Male-biased parasitism by common helminths is not explained by sex differences in body size or spleen mass of breeding cormorants Phalacrocorax auritus

In vertebrates, males are often more parasitised than conspecific females. This bias in parasitism might result from sex differences in parasite exposure and/or susceptibility to infection. Such information is important for testing hypotheses about allocation of resources to life histories of males and females and for testing hypotheses about factors thought to influence parasite fitness and parasite dynamics. We tested whether double-crested cormorants Phalacrocorax auritus exhibit male-biased parasitism by gut helminths. The prevalence of nematode Contracaecum spp. and trematode Drepanocaphalus spathans infections was ∼90% and 39%, respectively. Cestode, primarily Paradilepis caballeroi and acanthocephalan Andracantha gravida infections were less common (<10%). Male and female cormorants did not differ in prevalence of infection by any helminth species. However, males had twice the abundance and intensity of Contracaecum spp. infections and twice the intensity of D. spathans infections than found in females. For common parasites showing male-biased parasitism, degree of parasitism was also unrelated to body size or mass in either sex. Males and females did not differ in spleen mass and spleen mass was unrelated to abundance of common parasites. Furthermore, abundance of trematodes and nematodes was not correlated. At present, male biases in parasitism by nematodes and trematodes in cormorants are independent patterns that remain unexplained, but are most likely attributable to sex differences in exposure and/or immunological differences not yet assessed.     

Seasonal and demographic factors influencing gastrointestinal parasitism in ungulates of Etosha National Park

Host-parasite dynamics can be strongly affected by seasonality and age-related host immune responses. We investigated how observed variation in the prevalence and intensity of parasite egg or oocyst shedding in four co-occurring ungulate species may reflect underlying seasonal variation in transmission and host immunity. This study was conducted July 2005-October 2006 in Etosha National Park, Namibia, using indices of parasitism recorded from 1,022 fecal samples collected from plains zebra (Equus quagga), springbok (Antidorcas marsupialis), blue wildebeest (Connochaetes taurinus), and gemsbok (Oryx gazella). The presence and intensity of strongyle nematodes, Strongyloides spp. and Eimeria spp. parasites, were strongly seasonal for most host-parasite combinations, with more hosts infected in the wet season than the dry season. Strongyle intensity in zebra was significantly lower in juveniles than adults, and in springbok hosts, Eimeria spp. intensity was significantly greater in juveniles than adults. These results provide evidence that acquired immunity is less protective against strongyle nematodes than Eimeria spp. infections. The seasonal patterns in parasitism further indicate that the long dry season may limit development and survival of parasite stages in the environment and, as a result, host contact and parasite transmission.     

Host sex and ectoparasites choice: preference for, and higher survival on female hosts

1. Sex differences in levels of parasite infection are a common rule in a wide range of mammals, with males usually more susceptible than females. Sex-specific exposure to parasites, e.g. mediated through distinct modes of social aggregation between and within genders, as well as negative relationships between androgen levels and immune defences are thought to play a major role in this pattern. 2. Reproductive female bats live in close association within clusters at maternity roosts, whereas nonbreeding females and males generally occupy solitary roosts. Bats represent therefore an ideal model to study the consequences of sex-specific social and spatial aggregation on parasites' infection strategies. 3. We first compared prevalence and parasite intensities in a host-parasite system comprising closely related species of ectoparasitic mites (Spinturnix spp.) and their hosts, five European bat species. We then compared the level of parasitism between juvenile males and females in mixed colonies of greater and lesser mouse-eared bats Myotis myotis and M. blythii. Prevalence was higher in adult females than in adult males stemming from colonial aggregations in all five studied species. Parasite intensity was significantly higher in females in three of the five species studied. No difference in prevalence and mite numbers was found between male and female juveniles in colonial roosts. 4. To assess whether observed sex-biased parasitism results from differences in host exposure only, or, alternatively, from an active, selected choice made by the parasite, we performed lab experiments on short-term preferences and long-term survival of parasites on male and female Myotis daubentoni. When confronted with adult males and females, parasites preferentially selected female hosts, whereas no choice differences were observed between adult females and subadult males. Finally, we found significantly higher parasite survival on adult females compared with adult males. 5. Our study shows that social and spatial aggregation favours sex-biased parasitism that could be a mere consequence of an active and adaptive parasite choice for the more profitable host.     

