Tick Defense Strategies in Bison: The Role of Grooming and Hair Coat

Abstract

AbstractGrooming behaviour, and its effectiveness in controlling infestation by the winter tick (Dermacentor albipictus), was studied for plains bison (Bison bison bison) in Elk Island National Park, Alberta, Canada from October 1995 through June 1996. Bison had few ticks (mean, 133 ticks per animal; 0.009 ticks per cm2), particularly in comparison with smaller sympatric cervids (moose, Alees alees; elk, Cervus elaphus; and white-tailed deer, Odocoileus virginianus), suggesting that tick defense in bison is highly effective. Bison performed grooming or grooming-like behaviours (oral grooming, scratching, rubbing, and wallowing) at a high rate during October, when winter tick larvae were blood feeding, but groomed very little from November to April, when nymphal and adult ticks predominated. Grooming in October probably removed many larval ticks while they were still unattached and traversing the body surface in search of an attachment site. Because bison groomed at the highest rate during the larval feeding period, when they were subjected to the lowest intensity of tick stimulation, grooming in bison appears to be centrally programmed rather than stimulus driven. This might be the only time ticks are vulnerable to grooming activity because bison have an extremely thick hair coat (a morphological adaptation to extreme cold), which probably serves as a physical barrier to infestation by ticks. The tightly packed mat of primary hairs at the skin surface (the highest density of primary hairs among bovids) likely forced larval ticks to traverse much of the body surface on top of the hair coat, making them vulnerable to being removed through licking and other grooming activity. Little grooming throughout late autumn and winter (November-March) corresponded to the period of coldest temperatures and snow on the ground, and was probably due to the bison's 'thermal inertia' strategy of energy conservation in which physical activity is minimized during the times of greatest cold stress, when forage is least available and of poorest quality. Newly born bison calves, 2 months old or younger, delivered 15-20 times more oral grooming per hour and 6 times more episodes per bout than did adult cows. This result is in accordance with the prediction of the body size principle of the programmed grooming hypothesis, which maintains that smaller animals should groom more frequently in order to maintain fewer ticks. Programmed grooming, which removes most larval ticks before they can attach, and the physical barrier of the dense hair coat, are proposed as the major reasons that bison host few D. albipictus.