Although hibernation and daily torpor occur in nearly all mammalian orders and all over the world (Lyman et al., 1982; Carey et al., 2003; Heldmaier et al., 2004), among primates the occurrence of torpid states is so far only known in cheirogaleids. It is interesting to see how the different Cheirogaleidae species of the Kirindy forest have evolved different ways to cope with the marked seasonality of their habitat. As shown above, C. medius is capable of prolonged hibernation over many months. Microcebus berthae, at 30 g the smallest known primate (Schmid and Kappeler, 1994), shows short bouts of metabolic depression over a few hours (daily torpor) during the cold nights of the dry season (Schmid et al., 2000). Presumably this species is too small to undergo prolonged hibernation, as body size limits the amount of body fat that can be accumulated to fuel hibernation. Microcebus murinus (60 g), the second species of the Microcebus genera in Kirindy forest, seems to show a mixed strategy of staying active, showing daily torpor or going into hibernation for several days, depending on ambient temperature, body weight, and sex (Schmid and Kappeler, 1998; Schmid, 1999). No physiological parameters are yet known of Mirza coquereli (300 g). But as this lemur is found curled up and cold to the touch in traps after cold nights of the dry season, there is no doubt that it is capable of showing torpid stages at least occasionally. Phaner furcifer (330 g), the last of the Cheirogaleidae family in the Kirindy forest, is found active throughout the night until dawn all year long (Hladik et al., 1980; Schulke and Kappeler, 2003); it is not assumed to show stages of hypometabolism. Indeed, this lemur has a very specialized diet, feeding mainly on tree exudates that are self-maintained, and is therefore largely independent of seasonal environmental changes.
All lemur species occurring sympatrically with C. medius on the west coast have to cope with the strong seasonality of their habitat. Even though hibernation seems to be a very elegant method to survive this period of scarcity, only C. medius exhibits obligate hibernation. Life history parameters such as body size and feeding ecology, as well as phylogenetic constraints, may explain the occurrence or absence of hibernation and daily torpor of the lemurs living in the dry deciduous forest of western Madagascar.
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