Influence of blood parasites on the body mass of passeriform birds

Body masses of 3,739 birds representing immature and adult males and females of 15 species of passeriforms (both uninfected and infected with Haemoproteus spp. and Leucocytozoon spp.) were compared. There was some interaction among year, month and area of capture for several host species, but there was no discernible effect of either parasite genus on body mass. There were no effects due to high intensity parasitemia for eight host species examined. Either parasitism does not cause loss of body mass, or the techniques used were too insensitive to separate effects of parasitism from other natural causes.     

Seasonal and age-related changes in blood parasite prevalence in Dark-eyed Juncos (Junco hyemalis, Aves, Passeriformes)

We determined seasonal changes in blood parasite infections in a free-living population of Dark-eyed Juncos (Junco hyemalis) breeding in interior Alaska (65 degrees N; 148 degrees W). The common parasites found in blood smears were Leucocytozoon fringillinarum (56%), Trypanosoma avium (33%), and Haemoproteus fringillae (9%). In males, parasite prevalences were relatively high at arrival on breeding grounds and increased during the breeding season. Intensity of infection with Leucocytozoon also increased between spring and summer, and then decreased at the time of migration (September). This decrease did not occur in adult females. Elevated prevalences during the breeding season probably reflected the addition of new cases via vector activity to positive status resulting from spring relapse. We observed neither an association between parasite species nor a consistent relationship between parasite intensity and body condition. To further study relationships between reproductive system activity and parasite infections, we compared prevalences in adult males that were undergoing their first cycle of gonadal development and regression (males in their second calendar year, or SY) with those of older males (males in their third or more calendar year, i.e., after-second-year males or ASY). Circulating testosterone concentrations declined in both groups between arrival on breeding grounds (end of April-early May) and the end of the reproductive period (July), and they were higher in May in ASY than in SY males. At the peak of the breeding season (June), ASY males also had a higher parasite prevalence than SY males. This difference may have resulted from immunosuppressive effects of gonadal hormones and/or from behavioral differences between SY and ASY males such that older males were exposed to more insect vectors than younger males. .     

Weak relationships of parasite infection with sexual and life‐history traits in wild‐caught Texas field crickets (Gryllus texensis)

1. Sex differences in immune investment and infection rate are predicted due to the divergent life histories of males and females, where females invest more toward immunity due to the fitness consequences of a reduced lifespan and males allocate less toward immunity due to increased resource investment in traits critical to sexual selection. Consequently, males are expected to fight infection less adeptly, resulting in higher parasite loads relative to females across all taxa.
2. Wild animals rarely face a single parasite within their given environment, yet nearly all studies on sex‐biased infection rates have focused on a single host–parasite relationship. Here, we investigate how simultaneous natural infections of ecto‐ and endosymbionts (i.e. both parasitic and phoretic taxa) correlate with sex biases in host immune response and reproductive investment in a field‐caught cricket, Gryllus texensis.
3. Our comprehensive analysis found no significant sex differences in two measures of immune response (melanization and nodulation), and found no strong evidence of a sex bias in the prevalence or intensity of parasitism by the three most common parasites infecting wild G. texensis field crickets (Eutrombidiidae, gregarines, and nematodes).
4. Two traits related to female fitness, egg number and egg size, showed no relation to parasitic infection; however, males having wider heads and poorer body condition were significantly more infected by eutrombidiid mites, gregarines, and nematodes.
5. Despite frequent predictions of male‐biased parasitism in the literature, our results concur with many other studies indicating that the divergent life histories of males and females alone are not sufficient to explain natural infection rates in wild insects.

