Digestibilities of dry matter, energy, water, and nitrogen were determ
ined for four foods of desert tortoises (Gopherus agassizii) that were
kept individually in outdoor pens where food intake and feces output
could be measured quantitatively. Two native plants, the forb Malacoth
rix glabrata and the grass Achnatherum (Oryzopsis) hymenoides, and two
exotic plants, the forb Erodium cicutarium and the grass Schismus bar
batus, were collected in the field during the seasons that wild tortoi
ses consumed them (spring for the forbs, summer for the then-dead and
dry grasses), and were then offered to the penned tortoises. The diges
tibilities of the nutrients in the two forbs were similar, ranging fro
m 63-70% for dry matter, 69-73% for energy, 72-79% for nitrogen, and 7
0-75% for water. Which forb was the more nutritious depends on what de
termines feeding rate (appetite) in tortoises. If tortoises eat to obt
ain a given volume of food (''full stomach'') daily, then the exotic f
orb provides significantly more digestible energy and nitrogen, but if
tortoises eat to obtain a given amount of digestible energy (''meet e
nergy needs'') daily, then the native forb provides significantly more
nitrogen and water. The dry matter and energy digestibilities of the
two grasses were much lower than the forbs, but were similar to each o
ther, ranging from 46-50%. However, both grasses provided little or no
nitrogen, and the tortoises lost more water than they gained while pr
ocessing grasses. The type of food plant (forb or grass) and its pheno
logical stage, rather than its geographic origin (native or exotic), b
est predicted its nutritional value.