Grooming in desert bighorn sheep (Ovis canadensis mexicana) and the ghost of parasites past

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Abstract

Ectoparasites such as ticks have a negative effect on host fitness, whereas parasite-defense grooming is effective in removing ticks. The central control (programmed grooming) model proposes that animals engage in preventive tick-defense grooming in response to an internal timing mechanism, even in the absence of peripheral stimulation from parasites. This model predicts that smaller animals will groom more frequently than larger ones because of the higher cost of parasitism for a small animal (body size principle). The peripheral stimulation (stimulus driven) model predicts no size-related differences in grooming rate in the absence of tick bite irritation. We observed grooming behavior in a Chihuahuan desert population of bighorn sheep (Ovis canadensis mexicana), where ticks have been absent for perhaps thousands of years. Although not exposed to ticks, bighorns self groomed by means of oral and scratch grooming, albeit at very low rates compared to size-matched ungulates in both tick-infested and tick-free environments. Logistic regression and general linear models revealed both the probability that grooming was performed during a 10-min focal sample and the rate of grooming when it occurred was greater for younger, smaller age/sex categories of less body mass. Oral and scratch grooming were negatively associated with body mass during both years, with juveniles (X=15 kg) grooming the most frequently and the oldest males (X=70–85 kg) grooming the least. Assuming that programmed grooming evolved in a tick-infested environment, the current grooming behavior of this population is a relict of their ancestral environment, an adaptation to the “ghost of parasites past.”

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