction within Kedong, nestlings were heavier at hatching, but they did not grow faster over the first 10 days post-hatch. In contrast, rainfall, which did not relate to timing of breeding, had no effect on hatching mass, but more rain did coincide with faster growth post-hatch. Finally, we found no significant differences in nestling immune function, neither among locations nor with the year-round variation within Kedong.

Conclusion

Based on these results, we hypothesize that female body condition determines nestling mass and size at hatching, but other independent environmental conditions subsequently shape nestling growth. Overall, our results suggest that environmental conditions related to food availability for nestlings are relatively unimportant to the timing of breeding in equatorial regions, while these same conditions do have consequences for nestling size and growth.

Immune responsiveness to phytohemagglutinin displays species but not sex differences in three anuran species

Phytohemagglutinin (PHA)-induced skin swelling response is widely used as a rough surrogate of integrative cell-mediated and innate immunity across multiple vertebrate taxa due to its simplification and feasibility. However, little is known whether there are sex and interspecific differences of immune responsiveness to PHA in ectotherms, especially for anurans. Therefore, we studied sex and species differences of PHA response in three anurans, Asiatic toads (Bufo gargarizans), Dark-spotted frogs (Pelophylax nigromaculatus) and Mongolian toads (Pseudepidalea raddei), captured in northern regions of Anhui Province (China). Footpad thickness was measured prior to (0 h) and after (6, 12, 24, 48 and 72 h) a PHA injection and normalized against saline injection in the opposite footpad. Body mass was recorded at the beginning (0 h) and end of each assay (72 h). Results showed effects of PHA assay, sex and taxa on body mass. Relative maximum swelling response (PHA_max) ranged from 18.58–29.75%, 9.77 to 20.56% and 21.97 to 31.78% and its occurrence over time was apparent 10.6–19.72 h , 7.74–14.01 h and 17.39–23.94 h postinjection for Asiatic toads, Dark-spotted frogs and Mongolian toads, respectively. Finally, the magnitude or timing of PHA_max in Dark-spotted frogs was significantly thinner and faster than in Mongolian toads, and Asiatic toads had an in-between value, not different from the other two species. The magnitude of PHA_max was significantly positively correlated with the timing of PHA_max considering individuals altogether, but not when analyzed within species. Our results indicate that male and female anuran species respond similarly to PHA antigen stimulation, but the magnitude and timing of PHA_max is species-specific. Briefly, we provide new evidence for the suitability of PHA assay in non-model anuran species with different body sizes, and exhort the need to further investigate the nature of PHA assay at the hematological and histological levels in order to extend its application in ecoimmunological studies of amphibians.

Personality and innate immune defenses in a wild bird: Evidence for the pace-of-life hypothesis

We tested the two main evolutionary hypotheses for an association between immunity and personality. The risk-of-parasitism hypothesis predicts that more proactive (bold, exploratory, risk-taking) individuals have more vigorous immune defenses because of increased risk of parasite exposure. In contrast, the pace-of-life hypothesis argues that proactive behavioral styles are associated with shorter lifespans and reduced investment in immune function. Mechanistically, associations between immunity and personality can arise because personality differences are often associated with differences in condition and stress responsiveness, both of which are intricately linked with immunity. Here we investigate the association between personality (measured as proactive exploration of a novel environment) and three indices of innate immune function (the non-specific first line of defense against parasites) in wild superb fairy-wrens Malurus cyaneus. We also quantified body condition, hemoparasites (none detected), chronic stress (heterophil:lymphocyte ratio) and circulating corticosterone levels at the end of the behavioral test (CORT, in a subset of birds). We found that fast explorers had lower titers of natural antibodies. This result is consistent with the pace-of-life hypothesis, and with the previously documented higher mortality of fast explorers in this species. There was no interactive effect of exploration score and duration in captivity on immune indices. This suggests that personality-related differences in stress responsiveness did not underlie differences in immunity, even though behavioral style did modulate the effect of captivity on CORT. Taken together these results suggest reduced constitutive investment in innate immune function in more proactive individuals.

