Bruce T. Firth

Daily and seasonal rhythms in selected body temperatures in the Australian lizard Tiliqua rugosa (Scincidae): field and laboratory observations.

This study examined daily and seasonal activity and thermoregu‐latory behaviour of the sleepy lizard, Tiliqua rugosa, a large, diurnally active temperate‐dwelling Australian lizard, in the field and laboratory. Activity temperatures in the field were compared with those selected by lizards in laboratory thermal gradients in order to assess the extent to which endogenous versus exogenous factors contribute to seasonal variations in thermoregulatory behaviour. In the field, lizards are most active in late winter–spring (August –November), during which their activity varies from mostly unimodal on days of mild temperature to bimodal on hot days. In late spring–summer (November–January), activity is largely restricted to early morning, and at all other seasons sleepy lizards are rarely active. The winter–spring activity of sleepy lizards is constrained by low environmental temperatures, as lizards at these seasons have low body temperatures in the field but higher temperatures in laboratory thermal gradients. The lower temperatures selected in the laboratory in the summer–autumn months suggest the avoidance of high ambient temperatures and general inactivity in the field at these times. Thermal selection in the laboratory at the eight times of year tested showed that the phase of the minimum and maximum temperature selected and the amplitude of the rhythm of temperature selected varied continuously with the time of year. These daily and seasonal shifts in thermoregulatory behaviour may be regulated by endogenous physiological mechanisms coupled with seasonal ecological constraints such as food availability.

Melatonin content of the pineal, parietal eye and blood plasma of the lizard, Trachydosaurus rugosus: effect of constant and fluctuating temperature.

Lizards acclimated to a light cycle accompanied by a thermocycle of 30 degrees C/15 degrees C had a more robust rhythm in pineal and plasma melatonin levels than those acclimated to constant 30 degrees C. At constant 15 degrees C, the melatonin rhythm was abolished entirely. A similar thermosensitivity in melatonin content was found in the parietal eye, indicating that this photoreceptive structure may also be capable of synthesising methoxyindoles.

Plasma melatonin in the scincid lizard, Trachydosaurus rugosus: Diel rhythm, seasonality, and the effect of constant light and constant darkness

Abstract Melatonin was assayed in the plasma of the scincid lizard, Trachydosaurus rugosus , using a specific radioimmunoassay. The levels of this indole exhibited a daily fluctuation similar to that observed in many other vertebrates. Plasma melatonin titers were low during the light phase and elevated during the dark phase when exposed to a lighting regimen of 13 hr light and 11 hr dark. In lizards captured in spring and transferred to this regimen, the amplitude of the plasma melatonin oscillation appeared damped in comparison to those sampled in the autumn. This damping was attributed to higher light phase and lower dark phase concentrations in spring. In one experiment, a correlation between size and plasma melatonin concentration was evident, higher levels being present in smaller animals. In contrast to birds and mammals, where a rhythm in blood melatonin content persists in constant dark, the plasma melatonin rhythm of T. rugosus was abolished by both constant light and constant darkness under constant temperature conditions.

Effect of constant temperatures, darkness and light on the secretion of melatonin by pineal explants and retinas in the gecko Christinus marmoratus.

The effects of temperature and lighting conditions on the secretion of melatonin by the pineal organ of the nocturnal gecko Christinus marmoratus was studied using in vitro perifusion. In a 12L:12D lighting regime, a high-amplitude melatonin rhythm was detectable at a constant temperature of 20 and 30 degrees C but not at 10 or 37 degrees C. There were sustained high levels of melatonin in constant darkness and sustained low levels in constant light. No retinal melatonin was detected using static and perifusion culture techniques. These results show that the pineal organ of C. marmoratus maintains light sensitivity in vitro but does not contain an oscillator coupled to the melatonin synthetic pathway.

Polyunsaturated dietary lipids lower the selected body temperature of a lizard

SummaryCold acclimation lowers the selected body temperature (Tb) in many ectothermic vertebrates. This change in behavioural thermoregulation is accompanied by an increase in the proportion of polyunsaturated fatty acids in tissues and cellular membranes. We investigated how diets containing different fatty acids, known to significantly alter the fatty acid composition of animal tissues and membranes, affect the selected Tb of the lizard Tiliqua rugosa. Lizards on a diet containing many polyunsaturated fatty acids (10% sunflower oil) showed a 3–5°C decrease in Tb, whereas Tb in animals on a diet containing mainly saturated fatty acids (10% sheep fat) did not change. Our study suggests that the composition of dietary lipids influences thermoregulation in ectothermic vertebrates and may thus play a role in the seasonal adjustment of their physiology.

Thermoperiod and photoperiod interact to affect the phase of the plasma melatonin rhythm in the lizard, Tiliqua rugosa

Rhythms of plasma melatonin levels were determined in lizards (Tiliqua rugosa) subjected to 6 h thermocycles (6 h, 33 degrees C thermophase; 18 h, 15 degrees C cryophase) placed at 4 different phases of a 12 h photocycle (12 h light: 12 h dark). The peak of the melatonin rhythm was either shifted at different rates, or inhibited by the light phase of the photocycle, depending upon the phase relationship between the thermocycle and the photocycle. The results indicate that the pineal organ of ectotherms is part of a circadian pacemaker system, transducing photothermal environmental information into a neurochemical signal.

