Housing condition alters immunological and reproductive responses to day length in Siberian hamsters (Phodopus sungorus)

During winter, increased thermoregulatory demands coincide with limited food availability necessitating physiological tradeoffs among expensive physiological processes resulting in seasonal breeding among small mammals. In the laboratory, short winter-like day lengths induce regression of the reproductive tract, but also enhance many aspects of immune function. It remains unspecified the extent to which bolstered immune responses in short days represent enhanced immune function per se compared to long days or represents energetic disinhibition mediated by the regression of the reproductive tract. Cohabitation of male Siberian hamsters with intact female conspecifics can block short-day reproductive regression. We sought to determine whether female cohabitation could also block the enhanced immune function associated with short days. Adult male Siberian hamsters were housed in long or short day lengths in one of three housing conditions: (1) single-housed, (2) housed with a same sex littermate, or (3) housed with an ovariectomized female. Delayed-type hypersensitivity (DTH) responses were assessed after 8 weeks of photoperiod treatment. Housing with an ovariectomized female was not sufficient to block short-day reproductive regression, but prevented short-day enhancement of DTH responses. Housing with a male littermate did not alter reproductive or immune responses in either photoperiod. These data suggest that short day enhancement of immune function is independent of photoperiod-mediated changes in the reproductive system.     

Sex-specific effects of glucose deprivation on cell-mediated immunity and reproduction in Siberian hamsters (<Emphasis Type="Italic">Phodopus sungorus</Emphasis>)

In most species, sexes differ in levels of parasitism. These differences have traditionally been believed to be static, but a capacity for adjusting anti-parasite investments would allow sexes to allocate resources adaptively contingent on environmental conditions. During stressful periods, such as a food shortage, allocation decisions would be mandated in males and females, but the biasing of resources may differ depending on the value of various physiological alternatives to the fitness of each sex. To determine whether sexes sacrifice immune or reproductive capacity when stressed, male and female Siberian hamsters (Phodopus sungorus) were pharmacologically deprived of glucose. Glucose deprivation was expected to compromise immune activity (delayed-type hypersensitivity) more than reproductive capacity in males because male fitness is limited by reproductive opportunities. The opposite was predicted for females because of the greater value of surviving to breed in favorable conditions. Contrary to expectations, glucoprivation compromised immune activity in female, but not male, hamsters. Conversely, glucoprivation reduced male, but not female, reproductive organ masses. These results may reflect the adjustments made by wild hamsters during food shortages, or they may be influenced by the study design; neither sex was permitted to incur other behavioral and physiological costs, such as lactation and parental care. Regardless, our results indicate that sex differences in parasitism are likely to be plastic in many circumstances, but further work in free-living animals is critical to ascertain whether results of the present study are naturally representative.     

The effect of elevated reproductive effort onhumoral immune function in collared flycatcher females

In order to test whether high reproductive investments impair immune function in naturally breeding collared flycatchers, we performed a brood manipulation experiment and simultaneously induced an immune response by challenging birds with a non-pathogenic antigen – sheep red blood cells (SRBC). Females rearing experimentally enlarged number of nestlings showed significantly lower level of specific anti-SRBC antibodies than control females attending unaltered broods, but only in one of the two study years. The haemoconcentration of leukocytes did not differ between the two groups in both study years. The significant difference in immunological responsiveness between control and enlarged group coincided with differences in survival probability to the next breeding season: females attending enlarged broods showed lower probability of survival than control females, but there was no relationship between the level of immune response and survival probability. Our results indicate that reproduction may indeed trade for resources with immune functions at least in terms of specific antibody production. However, as in the other studies on reproductive costs, these costs seem not always to be pronounced.     

