The pineal complex and melatonin affect the expression of the daily rhythm of behavioral thermoregulation in the green iguana.

Daily variation in the body temperature of the green iguana (Iguana iguana) was studied by telemetry in laboratory photo-thermal enclosures under a 12Light:12Dark (L:D) photoperiod. The lizards showed robust daily rhythms of thermoregulation maintaining their body temperatures (Tb) at higher levels during the day than during the night. Some animals maintained rhythmicity when kept in constant darkness. On light:dark cycles parietalectomy produced only a transient increase of median Tb in the first or second night following the operation. Pinealectomized lizards on the other hand maintained their body temperatures as significantly lower levels during the day and at significantly higher levels during the night than did sham-operated or intact lizards. This effect was apparently permanent, since one month after pinealectomy lizards still displayed the altered pattern. Plasma melatonin levels in intact animals were high during the night and low during the day and were unaffected by parietalectomy. Pinealectomized lizards showed low levels of plasma melatonin during both the day and the night. A daily intraperitoneal injection of melatonin in pinealectomized animals given a few minutes after the light to dark transition decreased the body temperatures selected by the lizards during the night and increased the body temperatures selected during the following day. Control injections of saline solution had no effect. The significance of these results is discussed in relation to the role of the pineal complex and melatonin in the mediation of thermoregulatory behavior.