Nature does not "design" skeletons in the way that an engineer might design a bridge
That is, to build in
a substantial safety margin according to foreknowledge of the stresses to which
the structure might be subjected in its working lifetime. But blind evolution
produces essentially the same result. Any human or other animal species with weak
bones are more likely to suffer fractures.
In the absence of
modern medical procedures, they were at greater risk of dying before they could
transmit their genes to the next generation, or at least, lived shorter lives
in which they had fewer offspring than individuals with stronger bones.
This is a
misunderstood concept in evolution; the phrase "survival of the
fittest" implies that less fit animals die without reproducing, but
evolution also proceeds because less fit individuals, on average, leave fewer
descendants, changing the average genetic constitution of the population.
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| image:en.wikipedia.org |
Why do human, and women in particular, suffer from
osteoporosis?
One reason is that
osteoporosis is a disease of aging, usually manifesting itself only in the
sixth or seventh decade of life, long after a person has already passed his or
her genes to the next generation. The genes that build bones thus slip past
natural selection.
AFTER menopause women
synthesise less estrogen, a hormone that seems to have a role in maintaining
bone mass. Osteoporosis results in a severe loss of bone mass, chiefly from the
trabecular network, the honeycomb-like material that braces bones from within.
But it is now known
that for bones to resist stress, they must be subjected to stress. In the
skeleton of every vertebrate, bone mass is maintained (or increased in growing
young individuals) by a dynamic balance between the construction and breakdown
of bone.
That balance is at
least partly regulated by gravity and other sources of induced stress. Young
victims with serious injuries from road accidents can lose up to 50 percent of
their bone mass within a few weeks because their bones do not experience
mechanical stresses while they are immobilised in bed.
The same phenomenon is seen in astronauts, who are at risk
of breaking their legs when they return to earth gravity after prolonged
sojourns in space.
Experts regularly
warn that lack of physical exercise is an important cause of osteoporosis in
women. The sedentary Western lifestyle and the design of our cities, means that
many women undertake little strenuous exercise.
Most will drive the
several kilometers to a regional shopping centre because it is too far away to
reach on foot, and do not experience the benefit of the simple mechanical forces
imposed by walking and load-carrying that kept their great-grandmothers in good
shape.
Professor Andrew
Biewener of the Department of Organismal Biology and Anatomy at the University
of Chicago, says that during walking or running the limb bones of most mammals
experience skeletal stresses that are about a quarter to a half the force that
would be required to break them. There is a safety factor of two to four during
natural movement.
Modern mammals share
the same basic skeletal design because they all descend from a single ancestor.
Considering their shared ancestry, they come in an extraordinary range of sizes
that spans six orders of magnitude. A two-tonne African elephant weighs a
million times as much as the world's smallest mammal, the two gram bumblebee
bat of Thailand.
Professor Biewener
has studied the relationships between weight, bone strength and stresses of
mammals from all parts of this spectrum, and found that, for animals weighing
between 10g and 300kg, the safety margin between the everyday stresses of
locomotion and fracture-inducing stresses remains roughly constant.
The safety margin
appears to be determined by the forces that the animals muscles apply to the
limb bones. Good engineering principles are evident, the mid-range mammals tend
to have an upright posture and longer limb bones, an arrangement that maximises
the leverage the muscles apply to the bone, while minimising muscle mass. It
would be pointless to build long bones for rapid locomotion and then weigh
oneself down with slabs of surplus muscle.
VERY tiny mammals
have stiffer, more brittle bones - stiffness rather than outright strength is
required for a bumblebee bat to fly.
A subtle equation rules the design of animals of all sizes.
Doubling the size
roughly quadruples the weight, so that in theory, an animal the size of an
elephant should have bones and muscles four times as large as an animal half
its height
In practice, very
large animals must make compromises. In moving, they employ a gait that
minimises stresses on a relatively modest skeleton. And because the skeleton is
less massive, less muscle is required to move it around.
This problem of
scaling with size explains a puzzling feature in some fossils of giant
plant-eating dinosaurs, creatures that were many times heavier than elephants.
The massive bones at the base of the enormously long tail often showed signs of
having been fractured.
Paleontologists were
puzzled by the fractures, because species like Diplodocus were thought to have
been swamp dwellers that lived most of their lives immersed in water, with only
their necks visible. Buoyancy would minimise the stresses imposed on their
skeletons.
But a new theory
suggests that their long necks were an adaptation for grazing tall plants on
land. The animals may have sat down on their hind quarters, raising their
forelegs off the ground, to maximise their reach. An animal sitting down too
quickly, or one disturbed during feeding, may have been at risk of fracturing
its tail bones simply because of the enormous stresses applied by its weight.
Professor Biewener
points out that, in evolving their diverse skeletal sizes, modern mammals have
also evolved the capacity to remodel their bones during life, to adapt to
changes in loading during growth and during daily life.
Osteogenesis,
the process of new bone formation, is more efficiently stimulated by patterns
of activity in which the bones are alternately placed under load and then
relieved of pressure. Static loads, such as the act of standing up or sitting
at an office desk, are less effective at stimulating bone growth.
He says his studies
indicate it is not just the magnitude of the strain, but the number of loading
cycles, and the way the strain is distributed by different types of exercise,
that probably determine the rate and pattern of osteogenesis.
The greatest rate of
osteogenesis occurs when the body's normal functional strain pattern is
disrupted. This implies that abnormal strain generates a biological signal that
switches on bone synthesis, and the remodelling that takes place prepares the
animal for the possibility that the abnormal strain may be repeated.
Weightlifters undergo
this type of bone remodelling, often with unpleasant long-term effects. The
enormous forces placed on the ends of their limb bones can eventually result in
arthritis.
Conclusion
At the other end of
the stress spectrum are people who get too little exercise, and who suffer loss
of bone mass. Dietary measures to increase calcium intake, such as drinking the
calcium-enriched milk brands, may help protect against osteoporosis. But regular
exercise that puts pressure on the weight-bearing bones is likely to be a more
successful strategy.
References:
Author bio: Scarlett
Hilton, Diet/Nutrition Expert, Lifestyle Blogger having a deep passion for
writing and sharing articles on cardiac
diet,
cardiac health, and cardiac disease.
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