     

Seasonal Patterns of Hormones,Macroparasites, and Microparasites in Wild African Ungulates: The Interplay among Stress,Reproduction, and Disease

Sex hormones, reproductive status, and pathogen load all affect stress. Together with stress, these factors can modulate the immune system and affect disease incidence. Thus, it is important to concurrently measure these factors, along with their seasonal fluctuations, to better understand their complex interactions. Using steroid hormone metabolites from fecal samples, we examined seasonal correlations among zebra and springbok stress, reproduction, gastrointestinal (GI) parasite infections, and anthrax infection signatures in zebra and springbok in Etosha National Park (ENP), Namibia, and found strong seasonal effects. Infection intensities of all three GI macroparasites examined (strongyle helminths, Strongyloides helminths, and Eimeria coccidia) were highest in the wet season, concurrent with the timing of anthrax outbreaks. Parasites also declined with increased acquired immune responses. We found hormonal evidence that both mares and ewes are overwhelmingly seasonal breeders in ENP, and that reproductive hormones are correlated with immunosuppression and higher susceptibility to GI parasite infections. Stress hormones largely peak in the dry season, particularly in zebra, when parasite infection intensities are lowest, and are most strongly correlated with host mid-gestation rather than with parasite infection intensity. Given the evidence that GI parasites can cause host pathology, immunomodulation, and immunosuppression, their persistence in ENP hosts without inducing chronic stress responses supports the hypothesis that hosts are tolerant of their parasites. Such tolerance would help to explain the ubiquity of these organisms in ENP herbivores, even in the face of their potential immunomodulatory trade-offs with anti-anthrax immunity.     

Parasite mediated mortality and host immune response explain age-related differences in blood parasitism in birds

An important pattern in host-parasite assemblages is a higher intensity of parasites in juveniles than in adults, but the reasons for these differences remain obscure. Three non-mutually exclusive hypotheses have been proposed: (1) heavily parasitized juveniles die before being recruited into the adult population ('selection' hypothesis); (2) the development of an acquired immunity by the host in front of the parasite reduces the intensity of the parasite in adult hosts ('immunity' hypothesis); and (3) differences in behavior makes adults less exposed to the parasite than juveniles ('vector exposure' hypothesis). Having rejected the 'vector exposure' hypothesis in a previous study, here we tested the 'selection' and 'immunity' hypotheses in feral pigeons (Columba livia) infected by the blood parasite Haemoproteus columbae. In agreement with the 'selection' hypothesis, young (but not adult) pigeons that were highly parasitized had a lower probability of surviving until adulthood, independent of their body condition. However, selection was not strong enough to account for the observed differences in parasite intensity between age-classes, and after selection parasite intensity of survivors still remained 85% higher in juveniles than in adults. In contrast, the 'immunity' hypothesis offered a greater explanatory power. The intensity of blood parasites in young pigeons, but not in adults, decreased over time so dramatically that by the time they had become adults their intensities were indistinguishable from that typically seen in adults. Therefore, while selection against highly parasitized juveniles can contribute to some extent to a reduction in parasitism seen in the adult population, age-specific blood parasitism in feral pigeons is best explained as a transitory phase just before the host develops an effective immune response.     

Fecundity compensation in the three‐spined stickleback Gasterosteus aculeatus infected by the diphyllobothriidean cestode Schistocephalus solidus

