The effects of rainfall and sociality on host-parasite dynamics in the common mole-rat, Cryptomys hottentotus hottentotus (Lesson 1812)

Abstract

There are many factors which may influence the distribution of parasites and often parasites are not evenly distributed amongst their hosts. The development and survival of ectoparasites is usually dependent on abiotic factors such as temperature and rainfall. Therefore differences in ectoparasite load between host populations and seasonal fluctuations in abundance are often found to be associated with climatic variations. However, biotic (host-related) factors have also been found to play a role in these differential infection rates. Host social aggregations during the breeding season have been shown to enhance horizontal transfer of ectoparasites due to increased host density. As a consequence, the question arises as to whether an increase in parasitism comes as a cost to social species. Recently, a number of studies have started to assess whether particular individuals within social groups tend to suffer higher parasitic burdens as a consequence of their social status. Some of these studies have employed the analysis of testosterone and glucocorticoid concentrations in the host in relation to these factors. Both of these steroid hormones may have immunosuppressive effects when their circulating concentrations increase. Additionally, each can vary with the seasons and the social environment. To date universal trends have not been reported for either abiotic or biotic factors in association with parasite load. Therefore, this study aimed to provide a comprehensive report of the ectoparasitic fauna associated with the social, subterranean rodent Cryptomys hottentotus hottentotus. Differences in parasite prevalence and abundance were analysed across two habitats at the extremes of a rainfall gradient within which the host species is found. Once the inventory had been compiled, the impact of abiotic and biotic factors was assessed across the two populations to determine which was the most influential on parasite dynamics. The study also attempted to assess whether parasitism was costly to sociality and if certain members of the colony suffered greater parasitism due to their social status. Testosterone and glucocorticoid concentrations were also measured and compared against factors such as season, breeding status and parasitic infection. The ectoparasite communities were very similar between populations, inferring that the subterranean environment has led to specialisation of ectoparasites on C. h. hottentotus. The confinement of being underground also appears to have made the ectoparasite community more dependent on the host. Seasonal increases in parasite prevalence and abundance are thought to be linked to increased host dispersal and social interactions that occur during peak rainfall. However, there were distinct differences in parasite load between locations. Additionally, few common patterns were found between separate parasite taxa. Group size either did not influence or reduced mite abundance, suggesting sociality is beneficial to the host. However, differences between hosts of different social status were more difficult to assess, particularly as this may differ between locations. It is unlikely that any differences in parasite burden observed between individuals are a direct result of immunosuppression from steroid hormones. In conclusion there are many factors which may influence the host-parasite relationship simultaneously and therefore generate unique circumstances within each host population.