Sexually and Developmentally Dimorphic Grooming: A Comparative Survey of the Ungulata


Michael S. Mooring, Department of Biology, Point Loma Nazarene University, 3900 Lomaland Drive, San Diego, CA 92106, USA. E-mail:


Grooming is effective in removing fitness-compromising ectoparasites, such as ticks. Tick-removal grooming is regulated both by a central control mechanism (programmed grooming model) and by cutaneous stimulation from tick bites (stimulus-driven model). The programmed grooming model predicts that organismic factors that impact the cost-benefit ratio of grooming (e.g., sex and body size) will influence the rate of grooming. The ‘vigilance principle’ predicts that breeding males of sexually dimorphic species will groom less than females to maintain high levels of vigilance for rival males and oestrus females. The intraspecific body size principle predicts that juveniles will engage in more frequent grooming than larger-bodied adults to compensate for the higher cost of tick infestation for smaller animals. To examine the generalizability of these predictions, we surveyed the grooming rate of 53 species/subspecies of ungulates (primarily Bovidae and Cervidae) in tick-free zoological parks in which stimulus-driven grooming was controlled for. Matched-pair analysis supported both predictions. Males delivered fewer oral grooming episodes per hour than females in 85% of sexually dimorphic species (n = 40), but only 56% of monomorphic species (n = 11) exhibited sexually dimorphic grooming, which was not different from random. Juvenile oral episodes per hour exceeded that of adult females in 74% of surveyed species (n = 36). As predicted by the body size principle, the grooming rate of juveniles declined as juveniles grew larger and less vulnerable to tick infestation. In separate analyses of Bovidae and Cervidae to control for common ancestry, both families supported sexually dimorphic grooming, but developmentally dimorphic grooming was supported only for Bovidae. Our results indicate that sexually dimorphic grooming is widespread in the ungulate species surveyed, suggesting that programmed grooming evolved at least as early as the common ancestor to the Artiodactyla.