VII 



Some Adaptations of Survival Value in Spadefoot Toads^ 



ARTHUR N. BRAGG 



FOR several years I have been reporting on various phases oi ihe distri- 

 bution, behavior, and habits of spadefoot toads (genus Scaphiopus) as 

 observed principally in Oklahoma during periods of breeding and tad- 

 pole development Irom year to year. In the more recent papers, peculiar be- 

 havior of tadpoles of Scaphiopus hurterii Strecker has been emphasized 

 (Bragg, 1944, 1945, 1945a, 1948) and somewhat similar behavior noted in 

 some pools among other species of this genus ( Bragg, 1946, 1948). 



These studies have clearly demonstrated the following as tacts: (1) 

 Spadefoot tadpoles maintain a very fast intrinsic rate of development (cf. 

 Trowbridge and Trowbridge, 1937) but it is modified by factors in the en- 

 vironment so that the actual rate at any one time and place may be quite 

 different from that at any other time and place. (2) The rate of growth 

 (increase in size) is not always correlated with the rate of development 

 (progress toward metamorphosis), at least in Scaphiopus hurterii. This is 

 shown by the fact that the size of the animals of the same age at metamor- 

 phosis varies widely from pool to pool and, indeed, may be quite different 

 in different years at the same poolsites, although this is not so usual. (3) 

 During some developmental periods at a given pool, tadpoles of the spade- 

 foots show marked social behavior; during other developmental periods they 

 may not do so; during still others, their behavior, although still social, may 

 differ in detail from that shown in other pools at different times and places 

 or at the same place at different times. (4) This social behavior always in- 

 volves aggregation of the tadpoles, either during feeding (very commonly 

 seen) or at metamorphosis (more rare). (5) Some feeding aggregations in 

 S. hurterii at metamorphosis (but apparently not at other developmental 

 periods) take the form of active cannibalism based primarily on the attack 

 upon the shrivelling tails of transforming individuals. More rarely this phe- 

 nomenon also occurs in mixed groups of S. hammondii Baird and S. bombi- 

 jrons Cope (Bragg, 1946, 1948). (6) Metamorphic aggregations of the type 

 first described by Bragg (1944) in S. hurterii have never been known to 

 develop cannibalism despite the further fact that such aggregations occur 

 at about the same stage of development as cannibalistic aggregations at other 

 times and places. (See Ball, 1936, for cannibalistic behavior of Scaphiopus 



1 These studies were made possible through an arrangement by Dr. J. Teague Self, 

 Chairman, Department of Zoological Sciences, with the approval of Dean Laurence H. 

 Snyder, Graduate College, and Dean E. D. Meacham, College of Arts and Sciences, whereby 

 the author's teaching load was reduced. 



lOI 



