Stress biomarkers in rats submitted to swimming and treadmill running exercises

The objective of the present work was to compare stress biomarkers (serum ACTH and corticosterone hormones) during known intensity swimming and treadmill running exercises performed by rats. Adult Wistar rats (n=41) weighing 320-400 g at the beginning and 420-500 g at the end of the experiment, previously adapted to exercise and with Maximal Lactate Steady State (MLSS) already determined were used. The animals were divided into the following subgroups: (1) sacrificed shortly after session of 25 min of exercise (swimming or treadmill) at the MLSS intensity or (2) sacrificed after exhaustive exercise (swimming or treadmill) at intensity 25% higher than MLSS. For comparison, a control group C was sacrificed at rest. Two-way ANOVA was used to identify differences in the stress parameters (P<0.05). At both exercise intensities serum ACTH concentrations were significantly higher for the swimming group compared to running and control groups, while serum corticosterone concentrations in swimming and running groups were significantly higher than in the control group. The differences were more pronounced at the higher intensity (25% higher than MLSS). The swimming group showed higher concentrations for both hormones in relation to the running group. Only acute swimming exercise induced activity of the hypothalamic-pituitary-adrenal axis responses expected to stress: elevations in the serum ACTH and corticosterone concentrations.     

The effects of neonatal paternal deprivation on pair bonding, NAcc dopamine receptor mRNA expression and serum corticosterone in mandarin voles

High levels of paternal care are important for the development of social behavior in monogamous rodents. However, the effects of paternal care on the formation of pair bonding and underlying neuroendocrine mechanisms, especially the involvements of dopamine system and corticosterone, are not well understood. We investigated effects of paternal deprivation on pair bonding in mandarin voles (Microtus mandarinus), a socially monogamous rodent. Paternal deprivation was found to inhibit the formation of pair bonding in females according to partner preference tests (PPT). Paternal deprivation also reduced body contact behavior and increased aggression in males and females in PPT. During social interaction tests (SIT), paternal deprivation was found to reduce investigative and aggressive behaviors but increase body contact and self-grooming in females, and reduce staring, aggression, body contact and self-grooming in males when interacting with the opposite sex. Paternal deprivation reduced the expression of dopamine 1-type receptor (D1R) mRNA and dopamine 2-type receptor (D2R) mRNA in the nucleus accumbens of female offspring in later life, but enhanced mRNA expression of these two dopamine receptors in males. After three days of cohabitation the expression of D1R mRNA and D2R mRNA was negatively correlated for voles reared by two parents, but positively correlated in paternally deprived animals. Paternal deprivation reduced serum corticosterone levels in females but had the opposite effect in males. Three days of cohabitation did not alter corticosterone levels of PD females, but reduced it in PC females. Our results provide substantial evidence that paternal deprivation inhibits the formation of pair bonding in female mandarin voles and alters social behavior later in life. These behavioral variations were possibly associated with sex-specific alterations in the expression of two types of dopamine receptors and serum corticosterone levels induced by paternal deprivation.     

Social interactions unmask sex differences in humoral immunity in voles

Sex differences in immune function are well established among laboratory rodents, with males typically having lower immunity than females. This sex difference may reflect the suppressive effects of testosterone on immune function. Because polygynous males generally have higher circulating testosterone concentrations than monogamous males, sex differences in immune function are hypothesized to be more pronounced among polygynous as compared to monogamous species. Sex differences in immune function have not been consistently observed among individually housed Microtus in the laboratory; thus, social interactions are hypothesized to be necessary for the expression of sex differences in immune function. We assessed the effect of differential housing conditions on humoral immunity and steroid hormone concentrations in polygynous meadow voles Microtus pennsylvanicus, and monogamous prairie voles M. ochrogaster. We examined humoral immunity by immunizing voles with keyhole limpet haemocyanin (KLH) and measuring antibody production 5, 10, 15 and 30 days postimmunization. Overall, meadow voles mounted higher anti-KLH immunoglobulin (Ig)M and IgG responses than prairie voles, regardless of the housing condition. Sex differences in antibody production were only observed among meadow voles housed in pairs, in which females had higher anti-KLH IgM and IgG responses than males. Sex differences in antibody production were not observed among prairie voles or meadow voles housed individually. Sex and species differences in circulating oestradiol, testosterone, and corticosterone concentrations were not related to differences in humoral immunity. These data suggest that sex differences in immune function are more pronounced among polygynous species than monogamous species, but may be context dependent. Copyright 1999 The Association for the Study of Animal Behaviour.     

