How Humans Outlive Rats

Humans are built to last. Rats, not so much. A newly discovered body clock apparently ticks much faster in rats, snuffing out their lives tens of years earlier than humans.

This biological rhythm, which tends to cut short the lives of smaller animals and let big creatures live longer, should apply to all mammals, said researcher Timothy Bromage, a dental professor at New York University.

Bromage discovered the rhythm while looking at growth lines in tooth enamel and skeletal bones in rats, medium-sized monkeys and humans.

Unlike circadian rhythms, which follow a relatively strict 24-hour cycle and coordinate sleep-wake stints, the new biological clock ticks to a different beat depending upon the animal. In general, the clock operates on shorter time intervals for small mammals and longer ones for larger animals. For rats, every day meant a new growth ring, while the monkeys followed a four-day interval and humans showed eight-day patterns.

The same biological rhythm that controls tooth and bone growth also determines body processes, such as heart and respiration rates, Bromage said.

"In fact, the rhythm affects an organism's overall pace of life, and its life span," Bromage said. "So, a rat that grows teeth and bone in one-eighth the time of a human also lives faster and dies younger."

Depending on your perspective, Bromage says, rats do live as long as humans. "For instance, a rat can expect the same number of heart beats in their lifetime as you and me," Bromage told LiveScience.

The findings, presented today at a meeting of the American Association for Dental Research in Dallas, make biological sense. "In broad strokes, larger bodies are produced by slowing down growth and developing for longer," Bromage said.

This slower growth also means larger mammals in general will reach sexual maturity later than the half-pints. "Lifespan has to be longer if a species has a later age at sexual maturity," Bromage said. "Otherwise too many individuals will die before they can replace themselves." 

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