||In the Gray Mouse Lemur, a nocturnal Malagasy primate, nest sharing during the diurnal sleeping period provides significant thermoregulatory advantages. Using socially housed captive male mouse lemurs, we tested the hypothesis that males would present differential physiological parameters depending oil their nest sharing patterns at the beginning of the breeding season. Nest composition, and health status inferred from body weight, reproductive activity (testis width) and level of immune activity (lymphocyte number) were recorded at the beginning of the breeding season and after 4 weeks in newly formed 4 groups of 5 adult males. One nest was occupied by 3 males or more in 85% of the surveys, and 14 cases of males found alone in a nest were recorded, distributed over 65% of the surveys. Males found alone or in pairs lost significantly more weight after the 4 weeks of social grouping than males found in groups of 3 or more individuals in a nest. Additionally, males expelled from common nests had reduced immune activity prior to social grouping than nest associated males. Nest associated males had little change in their lymphocyte numbers after the 4 weeks of social grouping, whereas isolated males exhibited a pronounced increase in lymphocyte numbers. These results indicate that, in mouse lemurs, nest-sharing pattern at the beginning of the breeding season is a complex process, yielding diverse energetic and immunological consequences.