The role of parasite-induced immunodepression, rank and social environment in the modulation of behaviour and hormone concentration in male laboratory mice (Mus musculus)

Sex—the most underappreciated variable in research: insights from helminth-infected hosts

The sex of a host affects the intensity, prevalence, and severity of helminth infection. In many cases, one sex has been found to be more susceptible than the other, with the prevalence and intensity of helminth infections being generally higher among male than female hosts; however, many exceptions exist. This observed sex bias in parasitism results primarily from ecological, behavioural, and physiological differences between males and females. Complex interactions between these influences modulate the risk of infection. Indeed, an interplay among sex hormones, sex chromosomes, the microbiome and the immune system significantly contributes to the generation of sex bias among helminth-infected hosts. However, sex hormones not only can modulate the course of infection but also can be exploited by the parasites, and helminths appear to have developed molecules and pathways for this purpose. Furthermore, host sex may influence the efficacy of anti-helminth vaccines; however, although little data exist regarding this sex-dependent efficacy, host sex is known to influence the response to vaccines. Despite its importance, host sex is frequently overlooked in parasitological studies. This review focuses on the key contributors to sex bias in the case of helminth infection. The precise nature of the mechanisms/factors determining these sex-specific differences generally remains largely unknown, and this represents an obstacle in the development of control methods. There is an urgent need to identify any protective elements that could be targeted in future therapies to provide optimal disease management with regard to host sex. Hence, more research is needed into the impact of host sex on immunity and protection.

Galectins - Important players of the immune response to CNS parasitic infection

Galectins are a family of proteins that bind β-galactosides and play key roles in a variety of cellular processes including host defense and entry of parasites into the host cells. They have been well studied in hosts but less so in parasites. As both host and parasite galectins are highly upregulated proteins following infection, galectins are an area of increasing interest and their role in immune modulation has only recently become clear. Correlation of CNS parasitic diseases with mental disorders as a result of direct or indirect interaction has been observed. Therefore, galectins produced by the parasite should be taken into consideration as potential therapeutic agents.

Differential effects of progesterone on social recognition and the avoidance of pathogen threat by female mice

Although pathogen threat affects social and sexual responses across species, relatively little is known about the underlying neuroendocrine mechanisms. Progesterone has been speculated to be involved in the mediation of pathogen disgust in women, though with mixed experimental support. Here we considered the effects of acute progesterone on the disgust-like avoidance responses of female mice to pathogen threat. Estrous female mice discriminated and avoided the urinary and associated odors of males subclinically infected with the murine nematode parasite, Heligmosomoides polygyrus. These avoidance responses were not significantly affected by pre-treatment with progesterone. Likewise, brief (1 min) exposure to the odors of infected males attenuated the subsequent responses of females to the odors of the normally preferred unfamiliar males and enhanced their preferences for familiar males. Neither progesterone nor allopregnanolone, a central neurosteroid metabolite of progesterone, had any significant effects on the avoidance of unfamiliar males elicited by pre-exposure to a parasitized male. Progesterone and allopregnanolone, did, however, significantly attenuate the typical preferences of estrous females for unfamiliar uninfected males, suggestive of effects on social recognition. These findings with mice indicate that progesterone may have minimal effects on the responses to specific parasite threat and the expression of pathogen disgust but may influence more general social recognition and preferences.

Effects of Taenia Pisiformis Infection and Obesity on Clinical Parameters, Organometry and Fat Distribution in Male Rabbits

Taenia pisiformis infection causes important economic loss in farms. It is suggested that obesity has a major impact on infection and reproduction. We addressed the impact of T. pisiformis infection in normal and obese rabbits to evaluate its effect on parameters important in behavior and reproduction. T. pisiformis infection in obese rabbits decreased body weight. In the obese-infected rabbits, eosinophils and heterophiles were increased 23% by the infection (P ≤ 0.05). T. pisiformis decreased cholesterol by 13% in normal weight infected rabbits and 10% in obese group (P ≤ 0.05), while triglyceride and VLDL were increased by 23% and 45% in the non-infected obese group (P ≤ 0.05). The infection increased serum cortisol levels only in normal weight rabbits (P ≤ 0.05). Liver weight was 20% higher in obese and obese-infected rabbits (P ≤ 0.05). Testicular weight in obese-infected was 46% higher than normal weight (P ≤ 0.0001) and 20% more than the obese-non-infected (P ≤ 0.0001). Furthermore, the infection reduced the weight of submandibular glands in infected and obese-infected rabbits (P ≤ 0.05), body fat increased 10% in the obese-infected than in the obese, and infected group was 35% over the normal weight non-infected (P ≤ 0.01). Our results show that T. pisiformis alters metabolic characteristics in rabbits, which can impact on the production and welfare of animals.

