The small intestine of mammals and birds exhibits fascinating variation across taxa, body size, and life history features such as locomotion and diet. In the intestine's brush border membrane (BBM), hydrolases are more abundant than transporters in both mammals and birds, but there are differences among the groups in abundance of certain hydrolases and possibly in transporters. For example, mammals express two α-glucosidases, sucrase-isomaltase (SI) and maltase glucoamylase (MGAM), whereas songbirds we studied have only SI, and the chicken expresses SI plus another α-glucosidase that functions similarly to MGAM but is not a true ortholog. For intestinal absorption of sugars and amino acids, small fliers rely on a paracellular pathway to a greater extent than do nonflying mammals, which rely more on transporters. Possibly having evolved in fliers as compensation for lower intestinal nominal surface area (NSA), the fliers' reliance on paracellular absorption is supported by their greater villous surface enlargement that leads to more (per cm² NSA) tight junctions and greater clearance of passively absorbed compounds. To match digestive capacity to nutrient load, a positive relationship is often observed between dietary intake of macronutrients and intestinal activity of the enzymes and transporters of their respective constituents. In enterocytes, rapid, fine-tuned adjustment to high dietary carbohydrate and protein involves rapid, specific correlated increase in activity and abundance of hydrolases and transporters in the BBM and increases in their mRNA.

哺乳动物和鸟类的小肠在分类群、体型和生活史特征（如运动和饮食）方面表现出迷人的变化。在肠道刷状缘膜 (BBM) 中，哺乳动物和鸟类中的水解酶比转运蛋白更丰富，但在某些水解酶和转运蛋白的丰度方面，各组之间存在差异。例如，哺乳动物表达两种 α-葡糖苷酶，蔗糖酶-异麦芽糖酶 ( SI ) 和麦芽糖酶葡糖淀粉酶 ( MGAM )，而我们研究的鸣禽只有SI，而鸡表达SI加上另一种功能类似于MGAM 的α-葡糖苷酶但不是真正的直系同源。对于糖和氨基酸的肠道吸收，与非飞行哺乳动物相比，小型飞行器在更大程度上依赖细胞旁通路，后者更多地依赖转运蛋白。可能是在传单中进化作为对较低肠道标称表面积 (NSA) 的补偿，传单对细胞旁吸收的依赖得到了它们更大的绒毛表面扩大的支持，这导致更多（每厘米²NSA) 紧密连接和更大程度地清除被动吸收的化合物。为了使消化能力与营养负荷相匹配，经常观察到宏量营养素的膳食摄入量与酶的肠道活性和其各自成分的转运蛋白之间的正相关关系。在肠细胞中，对高膳食碳水化合物和蛋白质的快速、微调调整涉及 BBM 中水解酶和转运蛋白的活性和丰度的快速、特异性相关增加，以及它们的 mRNA 增加。