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.     

Sex differences in hormonal responses to social conflict in the monogamous California mouse

Monogamous species are usually considered to be less likely to exhibit sex differences in behavior or brain structure. Most previous studies examining sex differences in stress hormone responses have used relatively sexually dimorphic species such as rats. We examined the stress hormone responses of monogamous California mice (Peromyscus californicus) to resident-intruder tests. We also tested males and females under different photoperiods, because photoperiod has been shown to affect both aggression and stress hormone responses. Females, but not males showed a significant increase in corticosterone levels immediately following a resident-intruder test. Males but not females showed elevated corticosterone levels under short days. Females tested in aggression tests also showed a significant increase in plasma oxytocin levels, but only when housed in long days. This was consistent with our observation that females but not males had more oxytocin positive cells in the paraventricular nucleus (PVN) when housed under long days. Our data show that sex differences in glucocorticoid responses identified in other rodents are present in a monogamous species.     

Local variation in endoparasite intensities of bank voles (Clethrionomys glareolus) from ecologically similar sites: morphometric and endocrine correlates

Much interest has centred recently on the role of adaptive trade-offs between the immune system and other components of life history in determining resistance and parasite intensities among hosts. Steroid hormones, particularly glucocorticoids and sex steroids, provide a plausible mechanism for mediating such trade-offs. A basic assumption behind the hypothesis, however, is that steroid activity will generally correlate with reduced resistance and thus greater parasite intensities. Here, we present some findings from a field study of bank voles (Clethrionomys glareolus) in which we have looked at associations between parasite intensities, anatomical and morphometric measures relating to endocrine function and life history variation in three local populations inhabiting similar but mutually isolated woodland habitats. In general, sites with greater parasite intensities were those in which male C. glareolus had significantly larger adrenal glands, testes and seminal vesicles for their age and body size. Females also showed a site difference in adrenal gland weight. Some aspects of site-related parasite intensity were associated with asymmetry in adrenal gland weight and hind foot length, which may have reflected developmental effects on glucocorticoid activity.     

Seoul virus infection increases aggressive behaviour in male Norway rats

In natural populations of rodents, males are more likely to engage in aggression and be infected with hantaviruses than females. Whether the relationship between hantavirus infection and aggression is due to host- or parasite-mediated mechanisms is unknown. The aim of this study was to determine whether hantavirus infection causes an increase in aggression in male rats and whether these behavioural changes are due to infection of the central nervous system or peripheral tissues. Male laboratory rats were infected with Seoul virus and tested for aggression in a resident-intruder paradigm 15 and 30 days postinoculation (p.i.). Males tested 30 days p.i. (i.e. during the persistent phase of infection) spent more time engaged in aggression than either uninfected males or males tested during the acute phase of infection (i.e. 15 days p.i.). Males that engaged in aggression for a longer duration had more virus present in lung, kidney and testis than males that spent less time engaged in aggression. Infected males shed virus in saliva, faeces, and urine; virus shedding, however, was not correlated with aggression and neither wounding nor transmission of virus to intruder males occurred during behavioural tests. Infection with Seoul virus did not alter either testosterone or corticosterone concentrations. Seoul virus antigens were not detected in the brains of infected rats. These data suggest that hantavirus infection leads to elevated aggression in infected males and may be a by-product of increased virus replication in peripheral tissues.     

