Behavioural Thermoregulation and Energetics in Two Intermediate Hosts of Trematode Parasites

Abstract

Infection by macroparasites, such as trematodes (flatworms), can negatively impact survival of hosts such as larval amphibians, potentially altering host energy use in response to infection, and also through alterations of host behaviour that may increase infection tolerance or instead benefit the parasite. However, physiological consequences of macroparasite infections are not well studied, despite heavy parasite burdens in the field. The purpose of this work was to examine altered thermoregulatory behaviours in two taxa (snails and larval amphibians) used as intermediate hosts by trematodes, as well as to study the metabolism of naturally-infected tadpoles. Both infected and uninfected tadpoles (Lithobates sylvaticus and L. pipiens) and snails (Helisoma trivolvis) were placed in thermal gradients to observe thermal preferences in hosts. Oxygen consumption in naturally-infected bullfrog tadpoles (L. catesbeiana) was measured to determine whether macroparasites could impact host metabolism. The trematode-infected, L. sylvaticus tadpoles exhibited “behavioural fever” by choosing warmer temperatures by the end of the experiment compared to uninfected tadpoles, but this did not occur in L. pipiens. Active, infected snails also selected warmer temperatures relative to inactive snails and active uninfected snails. Trematode infection intensity did not affect respiration in L. catesbeiana tadpoles, but those with higher metabolic rates and larger fat bodies had lower parasite counts. These results suggest that behavioural fever may occur in ectotherms infected with macroparasites, but may be more important for species which are relatively intolerant of infection given that fever was not seen in L. pipiens. As infected snails selected warmer temperatures, this may be a case of parasite manipulation to increase production and emergence of infectious stages in warm microhabitats to facilitate transmission. Metabolic rate increased with fat body content, and larger fat bodies were observed in tadpoles with lower parasite intensity, suggesting more heavily parasitized animals had lower energy stores. Globally, infectious diseases are known to contribute to amphibian declines, thus more research is needed to understand the possible consequences of parasitism and mechanisms by which hosts to may defend themselves.