Programmed versus stimulus-driven antiparasitic grooming in a desert rodent

We tested 2 hypotheses concerning regulation of grooming in flea-infested rodents and examined if 2 grooming components, scan and scratch grooming, are controlled by programmed and stimulus-driven regulation, respectively. The programmed grooming hypothesis proposes central programming that periodically evokes a bout of grooming to remove ectoparasites before they are attached and predicts that juvenile rodents 1) regardless of infestation status will invest more time in grooming (the body size principle) and 2) sustain lower flea densities than adults. The stimulus-driven grooming hypothesis postulates a direct response to irritation from ectoparasite bites and predicts that under flea-free conditions, 1) the stimulus-driven grooming regulation will not be activated, thereby neither juveniles nor adults will engage in grooming, but under flea infestation, 2) adults will invest more time in grooming than juveniles and sustain similar flea densities. We recorded the behavior of adult and juvenile flea-parasitized and nonparasitized rodents and quantified the frequency and duration of the 2 grooming components. Flea infestation increased the time devoted to grooming, supporting the existence of a regulation mechanism. However, the results did not support the dominance of neither hypothesis. Both forms of grooming were affected similarly by flea infestation and host age, hence may not necessarily be linked to a given regulation mechanism. Regardless of infestation status, time devoted to grooming was lower in juveniles, and both age groups sustained fleas at similar densities. We suggest that the assumptions and predictions of the 2 hypotheses should take into account the morphology and natural history of the host organism.