Diversification in small mammals is associated with climatic specialisation, whereas species accumulation reflects evolutionary time.

Climatic specialisation, the restriction of species to particular climate regimes, may promote diversification via ecological divergence and dispersal limitation, but also increase extinction risk. To evaluate how climatic specialisation relates to diversification patterns in small mammals, a vertebrate group widely used in palaeoecological research, we analyse 4231 species belonging to the orders Rodentia, Chiroptera, Eulipotyphla, and Lagomorpha worldwide, combining climatic, geographic, and phylogenetic data. We find that nearly 40% of small mammal species are climatic specialists (restricted to one of five major climatic zones), and that half of these are climatic superspecialists (restricted to one of 30 sub-climatic zones), revealing widespread but uneven niche restriction across taxa and geography. At the family level, climatic superspecialisation is strongly associated with diversification, particularly for families consisting of species with shorter lifespans and smaller ranges. In contrast, family age is the main predictor of species richness, indicating that older families gradually accumulate species through time rather than through intrinsically faster diversification. Together, these findings support a dual macroevolutionary pattern linking ecology and evolution: levels of climatic specialisation are seemingly driven by diversification rates within lineages, whereas time largely explains the accumulation of species in small mammal families.