Patterns and variation in the mammal parasite-glucocorticoid relationship

Parasites are ubiquitous and can strongly affect their hosts through mechanisms such as behavioural changes, increased energetic costs and/or immunomodulation. When parasites are detrimental to their hosts, they should act as physiological stressors and elicit the release of glucocorticoids. Alternatively, previously elevated glucocorticoid levels could facilitate parasite infection due to neuroimmunomodulation. However, results are equivocal, with studies showing either positive, negative or no relationship between parasite infection and glucocorticoid levels. Since factors such as parasite type, infection severity or host age and sex can influence the parasite-glucocorticoid relationship, we review the main mechanisms driving this relationship. We then perform a phylogenetic meta-analysis of 110 records from 65 studies in mammalian hosts from experimental and observational studies to quantify the general direction of this relationship and to identify ecological and methodological drivers of the observed variability. Our review produced equivocal results concerning the direction of the relationship, but there was stronger support for a positive relationship, although causality remained unclear. Mechanisms such as host manipulation for parasite survival, host response to infection, cumulative effects of multiple stressors, and neuro-immunomodulatory effects of glucocorticoids could explain the positive relationship. Our meta-analysis results revealed an overall positive relationship between glucocorticoids and parasitism among both experimental and observational studies. Because all experimental studies included were parasite manipulations, we conclude that parasites caused in general an increase in glucocorticoid levels. To obtain a better understanding of the directionality of this link, experimental manipulation of glucocorticoid levels is now required to assess the causal effects of high glucocorticoid levels on parasite infection. Neither parasite type, the method used to assess parasite infection nor phylogeny influenced the relationship, and there was no evidence for publication bias. Future studies should attempt to be as comprehensive as possible, including moderators potentially influencing the parasite-glucocorticoid relationship. We particularly emphasise the importance of testing hosts of a broad age range, concomitantly measuring sex hormone levels or at least reproductive status, and for observational studies, also considering food availability, host body condition and social stressors to obtain a better understanding of the parasite-glucocorticoid relationship.

Conspecific infection threat rapidly biases the social responses of female mice: Involvement of oxytocin

Pathogen threat affects social preferences and responses across species. Here we examined the effects of social context and the infection status of conspecific females and males on the social and mate responses of female mice. The responses of female mice to males were rapidly affected by the presence of infected female conspecifics and infected males. In mice odor cues drive appetitive and aversive social and mate responses. Brief (1 min) exposure to the fresh urinary odors of females infected with the murine nematode parasite, Heligmosomoides polygyrus, attenuated the responses of other uninfected females to the odors of naturally preferred unfamiliar males and enhanced their preferences for familiar males. Likewise exposure to the odors of a male either infected with H. polygyrus or treated with the bacterial endotoxin, lipopolysaccharide, reduced the responses of females to the odors of unfamiliar males. In addition, females displayed an avoidance of, and discrimination against, male mice whose odors had been associated with that of an infected female ("guilt by association") and a preference for the odors associated with an uninfected female ("mate copying"). These shifts in preferences for female associated male odors were attenuated in a dose-related manner by pre-treatment with the oxytocin receptor antagonist, L-368,899. These findings show that social information associated with the infection status of conspecifics can rapidly bias the mate preferences of female mice in an oxytocin receptor dependent manner.

Atlantic salmon infected with salmon lice are more susceptible to new lice infections

Aggregation is commonly observed for macroparasites, but its adaptive value remains unclear. Heavy infestations intensities may lead to a decrease in some fitness-related traits of parasites (e.g. parasite fecundity or survival). However, to a dioecious parasite, increased aggregation could also increase the chance of finding individuals of the opposite sex. In a laboratory experiment, we tested if previous experience with salmon lice (Lepeophtheirus salmonis) affected susceptibility of Atlantic salmon (Salmo salar) to later exposure to the same parasite species. We found that currently infected fish got higher intensities of new lice than naive fish. This suggests that hosts already carrying parasites are more susceptible to new lice infections. For this dioecious parasite, such positive density dependence might be adaptive, ensuring successful reproduction under conditions of low lice densities by increasing the probability of both sexes infecting the same host.

