Two studies examine the effects of energy contents (calorie per unit weight) of
tree leaves but report no effects. Four studies report the effects of leaf containing
sugars and the positive effect is detected only in the black colobus monkey.
The contents of acid and neutral detergent fibers, celluloses, hemicelluloses,
lignins, and their mixes have a negative effect on food selection in most (11 of 16)
cases. Easily digestible leaves are preferred in all of three examinations. Protein (also
N) contents of leaves have usually positive effects (10 of 12 cases). Therefore, the
ratio of preferred proteins to avoid fibers is also positive (3 of 4 cases).
Of 31 examinations of secondary metabolites, 13 have negative effects on food
choice, suggesting this factor important for their food selection, although 18 have no
effects. Particularly in arboreal marsupials such as koalas, possums, and greater
gliders, clear avoidance of the trees whose leaves include high phenolic concentrations is demonstrated [23, 74, 76, 78–82]. In these marsupials, laboratory experiments using artificial foods with different phenolic concentrations also confirm their
food selection according to particular chemicals [74, 83].
Leaves including more water are preferable in all the four examinations. Effects
of ashes and P are not still examined for arboreal herbivores.
2.3
Which Parts of Leaves They Eat
Petioles are generally the toughest part of the leaf, followed by the midribs and
laminae, in 11 species of trees [47]. For both midrib and lamina, there is a positive
correlation between toughness and fiber contents, and Japanese macaques tend to eat
the soft parts. Chemical contents are also compared between leaf laminae and
petioles of Cullenia exarillata leaves as shown in Table 2 [46]. In this tree, however,
the petioles have more water, less crude protein, and lower acid and neutral detergent
fibers than the laminae. No alkaloid reagents are detected in the petioles. These
results suggest the petioles’ easy chewing for the leaf-monkey.
Leaf margins have significantly greater phenolic content than the central parts
of the leaf in several plant species [27, 85] because many herbivorous insects
initiate feeding at leaf edges [27, 28]. In the tree Quercus acutissima, total
phenolic concentrations are often lower in the central part than the margin of
the single leaf and giant flying squirrels prefer the center to the margin ([73], also
see Table 2 and the following Sect. 4). In contrast, total phenolics are distributed
homogenously in the single leaf of Quercus sessilifolia and the squirrels seldom
eat the leaves at only the central part [39]. Many plants employ also structural
defenses such as spines, hairs, and thickened leaves [3, 86], and some herbivores
have developed counter-adaptations to spinescent plants. The caterpillar
Hyphantria cunea consumes the central part of spinescent holly leaves [87], and
the woodrat Neotoma albigula removes the spines when feeding on spinescent
cactus leaves [88]. Spinescent tree species contain significantly less total phenolics and condensed tannins than spineless ones among six species of African
savanna trees [89]. Thus, there is a trade-off between chemical and structural
defenses.
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M. Ito and F. Hayashi
tree leaves but report no effects. Four studies report the effects of leaf containing
sugars and the positive effect is detected only in the black colobus monkey.
The contents of acid and neutral detergent fibers, celluloses, hemicelluloses,
lignins, and their mixes have a negative effect on food selection in most (11 of 16)
cases. Easily digestible leaves are preferred in all of three examinations. Protein (also
N) contents of leaves have usually positive effects (10 of 12 cases). Therefore, the
ratio of preferred proteins to avoid fibers is also positive (3 of 4 cases).
Of 31 examinations of secondary metabolites, 13 have negative effects on food
choice, suggesting this factor important for their food selection, although 18 have no
effects. Particularly in arboreal marsupials such as koalas, possums, and greater
gliders, clear avoidance of the trees whose leaves include high phenolic concentrations is demonstrated [23, 74, 76, 78–82]. In these marsupials, laboratory experiments using artificial foods with different phenolic concentrations also confirm their
food selection according to particular chemicals [74, 83].
Leaves including more water are preferable in all the four examinations. Effects
of ashes and P are not still examined for arboreal herbivores.
2.3
Which Parts of Leaves They Eat
Petioles are generally the toughest part of the leaf, followed by the midribs and
laminae, in 11 species of trees [47]. For both midrib and lamina, there is a positive
correlation between toughness and fiber contents, and Japanese macaques tend to eat
the soft parts. Chemical contents are also compared between leaf laminae and
petioles of Cullenia exarillata leaves as shown in Table 2 [46]. In this tree, however,
the petioles have more water, less crude protein, and lower acid and neutral detergent
fibers than the laminae. No alkaloid reagents are detected in the petioles. These
results suggest the petioles’ easy chewing for the leaf-monkey.
Leaf margins have significantly greater phenolic content than the central parts
of the leaf in several plant species [27, 85] because many herbivorous insects
initiate feeding at leaf edges [27, 28]. In the tree Quercus acutissima, total
phenolic concentrations are often lower in the central part than the margin of
the single leaf and giant flying squirrels prefer the center to the margin ([73], also
see Table 2 and the following Sect. 4). In contrast, total phenolics are distributed
homogenously in the single leaf of Quercus sessilifolia and the squirrels seldom
eat the leaves at only the central part [39]. Many plants employ also structural
defenses such as spines, hairs, and thickened leaves [3, 86], and some herbivores
have developed counter-adaptations to spinescent plants. The caterpillar
Hyphantria cunea consumes the central part of spinescent holly leaves [87], and
the woodrat Neotoma albigula removes the spines when feeding on spinescent
cactus leaves [88]. Spinescent tree species contain significantly less total phenolics and condensed tannins than spineless ones among six species of African
savanna trees [89]. Thus, there is a trade-off between chemical and structural
defenses.
362
M. Ito and F. Hayashi