Whiteman John P, Harlow Henry J, Durner George M, Regehr Eric V, Rourke Bryan C, Robles Manuel, Amstrup Steven C, Ben-David Merav
Program in Ecology, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA.
Department of Zoology and Physiology, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA.
Conserv Physiol. 2017 Aug 9;5(1):cox049. doi: 10.1093/conphys/cox049. eCollection 2017.
When reducing activity and using stored energy during seasonal food shortages, animals risk degradation of skeletal muscles, although some species avoid or minimize the resulting atrophy while experiencing these conditions during hibernation. Polar bears may be food deprived and relatively inactive during winter (when pregnant females hibernate and hunting success declines for other demographic groups) as well as summer (when sea ice retreats from key foraging habitats). We investigated muscle atrophy in samples of biceps femoris collected from free-ranging polar bears in the Southern Beaufort Sea (SBS) throughout their annual cycle. Atrophy was most pronounced in April-May as a result of food deprivation during the previous winter, with muscles exhibiting reduced protein concentration, increased water content, and lower creatine kinase mRNA. These animals increased feeding and activity in spring (when seal prey becomes more available), initiating a period of muscle recovery. During the following ice melt of late summer, ~30% of SBS bears abandon retreating sea ice for land; in August, these 'shore' bears exhibited no muscle atrophy, indicating that they had fully recovered from winter food deprivation. These individuals subsequently scavenged whale carcasses deposited by humans and by October, had retained good muscle condition. In contrast, ~70% of SBS bears follow the ice north in late summer, into deep water with less prey. These 'ice' bears fast; by October, they exhibited muscle protein loss and rapid changes in myosin heavy-chain isoforms in response to reduced activity. These findings indicate that, unlike other bears during winter hibernation, polar bears without food in summer cannot mitigate atrophy. Consequently, prolonged summer fasting resulting from climate change-induced ice loss creates a risk of greater muscle atrophy and reduced abilities to travel and hunt.
在季节性食物短缺期间减少活动并利用储存能量时，动物面临骨骼肌退化的风险，尽管一些物种在冬眠期间经历这些情况时能够避免或尽量减少由此产生的萎缩。北极熊在冬季（怀孕雌性冬眠，其他种群的捕猎成功率下降）以及夏季（海冰从关键觅食栖息地退缩）可能会面临食物匮乏且活动相对较少的情况。我们调查了从南波弗特海（SBS）自由放养的北极熊全年周期中采集的股二头肌样本中的肌肉萎缩情况。由于前一个冬季的食物匮乏，萎缩在4月至5月最为明显，肌肉表现出蛋白质浓度降低、水分含量增加以及肌酸激酶mRNA水平降低。这些动物在春季（海豹猎物变得更容易获得时）增加进食和活动，开始了肌肉恢复阶段。在随后的夏末融冰期，约30%的SBS北极熊放弃退缩的海冰前往陆地；8月，这些“岸上”的北极熊没有表现出肌肉萎缩，表明它们已从冬季食物匮乏中完全恢复。这些个体随后 scavenged 人类丢弃的鲸鱼尸体，到10月时，肌肉状况良好。相比之下，约70%的SBS北极熊在夏末跟随冰层向北进入猎物较少的深水区。这些“冰上”的北极熊禁食；到10月时，它们表现出肌肉蛋白质流失以及肌球蛋白重链异构体因活动减少而迅速变化。这些发现表明，与其他熊在冬季冬眠不同，北极熊在夏季没有食物时无法减轻萎缩。因此，气候变化导致海冰流失造成的夏季长期禁食会带来更大的肌肉萎缩风险以及旅行和捕猎能力下降的风险。 （注：scavenged 此处根据语境推测可能是“ scavenged （ scavenge 意为搜寻、 scavenged 此处可能是搜寻人类丢弃的鲸鱼尸体）”，但原词拼写有误，正确拼写应为 scavenge ）