Corticosterone: a costly mediator of signal honesty in sand lizards

The mechanisms underlying honest signal expression remain elusive and may involve the integration of social and physiological costs. Corticosterone is a socially modulated metabolic hormone that mediates energy investment and behavior and may therefore function to deter dishonest signal expression. We examined the relationship between corticosterone and green badge coloration in male sand lizards (Lacerta agilis), hypothesizing that physiological and behavioral costs resulting from elevated baseline glucocorticoids function in maintenance of honest signal expression. We found that large‐badged males had higher corticosterone titer, with this relationship apparent at the end of the season and absent early in the season. Large‐badged males also suffered higher ectoparasite load (number of tick nymphs), despite being in better condition than small‐badged males. Ectoparasite load was positively related to corticosterone titer early in the season at the time of badge formation. High‐condition individuals had lower corticosterone and lower numbers of ectoparasites than low‐condition individuals, suggestive of conditional variation in ability to withstand costs of corticosterone. We found an opposing negative relationship between corticosterone titer and endoparasite load. Corticosterone titer was also negatively associated with male mobility, a fitness‐determining behavior in this species. Because badge size is involved in mediating agonistic social interactions in this species, our results suggest that badge‐dependent variation in corticosterone is likely reflective of variation in social conditions experienced over the course of the season. Our results implicate corticosterone in maintenance of signal honesty, both early in the season through enforcement of physiological costs (ectoparasite load) and during the season through behavioral costs (male mobility). We propose that socially modulated variation in corticosterone critically functions in mediation of signal honesty without requiring a direct role for corticosterone in trait expression.

Shaking the myth: Body mass, aggression, steroid hormones, and social dominance in wild house mouse

In social mammals, the position of a male in the group's hierarchy strongly affects his reproductive success. Since a high social rank is often gained through competition with other males, selection should favour bigger males over smaller ones. We may therefore predict faster growth and/or delayed sexual maturity in dominant males. Likewise, dominants should have higher levels of testosterone, hormone important in many aspects of male dominance. Less obvious is the relationship between dominance and levels of corticosterone but generally higher concentrations are expected in subordinate individuals. We studied body growth, sexual maturation and endocrinal changes in males of two house mouse subspecies, raised in fraternal pairs. Since Mus musculus domesticus is the subspecies which dominates mutual encounters with Mus musculus musculus we predicted higher growth rate, delayed puberty and aggression, and higher testosterone and corticosterone levels in domesticus males compared to musculus. In all comparisons, no differences were found between dominant and subordinate musculus brothers. On the other hand, in M. m. domesticus, dominant males revealed a different growth trajectory and lower corticosterone levels than subordinate males but not delayed puberty and higher testosterone concentrations, thus contradicting our predictions. In inter-subspecific comparisons, musculus males matured earlier but became aggressive at the same time as domesticus males. The musculus testosterone ontogeny suggests that social positions in this subspecies remain unfixed for an extended period and that the increasing levels probably reflect prolonged hierarchy contests. It appears that the ontogeny of behaviour and physiological traits diverge cryptically between the two subspecies.

Social status, immune response and parasitism in males: a meta-analysis

In male vertebrates, two conflicting paradigms--the energetic costs of high dominance rank and the chronic stress of low rank--have been proposed to explain patterns of immune function and parasitism. To date, neither paradigm has provided a complete explanation for status-related differences in male health. Here, we applied meta-analyses to test for correlations between male social status, immune responses and parasitism. We used an ecoimmunological framework, which proposes that males should re-allocate investment in different immune components depending on the costs of dominance or subordination. Spanning 297 analyses, from 77 studies on several vertebrate taxa, we found that most immune responses were similar between subordinate and dominant males, and neither dominant nor subordinate males consistently invested in predictable immune components. However, subordinate males displayed significantly lower delayed-type hypersensitivity and higher levels of some inflammatory cytokines than dominant males, while dominant males exhibited relatively lower immunoglobulin responses than subordinate males. Despite few differences in immunity, dominant males exhibited consistently higher parasitism than subordinate males, including protozoan blood parasites, ectoparasites and gastrointestinal helminths. We discuss our results in the context of the costs of dominance and subordination and advocate future work that measures both parasitism and immune responses in wild systems.

Towards an integrative model of sociality in caviomorph rodents

In the late 1990s and early 2000s it was recognized that behavioral ecologists needed to study the sociality of caviomorph rodents (New World hystricognaths) before generalizations about rodent sociality could be made. Researchers identified specific problems facing individuals interested in caviomorph sociality, including a lack of information on the proximate mechanisms of sociality, role of social environment in development, and geographical or intraspecific variation in social systems. Since then researchers have described the social systems of many previously understudied species, including some with broad geographical ranges. Researchers have done a good job of determining the role of social environments in development and identifying the costs and benefits of social living. However, relatively little is known about the proximate mechanisms of social behavior and fitness consequences, limiting progress toward the development of integrative (evolutionary-mechanistic) models for sociality. To develop integrative models behavioral ecologists studying caviomorph rodents must generate information on the fitness consequences of different types of social organization, brain mechanisms, and endocrine substrates of sociality. We review our current understanding and future directions for research in these conceptual areas. A greater understanding of disease ecology, particularly in species carrying Old World parasites, is needed before we can identify potential links between social phenotypes, mechanism, and fitness.

