It is approximately invariant. See for example Joseph Bronizino's "The Biomedical Engineering Handbook: biomedical engineering fundamentals", section 17.4 "Comparative Analysis of the Mammalian Circulatory System"
—R P.S. So has the Google broken down in your corner of New Mexico? If you Google "the total number of heart beats in a mammal's lifetime is invariant" you get the above *as the very first link*. How much easier does the interweb need to be for members of FRIAM? ;-)
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