478 TEXTBOOK OF ZOOLOGY 



the tiger salamander, Amhystoma tigrinum, which is found in Texas, 

 New Mexico, Colorado, and elsewhere, has been used extensively in 

 experiments for studying the nature of this regeneration. 



Means of Defense. — Amphibians have few ways of protecting 

 themselves from their enemies. Their coloration often blends in 

 with their surroundings and camouflages them, and their habit of 

 remaining immobile frequently causes them to be overlooked. Many 

 species practice death feints and some swell up by inflating their 

 lungs, making themselves more difficult to swallow. The mucous 

 glands of frogs and salamanders make them slippery, and, in the case 

 of salamanders particularly, their writhing and twisting movements 

 when captured make them hard to hold. A few salamanders have, 

 in addition, the ability to break off their tails and escape. 



One of the most protective weapons that amphibians have, how- 

 ever, is the secretion of their poison glands. This is especially 

 effective in the case of toads, many of which have large glands on 

 their shoulders, known as parotoid glands. An animal that has 

 attempted to bite or swallow a toad and felt the effects of the poi- 

 sonous secretion of the parotoids upon the mouth tissues will not 

 soon forget the experience. The largest known toad of the North 

 American continent, Bufo marinus, which ranges from Texas to 

 Patagonia, produces one of the most virulent poisons known among 

 amphibians. There are records of dogs which have been killed by 

 its secretions. Glandular secretions of certain South American 

 toads, Dendrohates, have been used by the Indians of Colombia for 

 poisoning their ari'ows. The secretions of toads are ordinarily quite 

 harmless to man, however, unless they happen to get into his mouth 

 or eyes. 



Voice. — The amphibians were probably the first vertebrates to de- 

 velop a voice. The calls of modern frogs and toads are very distinc- 

 tive, each species having its own particular call. Most of the 

 croaking is done by the males, and the primary function of these 

 calls seems to be to attract females and other males to the pond or 

 stream. It is during the breeding season that the air resounds at 

 night with their choruses, although certain species may croak at 

 other times. 



The croaking of frogs and toads is usually done with the mouth 

 and nostrils closed. The air is forced back and forth between the 