Female mate choice in mammals

Studies of mate choice in vertebrates have focused principally on birds, in which male ornaments are often highly developed, and have shown that females commonly select mates on the basis of particular phenotypic characteristics that may reflect their genetic quality. Studies of female mate choice in mammals are less highly developed and they have commonly focused on female mating preferences that are likely to be maintained by benefits to the female's own survival or breeding success. However, recent experimental studies of mate choice in mammals--especially rodents--provide increasing evidence of consistent female preferences that appear likely to generate benefits to the fitness of offspring. As yet, there is no compelling evidence that female mating preferences are less highly developed in female mammals than in female birds, although these preferences may more often be masked by the effects of male competition or of attempts by males to constrain female choice.

Animal model of Nippostrongylus brasiliensis and Heligmosomoides polygyrus

Animal models of Nippostrongylus brasiliensis and Heligmosomoides polygyrus infection are powerful tools for the investigation of the basic biology of immune responses and protective immunity. In particular, they model the induction and maintenance of Th2 type immune responses and exhibit all the requisite hallmarks of CD4 T cell-dependent IgE production, eosinophilia, mastocytosis, and mucus production. This chapter describes simple, cost-effective techniques for using and maintaining these easy-to-work-with parasites in the context of a modern laboratory.

Early social isolation alters behavioral and physiological responses to an endotoxin challenge in piglets

Psychosocial stress in the form of maternal deprivation and social isolation during early postnatal life induces persistent alterations in behavioral and physiological mechanisms of adaptation. One consequence may be an increased susceptibility to diseases in later life. Therefore, the aim of the present study was to investigate in domestic piglets the effects of a repeated social isolation (2 h daily from day 3 to day 11 of age) on behavioral, endocrine and immune responses to an endotoxin challenge with lipopolysaccharide (LPS) 1 day or 45 days after the isolation period. Peripheral LPS administration caused serious sickness behavior (somnolence, shivering, vomiting) and provoked profound increases in circulating tumor necrosis factor-alpha (TNF-alpha), ACTH and cortisol concentrations. The prior social isolation treatment enhanced signs of sickness and impaired suckling behavior. Early isolated piglets responded to LPS by an increase of shivering on day 12 and by increased vomiting on day 56 compared to controls. Further, there were considerable delays and reductions of time isolated piglets spent suckling on day 12. The repeated isolation stressor diminished TNF-alpha increases after LPS, whereas stress hormone levels were not significantly affected by isolation treatment. Finally, stronger relationships between signs of sickness and physiological measures were revealed in early isolated piglets. The duration of somnolence in isolated piglets was related to changes of cortisol and TNF-alpha concentrations, and the highest impact on duration of shivering was found for changes in cortisol and corticosteroid binding globulin levels. The present results suggest a sustained adaptive sensitization of coping with infection by social stress experience during early development in piglets.

Experimentally increased social competition compromises humoral immune responses in house finches

Although social behavior can substantially influence an individual's physiology, few studies have examined whether intraspecific competition compromises individual immunocompetence. We experimentally manipulated the intensity of social competition in captive non-breeding house finches (Carpodacus mexicanus) by supplying few (high competition) or many (low competition) feeding sites. We tested whether elevated levels of social competition caused individual changes in aggression rates, humoral immunity, body mass, and baseline and stress-induced corticosterone concentrations. We also examined whether physiological responses to social competition were related to an individual's social status. We found that house finches under high social competition had significantly higher aggression rates, lower antibody responses, and lost more body mass. Within flocks, dominant individuals mounted stronger immune responses in both competition treatments. Our statistical power to detect differences in circulating corticosterone concentrations was low, but we did not find any support for the hypothesis that corticosterone concentrations mediate immunosuppression among or within flocks: baseline and stress-induced corticosterone concentrations did not differ under high and low social competition, were unrelated to individual social status, and did not predict the extent of immunosuppression among individuals. Overall, we documented that two universal components of social behavior, intraspecific competition and social status, modulated the strength of a humoral immune response in house finches.

