METABOLISM
Reptiles have a much slower metabolism than mammals of similar size, with on average about one fifth to one seventh the metabolic rate at temperatures of 37° C (Bennett & Dawson 1976).
The metabolic rate is influenced by many factors: it increases exponentially with rise in body temperature and smaller reptiles have a faster metabolism than larger ones. It can also vary between species: for example, the tuatara has one of the lowest metabolic rates whereas some Varanid (e.g., Varanus gouldii) and Teiid lizards (e.g., Ameiva spp.) have higher metabolic rates than most other lizards (Bennett, AF 1972; Espinoza & Tracy 1997).Metabolism also depends on diet and predation behavior. Passive “sit and wait” reptiles, like pythons and boas, that wait to ambush their food as it passes by, have a much lower metabolism. In order to conserve energy their gastrointestinal tract shuts down in the months between each feed but they then experience a 7 to 17-fold (depending on species) increase in metabolic rate to help them digest their prey (Secor & Diamond 1995; Secor & Nagy 1994). Active predators, such as insectivorous lizards, that “seek and hunt” their prey, have higher metabolic rates and, as they feed daily, expend their energy maintaining their gut functions all the time (Secor & Diamond 1995; Secor & Nagy 1994).
Herbivorous species gain less energy from plant tissues and their digestive efficiency is 30-85% in contrast to 70-95% in carnivores. However, they spend a lot less time and energy acquiring their food than do the active foragers.
Anaerobic metabolism
Although aerobic metabolism utilizes energy more efficiently, the aerobic capacities of reptiles are much lower than endothermic mammal and birds. Reptiles switch to anaerobic metabolism for vigorous activities like diving, sprinting, chasing prey or escaping predation. This is independent of temperature but is a very high drain on energy reserves (up to 10 times).
During anaerobic exercise glycogen stored in the muscle is quickly broken down into lactate. As lactate is slow to be eliminated in reptiles they rapidly become fatigued and this is why reptiles can only sustain short bursts of intense activity. The increased lactate causes a drop in blood pH. This decreases the oxygen affinity of hemoglobin (Bohr effect) and consequently delays oxygen transport, further increasing the need for anaerobic metabolism (Bennett & Dawson 1976; Pough et al. 1998d).