Research on mammals provides the evidence base for their conservation, management and related policy. We assess the effect of study choices on the composition of this evidence base. We derived species h-index (SHI) for 67 breeding terrestrial mammals from 2305 publications based on field research carried out in England, Scotland and Wales. Native species had higher SHI than introduced species, but SHI was not related to the level of species conservation concern. SHI was positively related to geographical range and/or population size and to functional group: large herbivores had higher SHI, then carnivores, rodents, small herbivores, bats and insectivores.