Positive effects of testosterone and immunochallenge on energy allocation to reproductive organs

Relationships between avian malaria resilience and corticosterone, testosterone and prolactin in a Hawaiian songbird

Glucocorticoids, androgens, and prolactin regulate metabolism and reproduction, but they also play critical roles in immunomodulation. Since the introduction of avian malaria to Hawaii a century ago, low elevation populations of the Hawaii Amakihi (Chlorodrepanis virens) that have experienced strong selection by avian malaria have evolved increased resilience (the ability to recover from infection), while high elevation populations that have undergone weak selection remain less resilient. We investigated how variation in malaria selection has affected corticosterone, testosterone, and prolactin hormone levels in Amakihi during the breeding season. We predicted that baseline corticosterone and testosterone (which have immunosuppressive functions) would be reduced in low elevation and malaria-infected birds, while stress-induced corticosterone and prolactin (which have immunostimulatory functions) would be greater in low elevation and malaria-infected birds. As predicted, prolactin was significantly higher in malaria-infected than uninfected females (although more robust sample sizes would help to confirm this relationship), while testosterone trended higher in malaria-infected than uninfected males and, surprisingly, neither baseline nor stress-induced CORT varied with malaria infection. Contrary to our predictions, stress-induced corticosterone was significantly lower in low than high elevation birds while testosterone in males and prolactin in females did not vary by elevation, suggesting that Amakihi hormone modulation across elevation is determined by variables other than disease selection (e.g., timing of breeding, energetic challenges). Our results shed new light on relationships between introduced disease and hormone modulation, and they raise new questions that could be explored in experimental settings.

Male rock lizards may compensate reproductive costs of an immune challenge affecting sexual signals

Sexual signals can be evolutionarily stable if they are condition dependent or costly to the signaler. One of these costs may be the trade-off between maintaining the immune system and the elaboration of ornaments. Experimental immune challenges in captivity show a reduction in the expression of sexual signals, but it is not clear whether these detrimental effects are important in nature and, more importantly, whether they have reproductive consequences. We designed a field experiment to challenge the immune system of wild male Carpetan rock lizards, Iberolacerta cyreni, with a bacterial antigen (lipopolysaccharide). The immune challenge decreased relative reflectance of ultraviolet structural and melanin-dependent sexual coloration in the throat and the lateral ocelli, whereas the carotenoid-dependent dorsal green coloration was not affected. Immune activation also decreased proportions of ergosterol and cholesta-5,7-dien-3-ol in femoral secretions. These results support a trade-off between the immune system and both visual and chemical sexual ornaments. Moreover, the reproductive success of males, estimated with DNA microsatellites, depended on the expression of some color and chemical traits. However, the immune challenge did not cause overall differences in reproductive success, although it increased with body size/age in control but not in challenged males. This suggests the use of alternative reproductive strategies (e.g., forced matings) in challenged males, particularly in smaller ones. These males might consider that their survival probabilities are low and increase reproductive effort as a form of terminal investment in spite of their “low-quality” sexual signals and potential survival costs.

A dynamic threshold model for terminal investment

Although reproductive strategies can be influenced by a variety of intrinsic and extrinsic factors, life history theory provides a rigorous framework for explaining variation in reproductive effort. The terminal investment hypothesis proposes that a decreased expectation of future reproduction (as might arise from a mortality threat) should precipitate increased investment in current reproduction. Terminal investment has been widely studied, and a variety of intrinsic and extrinsic cues that elicit such a response have been identified across an array of taxa. Although terminal investment is often treated as a static strategy, the level at which a cue of decreased future reproduction is sufficient to trigger increased current reproductive effort (i.e., the terminal investment threshold) may depend on context, including the internal state of the organism or its current external environment, independent of the cue that triggers a shift in reproductive investment. Here, we review empirical studies that address the terminal investment hypothesis, exploring both the intrinsic and extrinsic factors that mediate its expression. Based on these studies, we propose a novel framework within which to view the strategy of terminal investment, incorporating factors that influence an individual's residual reproductive value beyond a terminal investment trigger - the dynamic terminal investment threshold.

