Thursday, January 18, 2007

 

Bats in flight reveal unexpected aerodynamics

The maneuverability of a bat in flight makes even Harry Potter's quidditch performance look downright clumsy. While many people may be content to simply watch these aerial acrobats in wonder, Kenneth Breuer and Sharon Swartz are determined to understand the detailed aerodynamics of bat flight - and ultimately the evolutionary path that created it.

They have taken a major step toward that goal by combining high-resolution, three-dimensional video recordings with precise measurements of the wake field generated by the bats' wing movements. Their study, published in the journal Bioinspiration and Biomimetics, marks the first such measurements made in bats and highlights ways in which bat flight appears to differ from bird and insect flight. The results suggest the possibility that a novel lift-generating mechanism may be at work in bats and point to the highly maneuverable mammals as a model for tiny flying machines.

Breuer, a professor of engineering at Brown University, who studied mechanical aerodynamics earlier in his career, is particularly intrigued by bats because 'they can generate different wing shapes and motions that other creatures can't.

Continued at "Bats in flight reveal unexpected aerodynamics"

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Based on the paper:

Direct measurements of the kinematics and dynamics of bat flight

Abstract/Full Text

Experimental measurements and analysis of the flight of bats are presented, including kinematic analysis of high-speed stereo videography of straight and turning flight, and measurements of the wake velocity field behind the bat. The kinematic data reveal that, at relatively slow flight speeds, wing motion is quite complex, including a sharp retraction of the wing during the upstroke and a broad sweep of the partially extended wing during the downstroke. The data also indicate that the flight speed and elevation are not constant, but oscillate in synchrony with both the horizontal and vertical movements of the wing. PIV measurements in the transverse (Trefftz) plane of the wake indicate a complex 'wake vortex' structure dominated by a strong wing tip vortex shed from the wing tip during the downstroke and either the wing tip or a more proximal joint during the upstroke. Data synthesis of several discrete realizations suggests a 'cartoon' of the wake structure during the entire wing beat cycle. Considerable work remains to be done to confirm and amplify these results.

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A Proceedings of the National Academy of Sciences (PNAS) paper published April 2006:

Development of bat flight: Morphologic and molecular evolution of bat wing digits

Abstract

The earliest fossil bats resemble their modern counterparts in possessing greatly elongated digits to support the wing membrane, which is an anatomical hallmark of powered flight. To quantitatively confirm these similarities, we performed a morphometric analysis of wing bones from fossil and modern bats. We found that the lengths of the third, fourth, and fifth digits (the primary supportive elements of the wing) have remained constant relative to body size over the last 50 million years. This absence of transitional forms in the fossil record led us to look elsewhere to understand bat wing evolution. Investigating embryonic development, we found that the digits in bats (Carollia perspicillata) are initially similar in size to those of mice (Mus musculus) but that, subsequently, bat digits greatly lengthen. The developmental timing of the change in wing digit length points to a change in longitudinal cartilage growth, a process that depends on the relative proliferation and differentiation of chondrocytes. We found that bat forelimb digits exhibit relatively high rates of chondrocyte proliferation and differentiation. We show that bone morphogenetic protein 2 (Bmp2) can stimulate cartilage proliferation and differentiation and increase digit length in the bat embryonic forelimb. Also, we show that Bmp2 expression and Bmp signaling are increased in bat forelimb embryonic digits relative to mouse or bat hind limb digits. Together, our results suggest that an up-regulation of the Bmp pathway is one of the major factors in the developmental elongation of bat forelimb digits, and it is potentially a key mechanism in their evolutionary elongation as well.

Associated article from the Howard Hughes Medical Institute:

A change in a single gene may be in large part responsible for the evolution of flight in bats, according to new studies by Howard Hughes Medical Institute researchers. The findings not only help explain the emergence of flight in these animals, but also illustrate how alterations in genes that govern development can lead to the abrupt, dramatic changes in body shape frequently seen throughout evolution.

The fossil record indicates that bats, the only mammals with powered flight, date back to the Eocene, an era that began approximately 55 million years ago. Notably, bat wing anatomy has not changed substantially over the past 50 million years - an observation that served as a starting point for the new work, which was published April 17, 2006, in an advanced online publication of the Proceedings of the National Academy of Sciences.

"We saw that the evolution of flight was quite sudden," said Lee A. Niswander, a Howard Hughes Medical Institute investigator at the University of Colorado Health Sciences Center who led the study. "That means there could be just a few key changes in limb development that resulted in more dramatic downstream consequences."

To find those key changes, Niswander and colleagues focused on the third, fourth, and fifth digits of the bat forelimb. These digits - equivalent to a human's middle, ring, and pinky fingers - are highly elongated and provide the support necessary for the wing membrane to be used for flight.

The group compared the embryonic development of bat forelimbs with that of bat hind limbs, which have much shorter digits than those in the wing. They also compared the bat forelimbs to mouse forelimbs so that they would have a similarly sized reference group.

During digit development in both species, cartilage cells (chondrocytes) divide and mature in areas called growth plates. The unique shape of the bat's forelimb is due to higher rates of both chondrocyte division and terminal maturation. Terminal chondrocyte maturation occurs in a part of the growth plate known as the hypertrophic zone, which is correspondingly larger in bat forelimbs than in mouse forelimbs. This difference in size, the researchers found, is due in large part to the expression of a single gene: bone morphogenetic protein 2, or Bmp2.

The researchers found that developing digits in the bat forelimb expressed more Bmp2 than those in either bat hind limbs or mouse forelimbs. The group tested several other genes associated with chondrocyte maturation, but didn't find differences in expression.

Then, Niswander and colleagues found that if they cultured a growing bat forelimb in a soup of Bmp2 protein, the hypertrophic zone was larger and the digits grew longer than forelimbs grown without extra Bmp2. Forelimbs cultured with a Bmp2 blocking protein, on the other hand, developed a smaller hypertrophic zone and shorter digits than those grown normally.

The group's findings have implications not only for bat evolution, but also for mammalian evolution in general.

"What we seem to see is punctuated changes in morphology over evolutionary time," said Karen E. Sears, the first author on the research, who is also at UCHSC. "Species will be in stasis for millions of years and then very quickly we get brand new species. That hints at just a few changes in key developmental genes."

That observation supports the theory of punctuated equilibrium, put forth in 1972 by Niles Eldredge and Stephen Jay Gould. Punctuated equilibrium states that evolutionary change is not gradual and geologically "slow"; instead, long periods of stability can be punctuated by periods of dramatic evolutionary change, and new species can appear relatively rapidly.

Other authors on the study are Richard R. Behringer, of the Department of Molecular Genetics at the University of Texas M.D. Anderson Cancer Center, Houston; and John J. Rasweiler IV, of the Department of Obstetrics and Gynecology, State University of New York Downstate Medical Center, Brooklyn.

Recent posts:

"New Research on the Co-evolution of Bats and Moths"

"Home and away: Bat uses magnetic compass for long flights"

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