Molting strategies of Arctic seals drive annual patterns in metabolism

Abstract

Arctic seals, including spotted (Phoca largha), ringed (Pusa hispida) and bearded (Erignathus barbatus) seals, are directly affected by sea ice loss. These species use sea ice as a haul-out substrate for various critical functions, including their annual molt. Continued environmental warming will inevitably alter the routine behavior and overall energy budgets of Arctic seals, but it is difficult to quantify these impacts as their metabolic requirements are not well known—due in part to the difficulty of studying wild individuals. Thus, data pertaining to species-specific energy demands are urgently needed to better understand the physiological consequences of rapid environmental change. We used open-flow respirometry over a four-year period to track fine-scale, longitudinal changes in the resting metabolic rate (RMR) of four spotted seals, three ringed seals and one bearded seal trained to participate in research. Simultaneously, we collected complementary physiological and environmental data. Species-specific metabolic demands followed expected patterns based on body size, with the largest species, the bearded seal, exhibiting the highest absolute RMR (0.48 ± 0.04 L O2 min−1) and the lowest mass-specific RMR (4.10 ± 0.47 ml O2 min−1 kg−1), followed by spotted (absolute: 0.33 ± 0.07 L O2 min−1; mass-specific: 6.13 ± 0.73 ml O2 min−1 kg−1) and ringed (absolute: 0.20 ± 0.04 L O2 min−1; mass-specific: 7.01 ± 1.38 ml O2 min−1 kg−1) seals. Further, we observed clear and consistent annual patterns in RMR that related to the distinct molting strategies of each species. For species that molted over relatively short intervals—spotted (33 ± 4 days) and ringed (28 ± 6 days) seals—metabolic demands increased markedly in association with molt. In contrast, the bearded seal exhibited a prolonged molting strategy (119 ± 2 days), which appeared to limit the overall cost of molting as indicated by a relatively stable annual RMR. These findings highlight energetic trade-offs associated with different molting strategies and provide quantitative data that can be used to assess species-specific vulnerabilities to changing conditions.

Molting phenology of a lacustrine ringed seal, Pusa hispida saimensis

“…However, if molting was studied on an individual level, the duration would probably be longer. Thometz et al (2021) reported that the molting process of ringed seal individuals may last, on average, for 28 days. Several other studies have also reported both individual and inter‐annual differences in the timing of the molt for phocids (Ashwell‐Erickson et al, 1986; Kelly & Quakenbush, 1990; Krafft et al, 2006).…”

“…However, if molting was studied on an individual level, the duration would probably be longer. Thometz et al (2021) reported that the molting process of ringed seal individuals may last, on average, for 28 days. Several…”

Variation in body mass and food intake of northern fur seals (Callorhinus ursinus)

“…There is some support for the other two explanations. For example, resting metabolic rates of many pinnipeds are elevated during their annual molt (Ladds et al, 2017; Thometz et al, 2021; Williams et al, 2007), and elevated resting and field metabolic rates have been detected in northern fur seals during the fall (Dalton et al, 2015; McHuron et al, 2019) when much of the molt occurs (Scheffer & Johnson, 1963). There also is some evidence to suggest that digestive efficiencies may change with meal size or energy ingested (Lawson et al, 1997), although digestive efficiency in northern fur seals was negatively related to meal size (Gomez et al, 2016).…”

“…Animals managed in human care (captivity) are a valuable resource to obtain basic physiological data, as they often retain similar physiological patterns despite living in physical and social environments that may be different from their natural habitats. Captive animals have been used to describe hormonal changes, food intake rates, digestive efficiency, changes in mass and body composition, and energy costs of many marine mammals (e.g., Gomez et al, 2016; Larson et al, 2003; Pagano et al, 2018; Thometz et al, 2021; Williams et al, 2007). Such data can then be applied to wild populations to understand, among other things, how individuals and populations are likely to interact with prey resources, human activities (e.g., fisheries, offshore wind farms), and the potential impact of a rapidly changing climate on population dynamics (McHuron et al, 2020; Noren et al, 2014; Pagano & Williams, 2019; Villegas‐Amtmann et al, 2015).…”

Modeling vital rates and age-sex structure of Pacific arctic phocids: influence on aerial survey correction factors

“…Bearded seal males also exhibit polygynous behavior and defend relatively stable territories that are advertised to females via underwater vocalization (Van Parijs and Clark, 2006); we might, then, expect adult females to spend considerably more time hauled out than adult males during pupping season, but for adults to be concentrated near sea-ice haul-out locations. By contrast, subadults of all species have no reproductive constraints, though all age classes use sea ice as a platform to undergo an annual spring molt that ranges from ≈30 days in spotted and ringed seals to ≈120 days for bearded seals (Thometz et al, 2021).…”