Costs of Growth in Tortoises

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 (208 ,2 58, and 308C) 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. The additional metabolic cost of growing is demonstrably high in both birds and mammals, averaging 25% of total food energy that is allocated to growth (Wieser, 1994). This metabolic cost of growth is additive to the costs of main- tenance and activity, thus increasing overall daily energy expenditure (Nagy, 2000). Juveniles grow faster than adults; consequently, juveniles should have relatively higher daily food requirements, which confers possible ecological disadvantages (e.g., longer foraging times, greater exposure to predators). Although well documented in birds and mammals, the cost of growth has been difficult to detect in most reptiles studied thus far (Nagy, 2000). However, these studies have not focused specifically on growth costs. Growth is an important biological process for most species and can affect everything from survivorship to lifetime reproductive fitness (Stearns, 1994). Growth rates of young reptiles can be particularly critical; Aresco and Guyer (1999) found that Gopher Tortoises (Gopherus polyphemus) living in disturbed habitats had slower growth rates than those living in intact habitats, which resulted in delayed sexual maturity. The overriding effect of body mass on meta- bolic rate often impairs investigation of other factors influencing energy expenditures of ani- mals. In this study, we had the opportunity to study simultaneously juvenile Desert Tortoises (Gopherus agassizii) and adult Parrot-Beaked Tortoises (Homopus areolatus). As adults, Homopus tortoises are some of the smallest tortoises in the world (Branch, 1994); thus, the two species had similar body masses, thereby minimizing the confounding factor of body mass on metabolic rates. These two species are also from the same family (Testudinidae), and testudines as a whole have relatively slow rates of microevolutionary change among vertebrates (Avise et al., 1992). These similarities gave us a better opportunity to detect metabolic increments associated with growth. Such increments should cause daily energy expenditures (field metabolic rates) to be higher in animals that are growing faster. However, standard metabolic rate (SMR, the energy expenditure of fasting, inactive animals at specified temperatures) may either increase or remain unchanged depending on how quickly the biochemical processes of growth cease after feeding ends.