The influence of sulfur and hair growth on stable isotope diet estimates for grizzly bears

Stable isotope ratios of grizzly bear (Ursus arctos) guard hair collected from bears on the lower Stikine River, British Columbia (BC) were analyzed to: 1) test whether measuring δ34S values improved the precision of the salmon (Oncorhynchus spp.) diet fraction estimate relative to δ15N as is conventionally done, 2) investigate whether measuring δ34S values improves the separation of diet contributions of moose (Alces alces), marmot (Marmota caligata), and mountain goat (Oreamnos americanus) and, 3) examine the relationship between collection date and length of hair and stable isotope values. Variation in isotope signatures among hair samples from the same bear and year were not trivial. The addition of δ34S values to mixing models used to estimate diet fractions generated small improvement in the precision of salmon and terrestrial prey diet fractions. Although the δ34S value for salmon is precise and appears general among species and areas, sulfur ratios were strongly correlated with nitrogen ratios and therefore added little new information to the mixing model regarding the consumption of salmon. Mean δ34S values for the three terrestrial herbivores of interest were similar and imprecise, so these data also added little new information to the mixing model. The addition of sulfur data did confirm that at least some bears in this system ate marmots during summer and fall. We show that there are bears with short hair that assimilate >20% salmon in their diet and bears with longer hair that eat no salmon living within a few kilometers of one another in a coastal ecosystem. Grizzly bears are thought to re-grow hair between June and October however our analysis of sectioned hair suggested at least some hairs begin growing in July or August, not June and, that hair of wild bears may grow faster than observed in captive bears. Our hair samples may have been from the year of sampling or the previous year because samples were collected in summer when bears were growing new hair. The salmon diet fraction increased with later hair collection dates, as expected if samples were from the year of sampling because salmon began to arrive in mid-summer. Bears that ate salmon had shorter hair and δ15N and δ34S values declined with hair length, also suggesting some hair samples were grown the year of sampling. To be sure to capture an entire hair growth period, samples must be collected in late fall. Early spring samples are also likely to be from the previous year but the date when hair begins to grow appears to vary. Choosing the longest hair available should increase the chance the hair was grown during the previous year and, maximize the period for which diet is measured.

对采集自不列颠哥伦比亚省（British Columbia，简称BC）斯蒂金河下游灰熊（Ursus arctos）的护毛（guard hair）开展稳定同位素比值（stable isotope ratios）分析，本研究旨在达成三项核心目标：其一，验证相较于常规采用的δ¹⁵N，测定δ³⁴S值是否可提升鲑鱼（Oncorhynchus spp.）饮食占比估算的精度；其二，探究测定δ³⁴S值能否改善驼鹿（Alces alces）、旱獭（Marmota caligata）与雪羊（Oreamnos americanus）的饮食贡献区分度；其三，分析采样日期、毛发长度与稳定同位素值之间的关联。

同一只熊同一年份的毛发样本间，同位素特征的差异并非微不足道。将δ³⁴S值纳入用于估算饮食占比的混合模型（mixing models）后，鲑鱼与陆生猎物（terrestrial prey）的饮食占比估算精度仅得到小幅提升。尽管鲑鱼的δ³⁴S值精准且在不同物种与区域间表现出通用性，但硫同位素比值与氮同位素比值存在强相关性，因此在针对鲑鱼摄食占比的混合模型中并未新增多少有效信息。本次研究关注的三种陆生植食动物，其平均δ³⁴S值相近且精度不足，因此此类数据同样未给混合模型带来多少新信息。不过，硫同位素数据确实证实了该研究区域内至少有部分灰熊在夏秋季以旱獭为食。

本研究发现，在同一片沿海生态系统中，相距仅数公里的灰熊种群存在两类截然不同的取食策略：一类护毛较短，其饮食中鲑鱼占比超过20%；另一类护毛较长，完全不摄食鲑鱼。尽管学界普遍认为灰熊的护毛生长期为6月至10月，但我们对分段毛发的分析显示，至少部分个体的毛发始于7月或8月生长，而非6月，且野生灰熊的毛发生长速度可能快于圈养个体。

由于样本采集于灰熊新生毛发生长的夏季，因此本次采集的护毛样本可能来自采样当年或前一年。若样本来自采样当年，那么鲑鱼饮食占比会随采样日期延后而升高，这符合预期，因为鲑鱼于仲夏时节洄游抵达该区域。摄食鲑鱼的灰熊护毛更短，且δ¹⁵N与δ³⁴S值随毛发长度增加而降低，这也佐证了部分样本确实生长于采样当年。若要完整覆盖毛发生长周期，样本需于晚秋时节采集。早春采集的样本同样可能来自前一年，但毛发开始生长的日期存在个体差异。选取可获得的最长护毛，可提升样本来自前一年的概率，同时最大化饮食记录的覆盖时长。