A review of molt in mammals, with an emphasis on marmots (Rodentia: Sciuridae: Marmota)

Abstract

Molting is an evolutionarily ancient trait in which the outermost layer of an organism is replenished, usually according to a regular circannual rhythm. It is a metabolically costly process and, in vertebrates, is generally timed around other energetically demanding events such as reproduction and migration. In mammals, molting involves replacement of the fur coat—one of the most distinct innovations of the mammalian lineage. Despite the obvious importance of hair to mammalian fitness, our knowledge of hair growth cycles, circannual molting patterns, and hair structure remains largely restricted to marine and domesticated mammals, and our ability to identify explicit adaptive advantages of molting strategies in any mammal is therefore limited. In this review, we summarize what is known of these topics in wild, terrestrial mammals with a particular emphasis on marmots (Marmota spp.). Marmots are the largest extant ground squirrels and are well adapted to seasonally cold environments. Molting may be particularly relevant to fitness in marmots given the presumed importance of a healthy, insulative coat for metabolic efficiency in cold environments. Moreover, marmots hibernate for 7 to 8 months each year, meaning the annual molt and all other energetically demanding life-history events (such as parturition, lactation, fat accumulation, and dispersal) are constrained to an active period of only 4 to 5 months. Because the energetics of hibernation, fat accumulation, reproduction, and social behavior are already well studied, examining how molt is timed with respect to other important events and how it is influenced by local conditions may inform how molting is prioritized and how molting strategies evolve under specific selective pressures.