Does clutch size evolve in response to parasites and immunocompetence?

Mate choice is affected by parasite infestation rate of the choosing individual as well as of potential mating partners

Parasites are known to be a key driving force in mate choice and are important for the expression and evolution of ornaments and behavioral traits being used. However, there is little experimental evidence on how the parasite's burden of the choosing individual is integrated into the mate-choice process and how it affects decision-making, especially in relation to parasite infestation of potential mates. Thus, the aim of our study was to determine whether female house sparrows Passer domesticus adjust their mate preference according to their own as well as the parasite load of prospective partners. To do this, we experimentally manipulated female parasite load and determined their mate preferences prior to and after parasite treatment. We manipulated the chronic coccidian parasite burden of females either by initiating the acute infection phase via re-infecting them with coccidian or by temporally reducing the parasite load of coccidia. We then measured the effect of this manipulation on mate preference by presenting females with a choice of four stimuli: three males with similar ornaments, but unmanipulated, naturally varying chronic coccidiosis levels, and an unmanipulated control female. Additionally, we recorded some males' behavior in relation to their infection status pointing toward an increased or reduced interest in mating. We found that females preferred highly infested males prior to manipulation, regardless of their own infestation level. However, after manipulation, infested females avoided highly infested males probably in response to the deterioration of their health condition by parasites. Our study suggests that mate-choice decisions are more complex when they are mediated by parasites. The implications of parasites for evolutionary theories of sexual signaling and mate choice are discussed.

Longitudinal dynamics and health impact of Hepatozoon procyonis (Apicomplexa: Hepatozoidae) on naturally infected ring-tailed coatis Nasua nasua (Carnivora: Procyonidae) from Midwestern Brazil

This study aimed to morphologically and molecularly detect Hepatozoon procyonis in ring-tailed coatis' (Nasua nasua) blood and associated ticks from central-western Brazil, Campo Grande, Mato Grosso do Sul state and also evaluate the impact of the protozoa in blood parameters and coati´s health. Samplings were performed in a conservation area Parque Estadual do Prosa (PEP) and in a Brazilian Air Force Private Area namely Vila da Base Aérea (VBA), between March 2018 and April 2019. We collected 165 blood samples, 61 from recaptured coatis. Peripheral blood smears were stained with Romanovsky-type stain for H. procyonis parasitemia assessment. DNA extracted from blood samples and ticks (Amblyomma spp.) were submitted to a nested PCR (nPCR) assay based on the 18S rRNA gene for Hepatozoon spp. Out of 104 individuals sampled, 80 (77%) were positive for H. procyonis in at least one capture. Overall, 67/165 (40.6%) blood smears showed H. procyonis gametocytes (PEP: 41/63 - 65%; VBA: 26/102 - 25.5%). Parasitemia based on 500 assessed leucocytes ranged from 1 (0.2%) to 50 (10%) and 1 (0.2%) to 25 (5%), from animals sampled in PEP and VBA, respectively. Fluctuation on the parasitemia was observed during recaptures. nPCR results showed higher positivity when compared to blood smears, i.e. 112/165 (68%) positive blood samples [PEP: 41/63 (65%), VBA: 26/102 (25.5%)]. In total, 63/248 (25.4%) tick DNA samples were positive at nPCR for Hepatozoon sp., including 32/87 (37%) pools (1 to 10 larvae) of Amblyomma larvae, 21/105 (20%) pools (1 to 5 nymphs) of Amblyomma sculptum nymphs, 9/43 (21%) pools (1 to 5 nymphs) of Amblyomma dubitatumnymphs, and 1/12 (8%) A. sculptum adult female. The partial 18S rRNA sequence from one coati's blood sample and one representative of each positive tick species randomly selected from each area for sequencing (1,000 bp) showed 100% identity with sequences of H. procyonis from GenBank previously detected in coatis. Regarding H. procyonis infection, no statistical differences were obtained when comparing males vs. females (p-value 0.67), immature animals vs. adults (p-value 0.31), rainy vs. dry season (p-value 0.51) and sampling location (p-value 0.42). No noticeable alteration in blood parameters or heath status was observed in parasite animals. H. procyonis circulates in a high prevalence in coatis from central-western Brazil. Parasitemia fluctuates among different coatis' recaptures and apparently the infection has no influence in coatis' hematological and clinical parameters.

