Maternal and developmental immune challenges alter behavior and learning ability of offspring

What do zebra finches learn besides singing? Systematic mapping of the literature and presentation of an efficient associative learning test

The process of learning in birds has been extensively studied, with a focus on species such as pigeons, parrots, chickens, and crows. In recent years, the zebra finch has emerged as a model species in avian cognition, particularly in song learning. However, other cognitive domains such as spatial memory and associative learning could also be critical to fitness and survival, particularly during the intensive juvenile period. In this systematic review, we provide an overview of cognitive studies on zebra finches, with a focus on domains other than song learning. Our findings indicate that spatial, associative, and social learning are the most frequently studied domains, while motoric learning and inhibitory control have been examined less frequently over 30 years of research. All of the 60 studies included in this review were conducted on captive birds, limiting the generalizability of the findings to wild populations. Moreover, only two of the studies were conducted on juveniles, highlighting the need for more research on this critical period of learning. To address this research gap, we propose a high-throughput method for testing associative learning performance in a large number of both juvenile and adult zebra finches. Our results demonstrate that learning can occur in both age groups, thus encouraging researchers to also perform cognitive tests on juveniles. We also note the heterogeneity of methodologies, protocols, and subject exclusion criteria applied by different researchers, which makes it difficult to compare results across studies. Therefore, we call for better communication among researchers to develop standardised methodologies for studying each cognitive domain at different life stages and also in their natural conditions.

Transgenerational effects of innate immune activation in broiler breeders on growth performance and immune responsiveness

The impact of transgenerational effects on growth performance and immunity has not yet been studied extensively within the poultry husbandry sector. An important factor is the impact of the hens on the physical well-being and fitness to the environment of the offspring. This study is the first to investigate the effect of stimulating the maternal innate immune system with lipopolysaccharides (LPS) or β-glucan on growth performance and immune responses in the next generation. Transgenerational effects and consequences of these maternal treatments were further examined using a necrotic enteritis (NE) challenge model in the offspring. We show that offspring of LPS-treated broiler breeders have a higher feed efficiency from 14 to 21 days of age, that is, the period just after the NE challenge. Moreover, more broiler chickens with intestinal lesions after the NE challenge were found in the offspring of the LPS-treated broiler breeders. Both the LPS and β-glucan maternal treatments resulted in transgenerational effects on blood-derived monocytes by showing a tendency of decreased IL1β mRNA levels after ex vivo LPS stimulation. These data are a first indication that broiler breeder hens can affect immune responsiveness and feeding efficiency of their offspring in a transgenerational manner.

Effects of maternal exposure to a bacterial antigen and altered post-hatching rearing conditions on avian offspring behaviour

Abstract

The early-life environment plays a crucial role in shaping morphological, physiological, and behavioural traits, with potential long-term consequences for fitness. Indeed, a set of factors experienced by offspring during prenatal and early post-natal development has been recognised to affect behavioural trait expression in later life. Several studies have shown that in birds, nutritional and social rearing conditions and maternal and/or neonatal immunisations may profoundly determine the development and establishment of behaviour in offspring. To our knowledge, no research has examined whether and how the interaction between immune-mediated maternal effects and post-hatching rearing conditions affects offspring behaviour. Here, we studied the effects of maternal exposure to a bacterial antigen and altered brood size on docility, breathing rate, and aggression in the offspring of great tit, Parus major. We used a 2 × 2 design to investigate the interactive effects of maternal immunisation and brood size manipulation on offspring behavioural development. We found no such interactive effect on offspring behaviour, although we observed it regarding to offspring body mass and tarsus length. Maternal immunisation itself did not affect offspring behaviour. However, we demonstrated that the offspring breathing rate and level of aggression were affected by brood size manipulation. Both breathing rate and aggression in offspring reared in enlarged broods were lower than those in offspring reared in non-manipulated broods. Our study did not confirm earlier reports that immune-mediated maternal effects modulate offspring behavioural development, but we showed that brood size during rearing might indeed be a factor that affects offspring behaviour.

Significance statement

The early environment experienced by offspring constitutes a significant source of developmental plasticity, which may profoundly affect the establishment of their behavioural traits. Food availability, social conditions, and maternal or offspring infection are crucial factors shaping various behavioural traits in birds. However, there remains a lack of studies emphasising the potential interactive effects of early-life conditions on behavioural trait development in natural bird populations. Here, to our knowledge for the first time, we experimentally examined how maternal immunisation and altered post-hatching rearing conditions interact to determine the behaviour of fledged offspring. We found that maternal treatment and brood size manipulation interactively affected offspring body mass and tarsus length, but this interaction had no effect on offspring behaviour. Our findings suggest that different mechanisms may underlie the development of morphological and behavioural traits.