Photoperiodic polyphenisms in rodents: neuroendocrine mechanisms, costs, and functions

Annual changes in daylength figure prominently in the generation of seasonal rhythms in reproduction, and a wide variety of mammals use ambient photoperiod as a proximate cue to time critical reproductive events. Nevertheless, within many reproductively photoperiodic mammalian species, there exist individuals--termed "photoperiod nonresponders"--that fail to adopt a seasonal breeding strategy and instead exhibit reproductive competence at a time of year when their conspecifics are reproductively quiescent. Photoperiod nonresponsiveness has been principally characterized by laboratory observations--over half of the species known to be reproductively photoperiodic contain a proportion of nonresponsive individuals. The study of nonresponders has generated basic insights regarding photic regulation of reproduction in mammals. The neuroendocrine mechanisms by which the short-day photoperiodic signal is degraded or lost in nonresponders varies between species: differences in features of the circadian pacemaker, which provides photoperiodic input to the reproductive neuroendocrine system, have been identified in hamsters; changes in the responsiveness of hypothalamic gonadotrophs to melatonin and as-yet-unspecified inhibitory signals have been implicated in voles and mice. Individuals that continue to breed when their conspecifics refrain might enjoy higher fitness under certain circumstances. Statements regarding the adaptive function of reproductive nonresponsiveness to photoperiod require additional information on the costs (metabolic and fitness) of sustaining reproductive function during the winter months and how these costs vary as a function of environmental conditions. Reproductive nonresponders thus continue to represent a challenge to theories that extol the adaptive function of seasonality. Several nonexclusive hypotheses are proposed to account for the maintenance of nonresponsive individuals in wild rodent populations.     

Anorexia nervosa,amenorrhea, and adaptation

Female mammals under stress or in poor physical condition often delay puberty or otherwise suppress reproduction. This capacity is adaptive because it enables females to avoid reproduction when environmental conditions are not conducive to the survival of offspring. Because amenorrhea is one of the key diagnostic criteria for this condition, anorexia nervosa is discussed in an evolutionary context. It is suggested that anorexia nervosa may delay puberty in girls who are disposed to early maturity. Since there is some evidence that early and late maturers have different life histories in modern societies, the reproductive delay associated with anorexia nervosa exhibits features compatible with an evolved reproductive strategy. Insofar as anorexia nervosa is a modern phenomenon, it is unlikely that the syndrome per se is the result of selection. Rather, anorexia nervosa appears to be a reflection of the female ability to alter maturation rates and reproductive function in response to environmental conditions.     

Female reproduction bears no survival cost in captivity for gray mouse lemurs

The survival cost of reproduction has been revealed in many free‐ranging vertebrates. However, recent studies on captive populations failed to detect this cost. Theoretically, this lack of survival/reproduction trade‐off is expected when resources are not limiting, but these studies may have failed to detect the cost, as they may not have fully accounted for potential confounding effects, in particular interindividual heterogeneity. Here, we investigated the effects of current and past reproductive effort on later survival in captive females of a small primate, the gray mouse lemur. Survival analyses showed no cost of reproduction in females; and the pattern was even in the opposite direction: the higher the reproductive effort, the higher the chances of survival until the next reproductive event. These conclusions hold even while accounting for interindividual heterogeneity. In agreement with aforementioned studies on captive vertebrates, these results remind us that reproduction is expected to be traded against body maintenance and the survival prospect only when resources are so limiting that they induce an allocation trade‐off. Thus, the cost of reproduction has a major extrinsic component driven by environmental conditions.     

Perinatal photoperiod organizes adult immune responses in Siberian hamsters (Phodopus sungorus)