Ontogeny of leukocyte profiles in a wild altricial passerine

Ecophysiological studies have highlighted the relevance of the avian immune system in individual fitness prospects in the wild. However, studies on the ontogeny of avian immunity are scarce. We analyse age-related changes in the cellular constitutive immunity throughout nestling development, as well as its relationship with sex and brood size. We found that cellular constitutive immunity could be affected by age, sex, brood size, or daily rhythm. Early-stage nestlings relied more on cells of the innate immunity rather than on cells linked to the adaptive immune system. Cellular immunity may not be fully mature in fledglings, as reflected by differences in phagocytic cell counts with regard to adults. Beyond the age-dependent effects, agranulocyte cell counts were affected by sibling competition while granulocyte cell counts showed a daily rhythm. We also show that the heterophil to lymphocyte ratio was negatively related to body weight when nestlings become more independent. Our study contributes knowledge to the fields of developmental immunology and ecological immunology based on essential components of the cellular immune system.

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.

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.

Alien Invasion: Biology of Philornis Flies Highlighting Philornis downsi, an Introduced Parasite of Galápagos Birds

The muscid genus Philornis comprises approximately 50 described species of flies, nearly all of which are obligate parasites of nestling birds. Philornis species are native to the Neotropics and widely distributed from Florida to Argentina. Most research on this group has focused on P. downsi, which was introduced to the Galápagos Islands in the late twentieth century. Although Philornis parasitism kills nestlings in several native host species, nowhere do the effects seem more severe than in P. downsi in the Galápagos. Here, we review studies of native and introduced Philornis in an attempt to identify factors that may influence virulence and consider implications for the conservation of hosts in the Galápagos.

Parasites favour intermediate nestling mass and brood size in cliff swallows

A challenge of life-history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life-history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first-year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first-year survival at the most common, intermediate brood-size and nestling-mass categories, but cliff swallows from nonparasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favour intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life-history evolution.

Life history, immunity, Peto's paradox and tumours in birds

Cancer and tumours may evolve in response to life-history trade-offs between growth and duration of development on one hand, and between growth and maintenance of immune function on the other. Here, we tested whether (i) bird species with slow developmental rates for their body size experience low incidence of tumours because slow development allows for detection of rapid proliferation of cell lineages. We also test whether (ii) species with stronger immune response during development are more efficient at detecting tumour cells and hence suffer lower incidence of tumours. Finally, we tested Peto's paradox, that there is a positive relationship between tumour incidence and body mass. We used information on developmental rates and body mass from the literature and of tumour incidence (8468 birds) and size of the bursa of Fabricius for 7659 birds brought to a taxidermist in Denmark. We found evidence of the expected negative relationship between incidence of tumours and developmental rates and immunity after controlling for the positive association between tumour incidence and body size. These results suggest that evolution has modified the incidence of tumours in response to life history and that Peto's paradox may be explained by covariation between body mass, developmental rates and immunity.

Coloniality and migration are related to selection on MHC genes in birds

The major histocompatibility complex (MHC) plays a key role in pathogen recognition as a part of the vertebrate adaptive immune system. The great diversity of MHC genes in natural populations is maintained by different forms of balancing selection and its strength should correlate with the diversity of pathogens to which a population is exposed and the rate of exposure. Despite this prediction, little is known about how life-history characteristics affect selection at the MHC. Here, we examined whether the strength of balancing selection on MHC class II genes in birds (as measured with nonsynonymous nucleotide substitutions, dN) was related to their social or migratory behavior, two life-history characteristics correlated with pathogen exposure. Our comparative analysis indicated that the rate of nonsynonymous substitutions was higher in colonial and migratory species than solitary and resident species, suggesting that the strength of balancing selection increases with coloniality and migratory status. These patterns could be attributed to: (1) elevated transmission rates of pathogens in species that breed in dense aggregations, or (2) exposure to a more diverse fauna of pathogens and parasites in migratory species. Our study suggests that differences in social structure and basic ecological traits influence MHC diversity in natural vertebrate populations.

