The Beat Goes On -- but not indefinitely

An interesting story on NPR yesterday, highlighting the work of a scientist from the Santa Fe Institute who has postulated a mathematical formula behind lifespan. The basic question addressed is why some species live longer than others. Why do elephants walk the earth for six or seven decades, while mice (of whom elephants are supposed to be afraid) kak out after only two or three years?

The answer seems to be in heartbeats. Smaller animals tend to have much faster heart rates than do larger mammals. But what's really interesting is that every species (save humans, who -- through medicine and hygiene -- have expanded our life spans) seems to get 1.5 billion heartbeats. The mice, whose hearts beat so much faster, use up their allotment in a few years, while elephants and whales spread their 1.5 billion heartbeat ration over decades.