Testing the interspecific body size principle in ungulates: the smaller they come, the harder they groom

Tick removal grooming may be centrally regulated by an internal timing mech anism operating to remove ticks before they attach and engorge (programmed grooming model) and/or evoked by cutaneous stimulation from tick bites (sti mulus-driven model). The programmed grooming model predicts that organismic and environmental factors that impact the cost-benefit ratio of grooming ( e.g. body size and habitat) will influence the rate of tick removal groomin g. The body size principle predicts that smaller-sized animals, because of their greater surface-to-mass ratio, should engage in more frequent tick re moval grooming than larger-bodied animals in order to compensate for higher costs of tick infestation. The body size principle may be tested intraspec ifically between young and adult animals, or interspecifically among specie s of contrasting body sizes. To rigorously test the interspecific body size prediction, we observed the programmed grooming (oral and scratch grooming ) of 25 species (or subspecies) of bovids at a tick-free zoological park in which stimulus-driven grooming was ruled out. Multiple correlation analysi s revealed highly significant negative correlations between species-typical mass and mean species grooming rates when habitat was controlled for in th e model. Species-typical habitat type (classified along a gradient from mas t open to most closed) was positively correlated with mean oral grooming ra te, indicating that species tended to groom at a higher rate in woodland an d forest habitats (where typical tick density would be high) compared with more open environments. Species mass accounted for up to two-thirds of the variation in grooming rate across species, whereas habitat accounted for ca . 20% of variation in oral grooming. Similar results were obtained when the analysis was expanded to include 36 species/subspecies of six different fa milies. The body size principle can therefore account for a large proportio n of species-typical differences in programmed grooming rate among ungulate s. However, to understand the tick defence adaptations of very large mammal s that rarely or never engage in oral or scratch grooming (e.g. elephants, giraffes, rhinoceros), alternative tick defence strategies must be consider ed, such as thick skin, wallowing, rubbing and tolerance of oxpeckers and o ther tick-eating birds. (C) 2000 The Association for the Study of Animal Be haviour.