Jan 26, 2017

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.
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.


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.

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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