Excretion patterns of coccidian oocysts and nematode eggs during the reproductive season in Northern Bald Ibis (Geronticus eremita)

Individual reproductive success largely depends on the ability to optimize behaviour, immune function and the physiological stress response. We have investigated correlations between behaviour, faecal steroid metabolites, immune parameters, parasite excretion patterns and reproductive output in a critically endangered avian species, the Northern Bald Ibis (Geronticus eremita). In particular, we related haematocrit, heterophil/lymphocyte ratio, excreted immune-reactive corticosterone metabolites and social behaviour with parasite excretion and two individual fitness parameters, namely, number of eggs laid and number of fledglings. We found that the frequency of excretion of parasites' oocysts and eggs tended to increase with ambient temperature. Paired individuals excreted significantly more samples containing nematode eggs than unpaired ones. The excretion of nematode eggs was also significantly more frequent in females than in males. Individuals with a high proportion of droppings containing coccidian oocysts were more often preened by their partners than individuals with lower excretion rates. We observed that the more eggs an individual incubated and the fewer offspring fledged, the higher the rates of excreted samples containing coccidian oocysts. Our results confirm that social behaviour, physiology and parasite burden are linked in a complex and context-dependent manner. They also contribute background information supporting future conservation programmes dealing with this critically endangered species.

Neonatal body condition, immune responsiveness, and hematocrit predict longevity in a wild bird population

Measures of body condition, immune function, and hematological health are widely used in ecological studies of vertebrate populations, predicated on the assumption that these traits are linked to fitness. However, compelling evidence that these traits actually predict long-term survival and reproductive success among individuals in the wild is lacking. Here, we show that body condition (i.e., size-adjusted body mass) and cutaneous immune responsiveness to phytohaemagglutinin (PHA) injection among neonates positively predict recruitment and subsequent longevity in a wild, migratory population of house wrens (Troglodytes aedon). However, neonates with intermediate hematocrit had the highest recruitment and longevity. Neonates with the highest PHA responsiveness and intermediate hematocrit prior to independence eventually produced the most offspring during their lifetime breeding on the study site. Importantly, the effects of PHA responsiveness and hematocrit were revealed while controlling for variation in body condition, sex, and environmental variation. Thus, our data demonstrate that body condition, cutaneous immune responsiveness, and hematocrit as a neonate are associated with individual fitness. Although hematocrit's effect is more complex than traditionally thought, our results suggest a previously underappreciated role for this trait in influencing survival in the wild.

No selection on immunological markers in response to a highly virulent pathogen in an Arctic breeding bird

In natural populations, epidemics provide opportunities to look for intense natural selection on genes coding for life history and immune or other physiological traits. If the populations being considered are of management or conservation concern, then identifying the traits under selection (or 'markers') might provide insights into possible intervention strategies during epidemics. We assessed potential for selection on multiple immune and life history traits of Arctic breeding common eiders (Somateria mollissima) during annual avian cholera outbreaks (summers of 2006, 2007 & 2008). We measured prelaying body condition, immune traits, and subsequent reproductive investment (i.e., clutch size) and survival of female common eiders and whether they were infected with Pasteurella multocida, the causative agent of avian cholera. We found no clear and consistent evidence of directional selection on immune traits; however, infected birds had higher levels of haptoglobin than uninfected birds. Also, females that laid larger clutches had slightly lower immune responses during the prelaying period reflecting possible downregulation of the immune system to support higher costs of reproduction. This supports a recent study indicating that birds investing in larger clutches were more likely to die from avian cholera and points to a possible management option to maximize female survival during outbreaks.

