226 



ANALYSIS OF THE ENVIRONMENT 



good drainage extending across the coastal 

 plain to the Gulf of Mexico somewhat east 

 of Mobile Bay apparently is a western bar- 

 rier to four t'londa species of Procambanis 

 and an eastern barrier to three other species. 

 Of the five species known to Uve on both 

 sides of this soil barrier, three or perhaps 

 four have a present range that extends 

 north of the northern limits of the barrier, 

 which indicates the probable means of 

 transgression Hobbs (1942) describes. 



It is in the open country of the grass- 

 lands, savannahs, or parklands of the tropics 

 and temperate latitudes that the burrowing 

 habit is most fully developed. Termites and 

 ants may or may not build tunnelled 

 mounds above subterranean nests. In the 

 tropical savannahs of the world, termite 

 mounds may assume the size of hillocks. 

 They are made by cementing together bits 

 of excavated material with a sticky saUvary 

 secretion supplemented by pellets of excre- 

 ment. On the other hand, the nest may be 

 entirely underground, and many interme- 

 diate stages are realized. 



Reptiles of the open country are great 

 burrowers; land turtles and many Uzards 

 and snakes have this habit. Mammals are 

 represented underground by insectivores 

 such as various moles (Talpidae) and 

 particularly by rodents. Moles and pocket 

 gophers (Geomyidae) are the only North 

 American mammals restricted to fossorial 

 life. Prairie dog "towns" of the western 

 United States are duplicated by those of 

 whistling hares in MongoUa, and by the not 

 closely related long-tailed jumping "hare" 

 (Pedetes), the Abyssinian spiny squirrel 

 IXerus), and by the octodont rodent 

 (Ctenomys), of Patagonia. 



The burrowing owls may dig their own 

 bmrows or may inhabit abandoned rodent 

 holes, as do a number of insects, smaller 

 rodents, some snakes, and a variety of other 

 animals. Numerous birds, such as king- 

 fishers and bank swallows, make their nests 

 in holes dug in banks, and other birds, the 

 petrel among them, excavate nesting holes 

 in more level ground. 



Sand of dunes or deserts is a much-bur- 

 rowed substratum. Insects like the digger 

 wasp, Bembex, make shallow cavities for 

 resting or deeper burrows for their eggs. 

 The tiger beetle larvae (Cicindela) have 

 species that burrow only in the moist sand, 

 and other diggers-the burrowing spider, 

 Geolycosa, for *^xample— restrain loose sand 



by silk webbing. Larval ant lions (Myrme- 

 leonidae) and dipterous worm lions (Ver- 

 jnileo) dig conical pits and traps. This 

 habit is possible only on a substratum ol 

 dry sand or dust. 



The burrowing methods and equipment 

 of animals differ widely. Many forms, both 

 insects and mammals, ranging from digger 

 wasps to dogs, dig with their forelegs and 

 throw the dirt backward between their pos- 

 terior appendages. The mole cricket, like 

 the mole, pocket gopher, and a whole con- 

 vergent series of other animals, has strong 

 shovel-like, well-muscled anterior digging 

 feet and claws. In fossorial mammals the 

 shoulder girdle and associated musculature 

 is enlarged, and the pelvic development is 

 relatively weak. Snakes and lizards show 

 other convergent series. Thus, unrelated 

 forms have a speciaUzed digging rostrum 

 on the snout. Short, flat hzards, such as the 

 "homed toad," Phrynosoma, and the not 

 closely related Phrynocephalus produce 

 horizontal movements with their bodies that 

 carry them quickly below a sandy surface. 

 Perhaps most extreme of all, some amphib- 

 ians, lizards, and burrowing snakes have a 

 smooth, cylindrical or annulate, earthworm- 

 Hke form. 



These burrowing animals show structural 

 and color modifications that do not neces- 

 sarily have positive adaptive value. The 

 elongated tail of ordinary snakes appears to 

 lack survival value among burrowers and 

 may be replaced by a short, abruptly ter- 

 minated tail, as in the blind snakes or in 

 the shield-tailed snakes of southern India. 



In many instances Gloger's rule, that ani- 

 mals in warm, humid regions tend to be 

 more melanic than those in arid or cool 

 climates, holds for burrowing foiTns as well 

 as for surface dwellers. This may be seen by 

 inspecting a series of burrowing rodents oi 

 of the same species from regions, as in 

 California, where moist areas grade into 

 regions of great aridity. 



Another principle is illustrated by some 

 burrowing animals of which the coral snake. 

 Micrurus, is an example. This poisonous 

 American snake is strongly banded in red, 

 black, and yellow. The two species in the 

 United States, from the southeast and from 

 Arizona, do not show any association of 

 color with humidity. The brilUant colors in 

 burrowing snakes are not readily explain- 

 able in terms of prevailing theories concern- 

 ing cryptic or warning coloration. They are 



