Non-invasive monitoring of adrenocortical activity in three sympatric desert gerbil species

The study of the endocrine status can be useful to understand wildlife responses to the changing environment. Here, we validated an enzyme immunoassay (EIA) to non-invasively monitor adrenocortical activity by measuring fecal corticosterone metabolites (FCM) in three sympatric gerbil species (Gerbillus andersoni, G. gerbillus and G. pyramidum) from the Northwestern Negev Desert’s sands (Israel). Animals included into treatment groups were injected with adrenocorticotropic hormone (ACTH) to stimulate adrenocortical activity, while control groups received a saline solution. Feces were collected at different intervals and FCM were quantified by an EIA. Basal FCM levels were similar in the three species. The ACTH effect was evidenced, but the time of FCM peak concentrations appearance differed between the species (6–24 h post-injection). Furthermore, FCM peak values were observed sooner in G. andersoni females than in males (6 h and 18 h post-injection, respectively). G. andersoni and G. gerbillus males in control groups also increased FCM levels (18 h and 48 h post-injection, respectively). Despite the small sample sizes, our results confirmed the EIA suitability for analyzing FCM in these species as a reliable indicator of the adrenocortical activity. This study also revealed that close species, and individuals within a species, can respond differently to the same stressor

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Path analyses of cross-sectional and longitudinal data suggest that variability in natural communities of blood-associated parasites is derived from host characteristics and not interspecific interactions

Abstract Background The parasite composition of wild host individuals often impacts their behavior and physiology, and the transmission dynamics of pathogenic species thereby determines disease risk in natural communities. Yet, the determinants of parasite composition in natural communities are still obscure. In particular, three fundamental questions remain open: (1) what are the relative roles of host and environmental characteristics compared with direct interactions between parasites in determining the community composition of parasites? (2) do these determinants affect parasites belonging to the same guild and those belonging to different guilds in similar manners? and (3) can cross-sectional and longitudinal analyses work interchangeably in detecting community determinants? Our study was designed to answer these three questions in a natural community of rodents and their fleas, ticks, and two vector-borne bacteria. Methods We sampled a natural population of Gerbillus andersoni rodents and their blood-associated parasites on two occasions. By combining path analysis and model selection approaches, we then explored multiple direct and indirect paths that connect (i) the environmental and host-related characteristics to the infection probability of a host by each of the four parasite species, and (ii) the infection probabilities of the four species by each other. Results Our results suggest that the majority of paths shaping the blood-associated communities are indirect, mostly determined by host characteristics and not by interspecific interactions or environmental conditions. The exact effects of host characteristics on infection probability by a given parasite depend on its life history and on the method of sampling, in which the cross-sectional and longitudinal methods are complementary. Conclusions Despite the awareness of the need of ecological investigations into natural host-vector-parasite communities in light of the emergence and re-emergence of vector-borne diseases, we lack sampling methods that are both practical and reliable. Here we illustrated how comprehensive patterns can be revealed from observational data by applying path analysis and model selection approaches and combining cross-sectional and longitudinal analyses. By employing this combined approach on blood-associated parasites, we were able to distinguish between direct and indirect effects and to predict the causal relationships between host-related characteristics and the parasite composition over time and space. We concluded that direct interactions within the community play only a minor role in determining community composition relative to host characteristics and the life history of the community members

Data from: Alternative paths to success in a parasite community: within-host competition can favor higher virulence or direct interference

Selection imposed by coinfection may vary with the mechanism of within-host competition between parasites. Exploitative competition is predicted to favor more virulent parasites, while interference competition may result in lower virulence. Here, we examine whether exploitative or interference competition determines the outcome of competition between two nematode species (Steinernema spp.), which in combination with their bacterial symbionts (Xenorhabdus spp.), infect and kill insect hosts. Multiple isolates of each nematode species, carrying their naturally associated bacteria, were characterized by (1) the rate at which they killed insect hosts, and by (2) the ability of their bacteria to interfere with each other’s growth via bacteriocidal toxins called "bacteriocins." We found that both exploitative and interference abilities were important in predicting which species had a selective advantage in pairwise competition experiments. When nematodes carried bacteria that did not interact via bacteriocins, the faster killing isolate had a competitive advantage. Alternatively, nematodes could gain a competitive advantage when they carried bacteria able to inhibit the bacteria of their competitor. Thus, the combination of nematode/bacterial traits that led to competitive success depended on which isolates were paired, suggesting that variation in competitive interactions may be important for maintaining species diversity in this community

