GROWTH RATi:S AND SKXUAL MATURITY 



ing to occur, large numbers ot eggs are likely to be jiroduced alter rains 

 from late May onward, just at the beginning ot a period ol high temperatures 

 and hot, dry winds. Under such circumstances all, or nearly all, tadpoles may 

 be killed by evaporation of pools before they have had time to complete the 

 aquatic phase of their existence. Such a circumstance is by no means rare in 

 Oklahoma and about Norman two or three years may elapse without success- 

 ful breeding of B. cognatus. Part of this is a direct consequence of the selection 

 of none but shallow pools (up to 8-10") for breeding. In this, B. cognatus 

 differs from the other common toad here (B. w. tvoodhousii) which, by utiliz- 

 ing the deeper pools, has never failed completely to produce young during 

 the past thirteen years. It is important to understand these relationships to see 

 how we are better able to judge the ages of toads collected here, why we know 

 in some cases that a small toad {B. cognatus) is older than its size would seem 

 to indicate and why we cannot be so sure about B. w. woodhousii. During 

 the period under observation, 1935-1947, the degree of success in breeding of 

 B. cognatus has been as iollows: 



1935. Moderately successful breeding. 



1936. Very successful breeding. Thousands of young toads emerging 

 from pools in May. 



1937. Breeding largely unsuccessful. A very few tadpoles, probably not 

 more than 50, in one pool succeeded in the race with evaporation. 

 All others, even in this one pool, were killed. 



1938. Very successful breeding (Bragg, 1940), starting in late March. 



1939. No successful breeding. 



1940. No successful breeding in spring. Congresses in early July may 

 have produced some young. 



1941. Very successful breeding. Distribution of rainfall and tempera- 

 ture practically ideal for these toads (Bragg, 1942). Most toads 

 breeding at this time have been at least three years old. 



1942. Poor success in breeding — a few succeeded in metamorphosis 

 (about like 1937). 



1943. No successful breeding. 



1944. Poor success in breeding. A few tadpoles ertierged, however. 



1945. No successful breeding. 



1946. No successful breeding. 



1947. Successful breeding but not equalling 1938 and 1941. In some 

 pools, all tadpoles were killed but in others many survived. Young 

 toads abundant in early July, but few seen the following spring. 



Our data came largely from the population of young toads resulting 

 from the most successful breeding year ever observed in this region, i.e., 1941. 

 A glance at the foregoing list will show that two full years had elapsed since 

 the last markedly successful breeding had occurred and that this season was 



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