Individuals of the same species may look the same, but they are genetically somewhat different. Different species may look different, but share many common features. The genetic difference between a chicken and a pig is roughly 25%, which may seem considerable. However, it often reflects genetic differences to ensure that the same function can be realized in a (somewhat) different way. As animal nutritionists, we often focus on the differences between species and ignore (or: take for granted) the commonalities. For example, arginine is an essential amino acid for birds but not for mammals because birds lack an enzyme to enter carbamoyl-phosphate into the urea cycle. However, birds possess the other steps of the urea cycle allowing them to synthesize arginine from citrulline, and to catabolize arginine to proline or glutamate. This single missing step causes birds to excess nitrogen via the uric acid cycle rather than via the urea cycle, as in mammals. Mammals also use the uric acid cycle to metabolize purines (e.g., adenine), which is a building block of DNA and the core of ATP. Both species thus use (parts of) both cycles. The cost of synthesizing and excreting excess nitrogen is approximately 40.3 kJ/g N for urea and 60.7 kJ/g N for uric acid. In both cases, 56% is retained in excretion product and 44% is lost as heat with implications for the protein value in energy systems. A better understanding of the metabolic commonalities between species may help us in appreciating the practical differences.