Plasma corticosterone levels of female lizards (Anolis carolinensis) were sampled at 4-h intervals before and after a photothermal regimen known to stimulate ovarian growth in January and February, respectively. Female A. carolinensis have a diurnal cycle in plasma corticosterone with a daily peak at 1700 h in lizards with unstimulated ovaries exposed to short photophase and low temperature (12L 20°C:12D 20°C). Stimulation of ovarian growth by increasing the temperature and lengthening the photophase (14L 32°C: 10D 20°C; 80% relatively humidity) caused an 8-hr shift in the phase of the corticosterone peak to 0900 h. In lizards exposed to stimulatory temperature and photoperiod but also to low humidity (<35% j corticosterone exhibited a biphasic rhythm, with peaks at both 0900 and 1700 h. These results suggest that conditions which stimulate reproductive activity also cause a phase shift in the daily rhythm of plasma corticosterone. The results also suggest that chronic low humidity during these stimulatory conditions creates a biphasic rhythm of plasma corticosterone in this species. Therefore, a natural condition, low humidity, may be a chronic stressor of the lizard Anolis carolinensis.
Although there have been several studies of the effects of acute stress on the biological rhythms of stress hormones (Takebe et al., '66; Zimmerman and Critchlow, '67; Gibbs, '70; Dunn et al., '72; Torellas et al., '81; Kant et al., '86; Dauphin-Villemant and Xavier, '871, including administration of repetitive acute stressors (Paris and Ramaley, ,741, the effect of chronic stress on circadian or diurnal rhythms remains enigmatic. Also, although many kinds of stressors have been applied in the various studies of stress (see Rivier and Vale, '871, stimuli from the animal's normal and natural realm of experience have been used as stressors only infrequently. Therefore, the present experiments were to elucidate the effects of a possible chronic environmental stressor, in this case low humidity, on the diurnal corticosterone rhythm in plasma of the lizard Anolis carolinensis.
Anolis carolinensis is found in the humid south- eastern United States. Low humidity may be a biologically relevant stressor for this species because, although A. carolinensis adjusts its rate of cutaneous evaporation to protect its water balance at 30% relative humidity (Kobayashi et al., ,831, sporadic drought conditions in regions of the southeast may result in the total drying of many relatively permanent aquatic areas (Gibbons et al., '83). Water levels are strongly influenced by precipitation and 0 1988 ALAN R. LISS, INC.