Evolution of Rattus (mammalia, Rodentia) during the plio‐pleistocene in Thailand

Abstract

The adaptive radiation of Rattus is examined at two levels, the divergence of this genus relative to its sister genera (Maxomys, Niviventer, Leopoldamys, Berylmys, Sundamys and Bandicota) and the diversification of numerous species within the genus Rattus in Southeast Asia and Australia. Molecular data suggest a recent origin for Rattus (8–6 Ma), followed much later by an exceptionally rapid diversification of species (since 2.5 Ma). According to these data, Maxomys represents the most distant sister group, followed by a Niviventer‐Leopoldamys group, then by a Sundamys‐Berylmys group. Bandicota is only as distinctive as a species of Rattus. We have reconstructed the phylogenetic relationships of Thai representatives of these taxa using molar characters. Our cladogram, based on 65 characters, is largely congruent with the molecular data. Here, Rattus appears earlier, relative to its sister group consisting of Bandicota‐Berylmys and Niviventer‐Leopoldamys. The early divergence of Maxomys and the close relationship between Niviventer and Leopoldamys are consistent with the molecular data. Palaeontological data from 20 karst or cave localities in Thailand indicate that Rattus s.s. was represented during the Pliocene by a single species. The Pliocene community of murid rodents was significantly different from the living one, with several genera but only one species of Rattus. The number of Rattus species increased rapidly during the Pleistocene. During the latest Tertiary, grasslands were more extensive compared with the present day, indicating more pronounced seasonality. The climate became wetter, with less seasonality and more widespread evergreen forests, during the Pleistocene. There was a downward shift in elevation of at least 1,000 meters in vegetational zones. We relate the radiation of Rattus which seems to have spread northwards from a southern refugium, to this development of evergreen forests. This climatic history is in good agreement with global climate models, in which cooling and increasing humidity through the Plio‐Pleistocene of Southeast Asia is correlated with uplift of the Tibetan plateau, and also with the effects of rising sea level.