How can louse species provide clues about the evolutionary relationships of birds and mammals?

Louse species can provide insights into the evolutionary relationships of birds and mammals because they often exhibit high levels of host specificity, meaning they are restricted to one host species or a group of closely related hosts.12

This relationship suggests that ancestral louse populations diverged and evolved in tandem with their hosts. By studying these parasites, researchers can sometimes infer the evolutionary history of the hosts themselves. For example, the presence of specific Mallophaga on flamingos—genera otherwise found only on ducks, geese, and swans—suggests a closer evolutionary link between those groups than with storks. Similarly, the close relationship between the lice of humans and those of chimpanzees or gorillas mirrors the evolutionary proximity of the hosts.1

However, these clues are not always definitive. The process of secondary infestation, where a louse species moves to a new, unrelated host, can obscure the original evolutionary history, making it necessary to account for such transfers when using parasites to map host phylogeny.12