Insect-repelling behaviour in bovids: role of mass, tail length, and group size

Biting insects are costly to hosts, and insect-repelling movements of the tail, ears, head, and feet are widespread in mammals and effective in reducing bites. We investigate whether the 'peripheral stimulation model' can explain the regulation of this widespread behaviour pattern in a comparative study of bovids. The peripheral stimulation hypothesis predicts: (1) a positive association between insect-repelling rates and body size because larger hosts produce more of the sensory cues that attract biting insects; (2) that individuals in larger groups will exhibit a higher rate of insect defense behaviour if group size and insect attraction follows a linear function; and (3) larger species will evolve proportionately longer tails in response to higher rates of insect attack. To test these predictions, we observed insect-repelling behaviour in 26 species of bovids at a zoological park, and controlled for common ancestry with formal phylogenetic analyses (independent contrasts). Consistent with the peripheral stimulation hypothesis, rates of tail-switching and all insect-repelling behaviours combined were positively associated with body mass, whereas ear-flicking was positively associated with proportional tail length. Larger bovids had proportionately longer tails for more effective fly swatting. There was no significant association between insect-repelling rate and group size, suggesting that a nonlinear relationship exists between group size and insect attacks whereby individuals in larger groups do not experience an increased attack rate.