The history of ecoimmunology and its integration with disease ecology

Ecoimmunology is an example of how fruitful integrative approaches to biology can be. Since its emergence, ecoimmunology has sparked constructive debate on a wide range of topics, from the molecular mechanics of immune responses to the role of immunity in shaping the evolution of life histories. To complement the symposium Methods and Mechanisms in Ecoimmunology and commemorate the inception of the Division of Ecoimmunology and Disease Ecology within the Society for Integrative and Comparative Biology, we appraise the origins of ecoimmunology, with a focus on its continuing and valuable integration with disease ecology. Arguably, the greatest contribution of ecoimmunology to wider biology has been the establishment of immunity as an integral part of organismal biology, one that may be regulated to maximize fitness in the context of costs, constraints, and complex interactions. We discuss historical impediments and ongoing progress in ecoimmunology, in particular the thorny issue of what ecoimmunologists should, should not, or cannot measure, and what novel contributions ecoimmunologists have made to the understanding of host-parasite interactions. Finally, we highlight some areas to which ecoimmunology is likely to contribute in the near future.

Cognitive skills and bacterial load: comparative evidence of costs of cognitive proficiency in birds

Parasite-mediated selection may affect the evolution of cognitive abilities because parasites may influence development of the brain, but also learning capacity. Here, we tested some predictions of this hypothesis by analyzing the relationship between complex behaviours (feeding innovations (as a measure of behavioural flexibility) and ability to detect foreign eggs in their nests (i.e. a measure of discriminatory ability)) and abundance of microorganisms in different species of birds. A positive relationship would be predicted if these cognitive abilities implied a larger number of visited environments, while if these skills favoured detection and avoidance of risky environments, a negative relationship would be the prediction. Bacterial loads of eggshells, estimated for mesophilic and potentially pathogenic bacteria (i.e. Enterococcus, Staphylococcus and Enterobacteriaceae), were used as a surrogate of probability of contact with pathogenic bacteria. We found that bird species with higher feeding innovation rates and rejection rates of experimental brood parasitic eggs had higher density of bacteria on their eggshells than the average species. Since the analysed groups of microorganisms include pathogenic bacteria, these results suggest that both feeding innovation and ability to recognize foreign eggs are costly and highlight the importance of parasite-mediated selection in explaining the evolution of cognitive abilities in animals.

Feeding performance of fleas on different host species: is phylogenetic distance between hosts important?

We asked if and how feeding performance of fleas on an auxiliary host is affected by the phylogenetic distance between this host and the principal host of a flea. We investigated the feeding of 2 flea species, Parapulex chephrenis and Xenopsylla ramesis, on a principal (Acomys cahirinus and Meriones crassus, respectively) and 8 auxiliary host species. We predicted that fleas would perform better (higher proportion of fleas would feed and take larger bloodmeals) on (a) a principal rather than an auxiliary host and (b) auxiliary hosts phylogenetically closer to a principal host. Although feeding performance of fleas differed among different hosts, we found that: (1) fleas did not always perform better on a principal host than on an auxiliary host; and (2) flea performance on an auxiliary host was not negatively correlated with phylogenetic distance of this host from the principal host. In some cases, fleas fed better on hosts that were phylogenetically distant from their principal host. We concluded that variation in flea feeding performance among host species results from interplay between (a) inherent species-specific host defence abilities, (b) inherent species-specific flea abilities to withstand host defences and (c) evolutionary tightness of association between a particular host species and a particular flea species.

Incorporating parasite systematics in comparative analyses of variation in spleen mass and testes sizes of rodents

Parasite diversity is hypothesized to act on host life-history traits through investment in immunity. In order to incorporate the diversity of the parasite community that an individual host or a host species may face, two indices can be used: Taxonomic Species Richness and Taxonomic Entropy, where the taxonomic information is incorporated with the taxonomic weight. We tested whether these indices correlate with several morphological traits potentially implicated in immune defence and in reproduction, using data on gastrointestinal helminths and their rodent hosts sampled in Southeast Asia. We found no relationship between parasite diversity indices and either spleen mass or testes size at the intraspecific level, i.e. at the level of individuals. At the interspecific level, we found no relationship between the parasite diversity indices and testes size. However, we found that female spleen mass is significantly influenced by the specific species richness of parasites, whereas male spleen mass is influenced by individual mean parasite diversity indices. We concluded that female spleen mass may have evolved in response to gastrointestinal helminth pressure acting at species levels, while in males, the individual spleen mass could be constrained by other factors, such as the blood storage function of the spleen.