Individuals of many nontropical rodent species display reproductive, immunological, and somatic responses to day length. In general, short day (SD) lengths inhibit reproduction and enhance immune function in the laboratory when all other conditions are held constant. Most studies to date have focused on seasonal variation in immune function in adulthood. However, perinatal photoperiods also communicate critical day length information and serve to establish a developmental trajectory appropriate for the time of year. Nontropical rodents born early in the breeding season undergo rapid reproductive development, presumably to promote mating success during their first reproductive season. Rodents born late in the breeding season suspend somatic growth and puberty until the following vernal breeding season. We tested the hypothesis that perinatal day lengths have similar enduring effects on the immune system of rodents. Siberian hamsters (Phodopus sungorus) were maintained prenatally and until weaning (21 days) in either SDs (8 h light:16 h dark) or long days (LD) (16 h light:8 h dark), then they were weaned into either the opposite photoperiod or maintained in their natal photoperiod, forming four groups (LD-LD, LD-SD, SD-LD, and SD-SD). After 8-wk in these conditions, cell-mediated immune activity was compared among groups. SD-SD hamsters of both sexes enhanced immune function relative to all other groups. The reproductive effects of perinatal photoperiod were not evident by the end of the experiment; circulating testosterone and cortisol sampled at the end of the experiment reflected the postweaning, but not the perinatal photoperiod. This experiment demonstrates long-lasting organizational effects of perinatal photoperiod on the rodent immune system and indicates that photoperiod-induced changes in the immune system are dissociable from changes in the reproductive system.     

Recovery and immune priming modulate the evolutionary trajectory of infection‐induced reproductive strategies

In response to parasite exposure, organisms from a variety of taxa undergo a shift in reproductive investment that may trade off with other life‐history traits including survival and immunity. By suppressing reproduction in favour of somatic and immunological maintenance, hosts can enhance the probability of survival and recovery from infection. By plastically enhancing reproduction through terminal investment, on the other hand, hosts under the threat of disease‐induced mortality could enhance their lifetime reproductive fitness through reproduction rather than survival. However, we know little about the evolution of these strategies, particularly when hosts can recover and even bequeath protection to their offspring. In this study, we develop a stochastic agent‐based model that competes somatic maintenance and terminal investment strategies as they trade off differentially with lifespan, parasite resistance, recovery and transgenerational immune priming. Our results suggest that a trade‐off between reproduction and recovery can drive directional selection for either terminal investment or somatic maintenance, depending on the cost of reproduction to lifespan. However, some conditions, such as low virulence with a high cost of reproduction to lifespan, can favour diversifying selection for the coexistence of both strategies. The introduction of transgenerational priming into the model favours terminal investment when all strategies are equally likely to produce primed offspring, but favours somatic maintenance if it confers even a slight priming advantage over terminal investment. Our results suggest that both immune priming and recovery may modulate the evolution of reproductive shift diversity and magnitude upon exposure to parasites.     

Individual quality and age but not environmental or social conditions modulate costs of reproduction in a capital breeder

Costs associated with reproduction are widely known to play a role in the evolution of reproductive tactics with consequences to population and eco‐evolutionary dynamics. Evaluating these costs as they pertain to species in the wild remains an important goal of evolutionary ecology. Individual heterogeneity, including differences in individual quality (i.e., among‐individual differences in traits associated with survival and reproduction) or state, and variation in environmental and social conditions can modulate the costs of reproduction; however, few studies have considered effects of these factors simultaneously. Taking advantage of a detailed, long‐term dataset for a population of feral horses (Sable Island, Nova Scotia, Canada), we address the question of how intrinsic (quality, age), environmental (winter severity, location), and social conditions (group size, composition, sex ratio, density) influence the costs of reproduction on subsequent reproduction. Individual quality was measured using a multivariate analysis on a combination of four static and dynamic traits expected to depict heterogeneity in individual performance. Female quality and age interacted with reproductive status of the previous year to determine current reproductive effort, while no effect of social or environmental covariates was found. High‐quality females showed higher probabilities of giving birth and weaning their foal regardless of their reproductive status the previous year, while those of lower quality showed lower probabilities of producing foals in successive years. Middle‐aged (prime) females had the highest probability of giving birth when they had not reproduced the year before, but no such relationship with age was found among females that had reproduced the previous year, indicating that prime‐aged females bear higher costs of reproduction. We show that individual quality and age were key factors modulating the costs of reproduction in a capital breeder but that environmental or social conditions were not, highlighting the importance of considering multiple factors when studying costs of reproduction.     

