In marine mammals, muscular development has been identified as a rate-limiting factor in achieving adult dive capacities. This study investigates the rate that myosin heavy chain (MHC) composition matures in a postural and locomotor skeletal muscle for four pinniped species with different lactation lengths: hooded seals, Cystophora cristata; harp seals, Pagophilus groenlandicus; northern fur seals, Callorhinus ursinus, and Steller sea lions, Eumetopias jubatus. The ontogeny of MHC isoform expression was compared with developmental rates of myoglobin concentrations, and aerobic (citrate synthase, β-hydroxyacyl-CoA dehydrogenase) and anaerobic (lactate dehydrogenase) enzyme activities. Within taxonomic families, species with shorter lactation periods had more mature muscles biochemically at birth, and fiber types differentiated earlier during ontogeny (Phocidae: hooded > harp seals, Otariidae: northern fur seals > Steller sea lions). Northern fur seal neonates had the most phenotypically-mature muscles in this study, with no immature MHC isoforms. The relationship between muscle biochemistry and MHC composition became more pronounced with age, and developed to reflect swimming mode and activity levels. In adults, phocids had more slow-twitch oxidative protein in their primary locomotor muscle, the Longissimus dorsi (LD), than otariids which likely reflects oxygen-sparing strategies for the phocids’ longer dives. Conversely, northern fur seal muscles had higher proportions of fast-twitch MHCs in the Pectoralis and LD, likely indicative of this species’ smaller size and higher mass-specific metabolic rates. Thus, muscle phenotype is linked with species life history, and a mismatch between muscle biochemistry and MHC composition at weaning has important implications for the first year of independent foraging in pinniped pups.

中文翻译：

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肌肉表型和生活史之间的联系：肌球蛋白重链组成和肌肉生物化学在早熟和晚熟鳍足幼崽中的分化

在海洋哺乳动物中，肌肉发育已被确定为达到成人潜水能力的限速因素。本研究调查了四种具有不同泌乳长度的鳍足类动物的肌球蛋白重链 (MHC) 成分在姿势和运动骨骼肌中成熟的速率：罩盖海豹、Cystophora cristata；竖琴海豹，Pagophilus groenlandicus；北方海狗 Callorhinus ursinus 和 Steller 海狮 Eumetopias jubatus。将 MHC 同种型表达的个体发育与肌红蛋白浓度、需氧（柠檬酸合酶、β-羟酰基-CoA 脱氢酶）和厌氧（乳酸脱氢酶）酶活性的发育速率进行比较。在分类科中，泌乳期较短的物种在出生时具有更成熟的生化肌肉，和纤维类型在个体发育过程中早期分化（Phocidae：罩盖>竖琴海豹，Otariidae：北方海狗>Steller海狮）。在这项研究中，北方海狗新生儿的肌肉表型最成熟，没有未成熟的 MHC 亚型。肌肉生化和 MHC 组成之间的关系随着年龄的增长变得更加明显，并发展为反映游泳模式和活动水平。在成人中，phocids 在它们的初级运动肌肉背最长肌 (LD) 中具有更多的慢收缩氧化蛋白，而不是 otariids，这可能反映了 phocids 长时间潜水的氧气节约策略。相反，北方海狗的胸肌和 LD 中快肌 MHC 的比例更高，这可能表明该物种的体型较小，质量特异性代谢率较高。因此，