Dearing M D, Mangione A M, Karasov W H
Department of Wildlife Ecology, University of Wisconsin, Madison, WI 53706, USA, , , , , , US.
Oecologia. 2000 May;123(3):397-405. doi: 10.1007/s004420051027.
Two hypotheses, nutrient constraints and detoxification limitation, have been proposed to explain the lack of specialists among mammalian herbivores. The nutrient constraint hypothesis proposes that dietary specialization in mammalian herbivores is rare because no one plant can provide all requisite nutrients. The detoxification limitation hypothesis suggests that the mammalian detoxification system is incapable of detoxifying high doses of similar secondary compounds present in a diet of a single plant species. We experimentally tested these hypotheses by comparing the performance of specialist and generalist woodrats (Neotoma) on a variety of dietary challenges. Neotoma stephensi is a narrow dietary specialist with a single species, one-seeded juniper, Juniperus monosperma, comprising 85-95% of its diet. Compared with other plants available in the habitat, juniper is low in nitrogen and high in fiber, phenolics, and monoterpenes. The generalist woodrat, N. albigula, also consumes one-seeded juniper, but to a lesser degree. The nutrient constraint hypothesis was examined by feeding both species of woodrats a low-nitrogen, high-fiber diet similar to that found in juniper. We found no differences in body mass change, or apparent digestibility of dry matter or nitrogen between the two species of woodrats after 35 days on this diet. Moreover, both species were in positive nitrogen balance. We tested the detoxification limitation hypothesis by comparing the performance of the generalist and specialist on diets with and without juniper leaves, the preferred foliage of the specialist, as well as on diets with and without α-pinene, the predominant monoterpene in juniper. We found that on the juniper diet, compared with the specialist, the generalist consumed less juniper and lost more mass. Urine pH, a general indicator of overall detoxification processes, declined in both groups on the juniper diet. The generalist consumed half the toxin load of the specialist yet its urine pH was slightly lower. Moreover, the generalist consumed significantly less of the treatment with high concentrations of α-pinene compared to the control treatment, while the specialist consumed the same amount of food regardless of α-pinene concentration. For both groups, urine pH declined as levels of α-pinene in the diet increased. The generalist produced a significantly more acidic urine than the specialist on the treatment with the highest α-pinene concentration. Our results suggest that in this system, specialists detoxify plant secondary compounds differently than generalists and plant secondary compounds may be more important than low nutrient levels in maintaining dietary diversity in generalist herbivores.
有两种假说，即营养限制假说和解毒限制假说，被提出来解释食草哺乳动物中缺乏专食性动物这一现象。营养限制假说认为，食草哺乳动物的食性专业化很少见，因为没有一种植物能提供所有必需的营养物质。解毒限制假说则表明，哺乳动物的解毒系统无法对单一植物物种饮食中存在的高剂量类似次生化合物进行解毒。我们通过比较专食性和广食性林鼠（林鼠属）在各种饮食挑战下的表现，对这些假说进行了实验验证。斯蒂芬斯林鼠是一种食性狭窄的专食性动物，其饮食的85% - 95%由单一物种——单籽杜松（杜松）组成。与栖息地中其他可获得的植物相比，杜松的氮含量低，纤维、酚类和单萜含量高。广食性林鼠白喉林鼠也会食用单籽杜松，但食用量较少。通过给两种林鼠喂食与杜松中相似的低氮、高纤维饮食，对营养限制假说进行了检验。在这种饮食下35天后，我们发现两种林鼠在体重变化、干物质或氮的表观消化率方面没有差异。此外，两种林鼠都处于正氮平衡状态。我们通过比较广食性和专食性林鼠在含有和不含杜松叶（专食性林鼠偏好的叶子）的饮食以及含有和不含α - 蒎烯（杜松中主要的单萜）的饮食上的表现，对解毒限制假说进行了检验。我们发现，在杜松饮食中，与专食性林鼠相比，广食性林鼠食用的杜松较少，体重减轻更多。尿液pH值是整体解毒过程的一个一般指标，在杜松饮食组中，两组的尿液pH值都下降了。广食性林鼠摄入的毒素量是专食性林鼠的一半，但其尿液pH值略低。此外，与对照处理相比，广食性林鼠摄入的高浓度α - 蒎烯处理的食物量显著减少，而专食性林鼠无论α - 蒎烯浓度如何，食用的食物量相同。对于两组来说，随着饮食中α - 蒎烯水平的增加，尿液pH值下降。在α - 蒎烯浓度最高的处理中，广食性林鼠产生的尿液比专食性林鼠的尿液酸性明显更强。我们的结果表明，在这个系统中，专食性动物对植物次生化合物的解毒方式与广食性动物不同，并且在维持广食性食草动物的饮食多样性方面，植物次生化合物可能比低营养水平更重要。
