Convergence of biannual moulting strategies across birds and mammals

Abstract

Birds and mammals have developed numerous strategies for replacing worn feathers and hair. Moulting usually occurs on an annual basis; however, moults that take place twice per year (biannual moults) also occur. Here, we review the forces driving the evolution of various moult strategies, focusing on the special case of the complete biannual moult as a convergence of selection pressures across birds and mammals. Current evidence suggests that harsh environmental conditions or seasonality (e.g. larger variation in temperatures) drive evolution of a biannual moult. In turn, the biannual moult can respond to secondary selection that results in phenotypic alteration such as colour changes for mate choice dynamics (sexual selection) or camouflage requirements (natural selection). We discuss the contributions of natural and sexual selection to the evolution of biannual moulting strategies in the contexts of energetics, niche selection, functionality and physiological mechanisms. Finally, we suggest that moult strategies are directly related to species niche because environmental attributes drive the utility (e.g. thermoregulation, camouflage, social dynamics) of the hair or feathers. Functional efficiency of moult may be undermined if the pace of evolution fails to match that of the changing climate. Thus, future research should seek to understand the plasticity of moult duration and phenology, especially in the context of annual cycles.

Adaptive plumage wear for increased crypsis in the plumage of Palearctic larks (Alaudidae)

“…All adult birds with new tertials had been collected in just four months: August to November. The latter is typical of seasonally homogeneous areas, whereas a biannual molt is typical of temperate latitudes with cold winters and hot summers: precisely the environmental conditions experienced by a majority of Palearctic larks (Beltran et al 2018). In fact, this wearing pattern was previously suggested in Cramp (1988), who described consistent differences in the freshly molted autumn feathers of all 21 lark species in the Western Palearctic compared to their worn spring plumage.…”

“…In fact, this wearing pattern was previously suggested in Cramp (1988), who described consistent differences in the freshly molted autumn feathers of all 21 lark species in the Western Palearctic compared to their worn spring plumage. Outside the Sylvioidea, the species known to have a single annual molt in the Palearctic are all the Sturnidae, the Passeridae, Miliaria calandra in Emberizidae (Svensson 1992), and Pyrrhula pyrrhula and Carduelis flammea cabaret in Fringillidae (Beltran et al 2018). By contrast, the non-lark passerines in our sample, including the closely related Bearded Reedlings, presented fresh and unworn tertial feathers across the year.…”

Getting ready for the winter: Timing and determinants of molt in an alpine ungulate

“…The review by Ling () also reported that malnutrition delayed molt in various wild mammals (e.g., black‐tailed deer, caribou ( Rangifer tarandus groenlandicus ), and elephant seal ( Mirounga leonida )). Moreover, temperature is another external cue that can interact with photoperiod to modulate the timing of molt (Beltran et al, ; Zimova et al, ). Colder temperatures can delay molt completion, a tendency we observed for mountain goats.…”

“…Pelage provides concealment, thermoregulation, and protection, and is therefore a key component for mammals facing harsh climatic conditions in northern environments (Caro, ; Cowan & Raddi, ; Zimova, Mills, Lukacs, & Mitchell, ; Zimova, Mills, & Nowak, ). Several species exposed to large temperature variations in their environment grow and molt one or two annual coats (one for summer and one for winter; Maurel, Coutant, Boissin‐Agasse, & Boissin, ; Beltran, Burns, & Breed, ). Growing and molting new hairs involve energetic costs (Boyd, Arnbom, & Fedak, ; Neuhaus, ), particularly after winter, a period associated with high energetic expenses and limited resources for herbivores (Stewart, Bowyer, Dick, Johnson, & Kie, ).…”

“…Molt timing has often been studied in birds (Bridge, ; Rohwer, Ricklefs, Rohwer, & Copple, ), but fewer studies have been done in mammalian species. As a result, we know much less about the factors driving molt timing in mammals (Beltran et al, ). Most studies on mammals assessed histological processes and provided descriptive analyses of hair follicle growth (Blix et al, ; Cuyler & Øritsland; Nixon, Gurnseyb, Betteridgec, Mitchellc, & Welchc, ).…”