ADAPTATIONS AND DISTRIBUTION 



inhabiting the clear-cut xeric cnvironincMits of desert or semidesert character 

 farther to the southwest extend into the state. It is very signilicant, however, 

 that not a single western species showing the xeric breeding pattern ranges 

 out of Oklahoma to the cast. In all probability, the breeding pattern of the 

 organisms is not the limiting factor, but, rather, is a reflection of habits of 

 ancestors which have long inhabited a xeric region to the west where such 

 a pattern is an advantage. On the other hand, without exception these species 

 whose range merely extends from the east into Oklahoma, somewhere in the 

 state apparently are blocked from further expansion westward partly because 

 of the very habits which characterize their mesic breeding pattern. 



Expressed in more physiological terms, one can say that those frogs and 

 toads having the mesic breeding pattern have a physiological cyclic nature in- 

 volving internal factors such as hormonal control of the reproductive cycle. 

 By contrast, in those having a xeric pattern such internal control is modified. 

 The xeric forms are actually physiologically adjusted to the sudden changes 

 in conditions of moisture so characteristic of xeric regions. The contrast in 

 the two principal breeding patterns is, therefore, a difiference in cyclic and 

 non-cyclic rhythms, the former fitting the organism to life in more moist 

 (mesic), the latter to life in drier (xeric) regions. On this view, the inter- 

 mediate breeding pattern may be considered as a partial adaptation to xeric 

 conditions through an incomplete (or at least different) change in the cyclic 

 rhythin; and thus, in evolution, may be interpreted as physiological adjust- 

 ment to intermediate ecological conditions. 



For convenience of reference, one may divide the reproductive period 

 of individual frogs and toads into the following periods: (1) the initiation 

 of breeding behavior, (2) migration to breeding sites, (3) sex recognition 

 and mating, (4) reproduction and fertilization of eggs, (5) tadpole develop- 

 ment. (6) metamorphosis, and (7) dispersal of the young from their natal 

 aquatic environment. Each of these is discussed below in relation to the three 

 types of breeding patterns given above. 

 (1) The Initiation of Breeding Behavior 



Three principal factors seem clearly to be involved in the initiation of 

 the breeding in frogs and toads in general: (a) hormonal control, (b) the 

 cytological cycle in the gonads, and (c) environmental factors, especially pre- 

 vailing temperature and amounts and distribution of rainfall. The mesically 

 adapted species emerge to breed within a definite seasonal range, usually in 

 early to mid-spring (Bragg and Smith, 1943). The breeding season is, there- 

 fore, clear-cut and definite. In Oklahoma the following species are in this 

 group: Bujo terrestris americanus, B. woodhousii jotvleri , Hyla (all species), 

 Microhyla c. carolinensis, Rana catesbeiana, R. palustris, R. clamitans, both 

 subspecies of R. areolata and Pseudacris strec\eri. The xerically adapted 

 species, on the other hand, tend not to have a breeding season (except as 

 limited by temperature) but, instead, to follow the rains in breeding. This 

 group in Oklahoma is composed of Ps. clar^ii, B. cognatus, B. compactilis, 

 B. insidior, Microhyla c. olivacea, and all four species of Scaphiopus. The 

 intermediate group contains all of the other known species in Oklahoma. In 



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