PHA-induced inflammation is not energetically costly in the subterranean rodent Ctenomys talarum (tuco-tucos)

Immune activity has been proposed to be associated with substantial costs, due to trade-offs with other functions or activities that share common resources and contribute to an animal's fitness. However, direct estimates of the cost of mounting an immune response are few and have been performed mainly in birds. Thus, further work is needed to clarify the relative costs of different components of the immune system and the role of environmental and life-history traits in modulating the costs of resistance. Within the components of immunity, inflammation is considered to be associated with a larger energetic expenditure. Here, we evaluated the energetic cost of the inflammatory response to phytohemagglutinin (PHA) in a wild population of a subterranean rodent, Ctenomys talarum, and the trade-offs between immune activity and reproduction. C. talarum develops an inflammatory response to PHA, but contrary to our predictions, this response was not associated with an increase in oxygen consumption regardless of reproductive status or sex. Our study shows that an immune challenge may not always result in a detectable energetic cost. We discuss the possibility that other currencies could be underlying the cost, such as micro-or macronutrients requirements, autoimmunity or oxidative stress.

Social and nutritional stressors: agents for altered immune function in white-footed mice (Peromyscus leucopus)

As habitats become more fragmented, population densities and diets of wildlife can change dramatically, contributing to increased stress and incidence of infections and disease. To better understand effects of human disturbance on immunocompetence of wild mammals, we studied individual and combined effects of social and nutritional stress on the health of wild-captured adult male white-footed mice (Peromyscus leucopus (Rafinesque, 1818)), a species that commonly occurs in close proximity to humans. Paired mice had reduced body mass and circulating monocytes, higher serum corticosterone level, and a significantly weaker humoral immune response compared with mice housed individually. Mice fed a 5% protein diet had reduced body mass and circulating monocytes, but no differences in humoral or cellular immune responses compared with mice fed a 30% protein diet. The only interactive effect of the two stresses on immune-related parameters was on mass of the spleen. We hypothesize that reduced humoral immunocompetence in response to acute and chronic social stress likely contributes to increases in disease transmission in high-density populations associated with fragments of habitat. In addition, anthropogenic impacts that limit food availability may be of greater importance to immune function than impacts on food quality.

Does 2D:4D predict fitness in a wild mammal?

Environmental pressures present during critical periods in fetal development can have a strong influence on the long-term fitness of an individual. Susceptibility to parasites, immune function, and future reproductive success are all vulnerable to stressful events in utero. The causes and consequences of prenatal environmental stress are often difficult (if not impossible) to evaluate, especially in wild populations. Digit ratio, the ratio of 2nd digit to 4th digit length (2D:4D) has been identified as an index of fetal androgen exposure. Current techniques for assessing digit ratio pose severe limitations to the accuracy of digit measurement of wild animals owing to the constant movement of the feet and inaccessibility of laboratory equipment. Our study attempts a new indirect technique wherein subjects had an imprint taken of each foot that was then photographed and scanned for digital measurement. Using red squirrels ( Tamiasciurus hudsonicus (Erxleben, 1777)) as a model species, we hypothesized that higher degrees of fetal testosterone exposure (assumed from digit ratios) would result in reduced reproductive success and increased susceptibility to parasite infection. Digit ratio was negatively correlated with reproductive output and positively correlated with parasite loads. Although this may indicate that exposure to excess testosterone in utero may jeopardize future fitness, the relationships are inconsistent.

MHC variation, multiple simultaneous infections and physiological condition in the subterranean rodent Ctenomys talarum

Parasites and pathogens can play a significant role in shaping the genetic diversity of host populations, particularly at genes associated with host immune response. To explore this relationship in a natural population of vertebrates, we characterized Major Histocompatibility Complex (MHC) variation in the subterranean rodent Ctenomys talarum (the talas tuco-tuco) as a function of parasite load and ability to mount an adaptive immune response against a novel antigen. Specifically, we quantified genotypic diversity at the MHC class II DRB locus in relation to (1) natural variation in infection by multiple genera of parasites (potential agents of selection on MHC genes) and (2) antibody production in response to injection with sheep red blood cells (a measure of immunocompetence). Data were analyzed using co-inertia multivariate statistics, with epidemiological proxies for individual condition (hematocrit, leukocyte profile, body weight) and risk of parasite exposure (season of capture, sex). A significant excess of DRB heterozygotes was evident in the study population. Co-inertia analyses revealed significant associations between specific DRB alleles and both parasite load and intensity of humoral immune response against sheep red blood cells. The presence of specific DRB aminoacid sequences appeared to be more strongly associated with parasite load and response to a novel antigen than was heterozygosity at the DRB locus. These data suggest a role for parasite-driven balancing selection in maintaining MHC variation in natural populations of C. talarum. At the same time, these findings underscore the importance of using diverse parameters to study interactions among physiological conditions, immunocompetence, and MHC diversity in free-living animals that are confronted with multiple simultaneous immune challenges.

