Stephenson Thomas R, German David W, Cassirer E Frances, Walsh Daniel P, Blum Marcus E, Cox Mike, Stewart Kelley M, Monteith Kevin L
Sierra Nevada Bighorn Sheep Recovery Program, California Department of Fish and Wildlife, Bishop, CA, USA.
Idaho Department of Fish and Game, Lewiston, ID, USA.
J Mammal. 2020 Aug 17;101(5):1244-1256. doi: 10.1093/jmammal/gyaa091. eCollection 2020 Oct 5.
Bighorn sheep () can live in extremely harsh environments and subsist on submaintenance diets for much of the year. Under these conditions, energy stored as body fat serves as an essential reserve for supplementing dietary intake to meet metabolic demands of survival and reproduction. We developed equations to predict ingesta-free body fat in bighorn sheep using ultrasonography and condition scores in vivo and carcass measurements postmortem. We then used in vivo equations to investigate the relationships between body fat, pregnancy, overwinter survival, and population growth in free-ranging bighorn sheep in California and Nevada. Among 11 subpopulations that included alpine winter residents and migrants, mean ingesta-free body fat of lactating adult females during autumn ranged between 8.8% and 15.0%; mean body fat for nonlactating females ranged from 16.4% to 20.9%. In adult females, ingesta-free body fat > 7.7% during January (early in the second trimester) corresponded with a > 90% probability of pregnancy and ingesta-free body fat > 13.5% during autumn yielded a probability of overwinter survival > 90%. Mean ingesta-free body fat of lactating females in autumn was positively associated with finite rate of population increase (λ) over the subsequent year in bighorn sheep subpopulations that wintered in alpine landscapes. Bighorn sheep with ingesta-free body fat of 26% in autumn and living in alpine environments possess energy reserves sufficient to meet resting metabolism for 83 days on fat reserves alone. We demonstrated that nutritional condition can be a pervasive mechanism underlying demography in bighorn sheep and characterizes the nutritional value of their occupied ranges. Mountain sheep are capital survivors in addition to being capital breeders, and because they inhabit landscapes with extreme seasonal forage scarcity, they also can be fat reserve obligates. Quantifying nutritional condition is essential for understanding the quality of habitats, how it underpins demography, and the proximity of a population to a nutritional threshold.
大角羊（）能够生活在极其恶劣的环境中，一年中的大部分时间都以维持生存所需以下的食物为生。在这些条件下，储存为体脂的能量成为一种重要储备，用于补充饮食摄入，以满足生存和繁殖的代谢需求。我们利用超声检查、活体状态评分以及死后胴体测量数据，建立了预测大角羊无食糜体脂的方程。然后，我们使用活体方程来研究加利福尼亚州和内华达州自由放养的大角羊的体脂、怀孕、越冬存活和种群增长之间的关系。在包括高山冬季栖息地居民和迁徙者的11个亚种群中，秋季哺乳期成年雌性的平均无食糜体脂在8.8%至15.0%之间；非哺乳期雌性的平均体脂在16.4%至20.9%之间。在成年雌性中，1月份（妊娠中期早期）无食糜体脂>7.7%时，怀孕概率>90%，秋季无食糜体脂>13.5%时，越冬存活概率>90%。在高山地区越冬的大角羊亚种群中，秋季哺乳期雌性的平均无食糜体脂与次年种群的有限增长率（λ）呈正相关。秋季无食糜体脂为26%且生活在高山环境中的大角羊仅靠脂肪储备就拥有足以维持83天静息代谢的能量储备。我们证明，营养状况可能是大角羊种群统计学背后的一个普遍机制，并表征了它们所占据栖息地的营养价值。除了是资本繁殖者外，山地羊还是资本存活者，并且由于它们栖息在季节性草料极度匮乏的地区，它们也是脂肪储备依赖者。量化营养状况对于理解栖息地质量、它如何支撑种群统计学以及种群与营养阈值的接近程度至关重要。
