Experimental demonstration of the energetic cost of parasitism in free-ranging hosts

Although some parasites have obvious pathogenic effects, others appear to have subtle, indirect effects that are poorly understood, particularly in natural populations. Indirect effects may, result from parasites altering host metabolic rate and hence host energy needs, yet no experimental studies have shown this to be the case for non-laboratory hosts. We report the results of a long-term field experiment designed to test the impact of parasites on host energetics. We measured the energetics of feral rock doves (Columba livia) with populations of feather-feeding lice, traditionally considered to have little or no effect on host fitness. The lice reduced feather mass leading to increased thermal conductance and metabolic rate, as well as a steady reduction in host body mass over the course of the nine-month study. Our results demonstrate that even classically 'benign' parasites such as feather lice can reduce host condition through the accumulation of subtle energetic costs over time. We argue that experimental manipulations are a prerequisite for documenting such effects.