THE AMPHIBIA OF OKLAHOMA 



toward variability on the one hand and environmental pressure, selecting 

 only the most favorable individuals, on the other. Rigorous conditions any- 

 where almost always result in extermination of, or change in, the existing 

 fauna and flora of a region. 



There is a possibility, at least theoretically, that some organisms may 

 meet and cross a partial barrier to dispersal by change in habits without essen- 

 tial change in structure; and it should be noted that, should this occur, the 

 change need not be great in any one case. In passing to a progressively drier 

 country, for example, so simple a variation as an average slight increase in 

 developmental rate of tadpoles may make all the difiEerence between extinc- 

 tion and survival through many seasons. 



To determine such cases (or to test for their possible occurrence) involves 

 the gathering of multitudinous observations. Even the smallest detail may 

 be the key to the principle being sought although at the time when first ob- 

 served it may seem so trivial as not to be recorded or remembered. 



Habitat-Pattern s 



The salientians of Oklahoma, broadly considered, exhibit two general 

 habitat preferences. All of the frogs (Rana) and the cricket-frog ( Acris 

 crepitans) are semiaquatic in that they tend to be restricted to pond sides 

 or streams. On the other hand, all of the others range widely over the land 

 paying little attention to the presence or absence of water except during 

 breeding. Within each of these general types, however, there are varying 

 adaptations to specific types of habitats. 



Both subspecies of Rana areolata inhabit the burrows of crayfishes and 

 are, therefore, restricted mostly to low-lying fields and ditches wherein bur- 

 rowing crayfishes abound. Rana clamitans, here as elsewhere, is mostly re- 

 stricted to muddy stream banks, swampy areas, and lake margins; the bull- 

 frog (R. catesbeiana) is present most abundantly in and about permanent 

 pools or lakes. The southwestern leopard frog [R. bcrlandieri) and also the 

 pickerel frog {R. palustris), at least elsewhere, range widely from water in 

 order to feed, particularly during moist weather, or at night. All of these 

 semiaquatic organisms in Oklahoma seek other pools when the water upon 

 which they are dependent evaporates. It is, therefore, a common thing to find 

 frogs on roads at night. These movements tend to be seasonal; but this is more 

 the result of seasonal rhythms in rainfall and in hot weather than of inherent 

 drives in the animals, except during the time when they are migrating to 

 breeding sites. 



Members of the nonaquatic species tend in varying degrees toward both 

 nocturnal and fossorial habits, although these two phenomena are not neces- 

 sarily correlated. The individuals of various species of Hyla tend to rest in 

 the trees during the day and, so far as known, never burrow. Members of 

 Microhyla c. olivacea, M. c. carolinensis, and all species of Pscudacris rest 

 beneath objects during the day and range widely at night to feed; and this is 

 apparently true also of one of the toads (Bufo punctatus) since nearly all 

 adult specimens, as well as some juveniles, that have been taken in Oklahoma 



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