Modulating offspring responses: concerted effects of stress and immunogenic challenge in the parental generation

The perception, processing and response to environmental challenges involves the activation of the immuno-neuroendocrine (INE) interplay. Concerted environmental challenges might induce trade-off when resource allocation to one trait occurs at the expense of another, also producing potential transgenerational effects in the offspring. We evaluated whether concerted challenges, in the form of an immune inoculum against inactivated Salmonella enteritidis (immune challenge, ICH) and a chronic heat stress (CHS) exposure on adult Japanese quail, modulate the INE responses of the parental generation and their offspring. Adults were inoculated and later exposed to a CHS along nine consecutive days. For the last 5 days of the CHS, eggs were collected for incubation. Chicks were identified according to their parental treatments and remained undisturbed. Induced inflammatory response, heterophil/lymphocyte (H/L) ratio and specific humoral response against sheep red blood cells (SRBC) were evaluated in both generations. Regardless of the ICH, stressed adults showed a reduced inflammatory response and an elevated H/L ratio compared with controls. In offspring, the inflammatory response was elevated and the specific SRBC antibody titres were diminished in those chicks prenatally exposed to CHS, regardless of the ICH. No differences were found in the H/L ratio of the offspring. Together, our results suggest that CHS exposure influences the INE interplay of adult quail, establishing trade-offs within their immune system. Moreover, CHS not only affected parental INE responses but also modulated their offspring INE responses, probably affecting their potential to respond to future challenges. The adaptability of the developmental programming of offspring would depend on the environment encountered.

The effects of a bacterial challenge on reproductive success of fruit flies evolved under low or high sexual selection

The capacity of individuals to cope with stress, for example, from pathogen exposure, might decrease with increasing levels of sexual selection, although it remains unclear which sex should be more sensitive. Here, we measured the ability of each sex to maintain high reproductive success following challenges with either heat-killed bacteria or procedural control, across replicate populations of Drosophila melanogaster evolved under either high or low levels of sexual selection. Our experiment was run across four separate sampling blocks. We found an interaction between bacterial treatment, sexual selection treatment, and sampling block on female reproductive success. Specifically, and only in the fourth block, we observed that bacterial-challenged females that had evolved under high sexual selection, exhibited lower reproductive success than bacterial-challenged females that had evolved under low sexual selection. Furthermore, we could trace this block-specific effect to a reduction in viscosity of the ovipositioning substrate in the fourth block, in which females laid around 50% more eggs than in previous blocks. In contrast, patterns of male reproductive success were consistent across blocks. Males that evolved under high sexual selection exhibited higher reproductive success than their low-selection counterparts, regardless of whether they were subjected to a bacterial challenge or not. Our results are consistent with the prediction that heightened sexual selection will invoke male-specific evolutionary increases in reproductive fitness. Furthermore, our findings suggest that females might pay fitness costs when exposed to high levels of sexual selection, but that these costs may lie cryptic, and only be revealed under certain environmental contexts.

Simulated viral infection in early-life alters brain morphology, activity and behavior in zebra finches (Taeniopygia guttata)

Early-life immune challenges (ELIC) have long-term effects on adult behavior and brain development. ELIC studies on birds are still few, but they are epidemiologically crucial since birds are important hosts of many mosquito-borne viruses. In this study, we administered a viral infection mimicking agent, Polyinosinic: polycytidylic acid (Poly I:C), to nestling zebra finches on post-hatch day 14. When birds became sexually mature, their general activity (i.e., hopping, feeding behavior) and mosquito defense behaviors (i.e., hops, head movements, pecks, wing movements, foot movements, and scratches) were measured. Following behavioral trials, brains of male birds were collected for anatomical and histochemical analyses. Poly I:C challenge had sex-dependent effects on general activity and mosquito defense behaviors. When compared to control females, Poly I:C challenged females hopped and fed less often in their general activities, but hopped more often in the presence of mosquitoes. Poly I:C challenged males did not differ from control males in any behaviors. Brain analysis revealed that the nucleus taeniae of the amygdala (TnA) of Poly I:C challenged males were smaller in volume yet had more neurons expressing immediate-early gene proteins compared with controls, suggesting a more active TnA. These results suggest that immune challenges early in the life could have long-term effects on behaviors and brains of zebra finches, which may influence disease spread and fitness of individual birds.

