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Nobody knows, but I like to think that they're survivors, and are well conditioned to do well with the minimum.
In a lot of ways, they're the perfect predators. They kill, when they need to kill, and only when they need to, allowing the prey to thrive. Perfect ecological balance.
No data to back up my theory, just they way I like to see it.
you usually see fasting in desert species, were food and water availability is scarce. so I think its not so much fasting as is they just do not need to eat as much as tropical species. another way to look at it is that some are describe as pet rocks, which means they expelled less energy then say an arboreal would so they store up more energy for long hauls and thus need to eat less, but like kenny said nobody knows for sure.