Peripubertal immune challenges attenuate reproductive development in male Siberian hamsters (Phodopus sungorus)

Differential allocation of energy to reproduction versus host defense is assumed to drive the seasonal antiphase relation between peak reproductive function and immunocompetence; however, evidence supporting this assumption is only correlational. These experiments tested whether photoperiod affects immune responses to antigens in peripubertal Siberian hamsters, whether such activation of the immune system exacts energetic and reproductive costs, and whether such costs vary seasonally. Male Siberian hamsters were raised from birth in long (LD) or short days (SD), which respectively initiate or inhibit the onset of puberty. To elicit a specific immune response, hamsters were injected with a novel antigen (keyhole limpet hemocyanin [KLH]) as juveniles. Reproductive development was attenuated and body temperature was elevated in LD hamsters relative to saline-injected control animals. In contrast, KLH treatments affected neither thermoregulation nor reproductive development in photoinhibited SD hamsters. In experiment 2, juvenile male hamsters were challenged with bacterial lipopolysaccharide (LPS) in order to elicit an innate immune response. Febrile and anorexic responses to LPS were greater in reproductively stimulated LD hamsters relative to reproductively inhibited SD hamsters. LPS treatments attenuated somatic and testicular development in LD hamsters, but did not significantly affect circulating testosterone concentrations. In contrast, LPS treatments were without effect on somatic and reproductive development in SD hamsters. These experiments indicate that photoperiod affects antigen-specific antibody production, febrile responses to LPS, and sickness behaviors in juvenile Siberian hamsters, and that peripubertal activation of the immune system exacts energetic and metabolic costs that can diminish the magnitude of somatic and reproductive maturation in LD. The data also underscore the importance of seasonally dependent life history factors in assessing physiological tradeoffs.     

Mass- and density-dependent reproductive success and reproductive costs in a capital breeder

For capital breeders, mass may affect reproductive potential. Reproductive expenditure may reduce future reproductive potential, particularly when resources are scarce. To test the hypothesis that reproductive success and the costs of reproduction vary according to mass and population density, we analyzed 25 yr of data on bighorn ewes (Ovis canadensis). The number of adult females was first limited by yearly removals, then allowed to triple. We found no survival costs of reproduction for ewes aged 4-7 yr. For ewes aged 8-14 yr, survival was density dependent for barren ewes but not for ewes that weaned lambs. Failure to lamb was rare and negatively correlated with fertility the following year. At low population density, lactation had a negative effect on mass gain but had a limited reproductive cost. At high density, heavy ewes had higher reproductive success than light ewes, and the reproductive cost and somatic costs of reproduction increased. The cost of reproduction was greater for light than for heavy ewes. Survival of weaned lambs to 1 yr was affected by population density but not by maternal mass or previous reproductive success. In large mammals, manipulations of reproductive effort are problematic, but long-term monitoring of individual mass and reproductive success under varying conditions of resource availability can provide insights into the evolution of life histories.     

Leptin, but not immune function, is linked to reproductive responsiveness to photoperiod