Adaptive immune responses are linked to the mating system of arvicoline rodents

ABSTRACT Males generally exhibit reduced immune responses and greater susceptibility to disease than females. The suppressive effect of testosterone on immune function is hypothesized to be one reason why males have lower immune responses than females. Presumably, this effect of testosterone should be more pronounced among polygynous than monogamous species because circulating testosterone is higher among polygynous than monogamous males. The present study examined the extent to which sex differences in specific humoral immunity are related to the endocrine status and mating system of two arvicoline rodents. Humoral immunity was evaluated among polygynous meadow voles (Microtus pennsylvanicus) and monogamous prairie voles (Microtus ochrogaster) by challenging them with the novel antigen keyhole limpet hemocyanin (KLH) and assessing specific immune responses 5, 10, and 15 d following immunization. Overall, meadow voles mounted higher anti-KLH IgM and IgG responses than prairie voles did. Sex differences were also apparent for anti-KLH IgM responses; male meadow voles mounted higher antibody responses than conspecific females, whereas female prairie voles mounted greater responses to KLH than did conspecific males. Male meadow voles had significantly higher testosterone concentrations and reproductive organ mass than male prairie voles did but had elevated immune responses, suggesting that testosterone may not be the primary factor involved in the observed sex and species differences in immune responses. Species and sex differences in corticosterone concentrations were also evident and may contribute to the observed differences in immune function. The influence of extrinsic factors on immune function is also discussed. Taken together, these data provide evidence that the mating system may influence endocrine-immune interactions.     

Helminth community structure of Tropidurus torquatus (Squamata: Tropiduridae) in a rocky outcrop area of Minas Gerais state, southeastern Brazil

The helminth community of Tropidurus torquatus (Squamata: Tropiduridae) from a rocky outcrop area located in the state of Minas Gerais, southeastern Brazil, was studied. Ninety-two of the 110 individuals examined (83.6%) harbored helminths. Five species were found, including 3 nematodes ( Physaloptera lutzi , Parapharyngodon bainae , and Oswaldofilaria chabaudi ), 1 unidentified cestode species, and 1 acanthocephalan cystacanth also not identified. Only the nematode species had sufficient data to perform ecological analysis, with P. lutzi exhibiting the highest prevalence (67.3%). Prevalence between male and female hosts differed only for Oswaldofilaria chabaudi , with males exhibiting the highest values. The intensities of infection by P. lutzi and O. chabaudi were different among male and female hosts, with males also exhibiting the highest values. The host body size was positively related to intensity of infection for all nematode species. Local seasonality had some influence on the helminth community structure. Host diet, sexual dimorphism, and behavior (territorialism, forage strategy) represented important factors for the structure of this parasite community. In general, the helminth community was species poor, depauperate, and non-interactive, representing a typical structure observed in lizard hosts.     

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

Peripheral immune responsiveness in male laboratory mice was reduced by infection with the trichostrongyloid nematode Heligmosomoides polygyrus. Responsiveness was also lower among high-ranking (aggressive) males regardless of infection status. Reduced responsiveness in both infected animals and high rankers was associated with elevated serum corticosterone concentration (a potential immunodepressant) and was compounded among high-ranking males by subsequent high aggressiveness. As in previous experiments, only low rankers modulated testosterone secretion in relation to current immunocompetence and corticosterone concentration. The lack of any downregulation of aggression in response to parasite-induced immunodepression contrasted with previous results using antithymocyte serum and may be due to the more localized nature of immunodepression during H. polygyrus infection. However, the additional increase in corticosterone concentration resulting from exposure to female odour and destabilized aggressive social relationships did result in downregulation of aggression among high rankers and of testosterone among mice generally, suggesting that modulation rules of thumb are at least partly dependent on the proximate cues associated with immunodepression.     

Host-parasite relationships of longnose dace, Rhinichthys cataractae, from the Ford River, Michigan.

A total of 1,115 longnose dace, Rhinichthys cataractae (family Cyprinidae), were examined for parasites from May 1983 through October 1986 from 3 localities in the Ford River in Michigan's Upper Peninsula. Thirteen parasite species (1 Monogenea, 2 Digenea, 2 Cestoda, 4 Nematoda, 1 Acanthocephala, 3 Protozoa) infected dace. The parasite faunas of dace, taxonomically and in species number, were similar between localities. Posthodiplostomum minimum minimum, Neascus sp., and Rhabdochona canadensis were the most common helminths infecting dace from each locality. The first 2 species did not exhibit consistent seasonal infection patterns between years, whereas the prevalence and mean intensity of R. canadensis in dace from the downriver locality were higher in summer 1983, 1984, and 1985. The intensity of infection of each of these helminth species significantly increased with host length. The prevalences and mean intensities of P. m. minimum, Neascus sp., and R. canadensis as well as the helminth infracommunity diversity were highest in dace from the upriver locality. The major factors that influenced parasite intensity were environmental factors that occurred when and where a fish began its life, the sequence of events that occurred in each habitat the fish encountered during its life, and the length of exposure (age of fish). Dace have isolationist helminth infracommunities arising from factors including ectothermy, a simple enteric system, restricted vagility, and being gape-limited. Allogenic helminths with indirect life cycles predominate in the depauperate helminth fauna of dace.     