Molecular evidence that Heligmosomoides polygyrus from laboratory mice and wood mice are separate species

The gastro-intestinal (GI) nematode Heligmosomoides polygyrus is an important experimental model in laboratory mice and a well-studied parasite of wood mice in the field. Despite an extensive literature, the taxonomy of this parasite in different hosts is confused, and it is unclear whether laboratory and field systems represent the same or different Operational Taxonomic Units (OTUs). Molecular analyses reveal high sequence divergence between H. p. bakeri (laboratory) and H. p. polygyrus (field); 3% difference in the ribosomal DNA Internal Transcribed Spacers (ITS) and 8.6% variation in the more rapidly evolving mitochondrial cytochrome c oxidase I (COI) gene. The COI sequence of U.K. H. p. polygyrus is more similar to H. glareoli from voles than to H. p. bakeri, while a single isolate of H. p. polygyrus from Guernsey confirms the extent of genetic variation between H. p. polygyrus populations. Analysis of molecular variance demonstrated that mtCOI sequence variation is associated primarily with groups with distinct ITS2 sequences, and with host identity, but is not partitioned significantly with a single combined taxon H. polygyrus incorporating European and North American isolates. We conclude therefore that the laboratory OTU should be raised to the level of a distinct species, as H. bakeri from the laboratory mouse Mus musculus, and we reject the hypothesis that H. bakeri has diverged from H. polygyrus in the recent past following introduction into America. However, we are unable to reject the hypothesis that H. polygyrus and H. bakeri are sister taxa, and it may be that H. polygyrus is polyphyletic or paraphyletic.

Social status does not predict responses to Seoul virus infection or reproductive success among male Norway rats

Trade-offs exist among life history strategies that are used to increase survival and reproduction; such that, males that engage in more competitive behaviors may be more susceptible to infection. Hantaviruses are transmitted horizontally between rodents through the passage of virus in saliva during wounding and male rodents are more likely to be infected with hantaviruses than females. To determine whether a trade-off exists between dominance and susceptibility to Seoul virus infection, male Long Evans rats were group housed (3/cage) with a female rat and aggressive and subordinate behaviors were monitored during a 10 day group housing condition. After behavioral testing, males were individually housed, inoculated with Seoul virus, and blood, saliva, and fecal samples were collected. Dominant males initiated more aggressive encounters than subordinate males. Dominant and subordinate males, however, had similar steroid hormone concentrations, anti-Seoul virus IgG responses, and weight gain over the course of infection. A similar proportion of dominant and subordinate males shed virus in saliva and feces during infection. Using microsatellite DNA markers paternity was assigned to pups derived during the group housing period. In contrast to our initial hypothesis, dominant and subordinate males sired a similar percentage of pups. Taken together, host social status may not predict reproductive success or susceptibility to hantaviruses in rodent reservoir populations.

Parasite regulation by host hormones: an old mechanism of host exploitation?

Recent experimental evidence suggests that parasites can not only evade immune responses actively but also exploit the hormonal microenvironment within the host to favor their establishment, growth and reproduction. The benefit for parasites of hormonal exploitation is so great that they have evolved structures similar to the steroid and protein hormone receptors expressed in upper vertebrates that can bind to the hormonal metabolites synthesized by the host. This strategy is exemplified by two parasites that respond to adrenal steroids and sexual steroids, respectively: Schistosoma mansoni and Taenia crassiceps. Understanding how the host endocrine system can, under certain circumstances, favor the establishment of a parasite, and characterizing the parasite hormone receptors that are involved might aid the design of hormonal analogs and drugs that affect the parasite exclusively.

Genes, odours and the recognition of parasitized individuals by rodents

Social recognition, whereby animals identify and recognize other individual conspecifics, is a crucial prerequisite for a wide range of social behaviours. There are relationships among social odours (chemical signals), parasite recognition and avoidance that are associated with hormonal, neural and genomic mechanisms in rodents. Rodents use social odours to: (i) distinguish between infected and uninfected individuals; (ii) recognize specific infected individuals; and (iii) avoid and display aversive responses to infected individuals. There are genomic correlates of this parasite recognition and avoidance in which genes expressing the neuropeptide oxytocin have roles. In this article, we provide a framework ("micronet") by which the genetic, hormonal and neural interactions associated with social behaviours and recognition and avoidance of parasitized individuals can be explored.

House sparrows (Passer domesticus) adjust their social status position to their physiological costs

For group-living animals, the maintenance of a position in the social hierarchy may be associated with physiological costs such as increased stress and energy expenditure or suppressed immune functions. In this study, we experimentally manipulated the social status of house sparrows so that each bird experienced two social environments in random sequence: being dominant and subordinate. For 14 males, we investigated how corticosterone concentrations, energy expenditure and immune functions were affected by these changes in social status position. We found that the cost of maintaining a social status position differed between individuals and were related to individual body size. Birds with small body size had increased costs in terms of increased stress responses and reduced cell-mediated immune responses while being experimentally kept as dominants, while birds with large body size had increased costs while they were subordinates. We also found that birds with increased energetic and immunological costs as dominants obtained a low status position in the large group, while birds with increased costs as subordinates obtained a high status position in the large group. In summary, we found that the costs associated with the maintenance of social status position differed between individuals and was related to the individuals' body size. Furthermore, in a large group, individuals maintained a social status position that minimized energetic and immunological costs.

