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Introduction The genus Arvicanthis (Lesson 1842) (Rodentia: Murinae), usually referred to as the unstriped grass rat, is mainly distributed in savanna and grassland habitats of Sub-Saharan Africa. Among the four chromosomal forms of Arvicanthis recently differentiated in Western and Central Africa, the one with a diploid chromosomal number (2n) of 62 and an autosomal fundamental number (NFa) of 62 or 64 is ascribed to Arvicanthis niloticus (Demarest 1822), while the one with 2n = 62 and a NFa between 74 and 76 is referred to A. ansorgei (Thomas 1910). Despite the broad area of sympatry recently uncovered along the inner delta of the Niger river in Mali [details in Volobouev et al. (2002) Cytogenetics and Genome Research, 96, 250-260], the distribution of the two species is largely parapatric and follows the latitudinal patterns of the West-African biogeographical domains, which are related to the latitudinal patterns of annual rainfall in this region. Here, we analyse the suggestion that the two species show specific adaptations to differences in climate aridity.

Methods Karyologically screened animals were sampled in 19 localities in seasonally flooded regions located along the ’Niger’ river in Mali and extending from 1100 to 200 mm of mean annual rainfall. The analysis of trapping success (TS) data allowed us to investigate the respective effects of climate (i.e. annual rainfall) and local (i.e. duration of the green herbaceous vegetation) aridity on the latitudinal and habitat distribution of the two species.

Conclusions The broad zone of sympatry was found to correspond to a northward expansion of the recognized distribution area of A. ansorgei. TS values indicated that the two species responded very differently to climatic and local conditions of aridity. Arvicanthis ansorgei decreased in TS as regional conditions became more arid; a similar trend was also observed within regions where habitat occupancy decreased with local aridity. The higher TS observed in the most humid habitat relative to the others persisted throughout the latitudinal rainfall gradient. In contrast, TS of A. niloticus increased with latitudinal aridity. This species was present in more arid habitats than A. ansorgei from 1000 mm down to 400 mm of mean annual rainfall where a shift to the most humid habitat occurred. These opposite trends in TS distribution between species suggest that A. ansorgei is less adapted than A. niloticus to arid environments at both a regional and habitat level; thus, A. ansorgei would be able to invade dry regions only along the extensive floodplains bordering the inner delta of the ’Niger’ river. Several biological traits that may be involved in limiting the southward distribution of A. niloticus are discussed.