Elevated testosterone and reduced 5-HIAA concentrations are associated with wounding and hantavirus infection in male Norway rats

Among rodents that carry hantaviruses, males are more likely to engage in aggression and to be infected than females. One mode of hantavirus transmission is via the passage of virus in saliva during wounding. The extent to which hantaviruses cause physiological changes in their rodent host that increase aggression and, therefore, virus transmission has not been fully documented. To assess whether steroid hormones and neurotransmitters contribute to the correlation between aggression and Seoul virus infection, Norway rats were trapped in Baltimore, Maryland and wounding, infection status, steroid hormones, and concentrations of neurotransmitters, including norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenol acetic acid (DOPAC), serotonin (5-HT), and 5-hydroxyindole-3-acetic acid (5-HIAA) in select brain regions were examined. Older males and males with high-grade wounds were more likely to have anti-Seoul virus IgG and viral RNA in organs than either juveniles or adult males with less severe wounds. Wounded males had higher circulating testosterone, lower hypothalamic 5-HIAA, and lower NE in the amygdala than males with no wounds. Infected males had higher concentrations of testosterone, corticosterone, NE in the hypothalamus, and DOPAC in the amygdala than uninfected males, regardless of wounding status. In the present study, wounded males that were infected with Seoul virus had elevated testosterone and reduced 5-HIAA concentrations, suggesting that these neuroendocrine mechanisms may contribute to aggression and the likelihood of transmission of hantavirus in natural populations of male Norway rats.     

Central vasopressin administration regulates the onset of facultative paternal behavior in microtus pennsylvanicus (meadow voles)

Pharmacological experiments have implicated a role for central arginine vasopressin (AVP) in regulating paternal behavior in monogamous prairie voles. Although nonmonogamous meadow voles exhibit appreciable paternal care when housed under winter, short day lengths (SD), no research has examined whether the same neurobiological systems are involved in regulating paternal behavior in a nonmonogamous species when it behaves paternally. The goal of these experiments was to determine whether central administration of AVP, but not cerebrospinal fluid (CSF), affected the suppression of pup-directed aggression and/or the onset of paternal behavior in meadow voles. Data from experiment 1 implicated a role for AVP in facilitating changes in male behavior: central administration of 1 ng of AVP (but not 3 ng or CSF) inhibited pup-directed aggression in previously pup-aggressive males, and 3 ng of AVP (but not 1 ng or CSF) induced paternal behavior in previously nonpaternal males. In contrast, AVP (1 and 3 ng) did not enhance paternal behavior in already paternal males. Experiment 2 tested the specificity of AVP. Previous research indicated that 24 h of unmated cohabitation with a female reliably induced paternal behavior in SD males. Hence, experiment 2 examined whether administration of a V(1a) AVP antagonist (AVPA), but not CSF, prior to 24 h of unmated cohabitation would block the onset of paternal behavior. Males that received CSF displayed paternal behavior faster and engaged in more investigatory and paternal behaviors than males that received AVPA. Thus, pharmacological experiments support the hypothesis that AVP likely regulates paternal behavior in both facultatively and consistently paternal vole species.     

Recapture of male and female dragonflies in relation to parasitism by mites, time of season, wing length and wing cell symmetry

For aquatic mites parasitic on dragonflies, completion of their life cycle depends on their being returned to appropriate water bodies by their hosts, after completion of engorgement. We examined whether differences among hosts in timing of emergence or phenotypic attributes might affect their probability of return to an emergence pond, and hence success of mites. Parasitized males and females of the dragonfly Sympetrum obtrusum (Hagen) did not differ in overall recapture rates. Females that had wing cell symmetry and emerged early were more likely to be recaptured than females that emerged later or had wing cell asymmetry, but there were no consistent relations between these variables and parasitism by mites. No such relations between wing cell asymmetry, emergence date, and recapture likelihood were found for males. Using randomization tests, we found that mean intensities of Arrenurus planus (Marshall) mites at host emergence were the same for recaptured females and females not recaptured; however, males that were recaptured had lower mean intensities of mites at emergence than males not recaptured. Further, mature females carried more mites than mature males, and the latter had fewer mites than newlyemerged males not recaptured. Biases in detachment of engorging mites do not explain the differences in parasitism between mature males and females, nor the differences in mite numbers between mature males and newly emerged males that were not recaptured. Rather, heavily parasitized males appear to disperse or die and are not recaptured, which should have implications for dispersal of mites and fitness of male hosts.     

