Why are many louse species limited to one host species or closely related hosts?

Many louse species exhibit host specificity—being restricted to one or a few closely related host species—primarily due to the infrequency of transfer between hosts and the specialized adaptations required for survival.1

Lice are highly dependent on their host for survival, as they cannot live away from them for long periods. They are adapted to specific body regions, relying on host body heat, darkness, and the physical structure of feathers or hairs for orientation and egg-laying. When a louse is transferred to a new host, it often fails to establish a population due to chemical or physical incompatibilities with the new host as a source of food or habitat.1

Furthermore, this specificity is often a result of coevolution. Ancestral louse populations likely diverged and evolved alongside their hosts as the hosts themselves diverged. This long-term association is so consistent that the distribution of louse species can sometimes provide insights into the evolutionary relationships of the hosts themselves. While secondary infestations on new, unrelated hosts can occur, the general pattern of host restriction remains a defining feature of louse ecology and evolution.21