Influence of biological and environmental conditions on winter mortality risk of a northern ungulate: evidence for a late-winter survival bottleneck

1. A relationship between winter weather and survival of northern ungulates has long been established, yet the possible roles of biological (e.g., nutritional status) and environmental (e.g., weather) conditions make it important to determine which potential limiting factors are most influential.

2. Our objective was to examine the potential effects of individual (body mass and age) and extrinsic (winter severity and snow melt conditions) factors on the magnitude and timing of mortality for adult (>2.5 years old) female white-tailed deer (Odocoileus virginianus [Zimmerman, 1780] during February–May in the Upper Peninsula of Michigan, USA.

3. One hundred and fifty deer were captured and monitored during 2009–2015 in two areas with varying snowfall. February–May survival ranged from 0.24–0.89 (mean = 0.69) across years. Mortality risk increased 1.9% with each unit increase of cumulative winter severity index, decreased 8.2% with each cumulative snow free day, and decreased 4.3% with each kg increase in body mass. Age and weekly snow depth did not influence weekly deer survival. Predation, primarily from coyote (Canis latrans [Say, 1823]) and wolves (Canis lupus [L., 1758]), accounted for 78% of known-cause mortalities.

4. Our results suggest that cumulative winter severity, and possibly to a lesser degree deer condition entering winter, impacted deer winter survival. However, the timing of spring snow melt appeared to be the most influential factor determining late winter mortality of deer in our study. This supports the hypothesis that nutrition and energetic demands from weather conditions are both important to northern ungulate winter ecology. Under this model, a delay of several weeks in the timing of spring snow melt could exert a large influence on deer survival, resulting in a survival bottleneck.

1. 长期以来，冬季气候与北方有蹄类动物（ungulate）存活率之间的关联已得到广泛证实；但由于生物学（如营养状态）与环境（如气候）条件可能存在的潜在作用，明确其中影响最为显著的限制因子仍具有重要研究意义。
2. 本研究旨在探究个体层面（体重与年龄）以及外界因素（冬季严酷程度与融雪条件），对美国密歇根州上半岛2009-2015年2月至5月期间，成年（年龄大于2.5岁）雌性白尾鹿（Odocoileus virginianus [Zimmerman, 1780]）死亡率的强度与发生时间的潜在影响。
3. 2009-2015年间，研究人员在两个降雪量存在差异的区域共捕获并监测了150只白尾鹿。各年度2月至5月的存活率介于0.24至0.89之间（平均值为0.69）。随着累积冬季严酷度指数每提升1个单位，死亡风险上升1.9%；每增加1个累积无雪日数，死亡风险下降8.2%；而体重每增加1千克，死亡风险下降4.3%。年龄与周积雪深度对鹿群的周存活率无显著影响。已知死因的死亡个体中，78%为捕食所致，主要捕食者为郊狼（Canis latrans [Say, 1823]）与灰狼（Canis lupus [L., 1758]）。
4. 本研究结果表明，累积冬季严酷程度以及程度相对较轻的越冬前鹿群身体状况，均会对白尾鹿的冬季存活率产生影响。然而在本研究中，春季融雪时间似乎是决定鹿群晚冬死亡率的最关键因素。该结果支持了如下假说：营养状态与气候条件带来的能量需求，均对北方有蹄类动物的冬季生态学具有重要意义。基于该模型，春季融雪时间延迟数周可对鹿群存活率产生显著负面影响，进而引发存活瓶颈效应。