Central Vasopressin Administration Regulates the Onset of Facultative Paternal Behavior in Microtus pennsylvanicus (Meadow Voles)

Pharmacological experiments have implicated a role for central arginine vasopressin (AVP) in regulating paternal behavior in monogamous prairie voles. Although nonmonogamous meadow voles exhibit appreciable paternal care when housed under winter, short day lengths (SD), no research has examined whether the same neurobiological systems are involved in regulating paternal behavior in a nonmonogamous species when it behaves paternally. The goal of these experiments was to determine whether central administration of AVP, but not cerebrospinal fluid (CSF), affected the suppression of pup-directed aggression and/or the onset of paternal behavior in meadow voles. Data from experiment 1 implicated a role for AVP in facilitating changes in male behavior: central administration of 1 ng of AVP (but not 3 ng or CSF) inhibited pup-directed aggression in previously pup-aggressive males, and 3 ng of AVP (but not 1 ng or CSF) induced paternal behavior in previously nonpaternal males. In contrast, AVP (1 and 3 ng) did not enhance paternal behavior in already paternal males. Experiment 2 tested the specificity of AVP. Previous research indicated that 24 h of unmated cohabitation with a female reliably induced paternal behavior in SD males. Hence, experiment 2 examined whether administration of a V_1a AVP antagonist (AVPA), but not CSF, prior to 24 h of unmated cohabitation would block the onset of paternal behavior. Males that received CSF displayed paternal behavior faster and engaged in more investigatory and paternal behaviors than males that received AVPA. Thus, pharmacological experiments support the hypothesis that AVP likely regulates paternal behavior in both facultatively and consistently paternal vole species.     

HETEROZYGOSITY,AGGRESSION, AND POPULATION FLUCTUATIONS IN MEADOW VOLES (MICROTUS PENNSYLVANICUS)

We tested whether variation in heterozygosity could produce cyclic changes in population size in meadow voles (Microtus pennsylvanicus). For this to occur, three conditions must be met: (1) populations are more outbred (heterozygotic) at high than low population density, (2) heterozygotic voles are more aggressive than relatively inbred individuals, and (3) heterozygotic voles have lower reproductive fitness, though being superior at defending resources. We found no evidence that heterozygosity varied with population size or that reproductive success varied with heterozygosity. However, the former test was indirect and relatively weak. We directly measured aggression and heterozygosity of individual voles. Aggression was significantly related to heterozygosity: higher heterozygosity correlated with more aggression in males and less aggression in females. The proportion of variance in aggression that could be explained by heterozygosity was small. These results suggest that changes in population size of meadow voles could not be driven by systematic changes in heterozygosity with population size.     

Species affinity and infracommunity ordination of helminths of Leptodactylus chaquensis (Anura: Leptodactylidae) in two contrasting environments from northeastern Argentina

One hundred seventy-two frogs (Leptodactylus chaquensis) were collected from November 2002 to November 2003, in agricultural (n = 132) and nonagricultural (n = 40) areas. Both sites are near the city of Corrientes, Argentina. The main goals of this study were as follows: (1) to determine the helminth parasite community in agricultural and nonagricultural habitats; (2) to analyze the relationships between helminth parasites and site of infection, frog body size, and gender; (3) to identify and examine covariation and association of helminth communities; and (4) to determine the mean richness and diversity of parasite communities. The helminth compound community of this amphibian species consisted of 24 species: 19 in agricultural habitats and 18 in nonagricultural habitats. The mean richness, mean diversity, and evenness of helminths were significantly different between the habitats (P < 0.05). The body size of the host was the important factor in determining parasite richness. Both habitats exhibited differences in community ordination. The helminth species in the 2 habitats exhibited the same interspecific relationships, although differences were observed in the intensity of infection.     

