Great encounter rates of Ambarijeby mouse lemurs and Anjiamangirana sportive lemurs in the Anjajavy Private Reserve, northwestern Madagascar

Madagascar, famous for its unique biodiversity including the lemurs, comprises varied and heterogenous landscapes that establish regions of microendemism. In the northwest, rivers often delimit the geographic distribution of lemur species, including the mouse (Microcebus spp.) and sportive (Lepilemur spp.) lemurs. Within the Sofia Region, Inter-River System (IRS) III is the only home to the Ambarijeby mouse lemur (M. danfossi) and the Anjiamangirana sportive lemur (L. grewcockorum). In 2008, surveys of the nocturnal lemurs in IRS III highlighted Anjajavy as a promising site for these species, both of which were listed, at the time, as Data Deficient by the IUCN. Here, 15 years later, we conducted standard, nocturnal surveys at Anjajavy across the dry season to document encounter rates of mouse and sportive lemurs, with sporadic observations of sympatric dwarf lemurs (Cheirogaleus sp. cf. medius) in two areas of the Private Reserve. Overall, we encountered 17.6 mouse lemurs per kilometer, with significant variation by transect area and study month. We saw more mouse lemurs between August and October and a greater number of individuals per sighting in September, likely due to the onset of the reproductive season. We encountered 2.3 sportive lemurs per kilometer, with little variation by transect area or study month. A lack of dwarf lemur sightings from May through September loosely establishes an approx. 5-month hibernation season at this site. Compared to sportive and dwarf lemurs, we observed mouse lemurs at lower heights in the canopy, particularly when feeding. Our survey data of Ambarijeby mouse lemurs and Anjiamangirana sportive lemurs, now respectively listed as Vulnerable and Critically Endangered, reaffirms Anjajavy as a haven for the nocturnal lemurs of IRS III. We posit that Anjajavy is well positioned to become a regional hub of long-term lemur research and species safeguarding.