The effect of pre-laying maternal immunization on offspring growth and immunity differs across experimentally altered postnatal rearing conditions in a wild songbird

Background

Prenatal antibody transfer is an immune-mediated maternal effect by which females can shape postnatal offspring resistance to pathogens and parasites. Maternal antibodies passed on to offspring provide primary protection to neonates against diverse pathogenic antigens, but they may also affect offspring growth and influence the development of an offspring’s own immune response. The effects of maternal antibodies on offspring performance commonly require that the disease environment experienced by a mother prior to breeding matches the environment encountered by her offspring after hatching/birth. However, other circumstances, like postnatal rearing conditions that affect offspring food availability, may also determine the effects of maternal antibodies on offspring growth and immunity. To date, knowledge about how prenatal immune-mediated maternal effects interact with various postnatal rearing conditions to affect offspring development and phenotype in wild bird population remains elusive. Here we experimentally studied the interactive effects of pre-laying maternal immunization with a bacterial antigen (lipopolysaccharide) and post-hatching rearing conditions, altered by brood size manipulation, on offspring growth and humoral immunity of wild great tits (Parus major).

Results

We found that maternal immunization and brood size manipulation interactively affected the growth and specific humoral immune response of avian offspring. Among nestlings reared in enlarged broods, only those that originated from immunized mothers grew better and were heavier at fledging stage compared to those that originated from non-immunized mothers. In contrast, no such effects were observed among nestlings reared in non-manipulated (control) broods. Moreover, offspring of immunized females had a stronger humoral immune response to lipopolysaccharide during postnatal development than offspring of non-immunized females, but only when the nestling was reared in control broods.

Conclusions

This study demonstrates that offspring development and their ability to cope with pathogens after hatching are driven by mutual influences of pathogen-induced prenatal maternal effects and post-hatching rearing conditions. Our findings suggest that immune-mediated maternal effects may have context-dependent influences on offspring growth and immune function, related to the postnatal environmental conditions experienced by the progeny.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0272-y) contains supplementary material, which is available to authorized users.

Maternal immunization increases nestling energy expenditure, immune function, and fledging success in a passerine bird

Female birds transfer maternally derived antibodies (matAb) to their nestlings, via the egg yolk. These antibodies are thought to provide passive protection, and allow nestlings to avoid the costs associated with mounting an innate immune response. To test whether there is an energetic benefit to nestlings from receiving matAb, we challenged adult female tree swallows (Tachycineta bicolor) prior to clutch initiation with either lipopolysaccharide (LPS) or saline (Control). Following hatching, one half of each female's nestlings were immunized on day 8 post-hatch with LPS or saline, and the 4-h post-immunization nestling metabolic rate (MR) was measured. There was no difference in either LPS-reactive antibodies or total Ig levels between offspring of immunized and non-immunized mothers on day 6 or 14 post-hatch, possibly reflecting a relatively short half-life of matAbs in altricial birds. Additionally, we found no evidence that nestlings from LPS-immunized mothers could avoid the growth suppression that may result from activation of an inflammatory response. Unexpectedly, we found that control nestlings from LPS mothers had higher resting MR than control nestlings of control mothers. We attribute the increased MR to the costs associated with a general non-specific enhancement of immune function in nestlings from LPS-immunized mothers. Consistent with enhanced immune function, nestlings of immunized mothers had a more robust inflammatory response to phytohaemagglutinin and higher fledging success. Our results suggest that maternal antigen exposure pre-laying can result in increased fitness for both mothers and offspring, depending on food availability.

Summary: Exposure of female birds to a simulated pathogen prior to egg laying increases the metabolic rate, immune function, and fledging success of her nestlings.

