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Functional explanations for loud calling in nocturnal primates include territorial or sexual advertisement, maintenance of cohesiveness, and group coordination. It is generally accepted that loud calls of lesser galagos (genus Galago) are used for territorial advertisement and long-distance spacing. Field studies suggest that they are uttered at dusk and dawn, when the animals leave and reunite at their sleeping sites. However, empirical validation of these inferences is lacking. We conducted 16-night-long acoustic monitoring of a northern lesser galago (G. senegalensis) population in Senegal and quantified the occurrence of loud calls throughout the night. We hypothesized that significantly more of these calls would be emitted at dusk and dawn if they were used for territorial advertisement and long-distance spacing. This hypothesis was only partially supported, as we found an asymmetrical distribution of loud calls, which significantly increased only before and at dawn. The finding that the relatively early increase in vocal activity was not directly related to approaching and entering sleeping sites suggests that the northern lesser galagos' loud calls differ in function from reassembly calls described for other species of nocturnal primates. Furthermore, the early onset cannot be explained by changes in the intensity of sunlight, moonlight or starlight, which suggests that a different stimulus, most likely internal, elicits early-morning calling behavior in northern lesser galagos. [ABSTRACT FROM AUTHOR]