Causal explanations for host reproductive phenotypes influenced by parasitism fit into three broad evolutionary models: (1) non‐adaptive side effect; (2) adaptive parasitic manipulation; and (3) adaptive host defence. This study demonstrates fecundity compensation, an adaptive non‐immunological host defence, in the three‐spined stickleback fish (Gasterosteus aculeatus) infected by the diphyllobothriidean cestode Schistocephalus solidus. Both infected and uninfected female sticklebacks produced egg clutches at the same age and size. The reproductive capacity of infected females decreased rapidly with increased parasite : host body mass ratio. Body condition was lower in infected females than uninfected females and decreased with increasing parasite : host mass ratio. Females with clutches had greater body condition than those without clutches. A point biserial correlation showed that there was a body condition threshold necessary for clutch production to occur. Host females apparently had the capacity to produce egg clutches until the prolonged effects of nutrient theft by the parasite and the drain on resources from reproduction precluded clutch formation. Clutch mass, adjusted for female body mass, did not differ significantly between infected and uninfected females. Infected females apparently maintained the same level of reproductive allotment (egg mass as proportion of body mass) as uninfected females. Infected females produced larger clutches of smaller eggs than uninfected females, revealing a trade‐off between egg mass and egg number, consistent with the fecundity compensation hypothesis. The rapid loss of reproductive capacity with severity of infection probably reflects the influence of the parasite combined with a trade‐off between current and future reproduction in the host. Inter‐annual differences in reproductive performance may have reflected ecological influences on host pathology and/or intra‐annual seasonal changes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Are sexes equally parasitized in damselflies and dragonflies?

Parasitism plays an essential part in ecology and evolution of host species and understanding the reasons for differential parasitism within and among hosts species is therefore important. Among the very important factors potentially affecting parasitism is the gender of the host. Here, we studied whether either females or males are more likely to harbour parasites among Odonatan insects, by relying on an extensive literature review and new field data. We collected data on numerous dragonfly and damselfly species and their ectoparasites (water mites) and endoparasites (gregarines) to examine the generality of similarities and differences in prevalence, intensity and maximum number of parasites of male and female hosts. We found three main results. Firstly, most of the odonate host species showed no differences between sexes in either gregarine or water mite prevalence and intensity. The only exception was female damselflies’ higher gregarine prevalence and intensity compared to conspecific males. These inequalities in gregarine parasitism may be due to behavioral and physiological differences between conspecific males and females. In comparison, there were no differences in dragonflies between sexes in water mite or gregarine prevalence and intensity. Secondly, damselflies had higher prevalence and intensity levels of both gregarine and water mite parasites compared to dragonflies. Finally, we found a strong species level pattern between female and male parasitism: a certain level of gregarine or water mite parasitism in one sex was matched with a similar parasitism level for the other. This indicates similar exposure and susceptibility to parasites on both sexes. Even though significant differences of parasite levels between the sexes were observed within certain host species, our results strongly suggest that on a general level a more parasitized sex does not exist in the order, Odonata.     

Sex differences in parasitic infections among arthropod hosts: is there a male bias?

A higher susceptibility to diseases or parasites in males than females may be an ultimate consequence of the different reproductive strategies favored by selection in the two sexes. At the proximate level, the immunosuppressant effects of testosterone in vertebrates provide a mechanism that can cause male biases in parasite infections. Invertebrates, however, lack testosterone and other steroid hormones. We used a meta-analysis of published results to investigate whether sex biases in parasite infections were generally observed among arthropod hosts despite the absence of the immune-endocrine coupling provided by testosterone. Overall, male and female arthropods did not differ in prevalence or intensity of parasite infections. This is based on an analysis of sex differences corrected for sample size and, when possible, variability in the original data. Sex biases in parasite infection were not more likely to be observed in certain host or parasite taxa, and were not more pronounced in experimental studies than in surveys of naturally infected hosts. Our results suggest that because of the absence of endocrine-immune interactions in arthropods, males are not generally more prone to parasite infections than females despite the greater intensity of sexual selection acting on males.     

Ecological causes of sex-biased parasitism in threespine stickleback

Males and females can differ in levels of parasitism and such differences may be mediated by the costs of sexual selection or by ecological differences between the genders. In threespine stickleback, Gasterosteus aculeatus , males exhibit paternal care and territorial nest defence and the costs of reproduction may be particularly high for males relative to females. We monitored levels of parasitism for 15 years in a population of stickleback infected by four different parasite species. Consistent with general predictions, overall parasite prevalence (total parasitism) was greater in males than in females. However, this excess did not occur for each species of parasite. Males had higher prevalence of a cestode Cyathocephalus truncatus and a trematode Bunodera sp. relative to females, while females had higher prevalence of a cestode Schistocephalus solidus and nematodes. This suggested ecological sources to differences in parasitism rather than reproductive costs and therefore we examined diet of unparasitized stickleback, predicting that differences in dietary niche would influence relative parasitism. This was partially confirmed and showed that female stomach contents had increased frequency of pelagic items, the major habitat for the primary host of S. solidus whereas males exhibited increased frequency of benthic items, the dominant habitat of C. truncatus and Bunodera. Temporal shifts in the extent and direction of differential parasitism among years between the sexes were associated with temporal shifts in dietary differences. Our results, combined with those in the literature, suggest that ecological differences between genders could be a more important component to patterns of parasitic infection in natural populations than currently appreciated.     

