Abstract
We studied the temperature relations of wild and zoo Aldabra giant tortoises (Aldabrachelys gigantea) focusing on (1) the relationship between environmental temperature and tortoise activity patterns (n = 8 wild individuals) and (2) on tortoise body temperature fluctuations, including how their core and external body temperatures vary in relation to different environmental temperature ranges (seasons; n = 4 wild and n = 5 zoo individuals). In addition, we surveyed the literature to review the effect of body mass on core body temperature range in relation to environmental temperature in the Testudinidae. Diurnal activity of tortoises was bimodally distributed and influenced by environmental temperature and season. The mean air temperature at which activity is maximized was 27.9°C, with a range of 25.8–31.7°C. Furthermore, air temperature explained changes in the core body temperature better than did mass, and only during the coldest trial, did tortoises with higher mass show more stable temperatures. Our results, together with the overall Testudinidae overview, suggest that, once variation in environmental temperature has been taken into account, there is little effect of mass on the temperature stability of tortoises. Moreover, the presence of thermal inertia in an individual tortoise depends on the environmental temperatures, and we found no evidence for inertial homeothermy. Finally, patterns of core and external body temperatures in comparison with environmental temperatures suggest that Aldabra giant tortoises act as mixed conformer–regulators. Our study provides a baseline to manage the thermal environment of wild and rewilded populations of an important island ecosystem engineer species in an era of climate change.
Highlights
Activity and body temperature of reptiles depend on the external thermal fluctuations in the environment and are both drivers and consequences of their physiological and behavioral biology, which affects their ecology (Heatwole, 1976; Huey & Stevenson, 1979; Lailvaux & Irschick, 2007; Van Damme, Bauwens, & Verheyen, 1991)
We focus on (1) the relationship between environmental temperature (Ta) and activity patterns of wild tortoises to determine their optimal environmental temperature range (Ta-opt); and (2) the body temperature fluctuations of captive and wild tortoises, including how their core and external body temperatures vary in relation to environmental temperatures, and whether body mass influences the response of tortoise core body temperatures to environmental temperatures
Under similar Ta conditions as those experienced by Aldabra giant tortoises, leopard tortoises started becoming inactive when Ta increased above 30.5–32°C between 10:00 and 11:00 hr (McMaster & Downs, 2013a), when presumably tortoises seek shade to cool down
Summary
Activity and body temperature of reptiles depend on the external thermal fluctuations in the environment and are both drivers and consequences of their physiological and behavioral biology, which affects their ecology (Heatwole, 1976; Huey & Stevenson, 1979; Lailvaux & Irschick, 2007; Van Damme, Bauwens, & Verheyen, 1991). | 2109 of reptiles, including for example fluxes of radiative heat, convection, conduction, and wind (Cossins & Bowler, 1987; Willmer, Stone, & Johnston, 2005) Understanding this complex thermal environment alone does not allow an adequate description of a reptiles’ activity patterns and core body temperature (Tbc; Table 1 lists the terms adopted here and their definitions). Green iguanas (Iguana iguana) can display physiologically generated circadian rhythms in a constant environmental temperature, similar to those recorded in endotherms (Tosini & Menaker, 1995) Another factor considered important in influencing fluctuations of Tbc in reptiles is their body size, as the surface-to-volume ratio influences the rate at which heat is exchanged with the environment. We surveyed the literature to investigate the effect of body mass on the body temperature ranges of Testudinidae in relation to air temperature
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