ADAPTATIONS OF SURVIVAL VALUE 



poles ol S. hiirtcrii and they emer^^ed in large luimhers in I 'Ml ( Ikagg, 1944); 

 but tadpoles of this species, transferred here from Pool E in 1945, became 

 emaciated and weak. Many died and very few transformed (Bragg, 1946). 

 This pool again supported tadpoles in the spring of 1948, during its second 

 evaporation cycle of the season, the brief first one having caught and killed 

 all tadpoles of several species developing here. Similarly, transferred tadpoles 

 from Pool C developed poorly in Pool D in 1945 but this pool supported 

 thousands which transformed normally in 1948, alter a longer than usual 

 period of development, however. 



It also partially explains the difference in rates of development and 

 growth mentioned earlier. In Pool F in 1947 tadpoles increased in size at an 

 unusually rapid rate, eventually becoming twice the size of those of the same 

 age in Pool C. In the latter pool, however, the tadpoles transformed a week 

 earlier than those in F at the smallest size ever seen (mean length ::= 2 

 mm.) on the thirteenth day after water had first filled the pool (Bragg, 

 1948). This is the fastest developmental rate known to me among amphi- 

 bians of any type. It is significant to note that all tadpoles had been caught 

 and killed in Pool C by evaporation at nearly this same age in 1945 and again 

 in 1946. Obviously conditions in the pool had made the difference. The ob- 

 servations certainly suggest that the number of cycles during which tadpoles 

 have been killed is the deciding factor through the accumulation of organic 

 matter (and perhaps some special substance) in their bodies. It is, for ex- 

 ample, well known that the thyroid gland is active in metamorphosis. It is 

 conceivable that the dried glands of dead tadpoles or something that they 

 contain (iodine or thyroxine, for example) may so accumulate in the pool as 

 to allow the tadpoles of the proper cycle to emerge at a small size, just as the 

 water disappears. It should be noted, too, that this would apply only to the 

 spadefoots. Several other species have used Pool C for breeding but not a 

 single tadpole of other species here has succeeded in emerging from this pool 

 since at least 1944 when I started watching it through all of its evaporation 

 cycles. 



Having determined broadly the source of organic materials which these 

 animals secure from the bottom, I thought it wise to check some of the more 

 obvious physical and chemical conditions of the water for possible factors 

 which may effect the rates of development and growth. These data are not 

 presented in detail because they were consistently and unquestioningly nega- 

 tive. Variation in temperature in pools (and diurnal-nocturnal fluctuation 

 of temperature), in hydrogen ion concentration (pH and methyl orange and 

 phenolphthalein alkalinity) and total carbonates and bicarbonates in the 

 water' obviously were not the determining factors, singly or collectively, in 

 behavior or in growth and developmental rates of the tadpoles. For tempera- 

 ture this confirms the conclusion earlier reached (Bragg, 1948). It should 



^ Analyses were made by Mr. William Thompson and his assistants of the State 

 Game and Fish Department working in the laboratory of the Oklahoma Biological Survey, 

 with which both he and I are connected. I express my appreciation for this aid by Mr. 

 Thompson. 



105 



