Lavin Shana R, Karasov William H, Ives Anthony R, Middleton Kevin M, Garland Theodore
Department of Wildlife Ecology, University of Wisconsin, Madison, Wisconsin 53706, USA.
Physiol Biochem Zool. 2008 Sep-Oct;81(5):526-50. doi: 10.1086/590395.
Flying animals may experience a selective constraint on gut volume because the energetic cost of flight increases and maneuverability decreases with greater digesta load. The small intestine is the primary site of absorption of most nutrients (e.g., carbohydrates, proteins, fat) in both birds and mammals. Therefore, we used a phylogenetically informed approach to compare small intestine morphometric measurements of birds with those of nonflying mammals and to test for effects of diet within each clade. We also compared the fit of nonphylogenetic and phylogenetic models to test for phylogenetic signal after accounting for effects of body mass, clade, and/or diet. We provide a new MATLAB program (Regressionv2.m) that facilitates a flexible model-fitting approach in comparative studies. As compared with nonflying mammals, birds had 51% less nominal small intestine surface area (area of a smooth bore tube) and 32% less volume. For animals <365 g in body mass, birds also had significantly shorter small intestines (20%-33% shorter, depending on body mass). Diet was also a significant factor explaining variation in small intestine nominal surface area of both birds and nonflying mammals, small intestine mass of mammals, and small intestine volume of both birds and nonflying mammals. On the basis of the phylogenetic trees used in our analyses, small intestine length and nominal surface area exhibited statistically significant phylogenetic signal in birds but not in mammals. Thus, for birds, related species tended to be similar in small intestine length and nominal surface area, even after accounting for relations with body mass and diet. A reduced small intestine in birds may decrease the capacity for breakdown and active absorption of nutrients. Birds do not seem to compensate for reduced digestive and absorptive capacity via a longer gut retention time of food, but we found some evidence that birds have an increased mucosal surface area via a greater villus area, although not enough to compensate for reduced nominal surface area. We predict that without increased rate of enzyme hydrolysis and/or mediated transport and without increased passive absorption of water-soluble nutrients, birds may operate with a reduced digestive capacity, compared with that of nonflying mammals, to meet an increase in metabolic needs (i.e., a reduced spare capacity).
飞行动物的肠道容积可能会受到选择性限制，因为随着消化物负荷的增加，飞行的能量消耗会增加，机动性会降低。小肠是鸟类和哺乳动物吸收大多数营养物质（如碳水化合物、蛋白质、脂肪）的主要部位。因此，我们采用系统发育信息方法，将鸟类的小肠形态测量值与非飞行哺乳动物的进行比较，并测试每个类群内饮食的影响。我们还比较了非系统发育模型和系统发育模型的拟合情况，以检验在考虑体重、类群和/或饮食影响后的系统发育信号。我们提供了一个新的MATLAB程序（Regressionv2.m），该程序有助于在比较研究中采用灵活的模型拟合方法。与非飞行哺乳动物相比，鸟类的名义小肠表面积（光滑内径管的面积）少51%，容积少32%。对于体重<365克的动物，鸟类的小肠也明显更短（短20%-33%，取决于体重）。饮食也是解释鸟类和非飞行哺乳动物小肠名义表面积、哺乳动物小肠质量以及鸟类和非飞行哺乳动物小肠容积变化的一个重要因素。根据我们分析中使用的系统发育树，小肠长度和名义表面积在鸟类中表现出统计学上显著的系统发育信号，而在哺乳动物中则没有。因此，对于鸟类来说，即使在考虑了与体重和饮食的关系之后，相关物种在小肠长度和名义表面积上往往也很相似。鸟类小肠的缩小可能会降低营养物质分解和主动吸收的能力。鸟类似乎没有通过延长食物在肠道中的停留时间来弥补消化和吸收能力的降低，但我们发现一些证据表明，鸟类通过更大的绒毛面积增加了粘膜表面积，尽管不足以弥补名义表面积的减少。我们预测，如果没有增加酶水解速率和/或介导转运速率，也没有增加水溶性营养物质的被动吸收，与非飞行哺乳动物相比，鸟类可能会以降低的消化能力来满足代谢需求的增加（即备用能力降低）。
