Menstruation (mammal)
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Lemur
Lemurs time their mating and birth seasons so that all weaning periods are synchronized to match the time of highest food availability. Weaning occurs either before or shortly after the eruption of the first permanent molars in lemurs. Mouse lemurs are able to fit their entire breeding cycle into the wet season, whereas larger lemurs, such as sifakas, must lactate for two months during the dry season. Infant survival in some species, such as Milne-Edwards' sifaka, has been shown to be directly impacted by both environmental conditions and the rank, age, and health of the mother. The breeding season is also affected by geographical location. For example, mouse lemurs give birth between September and October in their native habitat in the Southern Hemisphere, but from May through June in the captive settings in the Northern Hemisphere.
Scent factors heavily into lemur reproduction. Scent-marking activity escalates during the mating season. Pheromones may coordinate reproductive timing for females coming into estrus. Mating can be either monogamous or promiscuous for both males and females, and mating can include individuals from outside the group. Monogamous lemurs include the red-bellied lemur ("Eulemur rubriventer") and the mongoose lemur ("E. mongoz"), although the mongoose lemur has been observed mating outside of its pair bond. Monogamy is most common among nocturnal species, although some exhibit scramble competition, sexual suppression of subordinates, or competitions between males that avoid direct fighting. In mouse lemurs, males utilize sperm plugs, developed enlarged testes during the mating season, and develop size dimorphism (likely due to the enlarged testes). These indicate a mating system known as scramble competition polygyny, where males cannot defend females or the resources that might attract them.