Early-life immune activation increases song complexity and alters phenotypic associations between sexual ornaments

Early-life adversity can have long-lasting effects on physiological, behavioural, cognitive, and somatic processes. Consequently, these effects may alter an organism's life-history strategy and reproductive tactics.In response to early-life immune activation, we quantified levels of the acute phase protein haptoglobin (Hp) during development in male zebra finches (Taeniopygia guttata). Then, we examined the long-term impacts of early-life immune activation on an important static sexual signal, song complexity, as well as effects of early-life immune activation on the relationship between song complexity and a dynamic sexual signal, beak colouration. Finally, we performed mate-choice trials to determine if male early-life experience impacted female preference.Challenge with keyhole limpet hemocyanin (KLH) resulted in increased song complexity compared to lipopolysaccharide (LPS) treatment or the control. Hp levels were inversely correlated with song complexity. Moreover, KLH-treatment resulted in negative associations between the two sexual signals (beak colouration and song complexity). Females demonstrated some preference for KLH-treated males over controls and for control males over LPS-treated males in mate choice trials.Developmental immune activation has variable effects on the expression of secondary sexual traits in adulthood, including enhancing the expression of some traits. Because developmental levels of Hp and adult song complexity were correlated, future studies should explore a potential role for exposure to inflammation during development on song learning.Early-life adversity may differentially impact static versus dynamic signals. The use of phenotypic correlations can be a powerful tool for examining the impact of early-life experience on the associations among different traits, including sexual signals.

Ketogenic diet improves behaviors in a maternal immune activation model of autism spectrum disorder

Prenatal factors influence autism spectrum disorder (ASD) incidence in children and can increase ASD symptoms in offspring of animal models. These may include maternal immune activation (MIA) due to viral or bacterial infection during the first trimesters. Unfortunately, regardless of ASD etiology, existing drugs are poorly effective against core symptoms. For nearly a century a ketogenic diet (KD) has been used to treat seizures, and recent insights into mechanisms of ASD and a growing recognition that immune/inflammatory conditions exacerbate ASD risk has increased interest in KD as a treatment for ASD. Here we studied the effects of KD on core ASD symptoms in offspring exposed to MIA. To produce MIA, pregnant C57Bl/6 mice were injected with the viral mimic polyinosinic-polycytidylic acid; after weaning offspring were fed KD or control diet for three weeks. Consistent with an ASD phenotype of a higher incidence in males, control diet-fed MIA male offspring were not social and exhibited high levels of repetitive self-directed behaviors; female offspring were unaffected. However, KD feeding partially or completely reversed all MIA-induced behavioral abnormalities in males; it had no effect on behavior in females. KD-induced metabolic changes of reduced blood glucose and elevated blood ketones were quantified in offspring of both sexes. Prior work from our laboratory and others demonstrate KDs improve relevant behaviors in several ASD models, and here we demonstrate clear benefits of KD in the MIA model of ASD. Together these studies suggest a broad utility for metabolic therapy in improving core ASD symptoms, and support further research to develop and apply ketogenic and/or metabolic strategies in patients with ASD.

Timing of Maternal Immunization Affects Immunological and Behavioral Outcomes of Adult Offspring in Siberian Hamsters (Phodopus sungorus)

Maternal influences are an important contributing factor to offspring survival, development, and behavior. Common environmental pathogens can induce maternal immune responses and affect subsequent development of offspring. There are likely sensitive periods during pregnancy when animals are particularly vulnerable to environmental disruption. Here we characterize the effects of maternal immunization across pregnancy and postpartum on offspring physiology and behavior in Siberian hamsters (Phodopus sungorus). Hamsters were injected with the antigen keyhole limpet hemocyanin (KLH) (1) prior to pairing with a male (premating), (2) at separation (postmating), (3) at midpregnancy, or (4) after birth (lactation). Maternal food intake, body mass, and immunity were monitored throughout gestation, and litters were measured weekly for growth until adulthood when social behavior, hormone concentrations, and immune responses were determined. We found that immunizations altered maternal immunity throughout pregnancy and lactation. The effects of maternal treatment differed between male and female offspring. Aggressive behavior was enhanced in offspring of both sexes born to mothers treated postmating and thus early in pregnancy relative to other stages. In contrast, maternal treatment and maternal stage differentially affected innate immunity in males and females. Offspring cortisol, however, was unaffected by maternal treatment. Collectively, these data demonstrate that maternal immunization affects offspring physiology and behavior in a time-dependent and sex-specific manner. More broadly, these findings contribute to our understanding of the effects of maternal immune activation, whether it be from environmental exposure or immunization, on immunological and behavioral responses of offspring.

