THE AMPHIBIA OF OKLAHOMA 



holbrooXji holbrookji in Connecticut at metamorphosis.) (7) All such phe- 

 nomena, whatever the variations in detail from place to place may be, occur 

 in pools often containing tadpoles of several other species (of Bufo, Rana, 

 Pseudacris, Hyla, Acris and Microphyla) none of which behaves in these 

 ways.- 



Such distinctive and unusual behavior of tadpoles, as well as the varia- 

 tions in it in place and time, must have some explanation. Elsewhere (1946) 

 I have pointed out that this is likely to be found in adaptations to xeric con- 

 ditions in the environment characteristic of most spadefoot species, these 

 adaptations supplementing the xeric breeding pattern of the adults (Bragg, 

 1944a, 1945), both types of phenomena probably developing throvigh Dar- 

 winian selection in the evolution of these amphibians in the dry plains and 

 xeric savannahs of the West. 



But to stop here is scientifically unsatisfactory. We need to know (1) 

 why the tadpoles vary in behavior in different times and places, (2) what 

 factor (or factors) in the environment accounts for differential rate of growth 

 and development, and, in general, (3) what the effect of each factor in the 

 environment is (in t^uantitative terms) in correlation with the behavior 

 observed. 



Data are not yet available for a complete analysis. This paper, therefore 

 emphasizes only one broad factor concerned. This is food. It was early noted 

 (1944) that tadpoles of S. hurteri'i eat large quantities of the bottom mate- 

 rials, especially in pools largely devoid of microscopic algal growths. Such 

 pools always have little "normal" food of other types (Bragg, 1946). This 

 suggested that a study of the bottom materials correlated with behavior of 

 the animals in given pools might throw further light on the problem. Could 

 the food supply be correlated, at least broadly, with the occurrence of aggre- 

 gational phenomena and cannibalism? 



Observations 



Mud from the bottom of pools, collected while animals (S. hiirterii) 

 were feeding, was examined under a microscope and materials from the 

 stomachs and intestines of tadpoles collected at the same times and places 

 were compared. Several hours of search of such materials revealed no visible 

 significant differences in the different types of samples. Each was an amor- 

 phous mass, the only recognizable items being these: (1) a single empty 

 diatom shell, (2) a single three-celled fragment of a filimentous alga, (3) a 

 few bits of clay, and (4) an occasional plant fragment. Staining with several 

 biological stains proved only that the main bulk of the materials was organic 

 in nature and that it contained many very tiny rod-like particles which I took 



- Weak, feeding aggregations in artificially confined tadpoles ot Rana hcrlaiidicri 

 Baird have been seen (Bragg, 1948) and one case of hastened metamorphosis accompanied 

 by a sort of aggregation in this same frog has been reported (Bragg, 1944). Neither of 

 these cases was quite the same, at least in intensity, as in the spadefoot toads; and, perhaps 

 more significant, neither has occurred through several years of observations in pools of 

 natural water when spadefoot tadpoles were present and exhibiting social behavior. 



102 



