Biology and Conservation of North American Tortoises

10 Water and Food Acquisition and Their Consequences for Life History and Metabolism of North American Tortoises

10 Acquisition of sufficient food and water and the physiological consequences that occur when resource availability fluctuates are key to understanding the maintenance (Peterson 1996a, b), growth (Medica et al. 2012), reproduction (Henen 2002b), and health of tortoises (Jacobson et al. 1991) and wildlife populations generally (Robbins 1983). Diet information can inform studies on the nutritional requirements of herbivorous ectotherms (Nagy 1977, Christian et al. 1984, Bjorndal 1987, Pough 1973, Zimmerman and Tracy 1989, Troyer 1991, Oftedal et al. 2002, Tracy et al. 2006). Translating this information into nutritional status can be challenging, however, because of the tortoise’s ability to persist for long periods without food or water and to amortize nutritional gains and losses over periods extending over years (Nagy and Medica 1986, Henen 1997, Peterson 1996b, Hellgren et al. 2000, Oftedal et al. 2002). In this chapter, we compare and contrast the acquisition of water, food, and their influence on nutrition in relation to tortoise life history and physiology. Because of the paucity of information on smaller life stages of tortoises, the information here focuses on adults, unless specified. This distinction is important because of the challenges associated with the small body size of neonate and juvenile tortoises (hereafter called juveniles). Smaller body size has morphological, physiological, behavioral, and ecological consequences for juvenile tortoises relative to adults. For example , juvenile tortoises are smaller and weaker, cannot travel as far, and have reduced fields of view and relatively large requirements for body-building nutrients. Smaller tortoises also have body temperatures and hydric status that conform rapidly to ambient conditions; thus, internal stasis is challenging (Morafka 1994, Morafka et al. 2000). These factors tremendously influence requirements for drinking and accessibility to feeding opportunities that juvenile tortoises respond to by modifying their behaviors and increasing efficiencies. Opportunities for water and food acquisition among North American tortoises are driven by environmental factors at global, regional, and site-specific scales (chapter 9), as the five species’ ranges nearly span the southern extent of the continent, encompassing the driest to wettest regions. For example, Gopherus agassizii near Barstow, California, experiences an average of ~104 mm of precipitation annually, while G. polyphemus in southwest Georgia experiences 1270 mm (Turner 1994, McRae et al. 1981). Gopherus agassizii experiences predominately winter rainfall, G. morafkai experiences 50:50 summer-to-winter rainfall in their north-central range (near Tucson, Arizona), and G. flavomarginatus experiences predominantly summer rainfall. Gopherus berlandieri has moisture availability that is intermediate between the three western-most species and G. polyphemus. The majority of rainfall for the eastern species occurs during summer months, enhanced by tropical storms, although the northern part of the range experiences relatively even amounts of precipitation throughout the year (Germano 1994). Precipitation variability is greater where the three western species live than for the other two species. Habitats spanning this range of conditions would seemingly result in divergent challenges for water and food acquisition. Closer inspection of the microenvironments that tortoises occupy reveals the similarities and differences in how tortoises acquire water and nutrition and in the strategies they employ to overcome environmental variability. WateR aCQUisition Hydration Tortoises require free water for hydration, and opportunities for water acquisition vary considerably among North American tortoises. Gopherus agassizii has the fewest drinking opportunities among North American tortoises. Extreme aridity (100–200 mm/y) and infrequent precipitation events demand that they drink whenever possible, including winter . Winter drinking for G. agassizii occurs at temperatures as low as 6°C (T. Esque and K. Nussear, unpublished data). It is striking to discover this ectotherm exiting hibernation todd C. Esque k. kristina Drake kenneth E. Nussear Water and Food Acquisition and Their Consequences for Life History and Metabolism of North American Tortoises todd C. Esque, k. kristina Drake, and kenneth E. Nussear 86 Todd C. Esque, K. Kristina Drake, and Kenneth E. Nussear drinking. In those areas, light rainfall percolates immediately as it falls and is largely unavailable as free water. In coastal areas, G. berlandieri occupies silt/clay-dominated lomas surrounded by bottomlands seasonally inundated by seawater. Further inland, the species occupies seasonally humid shrublands with sandy soils, where only heavy rains or patches of less permeable materials provide drinking sites (Rose and Judd 1982). Because of variable permeability among soil surfaces, intermittent puddling occurs in microsites that are patchy and widespread in the landscape. Tortoises are not only opportunistic about drinking, but they employ active water acquisition, including environmental engineering and using their own bodies to form water catchments. Gopherus agassizii locates water...