Photothermal effects on the testicular cycle in the musk turtle, Sternotherus odoratus

A series of photothermal treatments was initiated at different times of year to examine the relative contributions of exogenous and endogenous factors affecting the timing of the testicular cycle of the kinosternid turtle, Sternotherus odoratus. Elevated temperature appeared to be the major factor stimulating testicular recrudescence (spermatocytogenesis, spermiogenesis, and, to some extent, androgen secretion). However, depending on the time of year, short or long daylengths, in conjunction with elevated temperatures, enhanced testicular responses (recrudescence or regression). Also, high temperatures are not requisite for recrudescence. Spermatogenic activity and testis growth differ in their temperature sensitivity and may become dissociated, and an endogenous mechanism may have affected peak androgen secretion. Testicular regression exhibited a dependence on an interaction among temperature, photoperiod, and endogenous regulation. Turtles with testes in advanced stages of spermiogenesis underwent accelerated regression when exposed to even slightly reduced temperatures, especially at short daylengths. However, regression was delayed by a combination of high temperature and short daylength in fall. Results are compared to other experimental studies of photothermal effects on turtles and discussed in terms of the described latitudinal variations in the timing of the testicular cycle of S. odoratus.