Hormones and reproductive cycles in turtles

Reproductive cycles of cryptodire turtles and species from the Northern Hemisphere have been investigated extensively compared to those from the Southern Hemisphere particularly of pleurodire species. Radiography, laparoscopy, and ultrasonography have been used on a few species of turtles to describe a diversity of ovarian and testicular cycles. Radioimmunoassays (RIAs), on the other hand, have provided evidence of the dynamic cycling of peptides and steroid hormones within a reproductive season. All turtles appear to be cyclic breeders, at least at the level of individuals (year-round breeding can occur in some populations), and show a range of annual and multiannual reproductive patterns—regulated by nutritional status and environmental conditions—which are most often described as prenuptial or postnuptial. These cycles may be seasonally coordinated by an active pineal gland in the brain via melatonin secretion affecting the hypothalamus. Moreover, the steroid hormone cycles can be rapid and dynamic, and steroid hormones stimulate a carefully orchestrated mating receptivity period, a series of rapid ovulations in the case of multiclutched turtles, egg production, and oviposition events.