Circadian and Circannual Rhythms in the Metabolism and Ventilation of Red‐Eared Sliders (Trachemys scripta elegans)

Abstract

Endogenous circadian and circannual rhythms may exist in the metabolism, ventilation, and breathing pattern of turtles that could further prolong dive times during daily and seasonal periods of reduced activity. To test this hypothesis, turtles were held under seasonal or constant environmental conditions over a 1-yr period, and in each season, V(O)(2) and respiratory variables were measured in all animals under both the prevailing seasonal conditions and the constant conditions for 24 h. Endogenous circadian and circannual rhythms in metabolism and ventilation occurred independent of ambient temperature, photoperiod, and activity, although long-term entrainment to daily and seasonal changes in temperature and photoperiod were required for them to be expressed. Metabolism and ventilation were always higher during the photophase, and the day-night difference was greater at any given temperature when the photoperiod was provided. When corrected for temperature, turtles had elevated metabolic and ventilation rates in the fall and spring (corresponding to the reproductive seasons) and suppressed metabolism and ventilation during winter. The strength of the circadian rhythm varied seasonally, with proportionately larger day-night differences in colder seasons. Daily and seasonal cycles in ventilation largely followed metabolism, although daily and seasonal changes did occur in the breathing pattern independent of levels of total ventilation. These endogenous circadian and circannual changes in metabolism, ventilation, and breathing pattern prolonged dive times at night and in winter and may serve to reduce the costs of breathing and transport and risk of predation.

Daily and seasonal rhythms in the respiratory sensitivity of red-eared sliders (Trachemys scripta elegans)

“…We have previously shown that there are circadian rhythms in V O2 and V E in red-eared sliders breathing air (Reyes and Milsom, 2009) (Fig. 1D,E).…”

“…The tank was placed inside a wooden box to insulate the turtle from external stimuli (noise and light). Full spectrum lights were set with a timer to control the light and dark cycle as we have previously described (Reyes and Milsom, 2009), and turtles were acclimated for two days to adjust to the handling stress, the experimental tank and the lack of a thermal cycle. On the first day following acclimation turtles were given humidified air to breathe for 24 h. On the second day after acclimation turtles were given a H-H gas mixture (8% oxygen-3% CO 2 ) to breathe for 24 h. Water temperature in the experimental tank was recorded every hour for the length of the experimental treatment with a temperature logger (DS1921 Thermocron iButton, Dallas Semiconductor Corporation) to ensure that temperature remained steady during the experimental trial.…”

“…V O2 and V E were measured simultaneously using open-flow respirometry (for details, see Reyes and Milsom, 2009). Air or the H-H gas mixture was delivered through a port in the side of a breathing funnel at a rate of 350 ml min -1 .…”

“…Such temporal changes in physiological variables may be particularly crucial for the survival of temperate species, which are subjected to large daily and seasonal variations in their environment (changes in day length and temperature cycles). We previously showed that endogenous circadian and circannual oscillations were present in the metabolism and ventilation of red-eared sliders (Trachemys scripta elegans Wied) (Reyes and Milsom, 2009), and were accompanied by changes in breathing pattern that resulted in longer apneas at night and during colder seasons. We speculated that by reducing trips to the surface to breathe, turtles could reduce the cost of locomotion, risk of predation (Cagle, 1950) and, potentially, the cost of breathing (Vitalis and Milsom, 1986) during dormancy.…”

“…the change in air convection requirement; ⌬ACR=⌬V E /⌬V O2, where V E is ventilation and V O2 is oxygen consumption] could be involved in producing the daily and seasonal changes in ventilation and breathing pattern previously reported in red-eared sliders (Reyes and Milsom, 2009). We hypothesized that endogenous rhythms in chemosensitivity contribute to the daily and seasonal oscillations observed in the breathing pattern of red-eared sliders, allowing turtles to remain submerged longer at night and in the winter, which may have implications on their survival.…”