Sex differences and developmental effects of oxytocin on aggression and social behavior in prairie voles (Microtus ochrogaster)

Various hormones, including sex steroids and neuropeptides, have been implicated in aggression. In this study we examined (1) sex differences in intrasexual aggression in na?ve prairie voles; (2) the effects of developmental manipulations of oxytocin on intrasexual aggression; and (3) changes in patterns of intrasexual aggression after brief exposure to an animal of the opposite sex. Within 24 h of birth, infants were randomly assigned to receive either an injection of oxytocin (OT) or oxytocin antagonist (OTA) or to one of two control (CTL) groups receiving either isotonic saline or handling without injection. As adults, animals were tested twice in a neutral arena; before (Test 1) and 24 h after (Test 2) a 4-h exposure to an animal of the opposite sex. In Test 1, CTL males were more likely to show aggressive and less likely to show social behavior than CTL females. No significant treatment differences were observed within either sex in Test 1. In Test 2, after brief exposure to a male, females treated with OT became more aggressive and less social than OTA or CTL females. Male aggressive behavior did not change after exposure to a female. An increase in aggression and decline in social behavior toward other females, seen here in OT-treated females, is typically observed only following several days of female-male cohabitation. These findings demonstrate a sex difference in intrasexual aggression and suggest that neonatal exposure to OT may facilitate the onset of the mate-guarding component of pair bonding in female prairie voles.     

Crowding pregnant mice affects attack and threat behavior of male offspring

Attack and threat behavior of adult male offspring of female mice crowded during the final third of pregnancy was investigated. In 5-min test pairings with an anosmic "standard opponent" which had 50 microliter of male mouse urine applied to its fur, the prenatally stressed group of males showed significantly less attack behavior; attack latency was longer and number of attacks, bites, amount of time spent attacking, and composite aggression scores were all lower, compared with the control group. Similarly, less threat behavior was observed in offspring from crowded dams; there were lower frequencies of tail rattles, rough grooms, and upright threats. Additionally, proportionally fewer males in the prenatally stressed group attacked or displayed threats. A second experiment was designed to investigate the effects of exogenous androgen on the aggressiveness of males from crowded mice: testosterone propionate administration (500 micrograms/animal/day, for 5 days prior to testing) abolished differences both in the proportion of males from crowded mice that fought and also apparently abolished differences in intensities of attack and threat behavior between groups. However, trends toward reduced aggression in prenatally crowded males remained. More detailed analysis of these responses, based only on animals that displayed aggression, revealed significantly reduced intensity of aggression in offspring from females crowded during pregnancy, indicating that testosterone propionate therapy did not completely restore this behavior. In order to reduce postnatal effects due to possible differences in mothering, all offspring were fostered to untreated mothers at birth. The results are discussed in terms of in utero exposure of male fetuses of crowded dams to stress-liberated adrenal steroids of maternal origin, and the possible consequences for the endocrine integrity of these offspring.     

Recapture of male and female dragonflies in relation to parasitism by mites, time of season, wing length and wing cell symmetry

For aquatic mites parasitic on dragonflies, completion of their life cycle depends on their being returned to appropriate water bodies by their hosts, after completion of engorgement. We examined whether differences among hosts in timing of emergence or phenotypic attributes might affect their probability of return to an emergence pond, and hence success of mites. Parasitized males and females of the dragonfly Sympetrum obtrusum (Hagen) did not differ in overall recapture rates. Females that had wing cell symmetry and emerged early were more likely to be recaptured than females that emerged later or had wing cell asymmetry, but there were no consistent relations between these variables and parasitism by mites. No such relations between wing cell asymmetry, emergence date, and recapture likelihood were found for males. Using randomization tests, we found that mean intensities of Arrenurus planus (Marshall) mites at host emergence were the same for recaptured females and females not recaptured; however, males that were recaptured had lower mean intensities of mites at emergence than males not recaptured. Further, mature females carried more mites than mature males, and the latter had fewer mites than newly emerged males not recaptured. Biases in detachment of engorging mites do not explain the differences in parasitism between mature males and females, nor the differences in mite numbers between mature males and newly emerged males that were not recaptured. Rather, heavily parasitized males appear to disperse or die and are not recaptured, which should have implications for dispersal of mites and fitness of male hosts.