EFFECTS OF BODY SIZE AND PARASITE INFECTION ON THE LOCOMOTORY PERFORMANCE OF JUVENILE TOADS, BUFO-BUFO

We tested the effects of body size and nematode infection on the stamina and burst performance of juvenile toads, Bufo bufo. Tadpoles were reared in ponds at high (120 individuals/1000 l) and low (60 individuals/(1000 l) densities, resulting in a 29.0%. difference in mass at metamorphosis. Ten days after metamorphosis, metamorphs were exposed to 0, 10 or 80 infective larvae of the lung nematode. Rhabdias bufonis, resulting in mean a posteriori worm burdens of 0, 3.4 and 7.5 worms per host, respectively. The major determinant of performance was the size of metamorphs; those from low-density ponds always out-performed those from high-density ponds, even up to one month after metamorphosis. Parasite density in the lungs played a secondary role in affecting stamina performance but not burst performance. We suggest two mechanisms by which infection may reduce stamina performance. First, infection reduces normal growth rates of metamorphs by 25.8%, and size is directly correlated with all aspects of performance. Second, R. bufonis may interfere with oxygen consumption or other aspects of lung function which have a direct impact on the host's ability to sustain activity via aerobic respiration. The results provide strong evidence for the indirect effects of parasitic infection in a natural parasite-host system, in which even small differences in parasite burden may influence a host's ability to forage or to interact with competitors and predators. Also, because toads which emerged large were never surpassed in growth or performance by those that emerged small, the results illustrate the important role of larval history, and how size at metamorphosis can determine the subsequent fitness of organisms with complex life-cycles.