The Spadefoot Toads in Oklahoma with a Summary of Our Knowledge of the Group. II

Through seven years, the peculiar advantages of the climate, ecological associations and geographic location of Oklahoma have been used in a study of the habits and distribution of Scaphiopodid toads. Four of the eight known forms occur and every opportunity for their study in all parts of the state has been utilized. Most of this has been in the field at night. Thousands of miles have been traveled and eggs and tadpoles have been studied in the laboratory. This work is here summarized and correlated with that of others in such a way as to form a summary of our knowledge of the group at the present time. The principal conclusions are as follows: (1) Tanner's (1939) conception of the two species groups as representing two subgenera, Spea of the west and Scaphiopus of the East, is accepted and confirmed by details concerning tadpoles and habits. (2) Tanner's further conclusion that the Scaphiopodidae arose in the North American Southwest is confirmed, also principally by the study of habits. (3) It is clear that anuran amphibians in North America which are limited to grasslands and deserts in their distribution have a breeding pattern different from those not so limited. Two principal patterns are described, the xeric and the mesic. (4) The xeric pattern includes the following tendencies: (a) the lack of a definite breeding season; (b) use of temporary water only; (c) initiation of breeding behavior in nature only by the coming of rain; (d) a hastened development of larvae; (e) possession of rather loud voice by the male; (f) marked attraction of males as well as of females by the breeding call of the species, thus tending to build up large congresses very quickly. (5) The mesic pattern includes the opposite tendencies in various degrees and is the common one of eastern species in contrast to those limited to the western grasslands and deserts. (6) All spadefoots show the xeric pattern exclusively. (7) However, those in mesic environments vary slightly, but consistently, from those still in xeric environments (i.e., they have developed a subpattern still essentially xeric in nature). (8) Since a xeric pattern in a mesic environment would not likely be detrimental to the life of a frog or toad, whereas a mesic pattern in a xeric environment in all probability would be, it follows that (other things being equal) eastern forms can spread westward only slowly and with difficulty whereas western species can spread eastward more easily. This, of course, could be modified by other conditions. (9) Applying these things to the spadefoots, we infer that they arose in a xeric environment, became adjusted nicely to this through natural selection, and split into two subgenera. Then one of these (Scaphiopus) tended to spread eastward and northward (i.e., to mesic conditions), changing in reproduction habits only slightly; whereas the other (Spea) remained in xeric regions without essential change in habits. (10) The one exception (S. couchii) is one that proves the rule; for, although belonging to the subgenus Scaphiopus taxonomically it nevertheless remained in a xeric habitat with members of Spea and retained the original habit-pattern like these. (11) From this it follows that (so far as North American anuran amphibians are concerned) the habit-pattern in breeding does not necessarily follow taxonomic lines during speciation and dispersal to varied environments. While this is shown most clearly by the spadefoot toads, there is ample evidence of it in other genera as well. (12) The emphasis in habit-patterns above deals only with similarities; but interspecific differences should not be overlooked. Details given in the text show clearly that each species is different in some respects from all others (even in habits). The situation seems to be that (a) all species differ in details of habits, (b) each tends to be adjusted to reproduce in some one environment or several closely related environments, (c) those inhabiting the same ecological (not necessarily geographic) situation tend (regardless of taxonomy) to develop the same or essentially similar habit-patterns (convergent evolution) and (d) those even closely related taxonomically (and with widely overlapping geographic distribution) which are adjusted to essentially different ecological situations, tend to be different in habits and habit-patterns, even in the same geographic locality. (13) Spadefoot toads which inhabit the same region (ecologically as well as geographically) tend to be sexually isolated during breeding by two types of factors, (a) psychological and (b) ecological. Psychological isolation is the more important and is made effective through two phenomena, the differences in behavior of the males in securing mates and the marked interspecific attraction of spadefoot breeding calls. Ecological isolation is only partial and involves differential responses to amounts of rain as well as possible specific selection of breeding sites. The latter may be more psychological than ecological, however, due to the marked attraction for others of the call of the first spadefoot to arrive at a given pool.