Systematics of the black rat in Madagascar: consequences for the transmission and distribution of plague

The plague has been present for a century in Madagascar; the only known reservoir is the black rat, which is also the main victim of this disease. Whereas the two plague foci are restricted to an altitude above 800 m, the black rat is distributed over the whole island, the only morphological variation being a within-site variability in belly colour. To resolve these apparent contradictions, an analysis of the different belly colour morphs and of populations living in different habitats and altitudes was undertaken. An allozymic study demonstrated that sympatric white- and grey-bellied rats ( N  = 26) living within the plague foci, all belonged to the same species: Rattus rattus . This specific assignment was confirmed by a chromosomal analysis which showed that all 90 individuals studied presented the same diploid number, 2 n  = 38, whatever the habitat and altitude. This study indicates that all specimens investigated in Madagascar could be referred to the same black rat species. Survival of rat populations, despite a century of coexistence with the plague, is most likely related to their high reproductive and recolonization potential, as well as selection of plague resistant genotypes. As the altitudinal limit of the plague foci does not seem to be related to characteristics of the rodent reservoir, investigations concerning the vectors are now essential.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78 , 335–341.