Elevational variation in body-temperature response to immune challenge in a lizard

Immunocompetence benefits animal fitness by combating pathogens, but also entails some costs. One of its main components is fever, which in ectotherms involves two main types of costs: energy expenditure and predation risk. Whenever those costs of fever outweigh its benefits, ectotherms are expected not to develop fever, or even to show hypothermia, reducing costs of thermoregulation and diverting the energy saved to other components of the immune system. Environmental thermal quality, and therefore the thermoregulation cost/benefit balance, varies geographically. Hence, we hypothesize that, in alpine habitats, immune-challenged ectotherms should show no thermal response, given that (1) hypothermia would be very costly, as the temporal window for reproduction is extremely small, and (2) fever would have a prohibitive cost, as heat acquisition is limited in such habitat. However, in temperate habitats, immune-challenged ectotherms might show a febrile response, due to lower cost/benefit balance as a consequence of a more suitable thermal environment. We tested this hypothesis in Psammodromus algirus lizards from Sierra Nevada (SE Spain), by testing body temperature preferred by alpine and non-alpine lizards, before and after activating their immune system with a typical innocuous pyrogen. Surprisingly, non-alpine lizards responded to immune challenge by decreasing preferential body-temperature, presumably allowing them to save energy and reduce exposure to predators. On the contrary, as predicted, immune-challenged alpine lizards maintained their body-temperature preferences. These results match with increased costs of no thermoregulation with elevation, due to the reduced window of time for reproduction in alpine environment.

Imperfect past and present progressive: beak color reflects early-life and adult exposure to antigen

Secondary sexual traits may convey information about individual condition. We assessed the capacity for immune challenge with lipopolysaccharide (LPS) or keyhole limpet hemocyanin (KLH) during the prenatal and early postnatal stages to impact beak color development and expression in captive zebra finches. In addition, we tested whether adult immune challenge impacted beak color, and if early-life experience was influential. Immune challenge with KLH early in life slowed development of red beak coloration, and males challenged with KLH as nestlings had reduced red coloration as adults. Following adult KLH challenge, males exhibited a decline in beak redness. Birds challenged with KLH during development produced more anti-KLH antibodies after adult challenge. There was a significant interaction between young treatment and anti-KLH antibody production; for males not challenged with KLH early in life, individuals that mounted a weaker antibody response lost more red coloration after challenge than males mounting a stronger antibody response. Based on models of avian vision, these differences in beak coloration should be detectable to the finches. In contrast to previous studies, we found no effect of early-life or adult challenge with LPS on any aspects of beak coloration. These results provide evidence that beak color reflects developmental and current conditions, and that the signal is linked to critical physiological processes.

Developmental immune activation programs adult behavior: insight from research on birds

Immune activation early in life can program adult behavioral expression. Previous research on birds has documented effects of parasite exposure and immune challenges early in life on dispersal, song, personality, learning and feather pecking. However, the mechanisms responsible for mediating these programming effects are unknown. Candidate brain regions that may be most sensitive include the hippocampus and HVC. Without an understanding of mechanism, it is difficult to assess if programmed behaviors represent pathological side effects or behavioral modifications with benefits to either hosts or parasites. Future research on birds promises to provide novel insight into the adaptive value of programming effects of early life immune activation and the capacity for selection to buffer hosts against negative effects.

Developmental stress increases reproductive success in male zebra finches

There is increasing evidence that exposure to stress during development can have sustained effects on animal phenotype and performance across life-history stages. For example, developmental stress has been shown to decrease the quality of sexually selected traits (e.g. bird song), and therefore is thought to decrease reproductive success. However, animals exposed to developmental stress may compensate for poor quality sexually selected traits by pursuing alternative reproductive tactics. Here, we examine the effects of developmental stress on adult male reproductive investment and success in the zebra finch (Taeniopygia guttata). We tested the hypothesis that males exposed to developmental stress sire fewer offspring through extra-pair copulations (EPCs), but invest more in parental care. To test this hypothesis, we fed nestlings corticosterone (CORT; the dominant avian stress hormone) during the nestling period and measured their adult reproductive success using common garden breeding experiments. We found that nestlings reared by CORT-fed fathers received more parental care compared with nestlings reared by control fathers. Consequently, males fed CORT during development reared nestlings in better condition compared with control males. Contrary to the prediction that developmental stress decreases male reproductive success, we found that CORT-fed males also sired more offspring and were less likely to rear non-genetic offspring compared with control males, and thus had greater overall reproductive success. These data are the first to demonstrate that developmental stress can have a positive effect on fitness via changes in reproductive success and provide support for an adaptive role of developmental stress in shaping animal phenotype.

