Rapid maturation of the muscle biochemistry that supports diving in Pacific walruses (Odobenus rosmarus divergens)

Abstract

Physiological constraints dictate animals' ability to exploit habitats. For marine mammals, it is important to quantify physiological limits that influence diving and their ability to alter foraging behaviors. We characterized age-specific dive limits of walruses by measuring anaerobic (acid-buffering capacity) and aerobic (myoglobin content) capacities of the muscles that power hind (longissimus dorsi) and fore (supraspinatus) flipper propulsion. Mean buffering capacities were similar across muscles and age classes (a fetus, five neonatal calves, a 3 month old and 20 adults), ranging from 41.31 to 54.14 slykes and 42.00 to 46.93 slykes in the longissimus and supraspinatus, respectively. Mean myoglobin in the fetus and neonatal calves fell within a narrow range (longissimus: 0.92-1.68 g 100 g −1 wet muscle mass; supraspinatus: 0.88-1.64 g 100 g −1 wet muscle mass). By 3 months post-partum, myoglobin in the longissimus increased by 79%, but levels in the supraspinatus remained unaltered. From 3 months post-partum to adulthood, myoglobin increased by an additional 26% in the longissimus and increased by 126% in the supraspinatus; myoglobin remained greater in the longissimus compared with the supraspinatus. Walruses are unique among marine mammals because they are born with a mature muscle acid-buffering capacity and attain mature myoglobin content early in life. Despite rapid physiological development, small body size limits the diving capacity of immature walruses and extreme sexual dimorphism reduces the diving capacity of adult females compared with adult males. Thus, free-ranging immature walruses likely exhibit the shortest foraging dives while adult males are capable of the longest foraging dives.

Demography of the Pacific walrus (Odobenus rosmarus divergens) in a changing Arctic

“…Independent juvenile walruses consume similar prey as adults (Fay ), but have a lower breath‐hold capacity (Noren et al . ) and less experience foraging than do adult walruses, putting them at a competitive disadvantage. Yet it is not clear why reduced access to food would have resulted in decreased juvenile survival (but not decreased calf production) in recent years, when it is thought to be the mechanism that resulted in decreased calf production (but not decreased juvenile survival) in the early 1980s.…”

A strategy for recovering continuous behavioral telemetry data from Pacific walruses

“…Censuring spurious foraging records is problematic because foraging in deep water has been reported with mature male Atlantic walruses (Born and Knutsen , Freitas et al ) and mature adult Pacific walruses may be able to engage in deep forage diving (Noren et al ). Noren et al () modeled the physiological aerobic dive potential of Pacific walruses based on measures of dive‐muscle oxygen stores and found that Pacific walrus aerobic diving is limited to depths <355 m. Further, they found that at depths >200 m, adult female walruses have <4 min of bottom time, which is more than a 60% reduction of modeled aerobic dive bottom time relative to diving over typical continental‐shelf foraging regions (≤80 m; Fay , Jay et al ). Based on Noren et al (), spurious foraging records may be censured when they occur in waters >355 m deep, but remain plausible when they occur at intermediate depths.…”

“…Noren et al () modeled the physiological aerobic dive potential of Pacific walruses based on measures of dive‐muscle oxygen stores and found that Pacific walrus aerobic diving is limited to depths <355 m. Further, they found that at depths >200 m, adult female walruses have <4 min of bottom time, which is more than a 60% reduction of modeled aerobic dive bottom time relative to diving over typical continental‐shelf foraging regions (≤80 m; Fay , Jay et al ). Based on Noren et al (), spurious foraging records may be censured when they occur in waters >355 m deep, but remain plausible when they occur at intermediate depths. We selected the behavioral track of an adult female to illustrate the potential of spurious foraging records.…”

“…Argos ID 0% 0-5% 5-15% 15-25% 25-35% 35-45% 45-55% 55-65% 65-75% 75-85% 85-95% 95-100% 100% Knutsen 1992, Freitas et al 2009) and mature adult Pacific walruses may be able to engage in deep forage diving (Noren et al 2015). Noren et al (2015) modeled the physiological aerobic dive potential of Pacific walruses based on measures of dive-muscle oxygen stores and found that Pacific walrus aerobic diving is limited to depths <355 m. Further, they found that at depths >200 m, adult locations of 2 of these may be in error because of the sparse relocation records used to assign a location for the behavioral intervals. In the case of our example behavioral track, all foraging reports north of the continental shelf appear to be in waters >355 m and may be censured.…”

“…female walruses have <4 min of bottom time, which is more than a 60% reduction of modeled aerobic dive bottom time relative to diving over typical continental-shelf foraging regions ( 80 m;Fay 1982, Jay et al 2001). Based onNoren et al (2015), spurious foraging records may be censured when they occur in waters >355 m deep, but remain plausible when they occur at intermediate depths. We selected the behavioral track of an adult female to illustrate the potential of spurious foraging records.…”