High parasite infection level in non‐native invasive species: it is just a matter of time

The enemy release hypothesis is often used to explain the success of non‐native species invasions. Growing evidence indicates that parasite or pathogen species richness increases over time in invasive non‐native species; however, this increase should not directly translate into release from enemy pressure as infection intensity of parasites (number of parasites per host) has a more profound impact on host fitness. The changes in intensity of parasitic infections in invasive non‐native species have not yet been thoroughly analysed in newly colonized areas. The goal of this study was to determine whether gastrointestinal parasite (nematode and trematode) infection intensity has increased with time since the populations of American mink Neovison vison were established and how host demographic parameters affect infection intensity. We tested the enemy release hypothesis by substituting space for time, evaluating parasite abundance in American mink at six sites along a chronosequence of mink invasion history. Nematode and trematode abundance increased with time since mink introduction, from a few parasites on average per mink after 16 yr, to 200–250 parasites per mink after 34 yr. The rate of increase in parasite abundance varied among demographic groups of mink (sex and age). Both nematodes and trematodes were more abundant in males than in females, and in subadults than in adults. Higher nematode abundance negatively affected body condition of mink. Our results provide evidence that non‐native species are released from enemy pressure only in the first phase of invasion, and that infection is modulated by host demographics and season. These results contribute to the evaluation of the long‐term patterns of parasite accumulation in invasive non‐native species after their colonization of new territories.     

Is there sex-biased resistance and tolerance in Mediterranean wood mouse (Apodemus sylvaticus) populations facing multiple helminth infections?

Sex-biased parasitism is rarely investigated in relation to host tolerance and resistance, which are two defense strategies hosts can adopt when challenged by parasites. Health or fitness deteriorations in less tolerant individuals with increasing parasite burden would be faster than those in more tolerant ones. Hence, the body condition and reproductive potential of an infected individual host can be considered proxies for tolerance to parasitism. We studied Mediterranean populations of the wood mouse (Apodemus sylvaticus) and its helminth parasites. We assessed their resistance using the phytohemagglutinin test and spleen size, and their tolerance using body condition in males and females and testes mass in males. In order to avoid spurious correlations, we took into account the phylogeographic structure of the Mediterranean wood mouse populations. We used a mixed model adapted from the animal model used in quantitative genetics. While helminth infection did not differ between the two sexes, females and males differed in their measured defenses. Females seem to invest more in immune defense with increasing risk of parasite diversity, but also appear to be potentially more tolerant of parasitic diversity. These results suggest the existence of sexual differences in resistance and tolerance, and that measurements of parasitic loads alone could be insufficient to detect any underlying sexual differences in the two strategies that have evolved in response to multiple parasitic attacks.     

