Bill Beatty 



Not yet able to fly, a juvenile little brown bat, left, clings head-up 

 to a tree trunk. Wlien landing, older bats execute a flip that 

 allows them to hang from their feet. Magnified ten times, a 

 stained embryo of a little brown bat, below, about thirty flve days 

 old, shows early bone development. Cartilage appears in blue 

 and bone in red. Fingers have begun to elongate for their 

 eventual fiinction as wing struts. 



Rick A. Adams and Scott C. Pedersen 





then to the body, forming a broad wing 

 surface. A similar membrane spreads be- 

 tween the legs and tail, completing an air 

 foil that surrounds the entire body. Most 

 insect-eating bat species strike the insects 

 with their wings, then grasp the stunned 

 prey with their feet. 



Juveniles are not as agile or maneuver- 

 able as adult bats. One reason, of course, is 

 simply inexperience, but restrictions asso- 

 ciated with growth and development also 



handicap young fliers. Bats first attempt to 

 fly when they are about four weeks old, 

 but their wings are still underdeveloped. In 

 some species, including most insectivo- 

 rous bats, youngsters have only about 20 

 percent of the adult wingspan. Yet in four 

 weeks, the rest of the juvenile's body may 

 have reached 60 percent of the adult size. 

 This imbalanced development leaves the 

 young in a precarious situation, for their 

 early flights are awkward at best. In fact, it 



is not uncommon at our maternity site to 

 observe what appear to be very disgrun- 

 ded young bats walking back to the roost 

 after having apparently, for whatever rea- 

 son, given up on flying for that night. 

 Their wings reach full size about forty to 

 fifty days after birth. 



A bat's ability to fly is preceded by a 

 long process that begins well before it is 

 born. Although some researchers have 

 studied the development of flight in bats, 

 little work had been done on bone forma- 

 tion in their wings. By focusing on the 

 growth studies, we hope to shed light on 

 the diversity and plasticity of the ancestral 

 vertebrate body plan: four limbs, each 

 ending in five digits. We are interested in 

 the unique developmental events that 

 allow bats to transform an otherwise "stan- 

 dard issue" mammalian embryo into an 

 airborne SiCvo-bat. 



To observe growth rates and the differ- 

 entiation of anatomical structures in pre- 

 served embryos, we used special chemical 

 stains that migrate to difl'erent kinds of tis- 

 sues, a technique that had not previously 

 been appUed to the little brown bat. Alcian 

 blue combines with certain sugars (rnu- 

 copolysaccharides) in the developing car- 

 tilage, while aUzarine red lodges in the cal- 

 cium found in developing bone. After 

 staining, the embryo is "cleared" using an 

 enzyme (usually trypsin) that digests 

 much of the remaining skin, muscle, and 

 connective tissue. Now the embryo speci- 

 men becomes translucent, allowing a clear 

 view of the stained bones and cartilages. 

 The final preparation is rather like a three- 

 dimensional, color version of an X-ray 

 image. 



In mammals, most skeletal elements 

 begin as cartilage "models," or precursors 

 of adult structures. As each bone develops, 

 the cartilage becomes infused with cal- 

 cium salts that will eventually form a hard, 

 hollow matrix. As more salts are de- 

 posited, the cartilage is eventually re- 

 placed by ossified calcium, at which point 

 the bone stops growing. 



Most bats develop in utero for about 

 fifty to sixty days, but we began to see sig- 

 nificant developments in the skeleton 



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