Energetic demands are high while energy availability is minimum during winter. To cope with this energetic bottleneck, animals exhibit numerous energy-conserving adaptations during winter, including changes in immune and reproductive functions. A majority of individual rodents within a population inhibits reproductive function (responders) as winter approaches. A substantial proportion of small rodents within a species, however, fails to inhibit reproduction (nonresponders) during winter in the field or in the laboratory when maintained in winter-simulated day lengths. In contrast, immune function is bolstered by short day lengths in some species. The specific mechanisms that link reproductive and immune functions remain unspecified. Leptin is a hormone produced by adipose tissue, and several studies suggest that leptin modulates reproductive and immune functions. The present study sought to determine if photoperiodic alterations in reproductive function and leptin concentrations are linked to photoperiod-modulated changes in immune function. Siberian hamsters (Phodopus sungorus) were housed in either long (LD 16:8) or short (LD 8:16) day lengths for 9 wk. After 9 wk, blood samples were collected during the middle of the light and dark phase to assess leptin concentrations. One week later, animals were injected with keyhole limpet hemocyanin to evaluate humoral immunity. Body mass, body fat content, and serum leptin concentrations were correlated with reproductive responsiveness to photoperiod; short-day animals with regressed gonads exhibited a reduction in these measures, whereas short-day nonresponders resembled long-day animals. In contrast, immune function was influenced by photoperiod but not reproductive status. Taken together, these data suggest that humoral immune function in Siberian hamsters is independent of photoperiod-mediated changes in leptin concentrations.     

Variation in probability of first reproduction of Weddell seals

1. For many species, when to begin reproduction is an important life-history decision that varies by individual and can have substantial implications for lifetime reproductive success and fitness. 2. We estimated age-specific probabilities of first-time breeding and modelled variation in these rates to determine age at first reproduction and understand why it varies in a population of Weddell seals in Erebus Bay, Antarctica. We used multistate mark-recapture modelling methods and encounter histories of 4965 known-age female seals to test predictions about age-related variation in probability of first reproduction and the effects of annual variation, cohort and population density. 3. Mean age at first reproduction in this southerly located study population (7.62 years of age, SD=1.71) was greater than age at first reproduction for a Weddell seal population at a more northerly and typical latitude for breeding Weddell seals (mean=4-5 years of age). This difference suggests that age at first reproduction may be influenced by whether a population inhabits the core or periphery of its range. 4. Age at first reproduction varied from 4 to 14 years, but there was no age by which all seals recruited to the breeding population, suggesting that individual heterogeneity exists among females in this population. 5. In the best model, the probability of breeding for the first time varied by age and year, and the amount of annual variation varied with age (average variance ratio for age-specific rates=4.3%). 6. Our results affirmed the predictions of life-history theory that age at first reproduction in long-lived mammals will be sensitive to environmental variation. In terms of life-history evolution, this variability suggests that Weddell seals display flexibility in age at first reproduction in order to maximize reproductive output under varying environmental conditions. Future analyses will attempt to test predictions regarding relationships between environmental covariates and annual variation in age at first reproduction and evaluate the relationship between age at first reproduction and lifetime reproductive success.     

A pre‐breeding immune challenge delays reproduction in the female dark‐eyed junco Junco hyemalis

Precise timing of life‐history transitions in predictably changing environments is hypothesized to aid in individual survival and reproductive success, by appropriately matching an animal's physiology and behavior with prevailing environmental conditions. Therefore, it is imperative for individuals to time energetically costly life‐history stages (i.e. reproduction) so they overlap with seasonal peaks in food abundance and quality. Female lifetime reproductive fitness is affected by several factors that influence energy balance, including arrival date, timing of egg production, and energetic condition. Therefore, any extra energetic costs during reproduction may negatively affect timing of egg production, and ultimately a female's fitness. For example, mounting an immunological response elicits a high energetic cost, and this transfer of resources towards cell and immune system maintenance could have direct negative effects on reproductive timing. In order to determine whether an immune challenge delays onset of breeding (i.e. egg production), we administered either a humoral immune challenge (keyhole limpet hemocyanin (KLH)) (treatment) or physiological saline (control) to free‐living female dark‐eyed juncos Junco hyemalis in the period immediately prior to egg‐laying (～4 weeks). We found that KLH‐injected females artificially delayed clutch initiation when compared to control females. These data help to refine our understanding of how free‐living birds allocate resources between reproduction and self‐maintenance processes during the critical pre‐laying period of the annual cycle.     

