ADAPTATIONS OF SURVIVAL VALUE 



to be bacteria. It made no diilereiice whetber tbese materials were unhxed; 

 or fixed in any of several commonly used reagents ( P>ouin's picro-tormal, 

 65 per cent or 70 per cent ethyl alcohol, or formalin in various dilutions up to 

 10 per cent). Since the bulk of bacteria was obviously insufficient to serve as 

 the major food supply, it was concluded that the tadpoles were acting as 

 scavengers, the essential iood being obtained from organic materials accumu- 

 lated during cycles of evaporation on the pools' bottoms. 



Field observations were then made in the region east of Norman, Okla- 

 homa, described in earlier papers as containing pools designated A, B, C, D, 

 E, and F (Bragg, 1945, 1948) all of which at some time have been utilized 

 by tadpoles of S. hurtcrii. The observations were made during tadpole devel- 

 opment in the spring and early summer of 1948; and extensive notes taken 

 in this region during the past six years (ten years for Pool A) were also re- 

 viewed. Pools A, D, E, and F were each used by this species in 1948, Pool E 

 twice, and Pool F three times during the season. 



Differences in the six pools were sufficient to make possible the follow- 

 ing general classification into three types: 



In type 1 (pools A, B, D, and F) the water varies from 5 to 10 inches in 

 maximum depth after rains sufficient to stimulate breeding activity and it 

 totally evaporates in from three to five weeks (if not replenished) during 

 the usual season of tadpole development (May through June). Very few 

 algae occur and (except for a few sedges growing from the bottom of Pool D, 

 and sometimes of Pool F) there is little plant growth. Water is turbid at first 

 but gradually clears if undisturbed. Pool F is the smallest in extent and shal- 

 lowest of pools of type 1 and therefore tends to disappear soonest during any 

 given period. 



Pools of type 2 (Pool C) are very shallow (4 inches maximum) but 

 extensive. Pool C measures approximately 70 by 30 feet and has an essen- 

 tially flat and level bottom (small depressions, in the order of the size of one's 

 cupped hands, occur). Water has never been known to stand for more than 

 two weeks after a single rain. Almost no algae develop but seed plants (prin- 

 cipally grasses, sedges, and a few other coarser types) grow in great profusion 

 from the bottom, often hiding the water just before it completely evaporates. 

 In winter, the pool site is a tangle of dead plants which, each year, disinte- 

 grate as new growth occurs in the spring. Water is at first turbid but clears 

 within a week as the water level falls. Evaporation cycles pass rapidly; it is 

 not uncommon for two, three, or even more to occur during a single spring 

 and summer. This depends entirely upon the distribution of seasonal rain- 

 fall and upon evaporation rates. 



Pools of type 3 (Pool E) are c]uite different. The water is clear at all 

 times and its level depends upon seepage into and out of the pool as well as 

 upon run-off and evaporation. How long water will remain (as well as its 

 level at any one time) depends primarily upon the total precipitation of the 

 season prior to a given date, since seepage from above is controlled by the 

 ainount of water absorbed by a sandy hill. After periods of considerable rain- 

 fall, seepage from the pool and evaporation from the surface often counter- 



103 



