The biological basis of grooming in moose: programmed versus stimulus-driven grooming

Abstract

In domestic and wild mammals, tick infestation can be a significant fitness cost. Grooming behaviour has been shown to be effective in removing ticks. We studied grooming by moose,Alces alces, infested with winter ticks,Dermacentor albipictus, to determine which of two nonexclusive models for the regulation of tick-removal grooming, programmed or stimulus driven, best fit this host–parasite relationship. The programmed grooming model states that most grooming is driven by an internal timing mechanism which periodically evokes a bout a grooming independent of peripheral stimulation from tick bites. Because programmed grooming is preventive, the model predicts that those animals that groom the most will carry the fewest ticks, and a baseline level of programmed grooming is predicted even in a tick-sparse or tick-free environment. The stimulus-driven grooming model, on the other hand, states that grooming is a direct response to cutaneous irritation caused by tick bites and other sources of irritation. This stimulus-driven model predicts that (1) animals showing the highest rate of grooming will carry the most ticks and (2) animals will groom little when tick challenge is low. Both predictions of the stimulus-driven model were supported for moose: (1) calves oral-groomed three times more than cows, and tick densities on calves were three times higher than on cows; and (2) although all moose carried high densities of immature winter ticks (larvae, nymphs) from October through to February, grooming rate was very low until adult ticks started feeding in March–April. Peak grooming rates occurred during adult tick engorgement in March–April. Because an engorging adult female tick produces far more irritation than an engorging nymphal or larval tick, moose appeared to groom in direct proportion to the degree of cutaneous irritation and did not show a baseline level of grooming. The predominance of stimulus-driven grooming and apparent absence of programmed grooming may be the result of relaxed selection pressure for grooming in the evolutionary history of moose. Because the winter tick appears to have been introduced to moose from deer relatively recently, moose may not have had the time to adapt to winter ticks. The coevolutionary relationship between moose and winter ticks may be of insufficient duration for the evolution and/or maintenance of programmed grooming.