Bacterial and parasitic effects on the performance of lizards
An outstanding issue in disease ecology is understanding the relationship between parasites and host behaviour. The co-occurrence (or co-infections) of parasites is particularly important because different parasites may have either divergent effects on their host or compound effects, thereby particularly challenging the host’s physiological responses. Based on this notion, studying the co-infection of multiple parasites has become vital to understanding of this host-parasite relationship. The main aim of my thesis was to investigate how multiple parasites co-occur in the host and to what extend parasite diversity can affect the performance of the host (specifically locomotor performance). To answer these questions, I explored the consequences of diverse bacteria and the impact of parasite (i.e., endo- and ecto-parasite) coinfections in the Australian sleepy lizard (Tiliqua rugosa) and garden skink (Lampropholis guichenoti), respectively. In chapter one, I investigated how bacterial strains (Enterobacteriaceae) co-occurred in sleepy lizards. I examined bacterial diversity and bacterial co-occurrence patterns over the activity season (September – December). This chapter showed that bacterial occurrence (richness and prevalence) changed over time among the lizards and increased during the activity season of the host. However, bacterial co-occurrences did not differ from what we would expect in the absence of facilitation or inhibition processes. In chapter two, I investigated whether the community of Enterobacteriaceae affected host vigour in the sleepy lizard. I measured vigour as daily movement distance but did not find a relationship with bacterial diversity. In some species, infected individuals lose vigour and consequently interact less with other individuals, thereby altering the social network. Here, I did not find that sleepy lizards with diverse bacteria had lower vigour, which would suggest that the social network would be uninfluenced by bacterial diversity. In chapter three, I examined the effects of parasites on whole-organism performance in a different host, the garden skink. I tested whether endoparasites (roundworms) and ectoparasites (mites) impacted the lizards’ performance (sprint speed, endurance, and foraging efficiency) and tested the hypothesis that infected lizards modified their preferred body temperature through altered basking behaviour. The thermoregulatory response to infection alternated between inducing fever and hypothermia. Higher body temperature is thought to increase a host’s defence against parasites while the lower body temperature can reduce the activity or growth of parasites. Mites negatively affected lizard sprint speed, a performance measure. However, this negative effect was absent when lizards were coinfected with roundworms. Furthermore, in contrast to my prediction, I found no evidence of lizards selecting a warmer body temperature to induce fever. In chapter four, I explored the relationship between host sex and parasite load in breeding and non-breeding seasons. Seasonality can affect parasite infection rates by changing host physiology and host interactions. Similarly, the ecological conditions of one season may be more suitable for the parasite than the other season. To test the effect of sex and seasonality on infection intensity, I experimentally manipulated the sex composition of lizard groups and measured endoparasite loads during each season. I found a significant increase in endoparasite loads during the breeding season despite the sex. No difference in endoparasite loads in both sexes may suggest that the short-lived lizard tends to endure the infection rather than mounting an immune response. This study provides insight into the cooccurrence and co-infection of parasites in lizards, and extends our understanding of hostparasite relationships.