Similarities and seasonal variations in bacterial communities from the blood of rodents and from their flea vectors

Vector-borne microbes are subject to the ecological constraints of two distinct microenvironments: that in the arthropod vector and that in the blood of its vertebrate host. Because the structure of bacterial communities in these two microenvironments may substantially affect the abundance of vector-borne microbes, it is important to understand the relationship between bacterial communities in both microenvironments and the determinants that shape them. We used pyrosequencing analyses to compare the structure of bacterial communities in Synosternus cleopatrae fleas and in the blood of their Gerbillus andersoni hosts. We also monitored the interindividual and seasonal variability in these bacterial communities by sampling the same individual wild rodents during the spring and again during the summer. We show that the bacterial communities in each sample type (blood, female flea or male flea) had a similar phylotype composition among host individuals, but exhibited seasonal variability that was not directly associated with host characteristics. The structure of bacterial communities in male fleas and in the blood of their rodent hosts was remarkably similar and was dominated by flea-borne Bartonella and Mycoplasma phylotypes. A lower abundance of flea-borne bacteria and the presence of Wolbachia phylotypes distinguished bacterial communities in female fleas from those in male fleas and in rodent blood. These results suggest that the overall abundance of a certain vector-borne microbe is more likely to be determined by the abundance of endosymbiotic bacteria in the vector, abundance of other vector-borne microbes co-occurring in the vector and in the host blood and by seasonal changes, than by host characteristics

Programmed versus stimulus-driven antiparasitic grooming in a desert rodent

We tested 2 hypotheses concerning regulation of grooming in flea-infested rodents and examined if 2 grooming components, scan and scratch grooming, are controlled by programmed and stimulus-driven regulation, respectively. The programmed grooming hypothesis proposes central programming that periodically evokes a bout of grooming to remove ectoparasites before they are attached and predicts that juvenile rodents 1) regardless of infestation status will invest more time in grooming (the body size principle) and 2) sustain lower flea densities than adults. The stimulus-driven grooming hypothesis postulates a direct response to irritation from ectoparasite bites and predicts that under flea-free conditions, 1) the stimulus-driven grooming regulation will not be activated, thereby neither juveniles nor adults will engage in grooming, but under flea infestation, 2) adults will invest more time in grooming than juveniles and sustain similar flea densities. We recorded the behavior of adult and juvenile flea-parasitized and nonparasitized rodents and quantified the frequency and duration of the 2 grooming components. Flea infestation increased the time devoted to grooming, supporting the existence of a regulation mechanism. However, the results did not support the dominance of neither hypothesis. Both forms of grooming were affected similarly by flea infestation and host age, hence may not necessarily be linked to a given regulation mechanism. Regardless of infestation status, time devoted to grooming was lower in juveniles, and both age groups sustained fleas at similar densities. We suggest that the assumptions and predictions of the 2 hypotheses should take into account the morphology and natural history of the host organism

Aggregation and species coexistence in fleas parasitic on small mammals

International audienceThe aggregation model of coexistence states that species coexistence is facilitated if interspecific aggregation is reduced relative to intraspecific aggregation. We investigated the relationship between intraspecific and interspecific aggregation in 17 component communities (the flea assemblage of a host population) of fleas parasitic on small mammals and hypothesized that interspecific interactions should be reduced relative to intraspecific interactions, facilitating species coexistence. We predicted that the reduction of the level of interspecific aggregation in relation to the level of intraspecific aggregation would be positively correlated with total flea abundance and species richness of flea assemblages. We also expected that the higher degree of facilitation of flea coexistence would be affected by host parameters such as body mass, basal metabolic rate (BMR) and depth and complexity of burrows. Results of this study supported the aggregation model of coexistence and demonstrated that, in general, a) conspecific fleas were aggregated across their hosts; b) flea assemblages were not dominated by negative interspecific interactions; and c) the level of interspecific aggregation in flea assemblages was reduced in relation to the level of intraspecific aggregation. Intraspecific aggregation tended to be correlated positively to body mass, burrow complexity and mass-independent BMR of a host. Positive interspecific associations of fleas tended to occur more frequently in species-rich flea assemblages and/or in larger hosts possessing deep complex burrows. Intraspecific aggregation increased relative to interspecific aggregation when species richness of flea infracommunities (the flea assemblage of a host individual) and component communities increased. We conclude that the pattern of flea coexistence is related both to the structure of flea communities and affinities of host species