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 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 of Proceedings 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. Id like to see that change.”
Dumont measured the density of the cranium, the upper arm bone or humerus and the thigh or femur bones in song birds, rodents and bats by measuring bone mass and volume. “I found that, on average, these bones are densest in birds, followed closely by bats. Many other studies have shown that as bone density increases, so do bone stiffness and strength. Maximizing stiffness and strength relative to weight are optimization strategies that are used in the design of strong and stiff but lightweight man-made airframes,” she points out. Density is a measure of mass per unit of volume; dense bones are both heavier and stronger, much as a titanium toothpick would be stronger than a wooden one.
Over time bird bones have evolved specializations that maximize stiffness and strength, Dumont says. These specializations include high bone density, a reduction in the total number of bones, fusion of some bones, and changes in bone shape. For example, a long history of studies have shown that the main bone in the bird wing, the humerus, is quite round in cross-section. This makes it stiffer in the same way that a round toothpick is harder to snap than a flat one.
Galileo described bird bones as lightweight in 1683, Dumont says. Her new data help to dispel the common misconception that bird skeletons are lightweight relative to body mass. Instead, bird and bat skeletons only appear to be slender and delicate — because they are dense, they are also heavy. Being dense, strong and stiff is one more way that birds and bats bones are specialized for flight.
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Although this has long been known by bird biologists, Elizabeth Dumont of the University of Massachusetts Amherst, a contemporary bat researcher, had to explain how bird skeletons could appear so delicate yet still be heavy. The reason for this is that, despite being thin and occasionally hollow, bird bones are heavier than mammal bones because they are denser.
By measuring bone mass and volume, Dumont was able to determine the density of the skull, the humerus (upper arm bone), and the femur (thigh) bones in songbirds, rodents, and bats. “I discovered that birds have the densest bones on average, closely followed by bats.” Numerous other studies have demonstrated that bone strength and stiffness both rise with bone density. Strong, stiff, but lightweight man-made airframes are designed using optimization strategies that maximize stiffness and strength relative to weight, the author notes. Dense bones are heavier and more robust than less dense ones, similar to how a titanium toothpick would be more durable than a wooden one. Density is defined as mass per unit of volume.
Bone density measurements corroborate her findings, which are published in the Proceedings of the Royal Society B on March 17. Dumont says, “Birds are heavier for their size due to their denser bones than those of mammals, and they may also be stiffer and stronger as a result.” This offers a fresh perspective on how bird skeletons are made for flight and provides an explanation for why bird skeletons seem to be so light in spite of their relative weight. Since it has never been thoroughly explained, textbooks have never included this. Id like to see that change. “.
Bird skeletons are lightweight to counteract the high energy cost of flying, as even elementary school students are aware. Biologists have known for centuries that bird bones are hollow. Still, a lot of people are shocked to hear that the skeletons of birds weigh the same as those of mammals of comparable size. Stated differently, the weight of the skeleton of a two-ounce songbird is equivalent to that of a two-ounce rodent.
According to Dumont, bird bones have developed over time to maximize stiffness and strength. Among these specializations are high bone density, decreased total number of bones, fusion of some bones, and morphological changes to the bones. For instance, numerous studies have demonstrated that the humerus, the main bone in a bird’s wing, has a fairly round cross-section. This stiffens it in the same way that breaking a round toothpick is more difficult than breaking a flat one.
Thanks to this innovation, the researchers could test hypotheses about why certain birds’ bones contain more air than others. “Anyone with an interest in birds will be familiar with the axiom that birds have hollow bones,” stated Daniel Field, the paper’s senior author and a member of Cambridge’s Department of Earth Sciences. However, research on the degree to which bird bones are genuinely filled with air rather than marrow has never been done before. .
The new study offers a method that makes it possible to make the crucial distinction between the proportions of air and marrow found in bird skeletons, which has not been possible in previous studies. Focusing on the humerus, one of the bones in a bird’s skeleton that is most frequently filled with air, the researchers
The hypothesis that already exists gains further support from their findings, which show that larger birds typically have thinner bones and more air in their air-filled bones. She says that larger birds are still able to move quickly because air replaces their heavy bone tissue and marrow.
Nonetheless, even when taking into account relatively closely related birds, they did discover some complexities. For instance, they discovered that the Long-tailed Duck, the deepest diving sea duck, had thick, marrow-filled bones. Many aquatic diving birds exhibit this tendency because it adds weight to offset their buoyancy in the water, which demands additional energy for diving. However, other ducks, such as the Eurasian Teal, which is a dabbling duck that lives mostly in the shallows and does not submerge its entire body underwater, were found to have nearly entirely air-filled humeri.
“We were able to re-examine some of the long-standing hypotheses about why birds have this unusual characteristic,” Burton said, citing their new methodology. Researchers have previously hypothesized that a bird’s body size and the volume of air it contains in its skeleton are related, with larger, heavier birds having more air-filled bones to make up for this.
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