Sex‐dependent infection causes nonadditive effects on kissing bug fecundity

The influence of parasites on host reproduction has been widely studied in natural and experimental conditions. Most studies, however, have evaluated the parasite impact on female hosts only, neglecting the contribution of males for host reproduction. This omission is unfortunate as sex‐dependent infection may have important implications for host–parasite associations. Here, we evaluate for the first time the independent and nonindependent effects of gender infection on host reproductive success using the kissing bug Mepraia spinolai and the protozoan Trypanosoma cruzi as model system. We set up four crossing treatments including the following: (1) both genders infected, (2) both genders uninfected, (3) males infected—females uninfected, and (4) males uninfected—females infected, using fecundity measures as response variables. Interactive effects of infection between sexes were prevalent. Uninfected females produced more and heavier eggs when crossed with uninfected than infected males. Uninfected males, in turn, sired more eggs and nymphs when crossed with uninfected than infected females. Unexpectedly, infected males sired more nymphs when crossed with infected than uninfected females. These results can be explained by the effect of parasitism on host body size. As infection reduced size in both genders, infection on one sex only creates body size mismatches and mating constraints that are not present in pairs with the same infection status. Our results indicate the fitness impact of parasitism was contingent on the infection status of genders and mediated by body size. As the fecundity impact of parasitism cannot be estimated independently for each gender, inferences based only on female host infection run the risk of providing biased estimates of parasite‐mediated impact on host reproduction.     

Parasitism in a declining population of snowshoe hares

Prevalence and intensity of six endoparasites were determined in 346 snowshoe hares (Lepus americanus) obtained at Rochester, Alberta, during December-April 1981-1982, the second winter of a cyclic population decline. The data were analyzed for (1) differences among host sex and age classes, and among months and sample sources, and (2) evidence that parasitism was of demographic significance to the hare population. Prevalence and intensity of Obeliscoides cuniculi were consistently highest among adult hares, but rose most sharply from February to March among juveniles. In contrast, prevalence and intensity of Nematodirus triangularis were highest among juveniles; prevalence reached 90-100% by January, whereas intensity continued to rise through April. Prevalence and intensity of both Trichuris leporis and Protostrongylus boughtoni were highest also among juvenile hares; neither parameter exhibited a definite trend over time. Prevalences of Taenia pisiformis (cysticerci) and Eimeria spp. were unrelated to sex, age or month; but Taenia intensity was highest among juveniles, and Eimeria intensity tended to decrease from December to April. Intensities of Nematodirus, Protostrongylus and Eimeria were higher in male hares than in females. Prevalence and intensity were correlated directly in Obeliscoides, Nematodirus, Trichuris and Eimeria. Hares that died during trapping and handling, or from natural predation, had greater intensities of Obeliscoides than did animals killed on purpose. There was no indication, however, that risk of death was increased by the other parasitic infections. Age-related immune responses to parasitism (except Obeliscoides) were evidenced by reduced or stabilized prevalence and/or intensity among older hares. A multiple-regression model predicted depressed body weight with increasing intensities of Nematodirus, Trichuris or Protostrongylus. Other body-condition and reproductive indices were unassociated with parasite intensities. Within the hare population, Obeliscoides, Trichuris, Protostrongylus and Taenia had overdispersed distributions (typical of many endoparasites) that did not differ from a negative binomial. The frequency with which each possible combination of helminth species occurred within individual hares was consistent with the assumption that such infections occurred independently. There was no compelling reason to believe parasitism was a significant factor in the overwinter decline of this population of snowshoe hares.     

Host-size mediated trade-off in a parasitoid Sclerodermus harmandi

Optimality models of host-parasitoid relationships have traditionally assumed that host quality increases as a function of host size at parasitism. However, trade-offs may play a crucial role in species evolution and should be found in host-parasitoid interactions where the host quality may differ between different sizes. Here, we investigated the effects of host size differences of Monochamus alternatus larva on foraging decisions, parasitism and related fitness in a gregarious ectoparasitoid, Sclerodermus harmandi. Two-choice and non-choice experiments were conducted with M. alternatus larvae to evaluate preference and performance of S. harmandi, respectively. Results from two-choice test showed that adult females prefer to attack large larvae rather than small larvae. In no-choice tests, adult females needed more time to paralyze large larvae than small larvae before laying eggs on the body surface of M. alternatus larvae and had lower survival and parasitism rate on those large larvae. Foraging decisions of S. harmandi led to the selection of the most profitable host size for parasitoid development, which showed more offspring gained on large M. alternatus larvae than on small larvae and got higher body weight on those large hosts. This study demonstrates the existence of trade-off occurring during host-parasitoids interactions according to host size related quality.     

