The "Rate of Living Hypothesis" enjoyed prominence as one of the foremost theories of aging for nearly 50 years. The "Rate of Living Hypothesis" is undermined by the observation that a rat and a bat have similar metabolic rate, but a bat lives several times longer. More recently, further doubts have been raised on the "Rate of Living Hypothesis" by the demonstration that, when modern statistical methods for correcting for the effects of body size and phylogeny are employed, metabolic rate does not correlate with longevity in mammals or birds. (For a critique of the "Rate of Living Hypothesis" see "Living fast, dying when?".)