Thermocyclic entrainment of lizard blood plasma melatonin rhythms in constant and cyclic photic environments

We assessed how chronic exposure to 6-h cryophase temperatures of 15 degrees C in an otherwise 33 degrees C environment entrains the rhythm of blood plasma melatonin rhythms in lizards (Tiliqua rugosa) subjected to constant dark (DD), constant light (LL), and to 12:12-h light-dark cycles (12L:12D). The peak of the melatonin rhythm was entrained by the cryophase temperature of the thermocycle in DD and LL, irrespective of the time at which the cryophase temperature was applied. Comparable thermocycles of 6 h at 15 degrees C imposed on a 12L:12D photocycle, however, affected the amplitude and phase of the melatonin rhythm, depending on the phase relationship between light and temperature. Cold pulses in the early light period and at midday resulted, respectively, either in low amplitude or nonexistent melatonin rhythms, whereas those centered in or around the dark phase elicited rhythms of high amplitude. Supplementary experiments in 12L:12D using two intermittent 6-h 15 degrees C cryophases, one delivered in the midscotophase and another in the midphotophase, elicited melatonin rhythms comparable to those in lizards subjected to constant 33 degrees C and 12L:12D. In contrast, lizards subjected to 12L:12D and a 33 degrees C:15 degrees C thermocycle, whose thermophase was aligned with the photophase, produced a threefold increase in the amplitude of the melatonin rhythm. Taken together, these results support the notion that there is an interaction between the external light and temperature cycle and a circadian clock in determining melatonin rhythms in Tiliqua rugosa.We assessed how chronic exposure to 6-h cryophase temperatures of 15°C in an otherwise 33°C environment entrains the rhythm of blood plasma melatonin rhythms in lizards ( Tiliqua rugosa) subjected to constant dark (DD), constant light (LL), and to 12:12-h light-dark cycles (12L:12D). The peak of the melatonin rhythm was entrained by the cryophase temperature of the thermocycle in DD and LL, irrespective of the time at which the cryophase temperature was applied. Comparable thermocycles of 6 h at 15°C imposed on a 12L:12D photocycle, however, affected the amplitude and phase of the melatonin rhythm, depending on the phase relationship between light and temperature. Cold pulses in the early light period and at midday resulted, respectively, either in low amplitude or nonexistent melatonin rhythms, whereas those centered in or around the dark phase elicited rhythms of high amplitude. Supplementary experiments in 12L:12D using two intermittent 6-h 15°C cryophases, one delivered in the midscotophase and another in the midphotophase, elicited melatonin rhythms comparable to those in lizards subjected to constant 33°C and 12L:12D. In contrast, lizards subjected to 12L:12D and a 33°C:15°C thermocycle, whose thermophase was aligned with the photophase, produced a threefold increase in the amplitude of the melatonin rhythm. Taken together, these results support the notion that there is an interaction between the external light and temperature cycle and a circadian clock in determining melatonin rhythms in Tiliqua rugosa.

CIRCADIAN RHYTHM OF BEHAVIORAL THERMOREGULATION IN THE SLEEPY LIZARD (TILIQUA RUGOSA)

The daily rhythm of behavioral thermoregulation in sleepy lizards (Tiliqua rugosa) was studied in laboratory thermal gradients under a 12L∶12D photoperiod in autumn and spring. In both seasons, lizards showed distinctive daily rhythms of behavioral thermoregulation, selecting their highest body temperatures during the late photophase and lowest ones during the late scotophase or early photophase. When lizards were subjected to a period of constant darkness at both seasons, this rhythm persisted and free-ran with a period of 24.7 h, indicating that it is an endogenous circadian rhythm. Furthermore, the expression of this circadian rhythm varies seasonally in the amplitude of its free-running rhythm and in the phase at which the minimum Tb is selected during 12L∶12D. The seasonal variation in the expression of the circadian rhythm of behavioural thermoregulation in Tiliqua rugosa lizards is likely to reflect seasonal changes in the neural regulation of this rhythm.

Thermoperiodic influences on plasma melatonin rhythms in the lizard tiliqua rugosa effect of thermophase duration

Rhythms of plasma melatonin levels were determined in lizards (Tiliqua rugosa) subjected to a 12 h photocycle (12 h light: 12 h dark) at constant 33 degrees C, and at 7 different thermoperiods (33 degrees C thermophase and 15 degrees C cryophase) whose thermophase duration ranged from 1.5 to 21 h. The melatonin secretion rate, as measured by the amplitude and duration of elevated melatonin levels and the area under the curve, was maximal at thermoperiods whose thermophase was between 9 and 18 h in duration. The results indicate that in ectothermic vertebrates the prevailing thermoperiod as well as the photoperiod may influence melatonin rhythms and hence the timing of annual physiological cycles.

Interseasonal variation in the circadian rhythms of locomotor activity and temperature selection in sleepy lizards, Tiliqua rugosa.

Few studies in non-mammalian vertebrates have examined how various effectors of the circadian system interact. To determine if the daily locomotor and behavioural thermoregulatory rhythms of Tiliqua rugosa are both controlled by the circadian system in different seasons, lizards were tested in laboratory thermal gradients in four seasons and in constant darkness. Circadian rhythmicity for both rhythms was present in each season, being most pronounced in spring and summer and least evident in autumn. Most lizards displayed a unimodal locomotor activity pattern across all seasons. However, some individuals presented a bimodal locomotor activity pattern in spring and summer. Seasonal variations in the phase relationships of both rhythms to the light:dark (LD) cycle were demonstrated. No seasonal differences in the free-running period lengths of either rhythm were detected, raising the possibility that a single circadian pacemaker drives both rhythms in this species. Our present results demonstrate that both rhythms are similarly controlled by the circadian system in each season. Although seasonal variations in the thermal preferences of reptiles both in the field and laboratory have previously been well documented, this study is the first to demonstrate circadian rhythms of temperature selection in a reptile species in each season.