Disease, Predator Avoidance, and Vulnerability to Predation in Tadpoles

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Abstract
Many parasites alter the behaviour of their hosts. In most systems that have been studied, predation of the intermediate host by the definitive host is necessary for parasite transmission. Directly transmitted parasites have been studied less frequently and the fitness benefits from altering host behaviours that lead to the death of the host are not obvious. We examined the behavioural and ecological effects of the yeast Candida humicola, a directly transmitted parasite species in its natural host, the red-legged frog (Rana aurora). This parasite dies if its host dies. We used a mixture of field and laboratory experiments to test: 1) if infected tadpoles have altered thermal preferences (behavioural fevers), 2) the ability of infected tadpoles to detect and respond to chemical cues from predacious roughskinned newts (Taricha granulosa) and 3) the susceptibility of infected tadpoles to predation by newts. We found that infected tadpoles exhibited altered thermoregulatory behaviour, exhibited reduced ability to differentiate chemical cues of potentially dangerous from innocuous predators, and suffered increased predation. Behavioural by-products of the host response to infection may provide a mechanism by which behavioural alterations occur in some one and two-host parasite systems. Physiological changes associated with the host response to infection fight disease organisms, but behavioural aspects of the response may have negative effects on host survival.

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