Pre and post-natal antigen exposure can program the stress axis of adult zebra finches: evidence for environment matching

Both maternal exposure to stressors and exposure of offspring to stressors during early life can have lifelong effects on the physiology and behavior of offspring. Stress exposure can permanently shape an individual's phenotype by influencing the development of the hypothalamic-pituitary-adrenal (HPA) axis, which is responsible for the production and regulation of glucocorticoids such as corticosterone (CORT). In this study we used captive zebra finches (Taeniopygia guttata) to examine the effects of matching and mismatching maternal and early post-natal exposure to one of two types of antigens or a control on HPA axis reactivity in adult offspring. Prior to breeding, adult females were injected with lipopolysaccharide (LPS), keyhole limpet hemocyanin (KLH) or a control. Offspring of females in each of the three treatments were themselves exposed to LPS, KLH or a control injection at 5 and 28days post-hatch. When offspring were at least 18months of age, standardized capture and restraint stress tests were conducted to determine the impact of the treatments on adult stress responsiveness. We found significant interaction effects between maternal and offspring treatments on stress-induced CORT levels, and evidence in support of the environment matching hypothesis for KLH-treated birds, not LPS-treated birds. KLH-treated offspring of KLH-treated mothers exhibited reduced stress-induced CORT levels, whereas LPS-treated or control offspring of KLH-treated mothers exhibited elevated stress-induced CORT levels. Although the treatment effects on baseline CORT were non-significant, the overall pattern was similar to the effects observed on stress-induced CORT levels. Our results highlight the complex nature of HPA axis programming, and to our knowledge, provide the first evidence that a match or mismatch between pre and post-natal antigen exposure can have life-long consequences for HPA axis function.

Maternal antibody transfer can lead to suppression of humoral immunity in developing zebra finches (Taeniopygia guttata)

Maternally transferred antibodies have been documented in a wide range of taxa and are thought to adaptively provide protection against parasites and pathogens while the offspring immune system is developing. In most birds, transfer occurs when females deposit immunoglobulin Y into the egg yolk, and it is proportional to the amount in the female's plasma. Maternal antibodies can provide short-term passive protection as well as specific and nonspecific immunological priming, but high levels of maternal antibody can result in suppression of the offspring's humoral immune response. We injected adult female zebra finches (Taeniopygia guttata) with one of two antigens (lipopolysaccharide [LPS] or keyhole limpet hemocyanin [KLH]) or a control and then injected offspring with LPS, KLH, or a control on days 5 and 28 posthatch to examine the impact of maternally transferred antibodies on the ontogeny of the offspring's humoral immune system. We found that offspring of females exposed to KLH had elevated levels of KLH-reactive antibody over the first 17-28 days posthatch but reduced KLH-specific antibody production between days 28 and 36. We also found that offspring exposed to either LPS or KLH exhibited reduced total antibody levels, compared to offspring that received a control injection. These results indicate that high levels of maternal antibodies or antigen exposure during development can have negative repercussions on short-term antibody production and may have long-term fitness repercussions for the offspring.

Dose-dependent effects of an immune challenge at both ultimate and proximate levels in Drosophila melanogaster

Immune responses are highly dynamic. The magnitude and efficiency of an immune response to a pathogen can change markedly across individuals, and such changes may be influenced by variance in a range of intrinsic (e.g. age, genotype, sex) and external (e.g. abiotic stress, pathogen identity, strain) factors. Life history theory predicts that up-regulation of the immune system will come at a physiological cost, and studies have confirmed that increased investment in immunity can reduce reproductive output and survival. Furthermore, males and females often have divergent reproductive strategies, and this might drive the evolution of sex-specific life history trade-offs involving immunity, and sexual dimorphism in immune responses per se. Here, we employ an experiment design to elucidate dose-dependent and sex-specific responses to exposure to a nonpathogenic immune elicitor at two scales--the 'ultimate' life history and the underlying 'proximate' immune level in Drosophila melanogaster. We found dose-dependent effects of immune challenges on both male and female components of reproductive success, but not on survival, as well as a response in antimicrobial activity. These results indicate that even in the absence of the direct pathogenic effects that are associated with actual disease, individual life histories respond to a perceived immune challenge--but with the magnitude of this response being contingent on the initial dose of exposure. Furthermore, the results indicate that immune responses at the ultimate life history level may indeed reflect underlying processes that occur at the proximate level.