Reptiles have metabolic rates which are an order of magnitude lower than similar-sized mammals on the same diet; their rates of feeding are some ten times lower, as is the transit time of food through their gut. They have equal or lower extraction efficiencies, so their intestines absorb nutrients ten times more slowly than do those of mammals with their constant high temperatures and the larger surface area of their longer intestines (Diamond 1987). On top of all this, young reptiles start their existence with several other disadvantages compared with young mammals. While they will normally have a small reserve of yolk to supplement their nutrition for a short period, they are not fed on milk or helped in any other way by their parents. From the first moment they are on their own and must subsist as best they can with what food they can find for themselves. Furthermore, they are usually smaller as a fraction of their final adult size, so must ingest and assimilate nitrogen at a higher rate than adults just for daily maintenance, as well as for the higher additional requirement for rapid growth. If they are herbivorous the whole situation is exacerbated. Not surprisingly, therefore, most reptiles are carnivorous, or, at best, omnivorous. Very few are truly vegetarian in that at no stage of their life do they eat animal protein. There are a few apparently truly herbivorous turtles and lizards, but all depend on symbiotic microorganisms and an array of behavioural and physiological adaptations to gain enough nitrogen from their diet. Even so, the success of their reproduction — and thus of their abundance — is still precariously balanced on the edge of a varying insufficiency of protein.
KeywordsBiomass Cellulose Fermentation Assimilation Microbe
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