Effects of early horn growth on reproduction and hunting mortality in female chamois

1.?Environmental conditions during early development can affect the growth patterns of vertebrates, influencing future survival and reproduction. In long-lived mammals, females that experience poor environmental conditions early in life may delay primiparity. In female bovids, annual horn growth increments may provide a record of age-specific reproduction and body growth. Horn length, however, may also be a criterion used by hunters in selecting animals to harvest, possibly leading to artificial selection. 2.?We studied three populations of chamois (Rupicapra rupicapra) in the western Alps to explore the relationships between female horn length and early growth, age of primiparity and age-specific reproduction. We also compared the risk of harvest to reproductive status and horn length. 3.?Early horn growth was positively correlated with body mass in pre-reproductive females and with reproduction in very young and senescent adults. Females with strong early horn growth attained primiparity at an earlier age than those with weak early growth. Horn length did not affect hunter selection, but we found a strong hunter preference for nonlactating females. 4.?Our research highlights the persistent effects of early development on reproductive performance in mammals. Moderate sport harvests are unlikely to affect the evolution of phenotypic traits and reproductive strategies in female chamois. A policy of penalizing hunters that harvest lactating females, however, may increase the harvest of 2-year-old females, which have high reproductive potential.     

Reproductive resilience to food shortage in a small heterothermic primate

The massive energetic costs entailed by reproduction in most mammalian females may increase the vulnerability of reproductive success to food shortage. Unexpected events of unfavorable climatic conditions are expected to rise in frequency and intensity as climate changes. The extent to which physiological flexibility allows organisms to maintain reproductive output constant despite energetic bottlenecks has been poorly investigated. In mammals, reproductive resilience is predicted to be maximal during early stages of reproduction, due to the moderate energetic costs of ovulation and gestation relative to lactation. We experimentally tested the consequences of chronic-moderate and short-acute food shortages on the reproductive output of a small seasonally breeding primate, the grey mouse lemur (Microcebus murinus) under thermo-neutral conditions. These two food treatments were respectively designed to simulate the energetic constraints imposed by a lean year (40% caloric restriction over eight months) or by a sudden, severe climatic event occurring shortly before reproduction (80% caloric restriction over a month). Grey mouse lemurs evolved under the harsh, unpredictable climate of the dry forest of Madagascar and should thus display great potential for physiological adjustments to energetic bottlenecks. We assessed the resilience of the early stages of reproduction (mating success, fertility, and gestation) to these contrasted food treatments, and on the later stages (lactation and offspring growth) in response to the chronic food shortage only. Food deprived mouse lemurs managed to maintain constant most reproductive parameters, including oestrus timing, estrogenization level at oestrus, mating success, litter size, and litter mass as well as their overall number of surviving offspring at weaning. However, offspring growth was delayed in food restricted mothers. These results suggest that heterothermic, fattening-prone mammals display important reproductive resilience to energetic bottlenecks. More generally, species living in variable and unpredictable habitats may have evolved a flexible reproductive physiology that helps buffer environmental fluctuations.     

Cohort effects in red squirrels: the influence of density, food abundance and temperature on future survival and reproductive success

1. Environmental conditions experienced early in life may have long-lasting effects on individual performance, thereby creating 'silver-spoon effects'. 2. We used 15 years of data from a North American red squirrel (Tamiasciurus hudsonicus Erxleben) population to investigate influences of food availability, density and spring temperature experienced early in life on reproduction and survival of female squirrels during adulthood. 3. We found that spring temperature and food availability did not affect female survival after 1 year of age, whereas higher squirrel densities led to lower survival, thereby affecting longevity and lifetime fitness. 4. In addition, both food availability experienced between birth and weaning, and spring temperature in the year of birth, had long-lasting positive effects on female reproductive success. These results emphasize the critical effect environmental conditions during the early life stages can have on the lifetime performance of small mammals. 5. These long-term effects of early food and temperature were apparent only once we controlled for conditions experienced during adulthood. This suggests that silver-spoon effects can be masked when conditions experienced early in life are correlated to some environmental conditions experienced later in life. 6. The general importance of silver-spoon effects for adult demographic performance might therefore be underestimated, and taking adult environment into account appears to be necessary when studying long-term cohort effects.     

