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161 This conforms to Glasser's (1984) description of a faculta- tive strategist. Schluter (1981)v however, showed that in 15 studies of various species including fish, reptiles, birds* and mammals, only 4 showed an increase in diet breadth with diminishing food supplies, while 8 showed an increase in specialization. It may be that some facultative strategists perform in the opposite way from that proposed by Glasser (1984). The diet of herbivores is generally made up of a variety of food items varying in quality. Large generalist herbi- vores should optimize the nutrient mix within a given food supply which is constrained by availability of foods only at rather low levels of availability (Westoby 1974). However, if a specific nutrient is limiting in the diet, the herbi- vore's diet may reflect preferences for food items with high levels of that nutrient, regardless of other quality charac- teristics. Although nitrogen has been noted as a major fac- tor in determining food preferences (Sinclair 19759 White 19785 Mattson 1980), dietary habits of animals in arid eco- systems appear more often determined by water content of the food than by energy or protein content (Noy-Meir 1974). Desert tortoises in Utah obtain much of their water directly from their food or supplementally as metabolic water derived from food (Woodbury and Hardy 1948). Gal~apagos tortoises have been considered not very selec- tive in their feeding, eating almost any green vegetation they encounter (Slevin .1935, Hintz (1972?], Pritchard 1971,
162 1979a). The first quantitative study of their diet was done on Volcan Alcedo in 1980 (Fowler 1983, Fowler de Neira and Johnson 1985). Prior to that, the majority of descriptions of their diet consisted of general comments or lists of food items either seen eaten or found in their stomachs or feces (Table 5.1). Cactus (primarily Opuntia, though not always so noted) and grasses were often named as the principal components of the tortoise diet (Table 5.1). The importance of cactus as a source of water in arid areas and in times of drought was also stressed (Darwin 1860, Hemsley 1895, Beck 19039 Heller 19039 Slevin 19359 Dawson 1966, Hendrickson 1966, Pritchard 1979a, Fritts 1983). Tortoises are known to obtain a great deal of fluid from their plant diet (fahmoud and Klicka 1979). Cloudsley-Thompson (1971) suggested that all land tortoises can exist indefinitely on a vegetarian diet with- out free water. After grasses and cactus, fruit was the most mentioned food item for Galapagos tortoises (Table 5.1). The small fruits of the endemic Psidium alayaoeium (known locally as guayavilla) were a large portion of the tortoise diet on Volcan Alcedo during garua season (Fowler 1983, Fowler de Neira and Johnson 1985). Slevin (quoted in Van Denburgh 1914, and Fritts and Fritts 1962) thought that the fruits of HiD~omane mancinella were preferred over cactus. Tortois- es may spend up to 1 h feeding solely on this fruit (Rod- house et al. 1975). However, Slevin also stated (Fritts and Fritta 1962):