The metabolic cost of growth in reptiles has been difficult to detect in many previous studies. We designed experiments to detect and quantify added costs of growth by comparing daily energy expenditures of young, growing Desert Tortoises (Gopherus agassizii) with similar-sized, but adult, Parrot-Beaked Tortoises (Homopus areolatus). We measured both field (outdoors, semicaptive) and standard (20°, 25°, and 30°C) metabolic rates of each species in groups matched for body size to minimize body size effects. Mass-adjusted field metabolic rates of the juvenile tortoises were not higher but were, in fact, significantly lower (by 29%) than in adult tortoises. Standard metabolic rates did not differ between species at any temperature, thus failing to support the hypothesized higher energetic costs of growth in the juvenile tortoises. These results, although contrary to expectations, support a growing body of evidence, suggesting that the cost of growth in juvenile reptiles does not increase their field energy expenditures above those expected for adults.
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