A feature of crocodilian physiology is that most of their strenuous activity is carried out anaerobically, and must be followed by a period of rest so that the "oxygen debt" can be repaid. Crocodilians can struggle furiously when they are caught, or when they catch prey, but they become totally exhausted very quickly. The anaerobic activity results in a build-up of lactic acid in the blood, making it acidic. Although crocodilians can withstand high levels of blood acidity, well above those of most other animals, it can also be fatal. When very large crocodiles (over 5 m) are caught, they struggle to such an extent that they surpass their ability to pay back the oxygen debt during rest periods. This feature is probably the reason why very large crocodiles often die during capture operations.