A HISTORY Of VERTEBRATES: AMPHIBIANS AND REPTILES 



451 



In some other neotenic salamanders, the failure to metamorphose may 

 result from an inhibition of the mechanism that releases thyroxin. The 

 tiger salamander, Ambystoma tigrinum (Fig. 23.3 B), metamorphoses 

 under most conditions, but those living at high altitudes in the Rocky 

 Mountains fail to do so and remain permanent larvae known as ax- 

 olotls. Apparently cold inhibits the release of thyroxin, for when axolotls 

 are fed thyroxin or when they are brought to warmer climates, meta- 

 morphosis is normal. 



frogs and Toads. Most anurans are amphibious as adults, living 

 near water to which they frequently go to feed or escape danger, but 

 some are more terrestrial in habits, and others have become adapted to 

 an arboreal life. The terms frog, toad, and tree frog or tree toad or- 

 dinarily imply amphibious, terrestrial and arboreal modes of life, not 

 natural evolutionary groups. Members of several distinct families of 

 anurans, for example, have become adapted independently to an ar- 

 boreal life. 



Toads have adjusted to a terrestrial life by evolving structures and 

 patterns of behavior that reduce water loss. The epidermis of their skin 

 is more horny and less pervious to water than that of frogs. A thick, 

 dry skin reduces cutaneous respiration, but this is compensated for by 

 an increase in the respiratory surface of the lungs. The lining of toad 

 lungs is more complexly folded than that of frogs. Much of the water 

 lost through the kidneys is reabsorbed in the urinary bladder. Toads 

 are crepuscular in habits; they burrow or take shelter by day, and come 

 out in the moist evening to feed upon insects. 



Figure 23.4. The tree frog. Hyla versicolor, clings to trees by means of its expanded 

 digital pads. (Courtesy of the New York Zoological Society.) 