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.

Maternal effects mediated by antioxidants and the evolution of carotenoid-based signals in birds

Bright yellow to red signals used in mate choice or intrasexual competition are based on carotenoid pigments that are hypothesized to be traded between physiological functions and coloration. These signals have recently been shown to be influenced by maternal effects. Indeed, yolk-derived carotenoids are essential for embryos to develop efficient carotenoid metabolism in posthatching life. Maternal effects facilitate adaptation to environmental variability and influence the evolution of phenotypic traits such as secondary sexual signals. Here we propose that maternal investment in yolk carotenoids promotes the evolution of carotenoid-based ornaments. We conducted a comparative analysis of lipid-soluble antioxidants (carotenoids and vitamins A and E) in the eggs of 112 species of bird. Species with large clutch sizes deposited higher yolk concentrations of the three antioxidants. There was a significant positive relationship between yolk carotenoids and the expression of male carotenoid-based signals, but not between yolk carotenoids and sexual dichromatism in these signals. These relationships were specific to carotenoids, as they were not found for vitamins A and E. This provides evidence consistent with the hypothesis that maternal effects mediated by yolk carotenoids play a role in the evolution of carotenoid-based signals as a response to sexual selection, likely based on organizational effects of carotenoids during embryo development.

Effect of Electric Field During Incubation of Eggs on the Immune Responses of Hatched Chickens

The effects of electric field (EF) during incubation of eggs on the immunocompetence of chickens were investigated over a 42-day experimental period. Eggs from a meat-type breeder flock were incubated under EF of 30 kV/m, 60 Hz during the first 18 days of incubation as compared with the control incubation (C). Chickens from the two incubation treatments were fed ad libitum and their immune system were monitored. Measurements were made of body weight (BW), and lymphoid organs weight (thymus, spleen, and bursa of Fabricius (BOF)) of birds at 21 and 42 days of age. Immune systems of birds were tested for specific antibody responses to sheep red blood cell (SRBC) and Newcastle disease vaccine (NDV), in vivo T-lymphocyte proliferation responses to phytohemagglutinin (PHA), and in vitro to concanavalin A (Con-A). EF incubation of eggs did not significantly (P > 0.05) influence BW of bird, absolute weight of lymphoid organs and weight of thymus, and BOF as a percentage of BW of bird (% BW) at 21 and 42 days of age, humoral immune responses as measured by antibody responses to SRBC and NDV, and cell-mediated immune responses as measured by T-lymphocyte proliferation responses to PHA, and Con-A of birds when compared with those of the C treatment. EF incubation of eggs significantly (P < 0.05) increased spleen weight as a % BW at 21 and 42 days of age when compared with those incubated under the C treatment. Birds at 42 days of age had significantly (P < 0.01) higher BW, lymphoid organ weight, and weight of BOF as a % BW, and lower spleen weight as a % BW when compared with those of 21 days of age. It is concluded that the incubation of eggs under EF of 30 kV/m, 60 Hz increased spleen weight as a % BW, without altering cell-mediated and humoral immune responses and, consequently, immunocompetence of meat chickens during the rearing period of 42 days.

No energetic cost of anthropogenic disturbance in a songbird

Anthropogenic or natural disturbances can have a significant impact on wild animals. Therefore, understanding when, how and what type of human and natural events disturb animals is a central problem in wildlife conservation. However, it can be difficult to identify which particular environmental stressor affects an individual most. We use heart rate telemetry to quantify the energy expenditure associated with different types of human-mediated and natural disturbances in a breeding passerine, the white-eyed vireo (Vireo griseus). We fitted 0.5g heart rate transmitters to 14 male vireos and continuously recorded heart rate and activity for two days and three nights on a military installation. We calibrated heart rate to energy expenditure for five additional males using an open-flow, push-through respirometry system showing that heart rate predicted 74 per cent of energy expenditure. We conducted standardized disturbance trials in the field to experimentally simulate a natural stressor (predator presence) and two anthropogenic stressors. Although birds initially showed behavioural and heart rate reactions to some disturbances, we could not detect an overall increase in energy expenditure during 1- or 4-hours disturbances. Similarly, overall activity rates were unaltered between control and experimental periods, and birds continued to perform parental duties despite the experimental disturbances. We suggest that vireos quickly determined that disturbances were non-threatening and thus showed no (costly) physiological response. We hypothesize that the lack of a significant response to disturbance in vireos is adaptive and may be representative of animals with fast life histories (e.g. short lifespan, high reproductive output) so as to maximize energy allocation to reproduction. Conversely, we predict that energetic cost of human-mediated disturbances will be significant in slow-living animals.

