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First published online August 8, 2003
Inside JEB |
OLD BONES DON'T BUILD
kathryn{at}biologists.com
Daniel Lieberman admits that he's not your run-of-the-mill JEB author; he's a biological anthropologist, which makes his work with sheep even more intriguing. What have ancient hominids and modern day ungulates got in common? Not much. But it seems that sheep bones could have a thing or two to teach us about our ancestor's skeletons. Lieberman explains that many anthropological claims about early human activities are based on the assumption that bone structures are optimised for mechanical load bearing; thicker bones mean a harder life. But no one had ever tested whether real skeletons develop many of the features that anthropologists attribute to heavy labour, until Lieberman started putting sheep through their paces (p. 3125).
Most creatures' limbs taper toward the end, making them light to save energy while walking. `Thighs are thick and ankles are thin' says Lieberman. But no one knew for certain how the relatively slender bones in lower limbs respond to repeated stress. Conventional wisdom held that a bone that had been worked hard would be thicker. But would this wisdom stand up to closer scrutiny?
The team began working with sheep from three age groups, setting the animals a strict exercise regime; an hour treadmilltrotting every day, for 100 days. Lieberman remembers that the sheep were extremely cooperative, happily taking to the treadmill as soon as the pen's door was opened. At 30-day intervals, the sheep were injected with a fluorescent dye that was taken up by forming bone, so the team could monitor how the animal's skeleton had changed as they laboured on the treadmill.
When Lieberman looked at the lambs and pre-adult sheep, both group's bones had thickened, although the femurs had thickened more then the slender metatarsals. It was a different matter with the older animals; their bones hadn't built up at all! Although the bones were capable of rebuilding damage, no amount of exercise could stimulate the older bones to gain mass or increase the bone turnover rate.
But how much stress had the bones been under while the animals were working out? Lieberman and his colleagues fitted strain gauges around the tibia and metatarsal bones in the animal's legs, to measure the stress on the proximal and distal bones. While the relatively thick femur bones were strained a little as the animals trotted, the thinner metatarsal experienced enormous strains. Yet the metatarsal bones hardly thickened over the course of the experiment. Instead of growing, they increased their rate of boneturn over, while the tibia bones remodelled less and grew more.
Lieberman explains that `there is an energetic cost to adding mass' because it takes more energy to swing a bulky limb. He suspects that there is `a trade off between growth and repair' in juvenile bones, keeping the walls of proximal bones thin to save energy during locomotion, but at the risk of stress induced fractures.
So thick bones probably tell us less about the adult lives of our ancient ancestors than they do about their childhood antics; they just mean that the skeleton `took more loading as a kid' says Lieberman.
References
Lieberman, D. E., Pearson, O. M., Polk, J. D., Demes, B. and
Crompton, A. W. (2003). Optimization of bone growth and
remodeling in response to loading in tapered mammalian limbs. J.
Exp. Biol. 206,3125
-3138.
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