Linking disease and community ecology through behavioural indicators: immunochallenge of white-footed mice and its ecological impacts

1. Pathogens and immune challenges can induce changes in host phenotype in ways that indirectly impact important community interactions, including those that affect host-pathogen interactions. 2. To explore host behavioural response to immune challenge, we exposed wild white-footed mice (Peromyscus leucopus) to an immunogen from an endemic, zoonotic pathogen, the spirochete Borrelia burgdorferi. White-footed mice are a major reservoir host of Lyme disease (LD) spirochetes in northeastern USA and an abundant member of forest communities. The activity patterns, foraging behaviour, and space use of white-footed mice have implications for population growth rates of community members upon which mice incidentally prey (i.e. gypsy moths and native thrushes), as well as potentially determining host-vector encounter rates and human risk of LD. 3. Immunochallenge led to specific humoral (antibody) and cellular (i.e. elevated neutrophils and eosinophils) immune responses, supporting use of the immunogen as a surrogate for pathogenic infection. 4. Immunochallenged mice had reduced wheel-running activity early in the night when measured in the lab. However, mouse activity, as measured by track plates in natural field experiments, did not differ between mice exposed to the immunogen and unexposed mice. 5. Foraging behaviour of wild mice in the field - assessed with giving-up densities of seed at artificial feeding stations - was affected by exposure to the immunogen. Whereas immunochallenge did not influence whether foraging mice gained information on patch quality while foraging, it led to reductions in predator avoidance during foraging, suggesting that the proportion of space used by foraging mice may be greater as a result of immunochallenge. This increased space use is predicted to increase encounter rates with patchily distributed LD vectors (ticks) and with incidental prey items. 6. Thus, immunochallenge in white-footed mice, and potentially pathogenic infection, have the potential to indirectly impact community interactions, including those important for pathogen transmission.

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

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

Mounting a specific immune response increases energy expenditure of the subterranean rodent Ctenomys talarum (tuco-tuco): implications for intraspecific and interspecific variation in immunological traits

It was recently hypothesised that specific induced defences, which require substantial time and resources and are mostly beneficial against repeated infections, are more likely to be favoured in ‘slow-living-pace’ species. Therefore, understanding how different types of immune defences might vary with life history requires knowledge of the costs and benefits of defence components. Studies that have explored the energetic costs of immunity in vertebrates have done so with a focus primarily on birds and less so on mammals, particularly surface-dwelling rodents. In this study, we evaluated whether an experimental induction of the immune system with a non-pathogenic antigen elevates the energetic expenditure of a subterranean rodent: Ctenomys talarum (tuco-tucos). In both seasons studied, a significant increase in oxygen consumption was verified in immune-challenged tuco-tucos injected with sheep red blood cells (SRBC) compared with control animals. The increase in oxygen consumption 10 days after the exposure to SRBC was lower for female tuco-tucos monitored in the breeding season compared with females in the non-breeding season. Interestingly, antibody titres of female tuco-tucos did not decrease during the breeding season. Our results add new insight into the role of other factors such as basal metabolic rate or degree of parasite exposure besides ‘pace of life’ in modulating the interspecific immunological variation observed in natural populations of mammals.