Adaptive host-abandonment of ectoparasites before fledging? Within-brood distribution of nest mites in house sparrow broods

We studied the within-brood distribution of a haematophagous mite Pellonyssus reedi living on nestling house sparrows (Passer domesticus) near the time of fledging. We measured the natural level of infestation of individual nestlings, and determined the feeding efficiency of mites, by scoring their feeding status. Within-brood distribution of mite loads was unrelated to nestling body mass, tarsus length, or immunocompetence. These results did not support parasite preference for large or susceptible hosts. Mite feeding-efficiency was also unrelated to these nestling characteristics, confirming that large nestlings or nestlings with less-developed immunocompetence did not provide superior feeding conditions for mites. Therefore, our results do not support the hypothesis that within-brood distribution of avian ectoparasites is explained by the parasites' preferences for characteristics, such as large body size or low immunocompetence, that make nestlings suitable hosts. On the other hand, we found that mite loads were negatively correlated with nestling age and feather length, suggesting that nestlings closer to fledging harbored fewer mites then their less-developed nestmates. Furthermore, feather length had a stronger relationship with parasite distribution than did nestling age. We presume, therefore, that feather characteristics, i.e., length, may serve as a signal for mites to perceive the ready-to-fledge state of nestlings, inducing abandonment behavior. These results support another, largely neglected hypothesis, i.e., that the avoidance or abandonment of those nestlings that are close to fledging may also explain the parasites' distribution in a brood. This hypothesis is based on the argument that many nest-dwelling ectoparasites breed in the nest material and emerge only periodically to feed on nestlings. In such parasites, the ability to recognize and avoid mature fledglings can be adaptive because this may help the parasites to avoid their removal from the nest so they can continue to reproduce by feeding on unfledged chicks of the current or later broods. Our results suggest that adaptive host-abandonment by nest-dwelling ectoparasites can influence within-brood parasite distributions around the time of fledging.

Latitudinal variation of immune defense and sickness behavior in the white-crowned sparrow (Zonotrichia leucophrys)

There is a general trend that parasitism risk declines as latitude increases. Host populations breeding at high latitudes should therefore invest less in costly immune defenses than populations breeding in temperate or tropical zones, although it is unknown if such an effect is mediated by environmental (photoperiodic) or genetic factors or both. Acquired immune function (humoral, cell-mediated) and behavioral sickness responses to lipopolysaccharide (LPS; mimics bacterial infection) were assessed in two subspecies of white-crowned sparrow (Zonotrichia leucophrys) that breed at different latitudes in western North America. Zonotrichia l. gambelii (GWCS) is a high-latitude breeder (47-68 degrees N) while Z. l. pugetensis (PWCS) breeds at temperate latitudes (40-49 degrees N). Captive males of each subspecies were acclimated to (1) a short day (non-breeding) photoperiod (8L:16D), (2) the breeding photoperiod of PWCS (16L:8D), or (3) the breeding photoperiod of GWCS (20L:4D). Photoperiod was manipulated because shorter day lengths may enhance immune function. In support of a genetic effect, humoral responses to diphtheria-tetanus vaccination were significantly higher in PWCS compared to GWCS, regardless of photoperiod. There were no differences in cell-mediated responses to phytohemagglutinin (PHA) between subspecies or among photoperiods. For sickness responses to LPS, a significant interaction between photoperiod and subspecies was found, with long day GWCS producing stronger sickness responses (losing more weight, eating less) than short day GWCS and PWCS on all day lengths. However, these effects were influenced by photoperiodic changes in body condition. In conclusion, we find evidence for genetic control of immune responses across latitude, but no support for environmental (photoperiodic) regulation.

