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, particul arly in natural populations. Indirect effects may, result from parasit es altering host metabolic rate and hence host energy needs, yet no ex perimental studies have shown this to be the case for non-laboratory h osts. We report the results of a long-term field experiment designed t o test the impact of parasites on host energetics. We measured the ene rgetics of feral rock doves (Columba livia) with populations of feathe r-feeding lice, traditionally considered to have little or no effect o n host fitness. The lice reduced feather mass leading to increased the rmal conductance and metabolic rate, as well as a steady reduction in host body mass over the course of the nine-month study. Our results de monstrate that even classically 'benign' parasites such as feather lic e can reduce host condition through the accumulation of subtle energet ic costs over time. We argue that experimental manipulations are a pre requisite for documenting such effects.