Recognition and avoidance of the odors of parasitized conspecifics and predators: differential genomic correlates

In many species of animals chemical stimuli are an important source of information about the threats and dangers present in the social and non-social world. Olfactory cues play a fundamental role in modulating social recognition and interactions in a wide variety of mammals. Rodents, in particular, utilize chemical signals, to recognize and avoid conspecifics infected with parasites and other pathogens. Animals also respond to, and utilize, predator odor related information to assess and minimize their risk of predation. In this review, we briefly focus on the relations between odors, parasite recognition and avoidance and consider some of the associated hormonal, neural and genomic mechanisms. We describe how both male and female rodents distinguish between infected and uninfected males on the basis of odors, displaying aversive response to, and avoidance of, the urine odors of infected individuals. We further describe how the recognition and avoidance of the odors of infected individuals involves genes for the neuropeptide, oxytocin, (OT), and estrogenic mechanisms. We show that mice with deletions of the oxytocin gene (OT knockout mice (OTKO)) and mice whose genes for estrogen receptor (ER)-alpha or ER-beta [ER knockout mice (ERKO), alphaERKO and betaERKO] have been disrupted are specifically impaired in their recognition, avoidance, and memory of the odors of infected individuals. We contrast this with the recognition and display of aversive responses to predator (cat) odor that are insensitive to these genetic manipulations. These findings reveal some of the mechanisms associated with the olfactory mediated recognition of parasitized individuals and predators.

Hormonal and immunological mechanisms mediating sex differences in parasite infection

The prevalence and intensity of infections caused by protozoa, nematodes, trematodes, cestodes, and arthropods is higher in males than females. The primary thesis of this review is that immunological differences exist between the sexes that may underlie increased parasitism in males compared to females. Several field and laboratory studies link sex differences in immune function with circulating steroid hormones; thus, the roles of sex steroids, including testosterone, oestradiol, and progesterone, as well as glucocorticoids will be discussed. Not only can host hormones affect responses to infection, but parasites can both produce and alter hormone concentrations in their hosts. The extent to which changes in endocrine-immune interactions following infection are mediated by the host or the parasite will be considered. Although males are more susceptible than females to many parasites, there are parasites for which males are more resistant than females and endocrine-immune interactions may underlie this sex reversal. Finally, although immunological differences exist between the sexes, genetic and behavioural differences may explain some variability in response to infection and will be explored as alternative hypotheses for how differences between the sexes contribute to dimorphic responses to parasites.

A nonlinear model of stress hormone levels in rats-the interaction between pollution and parasites

The impact of an infection with a parasite and a simultaneous cadmium exposure on the stress hormone levels of rats was studied. To this end, we introduce a nonlinear heteroscedastic model, which is able to describe the temporal evolution of cortisol concentrations in groups of rats treated by cadmium or parasite infection. A thorough analysis gives strong evidence that parasitic infection and cadmium exposure affect the stress hormone level of rats in an additive manner. Therefore, the host's response to environmental pollution should be studied in relation to parasite infections.

Olfactory-mediated parasite recognition and avoidance: linking genes to behavior

A major cost of social behavior is the increased risk of exposure to parasites and infection. Animals utilize social information, including chemical signals, to recognize and avoid conspecifics infected with either endoparasites or ectoparasites. Here, we briefly discuss the relations among odors, parasite recognition, and avoidance, and consider some of the associated hormonal, neural, and genomic mechanisms. In rodents, odor cues mediate sexual and competitive interactions and are of major importance in individual recognition and mate detection and choice. Female mice distinguish between infected and uninfected males by urinary odors, displaying aversive response to, and avoidance of, the odors of infected individuals. This reduces both the likelihood of the transmission of parasites to themselves and allows females to select for parasite-free males. This set of olfactory and mate choice responses can be further modulated by social factors such as previous experience and exposure to infected males and the mate choices of other females. Male mice, who also face the threat of infection, similarly distinguish and avoid parasitized individuals by odor, thus reducing their likelihood of infection. This recognition and avoidance of the odors of infected individuals involves genes for the neuropeptide, oxytocin (OT), and estrogenic mechanisms. Mice with deletions of the oxytocin gene [OT knockout mice (OTKO)] and mice whose genes for estrogen receptor (ER)-alpha or ER-beta have been disrupted [ER knockout mice (ERKO), alpha-ERKO and beta-ERKO] are specifically impaired in their recognition of, aversion to, and memory of the odors of infected individuals. These findings reveal some of the genes involved in the mediation of social recognition in the ecologically relevant context of parasite recognition and avoidance.