Migratory common blackbirds have lower innate immune function during autumn migration than resident conspecifics

Animals need a well-functioning immune system to protect themselves against pathogens. The immune system, however, is costly and resource trade-offs with other demands exist. For migratory animals several (not mutually exclusive) hypotheses exist. First, migrants reduce immune function to be able to allocate resources to migration. Second, migrants boost immune function to cope with more and/or novel pathogens encountered during migration. Third, migrants reallocate resources within the immune system. We tested these hypotheses by comparing baseline immune function in resident and migratory common blackbirds (Turdus merula), both caught during the autumn migration season on the island of Helgoland, Germany. Indices of baseline innate immune function (microbial killing capacity and haptoglobin-like activity) were lower in migrants than in residents. There was no difference between the groups in total immunoglobulins, a measure of baseline acquired immune function. Our study on a short-distance avian migrant supports the hypothesis that innate immune function is compromised during migration.

Early developmental exposures shape trade-offs between acquired and innate immunity in humans

BACKGROUND AND OBJECTIVES

Life history theory predicts resource allocation trade-offs between competing functions and processes. We test the hypothesis that relative investment towards innate versus acquired immunity in humans is subject to such trade-offs and that three types of early developmental exposures are particularly salient in shaping adult immunophenotype: (i) pathogen exposure, (ii) nutritional resources; and (iii) extrinsic mortality cues.

METHODOLOGY

We quantified one aspect each of innate and acquired immune function, via C-reactive protein and Epstein-Barr virus antibodies, respectively, in a sample of 1248 men and women from the Philippines (ca. 21.5 years old). Early developmental exposures were assessed via long-term data collected prospectively since participants' birth (1983-4). We calculated a standardized ratio to assess relative bias towards acquired versus innate immune function and examined its relationship to a suite of predictors via multiple regression.

RESULTS

In partial support of our predictions, some of the measures of higher pathogen exposure, greater availability of nutritional resources, and lower extrinsic mortality cues in early life were associated with a bias toward acquired immunity in both men and women. The immune profile of women, in particular, appeared to be more sensitive to early life pathogen exposures than those of men. Finally, contrary to prediction, women exhibited a greater relative investment toward innate, not acquired, immunity.

CONCLUSIONS AND IMPLICATIONS

Early environments can exert considerable influence on the development of immunity. They affect trade-offs between innate and acquired immunity, which show adaptive plasticity and may differ in their influence in men and women.

Changes in variation at the MHC class II DQA locus during the final demise of the woolly mammoth

According to the nearly-neutral theory of evolution, the relative strengths of selection and drift shift in favour of drift at small population sizes. Numerous studies have analysed the effect of bottlenecks and small population sizes on genetic diversity in the MHC, which plays a central role in pathogen recognition and immune defense and is thus considered a model example for the study of adaptive evolution. However, to understand changes in genetic diversity at loci under selection, it is necessary to compare the genetic diversity of a population before and after the bottleneck. In this study, we analyse three fragments of the MHC DQA gene in woolly mammoth samples radiocarbon dated to before and after a well-documented bottleneck that took place about ten thousand years ago. Our results indicate a decrease in observed heterozygosity and number of alleles, suggesting that genetic drift had an impact on the variation on MHC. Based on coalescent simulations, we found no evidence of balancing selection maintaining MHC diversity during the Holocene. However, strong trans-species polymorphism among mammoths and elephants points to historical effects of balancing selection on the woolly mammoth lineage.

Trade-offs between acquired and innate immune defenses in humans

Immune defenses provide resistance against infectious disease that is critical to survival. But immune defenses are costly, and limited resources allocated to immunity are not available for other physiological or developmental processes. We propose a framework for explaining variation in patterns of investment in two important subsystems of anti-pathogen defense: innate (non-specific) and acquired (specific) immunity. The developmental costs of acquired immunity are high, but the costs of maintenance and activation are relatively low. Innate immunity imposes lower upfront developmental costs, but higher operating costs. Innate defenses are mobilized quickly and are effective against novel pathogens. Acquired responses are less effective against novel exposures, but more effective against secondary exposures due to immunological memory. Based on their distinct profiles of costs and effectiveness, we propose that the balance of investment in innate versus acquired immunity is variable, and that this balance is optimized in response to local ecological conditions early in development. Nutritional abundance, high pathogen exposure and low signals of extrinsic mortality risk during sensitive periods of immune development should all favor relatively higher levels of investment in acquired immunity. Undernutrition, low pathogen exposure, and high mortality risk should favor innate immune defenses. The hypothesis provides a framework for organizing prior empirical research on the impact of developmental environments on innate and acquired immunity, and suggests promising directions for future research in human ecological immunology.