REPRODUCTIVE ENDOCRINOLOGY OF THE EXPLOSIVELY BREEDING DESERT SPADEFOOT TOAD, SCAPHIOPUS-COUCHII

The spadefoot toad, Scaphiopus couchii, is an explosively breeding, de sert dwelling amphibian that shows two characteristics hypothesized to select for a dissociated breeding pattern: it lives in a harsh enviro nment and it has a very short, but predictable breeding period. We tes ted the hypothesis that these factors select for a dissociated breedin g cycle by measuring plasma steroid hormones and the gametogenic cycle s in a free-living population of S. couchii. Blood and tissue samples were obtained from prebreeding, calling (males only), amplexed, and po stbreeding toads. In males, plasma testosterone (T), dihydrotestostero ne (DHT), and corticosterone (B) differed significantly among reproduc tive groups, with calling and amplexing toads having the highest level s of each of these steroids. In females, plasma T, DHT, B, progesteron e (P), and estradiol (E(2)) were measured; all steroids except B chang ed significantly among mating groups. Plasma T and E(2) were greatest in prebreeding and amplexed females, while P and DHT were highest only in amplexed females. Primary spermatogonia (SPG I), secondary spermat ogonia (SPG II), spermatocytes, spermatids (SPT), and spermatozoa embe dded in Sertoli cells (SPS) all changed significantly among male breed ing groups. Although all stages were present in all breeding groups, e arly stages of spermatozoa (SPG I and SPG II) were greatest in postbre eding males, while late stages (SPT and SPS) were highest in prebreedi ng, calling, and amplexing males. In females, oocyte stages I, II, III , V, and VI and atretic follicles also differed significantly among ma ting groups. Prebreeding female ovaries were dominated by late-stage ( V and VI) oocytes which were subsequently ovulated during amplexus. In postbreeding females, oogenesis was well under way as indicated by th e rising numbers of stage II and III oocytes. These results indicate t hat aspects of gametogenesis occur during and after breeding in both s exes. Although the results demonstrate that gonadal activity was high during the period of sexual activity, suggesting that S. couchii most closely fits an associated pattern of reproduction as defined by Crews (1984, Horm. Behav. 18, 22-28), we also found that gonadal activity c ontinued past the breeding season. For this reason, S. couchii does no t completely conform to an associated pattern. (C) 1997 Academic Press