In previous Today In Mice posts, we've talked about teenage rats being 28 days old - the equivalent of human adolescence. Why do rats live faster and die younger than humans? NYU dental professor Timothy Bromage has discovered part of the reason while studying teeth.

The so-called biological clock controls many metabolic functions and is based on the circadian rhythm, a roughly 24-hour cycle that has been deemed crucial in determining patterns of sleeping and feeding, cell regeneration, and the body's biological processes. The newly discovered rhythm, like the circadian, originates in the hypothalamus, the part of the brain that controls the autonomic nervous system.

But unlike the circadian rhythm, this clock varies from one animal to another; in small mammals, the clock operates on shorter time intervals, and in larger animals, it ticks in larger intervals. Rats, for example, have a one-day interval; chimpanzees have six, and humans eight.

Bromage discovered the rhythm while studying incremental growth lines in tooth enamel, which serve a similar function to the annual rings on a tree. He also observed a related pattern of incremental growth in skeletal bone tissue - the first time this incremental rhythm has been observed in bone.

While reporting his findings during the 37th Annual Meeting of the American Association for Dental Research, Bromage said, "The same biological rhythm that controls incremental tooth and bone growth also affects bone and body size and many metabolic processes, including heart and respiration rates. In fact, the rhythm affects an organism's overall pace of life, and its life span. So, a rat that grows teeth and bone in one-eighth the time of a human also lives faster and dies younger."

Humans have the most variation in these long-term incremental growth rhythms, with some people clocking as few as five days, and others as many as 10. Future research will be needed to determine whether growth rhythms can be linked to variations in human behavior.