Introduction

The use of the small cheirogaleid, the gray mouse lemur (Microcebus murinus, which will be referred to as “mouse lemur” throughout this chapter), for laboratory studies of the brain and cognitive aging began in the early 1990s (Bons et al., 1991, 1994; Picq, 1992, 1993; Mestre and Bons, 1993). In the decades that followed, several research groups have become interested in M. murinus and carried out routine assessment of cognitive capacities on various behavioral tasks (see present review). Therefore, the mouse lemur represents an exception among the prosimians, which were largely ignored for a long time by researchers studying cognition. Indeed, when consulting a textbook, such as Primate Cognition (Tomasello and Call, 1997), the following type of remark is commonly encountered: “as usual, we know little about cognitive skills of prosimians …”. In fact, there was a longstanding lack of interest regarding the investigation of cognitive functions in prosimians, which resulted from two major ideas. First, prosimians were generally thought to be untrainable, as they were considered to be nervous, easily distracted, and non-cooperative (Wilkerson and Rumbaugh, 1979). Early attempts to work with these animals were very disappointing. For example, it was reported that “when removed from the home cage, lemurs grew hysterical, lorisoids sulked, and both refused to work” (Jolly, 1964b). Second, prosimians were believed to be less intelligent when compared to other primates. It was claimed that “Old and New World monkeys' intelligence outdistanced that of other mammals, including the prosimian primates” (Jolly, 1966). Two major explanations were proposed to explain the modest cognitive capacity of prosimians. First of all, it was suggested that they have found an ecological niche in which intelligence is not required (i.e., a niche with reduced opportunities for competition between species) (Andrew, 1962; Jolly, 1966). Second, they have been described as primitive, representing “surviving approximations of those ancestral primate forms from whose core components the higher primates evolved” (Wilkerson and Rumbaugh, 1979). Indeed, according to the view of Le Gros Clark (1959), the primate order is composed of a series of forms that are successive approximations of the evolutionary antecedents of man (characterized by high cognitive abilities), with prosimians at the bottom of the scale.