Food supplementation and testosterone interact to influence reproductive behavior and immune function in Sceloporus graciosus

The energetic resources in an organism's environment are essential for executing a wide range of life-history functions, including immunity and reproduction. Most energetic budgets, however, are limited, which can lead to trade-offs among competing functions. Increasing reproductive effort tends to decrease immunity in many cases, and increasing total energy via supplemental feedings can eliminate this effect. Testosterone (T), an important regulator of reproduction, and food availability are thus both potential factors regulating life-history processes, yet they are often tested in isolation of each other. In this study, we considered the effect of both food availability and elevated T on immune function and reproductive behavior in sagebrush lizards, Sceloporus graciosus, to assess how T and energy availability affect these trade-offs. We experimentally manipulated diet (via supplemental feedings) and T (via dermal patches) in males from a natural population. We determined innate immune response by calculating the bacterial killing capability of collected plasma exposed to Escherichia coli ex vivo. We measured reproductive behavior by counting the number of courtship displays produced in a 20-min sampling period. We observed an interactive effect of food availability and T-patch on immune function, with food supplementation increasing immunity in T-patch lizards. Additionally, T increased courtship displays in control food lizards. Lizards with supplemental food had higher circulating T than controls. Collectively, this study shows that the energetic state of the animal plays a critical role in modulating the interactions among T, behavior and immunity in sagebrush lizards and likely other species.

Humoral immune response suppresses reproductive physiology in male Brandt's voles (Lasiopodomys brandtii)

In order to evaluate the potential costs of humoral immune response, which is important for survival in small wild mammals, we studied the physiological function of adult male Brandt's voles (Lasiopodomys brandtii) challenged with human immunoglobulin G (IgG). Compared with controls, the immunochallenged voles showed significantly higher antibody levels 15 days after injection. Serum testosterone levels, and mass of testes and epididymides were lower in immunochallenged voles than in control animals. Body mass remained stable during the course of the experiment. Total and digestible energy intake showed a transient decrease following IgG injection, while resting metabolic rate (RMR) increased. Taken together, these data suggest a shift in metabolic priorities in response to immune challenge. Our results provide evidence that mounting a humoral immune response to an immunological challenge may have fitness costs in male Brandt's voles.

Hormonal and immunological changes in mice after spinal cord injury

Spinal cord injury (SCI) is associated with immune deficiencies and life-threatening infections. However, the specific mechanisms underlying this pathological condition remain unclear. In recent years, increasing evidence has suggested that anabolic hormones may be involved in immunological complications. Here, we monitored candidate hormone concentrations and immune cell counts, in CD1 mice, for 4 weeks after low-thoracic transection of the spinal cord (Tx). Serum dihydroepiandrosterone (DHEA), insulin, and parathyroid hormone (PTH) levels decreased throughout the time period studied compared with control, non-Tx mice. In turn, testosterone and growth hormone (GH) levels were only transiently changed, with a decrease of testosterone during the first 2 weeks and an increase of GH at 1 week post-Tx. A complete blood count revealed either unchanged or moderately decreased erythrocyte, platelet, hemoglobin and hematocrit levels. Total leukocyte, lymphocyte, and eosinophil counts also decreased, whereas neutrophils and monocytes did not change significantly. In the bone marrow, lymphocyte numbers decreased and neutrophils increased, whereas monocytes, eosinophils, and megakariocytes did not change significantly. These results revealed significant changes occurring rapidly (<1-2 weeks) after Tx in both hormonal and immunological systems, providing compelling evidence of a role for anabolic hormones in SCI-related immune deficiencies.

Immune challenge retards seasonal reproductive regression in rodents: evidence for terminal investment

Animals must balance investments in different physiological activities to allow them to maximize fitness in the environments they inhabit. These adjustments among reproduction, growth and survival are mandated because of the competing high costs of each process. Seasonally breeding rodents generally bias their investments towards reproduction when environmental conditions are benign, but shift these investments towards processes that promote survival, including immune activity, when environmental conditions deteriorate. Because survival probability of non-tropical small mammals is generally low in winter, under certain circumstances, these animals may not allocate resources to survival mechanisms in an effort to produce as many offspring as possible in the face of increased probability of death. Such 'terminal investments' have been described in passerines, but there are few examples of such phenomena in small mammals. Here, we show that male Siberian hamsters (Phodopus sungorus) challenged with lipopolysaccharide (a component of gram-negative bacteria that activates the immune system) induced a small, but significant, retardation of seasonal regression of the reproductive system relative to saline-injected hamsters. This delayed reproductive regression likely reflects a strategy to maintain reproductive function when survival prospects are compromised by infection.