Host gender in parasitic infections of mammals: an evaluation of the female host supremacy paradigm

A review of current literature on mammalian hosts' sexual dimorphism (SD) in parasitic infections revealed that (1) it is a scarcely and superficially studied biological phenomenon of considerable significance for individual health, behavior, and lifestyles and for the evolution of species; (2) there are many notable exceptions to the rule of a favorable female bias in susceptibility to infection; (3) a complex network of molecular and cellular reactions connecting the host's immuno-neuroendocrine systems with those of the parasite is responsible for the host-parasite relationship rather than just an adaptive immune response and sex hormones; (4) a lack of gender-specific immune profiles in response to different infections; (5) the direct effects of the host hormones on parasite physiology may significantly contribute to SD in parasitism; and (6) the need to enrich the reductionist approach to complex biological issues, like SD, with more penetrating approaches to the study of cause-effect relationships, i.e., network theory. The review concludes by advising against generalization regarding SD and parasitism and by pointing to some of the most promising lines of research.

Aerobic Performance of Wild‐Derived House Mice Does Not Change with Cold Exposure or Intestinal Parasite Infection

Aerobic performance is affected by numerous endogenous and exogenous factors. We investigated the effects of ambient temperature and parasite infection on resting metabolism and maximal exercise-induced oxygen consumption in wild-derived house mice (Mus musculus). We also collected preliminary data for effects of lactation on these measures of aerobic performance. Mice were experimentally infected with a naturally occurring intestinal nematode (Heligmosomoides polygyrus) and then exposed to cold temperatures for 10 d or allowed to mate and reproduce. Wild-derived house mice did not change their resting metabolism with H. polygyrus infection or cold exposure, which is in stark contrast to similar studies with laboratory mice. Preliminary data also showed no effect of lactation on aerobic performance. Similarly, maximal exercise-induced oxygen consumption and hematocrit and hemoglobin were unaffected by all experimental treatments. We conclude that resting metabolism, maximal oxygen consumption, and hematology of wild-derived house mice are unaffected by exogenous (temperature) and endogenous (H. polygyrus) demands and, therefore, wild-derived mice respond to these demands without incurring potential costs associated with changes in aerobic performance.

Hypotheses on how selection for some traits in rodents led to correlated responses in offspring sex ratios

Selective breeding of some species of rodent has sometimes occasioned unexpected and unexplained correlated changes in sex ratio (proportion male) at birth. Experiments in selective breeding for blood pH, sexual drive, body weight, sexual dimorphism in adult body weight, and for resistance to a dietary toxin have all been reportedly accompanied by this strange phenomenon. Here attempts are made to explain this. The overall form of explanation is that selection for any one of these characters is incidentally accompanied by selection for hormone concentrations which in turn are responsible for the variation in sex ratio. For example one may suppose that selection for sexual drive ipso facto selects for offspring sex ratios because, (1) Androgen levels are partially responsible for sexual drive and (2) Androgen levels are also (weakly) genetically determined and (3) Lastly, ex hypothesi, androgen concentrations are partially responsible for offspring sex ratios. If this line of reasoning were correct, the selection for sex drive would be expected to be accompanied by correlated changes in offspring sex ratio.

Local variation in helminth burdens of Egyptian spiny mice (Acomys cahirinus dimidiatus) from ecologically similar sites: relationships with hormone concentrations and social behaviour

Populations of Egyptian spiny mice (Acomys cahirinus dimidiatus) in a fragmented montane wadi system in the Sinai showed significant differences in the abundance of gut helminths. Differences in parasite load between populations were positively associated with measures of androgen activity but showed no significant relationship with glucocorticoid activity. Social discrimination tests with adult males from different wadis showed that those from sites with greater helminth abundance were less likely to investigate odours from other males and were less aggressive when subsequently interacting with the odour donors. Subjects showed markedly more investigation towards the odours of males from distant wadis compared with those from their own or immediately neighbouring wadi, but were less aggressive when confronted with odour donors from distant wadis. Despite this, there was a positive relationship between the amount of investigation towards distant male odour and subsequent aggression towards the male. While aggressiveness was positively associated with measures of androgen and glucocorticoid activity, no significant relationship emerged with individual helminth infection. Thus aggressiveness appeared to relate to overall local population levels of infection rather than individual challenge.