Immune defense and reproductive pace of life in Peromyscus mice

Immune activity is variable within and among vertebrates despite the potentially large fitness costs of pathogens to their hosts. From the perspective of life history theory, immunological variability may be the consequence of counterbalancing investments in immune defense against other expensive physiological processes, namely, reproduction. In the present study, we tested the hypothesis that immune defense among captive-bred, disease-free Peromyscus mice would be influenced by their reproductive life history strategies. Specifically, we expected that small species that reproduce prolifically and mature rapidly (i.e., fast pace of life) would favor inexpensive, nonspecific immune defenses to promote reproductive proclivity. Alternatively, we expected that large species that mature slowly and invest modestly in reproduction over multiple events (i.e., slow pace of life) would favor developmentally expensive, specific immune defenses and avoid cheap, nonspecific ones because such defenses are predisposed to self-damage. We found that species exhibited either strong ability to kill (gram-negative) bacteria, a developmentally inexpensive defense, or strong ability to produce antibodies against a novel protein, a developmentally expensive defense, but not both. Cell-mediated inflammation also varied significantly among species, but in a unique fashion relative to bacteria killing or antibody production; wound healing was comparatively similar among species. These results indicate that Peromyscus species use immune strategies that are constrained to a dominant axis, but this axis is not determined solely by reproductive pace of life. Further comparisons, ideally with broader phylogenetic coverage, could identify what ecological and evolutionary forces produce the pattern we detected. Importantly, our study indicates that species may not be differentially immunocompetent; rather, they use unique defense strategies to prevent infection.

Immunological change in a parasite-impoverished environment: divergent signals from four island taxa

Dramatic declines of native Hawaiian avifauna due to the human-mediated emergence of avian malaria and pox prompted an examination of whether island taxa share a common altered immunological signature, potentially driven by reduced genetic diversity and reduced exposure to parasites. We tested this hypothesis by characterizing parasite prevalence, genetic diversity and three measures of immune response in two recently-introduced species (Neochmia temporalis and Zosterops lateralis) and two island endemics (Acrocephalus aequinoctialis and A. rimitarae) and then comparing the results to those observed in closely-related mainland counterparts. The prevalence of blood parasites was significantly lower in 3 of 4 island taxa, due in part to the absence of certain parasite lineages represented in mainland populations. Indices of genetic diversity were unchanged in the island population of N. temporalis; however, allelic richness was significantly lower in the island population of Z. lateralis while both allelic richness and heterozygosity were significantly reduced in the two island-endemic species examined. Although parasite prevalence and genetic diversity generally conformed to expectations for an island system, we did not find evidence for a pattern of uniformly altered immune responses in island taxa, even amongst endemic taxa with the longest residence times. The island population of Z. lateralis exhibited a significantly reduced inflammatory cell-mediated response while levels of natural antibodies remained unchanged for this and the other recently introduced island taxon. In contrast, the island endemic A. rimitarae exhibited a significantly increased inflammatory response as well as higher levels of natural antibodies and complement. These measures were unchanged or lower in A. aequinoctialis. We suggest that small differences in the pathogenic landscape and the stochastic history of mutation and genetic drift are likely to be important in shaping the unique immunological profiles of small isolated populations. Consequently, predicting the impact of introduced disease on the many other endemic faunas of the remote Pacific will remain a challenge.

Phenotypic sorting in morphology and reproductive investment among sociable weaver colonies

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

Evolution of Yolk Androgens in Birds: Development, Coloniality, and Sexual Dichromatism

