In human-dominated landscapes throughout the world, wildlife seek out anthropogenic foods because they are high in nutritional value and are consistently available. To investigate this mode of foraging, some researchers use stable isotope analysis to detect these animals in populations and estimate their individual diets. In this study, we develop an integrative approach to measure the proportion of corn, a C₄ plant, in the diets of free-ranging mammalian omnivores in C₃-dominated ecosystems. We fed captive mice corn, C₃ plants, and meat until carbon stable isotopes (δ ¹³C) from each diet equilibrated in their hair. We then used carbon discrimination factors (Δ ¹³C; offsets between stable isotope values of consumer tissues and their foods) for mice from these feeding trials and a simple carbon stable isotope mixing model to estimate the corn-based diets of free-ranging American black bears in Wisconsin and brown bears in Slovenia. We used Δ ¹³C factors for mice to estimate the diets of bears because mouse models are used commonly to study mammalian diet and health, including humans and bears, and body mass has no effect on carbon discrimination factors in monogastric mammalian omnivores. In this study, we found that mice grew fastest, largest, and δ ¹³C values equilibrated quickest in the hair of mice fed meat versus plant-based diets, suggesting protein quantity (quality was the same) has an effect on Δ ¹³C. Evidence also suggests that Δ ¹³C did not increase with animal growth rate as all mice grew throughout the 109-day feeding trials, but isotopic equilibration occurred early while mice still were subadults and was maintained throughout their adult lives. We also found that Δ ¹³C was highest and most variable in the hair, serum, and liver, of mice fed a mixed diet of C₃ plants, supporting our mixed diet hypothesis that states that Δ ¹³C varies more among tissues of animals fed mixed diets than animals fed nonmixed diets because the former are composed of multiple foods, each with different macromolecular and isotopic compositions. Lastly, we found that corn may have been a more important component of bear diets in Wisconsin than previously thought (adults: $x¯$ = 29%; $x¯$ = 33%; subadults: $x¯$ = 22%; $x¯$ = 28%), and male brown bears may have fed on 50% more corn (⁠$x¯$ = 47% versus 31%) in autumn during a year when beechnut availability was low. In a world that is rapidly changing, it is more important than ever to develop the appropriate quantitative tools to measure the impact people have on wildlife. Here, we provide such a tool for monogastric mammalian omnivores and encourage other researchers to do the same for other taxa of interest.

中文翻译：

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测量玉米对哺乳动物杂食动物的影响

在世界各地以人为主导的景观中，野生动植物寻找人为食物，因为它们具有很高的营养价值并且可以持续获得。为了研究这种觅食方式，一些研究人员使用稳定同位素分析法来检测种群中的这些动物并估计它们的个体饮食。在这项研究中，我们开发了一种综合方法来测量以C ₃为主的生态系统中自由分布的杂食动物饮食中C ₄植物玉米的比例。我们给小鼠圈养玉米，C ₃植物，肉类，直到碳的稳定同位素（δ  ¹³从他们的头发平衡各饮食C）。然后，我们使用的碳判别因子（Δ  ¹³C; 这些喂养试验为小鼠提供的食用组织和食物的稳定同位素值之间的偏移量和一个简单的碳稳定同位素混合模型，用于估算威斯康星州自由放养的美国黑熊和斯洛文尼亚的棕熊的玉米饮食。我们用Δ  ¹³小鼠Ç因素来估算熊的饮食，因为小鼠模型被用于一般研究哺乳动物的饮食和健康，包括人类和熊，和体重对单胃哺乳动物杂食动物碳歧视的因素没有影响。在这项研究中，我们发现，小鼠增长最快，规模最大，δ  ^13个C值平衡的老鼠喂食肉类与蔬菜为主的饮食的头发最快，暗示蛋白量（质量是一样的）对Δ的影响 ¹³C.证据还表明，Δ  ¹³ Ç没有与动物的生长速度增加，因为所有小鼠长大整个109天的饲养试验，但同时仍小鼠分别为亚成体，并保持整个成年生活同位素平衡早期出现。我们还发现，Δ  ¹³只小鼠C组最高，头发，血清大多数变量，肝脏，喂℃的混合饲料₃植物，支持我们的混合饮食的假说，指出Δ  ¹³饲喂混合饮食的动物的组织中C的变化要比饲喂非混合饮食的动物的组织变化大，因为前者由多种食物组成，每种食物具有不同的大分子和同位素组成。最后，我们发现，威斯康星州的玉米饮食可能比以前认为的更重要的成分是玉米（成人：$X¯$ = 29％； $X¯$= 33％；亚成人：$X¯$ = 22％； $X¯$= 28％），而雄性棕熊则可能以多了50％的玉米为食（⁠$X¯$= 47％，而秋季则是在山毛榉供应量较低的一年中的31％。在这个瞬息万变的世界中，开发合适的定量工具来衡量人们对野生动植物的影响比以往任何时候都更为重要。在这里，我们为单胃哺乳动物杂食动物提供了这样的工具，并鼓励其他研究人员对其他感兴趣的类群也这样做。