Genetic analysis has revealed the existence of two new species of Madagascan mouse lemur, bringing the total number of recognised species to 20.

Weighing less than 100 g and rarely stretching more than 28 cm, tail included, mouse lemurs are the smallest primate in the world. Native to the forests of Madagascar, these strictly nocturnal omnivores come in either grey or a rich, rusty brown, and each one has a distinctive white splash running between the eyes and down the snout. Eighteen new mouse lemur species have been discovered since 1993, and their genus, Microcebus, boasts one of the highest species numbers of all primate groups. But because mouse lemur species all look so similar, distinguishing them can now only be done through genetic analysis.

During a field trip in December 2003, Rodin Rasoloarison from the University of Antananarivo in Madagascar caught two female mouse lemurs and one male in the Forêt de Marohita of the Toamasina province in eastern Madagascar. Four years later, in the Anosy region of south-eastern Madagascar, Rasoloarison caught six individuals in the Forêt de Manantantely and four more in the Forêt d’Ivorona. He performed a morphological analysis on the individuals, including weighing and measuring various external and internal parts, and fur colours were identified using colour charts. Tissue samples were then extracted and sent to Anne Yoder and Dave Weisrock at Duke University in Durham, North Carolina, for analysis.

Two mitochondrial and four nuclear loci were sequenced from the two new species, and then analysed alongside 279 mitochondrial and 209 nuclear sequences from different mouse lemur species found in 78 sites across Madagascar. The team used a technique called a Bayesian phylogenetic analysis for both the mitochondrial and nuclear loci, and a STRUCTURE analysis to infer the presence of distinct populations using multi-locus nuclear genotype data.