Current theory recognizes the adaptive value of maternal effects in shaping offspring phenotypes in response to selective pressures and vindicates the value of these traits in fostering adaptation and speciation. Yolk androgens in birds are a relatively well-known maternal effect and have been linked to adaptations related to development, coloniality life, and sexual selection. We tested whether interspecific patterns of yolk androgen levels (androstenedione and testosterone) were related to interspecific variation in development, sexual selection, and coloniality. First, we found no relationship between androgen levels and duration of development as reflected by incubation and nestling periods. However, androstenedione concentration was positively related to the relative duration of the incubation period and negatively related to the relative duration of the nestling period. These relationships were confirmed by analyses of phylogenetically independent contrasts. We suggest that androstenedione concentration may have evolved as a mechanism to shift the relative duration of development between the egg and nestling stages in response to selective pressures that differentially affect the duration of each stage. Second, neither plumage dichromatism nor mating system explained significant variation in yolk androgen levels after correction for similarity among species due to common descent. This finding indicates that sexual selection has not been an important selective pressure for this maternal effect. Third, we found a highly significant positive relationship between degree of breeding coloniality and concentration of androstenedione but not testosterone. These effects were confirmed in analyses of contrasts controlling for similarity due to common descent. Since the relationship with coloniality was different for each androgen, it is unlikely that increased levels of androgens in highly colonial species are a mere consequence of elevated androgen levels in mothers. Rather, our results suggest that high levels of androstenedione in eggs of colonial species are an adaptation to colony life, possibly related to the production of highly competitive phenotypes. In conclusion, from a comparative perspective, the results of this study support the role of maternal effects in promoting adaptation to certain environmental pressures.

Abundance patterns of two Oropsylla (Ceratophyllidae: Siphonaptera) species on black-tailed prairie dog (Cynomys ludovicianus) hosts

Behavioral, genetic, and immune variation within a host population may lead to aggregation of parasites whereby a small proportion of hosts harbor a majority of parasites. In situations where two or more parasite species infect the same host population there is the potential for interaction among parasites that could potentially influence patterns of aggregation through either competition or facilitation. We studied the occurrence and abundance patterns of two congeneric flea species on black-tailed prairie dog (Cynomys ludovicianus) hosts to test for interactions among parasite species. We live-trapped prairie dogs on ten sites in Boulder County, CO and collected their fleas. We found a non-random, positive association between the two flea species, Oropsylla hirsuta and O. tuberculata cynomuris; hosts with high loads of one flea species had high loads of the second species. This result suggests that there is no interspecific competition among fleas on prairie dog hosts. Host weight had a weak negative relationship to flea load and host sex did not influence flea load, though there were slight differences in flea prevalence and abundance between male and female C. ludovicianus. While genetic and behavioral variation among hosts may predispose certain individuals to infection, our results indicate apparent facilitation among flea species that may result from immune suppression or other flea-mediated factors.

To eject or to abandon? Life history traits of hosts and parasites interact to influence the fitness payoffs of alternative anti-parasite strategies

Hosts either tolerate avian brood parasitism or reject it by ejecting parasitic eggs, as seen in most rejecter hosts of common cuckoos, Cuculus canorus, or by abandoning parasitized clutches, as seen in most rejecter hosts of brown-headed cowbirds, Molothrus ater. What explains consistent variation between alternative rejection behaviours of hosts within the same species and across species when exposed to different types of parasites? Life history theory predicts that when parasites decrease the fitness of host offspring, but not the future reproductive success of host adults, optimal clutch size should decrease. Consistent with this prediction, evolutionarily old cowbird hosts, but not cuckoo hosts, have lower clutch sizes than related rarely- or newly parasitized species. We constructed a mathematical model to calculate the fitness payoffs of egg ejector vs. nest abandoner hosts to determine if various aspects of host life history traits and brood parasites' virulence on adult and young host fitness differentially influence the payoffs of alternative host defences. These calculations showed that in general egg ejection was a superior anti-parasite strategy to nest abandonment. Yet, increasing parasitism rates and increasing fitness values of hosts' eggs in both currently parasitized and future replacement nests led to switch points in fitness payoffs in favour of nest abandonment. Nonetheless, nest abandonment became selectively more favourable only at lower clutch sizes and only when hosts faced parasitism by a cowbird- rather than a cuckoo-type brood parasite. We suggest that, in addition to evolutionary lag and gape-size limitation, our estimated fitness differences based on life history trait variation provide new insights for the consistent differences observed in the anti-parasite rejection strategies between many cuckoo- and cowbird-hosts.