Individual heterogeneity in fitness in a long‐lived herbivore

Heterogeneity in the intrinsic quality and nutritional condition of individuals affects reproductive success and consequently fitness. Black brant (Branta bernicla nigricans) are long‐lived, migratory, specialist herbivores. Long migratory pathways and short summer breeding seasons constrain the time and energy available for reproduction, thus magnifying life‐history trade‐offs. These constraints, combined with long lifespans and trade‐offs between current and future reproductive value, provide a model system to examine the role of individual heterogeneity in driving life‐history strategies and individual heterogeneity in fitness. We used hierarchical Bayesian models to examine reproductive trade‐offs, modeling the relationships between within‐year measures of reproductive energy allocation and among‐year demographic rates of individual females breeding on the Yukon‐Kuskokwim Delta, Alaska, using capture–recapture and reproductive data from 1988 to 2014. We generally found that annual survival tended to be buffered against variation in reproductive investment, while breeding probability varied considerably over the range of clutch size‐laying date combinations. We provide evidence for relationships between breeding probability and clutch size, breeding probability and nest initiation date, and an interaction between clutch size and initiation date. Average lifetime clutch size also had a weak positive relationship with apparent survival probability. Our results support the use of demographic buffering strategies for black brant. These results also indirectly suggest associations among environmental conditions during growth, fitness, and energy allocation, highlighting the effects of early growth conditions on individual heterogeneity, and subsequently, lifetime reproductive investment.     

Social life-history response to individual immune challenge of workers of Bombus terrestris L.: a possible new cooperative phenomenon

Parasites typically reduce host survival or fecundity. To minimize fitness loss, hosts can make temporal adjustments of their reproductive effort. To date such plastic shifts of life‐history traits in response to parasitism are only known from solitary organisms where infected individuals can react by themselves. In the case of social insects, where brood care and reproductive effort is shared between reproductive individuals (typically the queen) and workers, adjustments of the reproductive effort would depend on collective decision‐making. We tested for this possibility by experimentally activating the immune response of individual workers in colonies of the bumblebee, Bombus terrestris L. This induction resulted, in combination with environmental conditions, in a reduction of fitness of the social unity (i.e. colony success, measured by number and biomass of offspring) and a collective response towards earlier reproduction. As both phenomena are expressed at the level of the colony, the result suggests that key elements of the use of immune defence have been maintained through the evolutionary transition to sociality.     

Filarial parasites develop faster and reproduce earlier in response to host immune effectors that determine filarial life expectancy

Humans and other mammals mount vigorous immune assaults against helminth parasites, yet there are intriguing reports that the immune response can enhance rather than impair parasite development. It has been hypothesized that helminths, like many free-living organisms, should optimize their development and reproduction in response to cues predicting future life expectancy. However, immune-dependent development by helminth parasites has so far eluded such evolutionary explanation. By manipulating various arms of the immune response of experimental hosts, we show that filarial nematodes, the parasites responsible for debilitating diseases in humans like river blindness and elephantiasis, accelerate their development in response to the IL-5 driven eosinophilia they encounter when infecting a host. Consequently they produce microfilariae, their transmission stages, earlier and in greater numbers. Eosinophilia is a primary host determinant of filarial life expectancy, operating both at larval and at late adult stages in anatomically and temporally separate locations, and is implicated in vaccine-mediated protection. Filarial nematodes are therefore able to adjust their reproductive schedules in response to an environmental predictor of their probability of survival, as proposed by evolutionary theory, thereby mitigating the effects of the immune attack to which helminths are most susceptible. Enhancing protective immunity against filarial nematodes, for example through vaccination, may be less effective at reducing transmission than would be expected and may, at worst, lead to increased transmission and, hence, pathology.