Cold entrainment of the annual cycle of ovarian activity in the lizard Lacerta vivipara: thermoperiodic rhythm versus hibernation.

A constant warm thermoperiod maintains ovarian quiescence in the lizard Lacerta vivipara, whereas a 4-month artificial hibernation rapidly induces synchronized vitellogenesis after transfer to warmth. The present study examined the possibility of a thermoperiodic regulation of the ovarian cycle and the formal properties of an internal temporal program. These questions were addressed using 24-hr thermoperiodic conditions that combined a long or a short thermophase (6 or 2 hr of basking) with a warm (19-21 degrees C), a cool (5-15 degrees C), or a cold (3-7 degrees C) cryophase. Lizards were exposed to the natural photocycle or to LD 12:12. Occurrence and timing of vitellogenesis completion were monitored using immunodetection of plasma vitellogenin and laparotomies. Cold remained stimulatory when given intermittently with a 24-hr periodicity. However, under long-thermophase conditions, lizards responded poorly to cool cryophases but fully to cold ones (72.7-100% vitellogenesis). Thus a certain amount of cold must be provided during each 24-hr cycle in order to be effective through the succession of thermocycles. Reduction of the daily heat input from 6 to 2 hr modulated the stimulating effects of cold cryophases: The median date for the beginning of vitellogenesis occurred 1 month earlier, but the number of responding females decreased from 100% to 40%. The thermoperiodic regulation of the ovarian cycle also relies upon a precise heat-cold balance per nycthemeral unit. This ensures the entrainment of an internal rhythm, since the timing of reproductive responses varies with the date of transfer from the inhibitory warm thermoperiod to the inducing thermoperiod (long thermophase, cold cryophase). At least half the females started vitellogenesis within 1-2 months after a late transfer (winter solstice) instead of 6 months after an early one (autumn equinox), and the median date for onset differed by 1 month between the two groups. However, autumn transfer was the only one to induce a group response in close agreement with the natural timing.