Effects of androgens on male sexual behavior and secondary sex characters in the explosively breeding spadefoot toad, Scaphiopus couchii.

In male Scaphiopus couchii, plasma elevations in androgens are associated with the seasonal expression of sexual behavior. To test the hypothesis that androgens are necessary for the expression of one aspect of sexual behavior, amplexus, and for the development of secondary sex characteristics in male S. couchii, the effects of testosterone (T) and dihydrotestosterone (DHT) on clasping behavior and thumb pad growth were investigated. Forty toads were divided into five treatment groups as follows: (1) intact, with no hormonal or surgical treatment; (2) sham-castrated, with silastic implants of cholesterol; (3) sham-castrated, with implants of T and DHT; (4) castrated, with implants of cholesterol; and (5) castrated, with implants of T and DHT. Displays of sexual behavior (amplexus) for each individual were counted during a 3-hr observation period at 2, 4, and 7 weeks after the surgical procedure. For each observation trial, there was no significant difference in the amount of clasping among treatment groups: virtually all individuals from each group displayed vigorous clasping. No calling was noted at any time. Thumb pad darkness of each individual was assessed at the 4- and 7-week time intervals. At both time periods, the thumb pads of androgen-treated males were significantly darker than those of cholesterol-treated males, while intact males remained intermediate. Measurement of plasma steroid levels from the treatment groups indicated that androgens were basal in the castrate group and higher in the implant treatment. These results suggest that in S. couchii, although androgens are associated temporally with the onset of reproductive behavior, they are not necessary for its expression. The elevation in androgens during the reproductive period may be instead important in the development and maintanence of secondary sex characteristics and possibly other reproductive functions.