Parasite co-infection and interaction as drivers of host heterogeneity

We examined the hypothesis that the interaction between concomitant infecting parasites modifies host susceptibility, parasite intensity and the pattern of parasite distribution within the host population. We used a 26 year time series of three common parasites in a natural population of rabbits: two gastrointestinal nematodes (Trichostrongylus retortaeformis and Graphidium strigosum) and the immunosuppressive myxoma virus. The frequency distribution of nematodes in the host population and the relationship between host age and nematode intensity were explored in rabbits with either single or dual nematode infections and rabbits infected with the nematodes and myxoma virus. The aggregation of T. retortaeformis and G. strigosum among the rabbits varied with the nature of the co-infection both in male and female hosts. The two nematodes exhibited different age-intensity profiles: G. strigosum intensity increased exponentially with host age while T. retortaeformis intensity exhibited a convex shape. The presence of a secondary infection did not change the age-intensity profile for G. strigosum but for T. retortaeformis co-infection (either both nematodes or myxoma-nematodes) resulted in significantly greater intensities in adult hosts. Results suggest that multi-species infections contributed to aggregation of parasites in the host population and to seasonal variation in intensity, but also enhanced differences in parasitism between sexes. This effect was apparent for T. retortaeformis, which appears to elicit a strong acquired immune response but not for G. strigosum which does not produce any evident immune reaction. We concluded that concomitant infections mediated by host immunity are important in modifying host susceptibility and influencing heterogeneity amongst individual hosts.     

Parasitism, immune response and reproductive success in the house martin Delichonurbica

Parasites often exert strong selection pressures on their hosts that have evolved anti-parasite defences to counter the negative effects of parasites. We studied the relationship between intensity of parasitism, one aspect of host immune response, and host reproductive success, using the house martin bug Oeciacushirundinis and its house martin Delichonurbica host as a model system. Experimental manipulation of parasite load of nests during laying of the first clutch altered the intensity of parasitism. Parasites reduced the reproductive success of their hosts measured in terms of body condition and survival of nestlings. Host immune response, measured as the concentration of gammaglobulins and total plasma proteins, was positively associated with parasite reproduction, estimated as the number of juvenile parasites, but was only weakly related to the intensity of adult parasites. The concentration of gammaglobulins was negatively related to nestling body mass, implying a trade-off between immune function and body condition. Parasite reproduction thus exerts a cost on hosts by increasing the immune response. Received: 25 August 1997 / Accepted: 3 November 1997     

Parasite‐associated mortality in a long‐lived mammal: Variation with host age,sex, and reproduction

Parasites can cause severe host morbidity and threaten survival. As parasites are generally aggregated within certain host demographics, they are likely to affect a small proportion of the entire population, with specific hosts being at particular risk. However, little is known as to whether increased host mortality from parasitic causes is experienced by specific host demographics. Outside of theoretical studies, there is a paucity of literature concerning dynamics of parasite‐associated host mortality. Empirical evidence mainly focuses on short‐lived hosts or model systems, with data lacking from long‐lived wild or semi‐wild vertebrate populations. We investigated parasite‐associated mortality utilizing a multigenerational database of mortality, health, and reproductive data for over 4,000 semi‐captive timber elephants (Elephas maximus), with known causes of death for mortality events. We determined variation in mortality according to a number of host traits that are commonly associated with variation in parasitism within mammals: age, sex, and reproductive investment in females. We found that potentially parasite‐associated mortality varied significantly across elephant ages, with individuals at extremes of lifespan (young and old) at highest risk. Mortality probability was significantly higher for males across all ages. Female reproducers experienced a lower probability of potentially parasite‐associated mortality than females who did not reproduce at any investigated time frame. Our results demonstrate increased potentially parasite‐associated mortality within particular demographic groups. These groups (males, juveniles, elderly adults) have been identified in other studies as susceptible to parasitism, stressing the need for further work investigating links between infection and mortality. Furthermore, we show variation between reproductive and non‐reproductive females, with mothers being less at risk of potentially parasite mortality than nonreproducers.