Os ossos das aves podem ser ocos, mas eles também são pesados

Bird Bones May Be Hollow, but They Are Also Heavy

ScienceDaily (Mar. 23, 2010) — For centuries biologists have known that bird bones are hollow, and even elementary school children know that bird skeletons are lightweight to offset the high energy cost of flying. Nevertheless, many people are surprised to learn that bird skeletons do not actually weigh any less than the skeletons of similarly sized mammals. In other words, the skeleton of a two-ounce songbird weighs just as much as the skeleton of a two-ounce rodent.

Bird bones have evolved special features to make them stiffer and stronger than mammal bones, including high bone density, fusion of some bones and altered shape. Studies show that the main bone in the bird wing, the humerus, is quite round in cross-section, making it stronger and more resistant than a flat-sided bone would be to twisting forces encountered in flying. (Credit: Betsy Dumont, UMass Amherst)

Bird biologists have known this for a long time, but it took a modern bat researcher, Elizabeth Dumont of the University of Massachusetts Amherst, to explain how bird skeletons can look so delicate and still be heavy. The answer is that bird bones are denser than mammal bones, which makes them heavy even though they are thin and sometimes even hollow.

Her findings, supported by bone density measurements, are published in the March 17 issue ofProceedings of the Royal Society B. As Dumont explains, "The fact that bird bones are denser than bones in mammals not only makes them heavier for their size, but it may also make them stiffer and stronger. This is a new way to think about how bird skeletons are specialized for flying and solves the riddle of why bird skeletons appear so lightweight and are still relatively heavy. This has never been explained fully and so has never gotten into the textbooks. I'd like to see that change."

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Read more here/eia mais aqui: Science Daily

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Bone density and the lightweight skeletons of birds

Elizabeth R. Dumont*

-Author Affiliations
Department of Biology, University of Massachusetts at Amherst, 221 Morrill Science Center, Amherst, MA 01003, USA
*bdumont@bio.umass.edu

Abstract

The skeletons of birds are universally described as lightweight as a result of selection for minimizing the energy required for flight. From a functional perspective, the weight (mass) of an animal relative to its lift-generating surfaces is a key determinant of the metabolic cost of flight. The evolution of birds has been characterized by many weight-saving adaptations that are reflected in bone shape, many of which strengthen and stiffen the skeleton. Although largely unstudied in birds, the material properties of bone tissue can also contribute to bone strength and stiffness. In this study, I calculated the density of the cranium, humerus and femur in passerine birds, rodents and bats by measuring bone mass and volume using helium displacement. I found that, on average, these bones are densest in birds, followed closely by bats. As bone density increases, so do bone stiffness and strength. Both of these optimization criteria are used in the design of strong and stiff, but lightweight, manmade airframes. By analogy, increased bone density in birds and bats may reflect adaptations for maximizing bone strength and stiffness while minimizing bone mass and volume. These data suggest that both bone shape and the material properties of bone tissue have played important roles in the evolution of flight. They also reconcile the conundrum of how bird skeletons can appear to be thin and delicate, yet contribute just as much to total body mass as do the skeletons of terrestrial mammals.

birds      bats     bone density      bone strength   

Footnotes

Received January 18, 2010.
Accepted February 23, 2010.
© 2010 The Royal Society

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