Behavioral traits can be used in order to reconstruct evolutionary scenarios and traits of common ancestors of species groups (e.g., Plavcan, 1999; Nunn and van Schaik, 2002). I will apply this principle to mouse lemur biology in order to detect universal traits in the different habitats of Madagascar that might allow us to learn about their common ancestor.
On the basis of our present knowledge, seasonal reproduction in mouse lemurs appears to be universal. The exact mechanism of reproductive activation seems to vary between different species (photoperiod, body condition, ambient temperatures), but gestation periods and births are not equally likely over the year. Mouse lemurs generally seem to produce litters of one to three offspring and more than one litter per year can be produced if the habitat is favorable. Seasonal reproduction with the potential for several successive litters per season and a litter size of about one to three young therefore seem to be the likely ancestral conditions for mouse lemurs. Such relatively high reproductive potential would have helped to establish populations during their radiation into new habitats.
Daily torpor is common in all mouse lemur species studied so far and may also be an ancient trait that evolved either in the common ancestor or even earlier in primate evolution. Prolonged seasonal torpor, on the other hand, has not been observed in all species and seems to occur only in harsh environments (low ambient temperatures) and in individuals/species with relatively high body mass (above 50 g; Schmid and Kappeler, 1998; Randrianambinina et al., 2003b). Therefore, the presence of this trait in the common ancestor would likely have been tied to its body mass. If it was small such as M. berthae (~ 30 g; Schwab, 2000), it is unlikely that it could have used prolonged seasonal torpor. If it was the size of M. murinus (~ 60 g; Zimmermann et al., 1998), prolonged seasonal torpor was more likely to occur. It has been suggested that the ancestral lemur crossed the Mozambique Channel on floating pieces of vegetation, surviving this period of presumably low food availability through prolonged periods of hibernation (Kappeler, 2000). This is a possible scenario only for an ancestor of moderate size and this would mean that M. berthae has decreased secondarily in size throughout its evolutionary history. Such secondary dwarfism is known from another branch of the primate phylogeny (callitrichids: Leutenegger, 1980; Martin, 1992) but its relevance in the lemur radiation has still to be verified.
Ancestral mouse lemurs were most likely omnivorous, since this mode of feeding can be found in all species studied today. Seasonally varying food availability characterizes all Malagasy ecosystems (e.g., Wright, 1999), therefore, a large variety of potential food sources could have been used: fruits, gum, insects, insect secretions, leaves, flowers, nectar, arthropods, and small vertebrates. Such flexibility allowed the quick colonization of new habitats in Madagascar and may have also facilitated survival during previous times of passage.
All mouse lemur species are arboreal, solitary foragers which scatter themselves spatially during their nocturnal activities. Nevertheless, they all show large degrees of home-range overlap, enabling individuals to regularly interact during their nocturnal activities. On the basis of current knowledge it is not possible to decide whether the berthae type (solitary sleeping mode), the murinus type (stable matrilinear female sleeping groups), or the ravelobensis type (stable mixed-sex sleeping groups) most closely reflects the ancestral condition for mouse lemurs. However, within this genus we see different types of sociality that may well serve as an interesting and suitable model for the evolution of sociality in primates, although these associations are continuously threatened by an enormous predation rate (Goodman et al., 1993) leading to the highest turnover rates known for primate populations (Cheney and Wrangham, 1987; Hill and Dunbar, 1998).
All described mouse lemur species live in a multimale/multi female mating system where monopolization of estrous females is not complete. Sperm competition seems to play a major role, but female interests should also shape the reproductive outcome considerably, since female dominance can be assumed to be an ancestral lemur trait (Radespiel and Zimmermann, 2001). Such a polygamous mating system could also be expected for the ancestral mouse lemur and perhaps even in earlier primates (Müller and Thalmann, 2000).
The comparison of interspecific similarities thus allows us to draw some conclusions about the ancestral mouse lemur condition. Whether this complex of traits may even hold for the ancestral lemur or the ancestral primate condition could only be revealed by further comparative studies (e.g., using small nocturnal galagos, lorises, and small mammalian nonprimate species as outgroups). These comparisons, however, lie beyond the scope of this present overview.
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