Comparative body size relationships in pocket gophers and their chewing lice

In this paper, we use the method of independent contrasts to study body siz e relationships between pocket gophers and their chewing lice, a host-paras ite system in which both host and parasite pilylogenies are well studied. T he evolution of body size of chewing lice appears to be dependent only on t he body size of their hosts, which confirms the 1991 findings of Harvey and Keymer. We show that there is a positive relationship between body size an d hair-shaft diameter in pocket gophers, and that there is also a positive relationship between body size and head-groove width in chewing lice. Final ly, we show a positive relationship between gopher hair-shaft diameter and louse head-groove width. We postulate that changes in body size of chewing lice are driven by a mechanical relationship between the parasite's head-gr oove dimension and the diameter of the hairs of its host. Louse species liv ing on larger host species may be larger simply because their hosts have th icker hairs, which requires that the lice have a wider head groove. Our stu dy of gopher hair-shaft diameter and louse head-groove dimensions suggest t hat there is a 'lock-and-key' relationship between these two anatomical fea tures. (C) 2000 The Linnean Society of London.