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To understand whether a parasite can exploit a novel invasive host species, we measured reproductive performance (number of eggs per female per day, egg size, development rate and size of new imagoes) of fleas from the Negev desert in Israel (two host generalists, Synosternus cleopatrae and Xenopsylla ramesis, and a host specialist, Parapulex chephrenis) when they exploited either a local murid host (Gerbillus andersoni, Meriones crassus and Acomys cahirinus) or two alien hosts (North American heteromyids, Chaetodipus penicillatus and Dipodomys merriami). We asked whether (1) reproductive performance of a flea differs between an alien and a characteristic hosts and (2) this difference is greater in a host specialist than in host generalists. The three fleas performed poorly on alien hosts as compared to local hosts, but the pattern of performance differed both among fleas and within fleas between alien hosts. The response to alien hosts did not depend on the degree of host specificity of a flea. We conclude that successful parasite colonisation of an invasive host is determined by some physiological, immunological and/or behavioural compatibility between a host and a parasite. This compatibility is unique for each host-parasite association, so that the success of a parasite to colonise an invasive host is unpredictable.

Parents may alter offspring phenotype depending on the type of environment they encounter. Parasitism is a common stressor; therefore, maternal reproductive investment could change in response to parasitic infection. However, few experiments have investigated the relationship between parasitism and maternal investment, whereas earlier field studies provided contradictory evidence. We investigated number, sex ratio, and growth of offspring in two rodent species, solitary altricial Meriones crassus and social precocial Acomys cahirinus, exposed to parasitism by fleas Xenopsylla ramesis and Parapulex chephrenis. No effect of treatment on litter size or sex ratio of a litter was found in either rodent species. Flea parasitism was found to affect pre-weaning body mass gain in M. crassus, but not in A. cahirinus pups. Furthermore, it appeared that female M. crassus invested resources into their offspring differently in dependence of litter size. In small litters (1-3 offspring), pups from infested females gained more body mass before weaning than pups from uninfested mothers. However, this trend was reversed in females with large litters indicating that parasitized females have a finite amount of resources with which to provision their young. Thus, M. crassus mothers parasitized by fleas seemed to receive some sort of external cues (e.g., stress caused by infestation) that prompted them to alter offspring provisioning, depending on species-specific possibilities and constraints. Therefore, parasites could be a mediator of environmentally induced maternal effects and offspring provisioning may have adaptive value against parasitism.

We investigated interspecific interactions between two flea species (Parapulex chephrenis and Xenopsylla ramesis) via evaluation of their feeding success (the size of a blood meal and time to death after a single blood meal) when they exploited rodent hosts [Acomys cahirinus (a characteristic host of the former) or Meriones crassus (a characteristic host of the latter)] in single-species or mixed-species groups. We predicted that the negative interactions between the two fleas will result in smaller blood meals and shorter survival time in mixed- versus single-species infestations. We also predicted that the negative effect of mixed-species infestation on feeding performance would be less pronounced when fleas exploited their characteristic host rather than a non-characteristic host. When exploiting a characteristic host, P. chephrenis took larger blood meals in single- than in mixed-species groups, whereas the blood meal size in X. ramesis did not differ between treatments. When exploiting a non-characteristic host, no effect of group composition was found in either flea species. Survival time after a single blood meal was not affected by co-infestation or host species in either flea. Our results suggest context-dependence of the negative effect of co-infestation on feeding performance in fleas with the manifestation of this effect varying in dependence of flea and host species identities.

To understand mechanisms behind positive interspecific co-occurrences in flea infracommunities, we asked whether co-infestation results in an increase of flea fitness (quantity and/or quality of the offspring). We studied reproductive performance of Xenopsylla ramesis and Parapulex chephrenis when they exploited their characteristic host (Meriones crassus and Acomys cahirinus, respectively) either alone or together with another species. We used egg production, the number of new imagoes, pre-imaginal survival and egg size as fitness-related variables and predicted that fitness will be higher in fleas feeding in mixed- than in single-species groups. In both fleas, mean number of eggs produced per female flea did not depend on experimental treatment. No effect of single- vs mixed-species infestation on the mean number of new imagoes per female and the number of emerged imagoes per egg was found for X. ramesis, whereas both these numbers were higher in mixed- than in single-species groups for P. chephrenis. X. ramesis produced eggs of similar size independently of treatment, whereas eggs produced by P. chephrenis in mixed-species groups were significantly larger than eggs produced in single-species groups. We conclude that an increase in reproductive performance as a response to co-infestation may be one of the mechanisms behind aggregative structure of flea infracommunities. However, this response may vary among flea species.

We studied the effects of early weaning on immunocompetence and parasite resistance in a precocial rodent Acomys cahirinus. We hypothesized that if parasite resistance is energetically expensive and nutritional and immunological support from mothers are necessary for the long-term health of offspring, then early weaned animals would be immunologically weaker and less able to defend themselves against parasites than later weaned animals. We weaned pups at 14, 21 or 28 days after birth and assessed their immunocompetence and resistance against fleas Parapulex chephrenis when they attained adulthood. Immunocompetence was assessed using leukocyte concentration (LC) and a phytohaemagglutinin injection assay (PHA test). To estimate resistance against fleas, we measured performance of fleas via the number of produced eggs and duration of development and resistance to starvation of the flea offspring. We found a significant positive effect of weaning age on the PHA response but not on LC. The effect of age at weaning on flea egg production was manifested in male but not female hosts, with egg production being higher if a host was weaned at 14 than at 28 days. Weaning age of the host did not affect either duration of development or resistance to starvation of fleas produced by mothers fed on these hosts. We conclude that even in relatively precocial mammals, weaning age is an important indicator of future immunological responses and the ability of an animal to resist parasite infestations. Hosts weaned at an earlier age make easier, less-resistant targets for parasite infestations than hosts weaned later in life., (© 2014. Published by The Company of Biologists Ltd.)