Syphacia obvelata infections and reproduction of male domestic mice Mus musculus domesticus on a sub-Antarctic Island

The reproductive activity of feral male mice on an island of the sub-Antarctic Kerguelen archipelago was influenced by biological factors depending on periods within the breeding season. After having controlled host reproductive activity indices for body size, i.e. age, and body condition effects, Syphacia obvelata prevalence did not vary with host reproductive status or age either during the beginning or the middle-end of the reproductive season. Considering the beginning of the breeding season, worm abundance was more pronounced in males the year following a strong winter crash of the population than in years when high over wintering survival occurred. During the middle-end of the breeding season, males with the highest reproductive status were more infected than males with a lower reproductive status in years when oldest individuals dominated the population. It is suggested that this situation was due to an endocrine related increased host susceptibility partly influenced by a change in the age structure of the population, and that an increase in worm transmission was not directly related to male activity concurrent with reproductive status, nor to population density.

IFN-gamma production in rabbits: behavioral and endocrine correlates

Freely interacting male rabbits were studied to establish the effect of exogenous testosterone on interferon-gamma (IFN-gamma) production in peripheral blood mononuclear cells (PBMCs) and to evaluate if this effect is related to season, social rank, plasma corticosterone and glucocorticoid receptors (GcR) in PBMCs. Dominance behavior increases after testosterone propionate (TP) administration only in rank 1 animals, while submission behavior increases after TP only in rank 4 animals, indicating a reinforcing effect of TP on the behavior. Corticosterone and IFN-gamma production are higher and GcR binding capacity is lower in spring than in autumn, suggesting that seasonal fluctuations in the immune system may be related to the pattern of secretion of immunomodulatory hormones. In autumn, corticosterone decreases after TP treatment and increases after social interaction, while GcR binding capacity decreases after TP treatment and social interaction. IFN-gamma production decreases in spring and increases in autumn after TP treatment plus social interaction, indicating that the modulating action of testosterone is related to the current immune status. The relationship between dominance, testosterone and the immune system in spring is suggested by the finding that GcR binding capacity after TP treatment is directly related to social rank, as confirmed by the positive correlation with dominance behavior frequency. The dominance index is positively correlated with GcR binding capacity and negatively with IFN-gamma production before TP treatment, indicating that high receptor activity in immunocompetent cells and low immunoreactivity could be prerequisites for dominance behavior. The immunosuppressive effect of corticosterone and the mechanism of down-regulation on GcR are confirmed by the negative correlations with IFN-gamma production and GcR binding capacity.

Testosterone depresses innate and acquired resistance to ticks in natural rodent hosts: a force for aggregated distributions of parasites

The effects of testosterone on acquired resistance to ticks, Ixodes ricinus, in their natural rodent hosts (voles, Clethrionomys glareolus, and wood-mice, Apodemus sylvaticus) were investigated by manipulating testosterone levels and exposing the hosts to repeated tick infestations. Testosterone reduced both innate and acquired resistance to tick feeding. During primary infestations, attachment rates were higher on rodents with high testosterone levels than on oil-implanted controls. Successive infestations on voles were accompanied by a decrease in tick feeding success and survival, but this decrease was significantly greater in ticks fed on control voles than in those fed on voles implanted with testosterone. When reduced feeding success had been induced, either by vaccination with tick salivary gland extract or by 4 successive infestations, implantation with testosterone partially reversed the acquired resistance. These effects of testosterone will generate heterogeneities within the rodent population with respect to tick distribution and microparasite transmission. The lowest innate and acquired resistance to tick feeding occurs in that fraction of the host population, i.e., sexually active males, most actively involved in the transmission of both Babesia microti and Borrelia burgdorferi s.l.

The effects of hormones on sex differences in infection: from genes to behavior

Males of many species are more susceptible than females to infections caused by parasites, fungi, bacteria, and viruses. One proximate cause of sex differences in infection is differences in endocrine-immune interactions. Specifically, males may be more susceptible to infection than females because sex steroids, specifically androgens in males and estrogens in females, modulate several aspects of host immunity. It is, however, becoming increasingly more apparent that in addition to affecting host immunity, sex steroid hormones alter genes and behaviors that influence susceptibility and resistance to infection. Thus, males may be more susceptible to infection than females not only because androgens reduce immunocompetence, but because sex steroid hormones affect disease resistance genes and behaviors that make males more susceptible to infection. Consideration of the cumulative effects of sex steroid hormones on susceptibility to infection may serve to clarify current discrepancies in the literature and offer alternative hypotheses to the view that sex steroid hormones only alter susceptibility to infection via changes in host immune function.