containing secondary metabolites by their bitter or unpalatable taste through experiences of toxicity or postingestive effects [4, 22, 65, 66]. However, only a few (5 of
42) examinations show negative effects, but most (33 of 42) show no effects and
some (4 of 42) show positive effects on food selection (Table 1). Clear negative
effects are shown particularly in food selection by arboreal marsupials among
conspecific trees variable greatly in their leaf phenolic contents (also see Sect. 2.2).
Thus, interspecific comparisons do not support the general prediction of the
negative effects of the secondary metabolites on their food choice. This is probably
because correlations between secondary metabolite and other leaf chemical concentrations mask the effects of each factor on herbivore’s food selection. For example,
there is a positive correlation between sugar and phenolic contents between Quercus
tree species (see Sect. 4). If the herbivores select leaves with higher sugar concentrations, they also select leaves with higher phenolic concentrations, resulting positive selection by phenolics in Table 1 [39]. Besides such methodological difficulties,
this is partly caused by herbivore’s feeding behavior that they often consume a small
amount of leaves from a variety of taxonomically distinct tree species. This type of
feeding may minimize a risk of toxic damages by the leaves with unknown toxic
substrates and/or have a role for continuous sampling to learn their toxicity [1, 67,
68]. Learning is emphasized to be important for herbivorous mammals to select
appropriate food in the field because they live long and establish their home ranges in
the spatially structured forest [5, 69, 70]. Repeated experiences enable them to learn
appropriate feeding sites, leaf selection, and feeding manners. On the other hand,
some mammals can produce salivary proteins as a defense against dietary tannins
[64]. In black howler monkeys, they always secrete tannin-binding salivary proteins
and therefore they obtain nutrients from leaves even containing high levels of
tannins [51].
Leaves containing more water are preferable by giant flying squirrels, but this
factor is not seemed to affect food selection of primates (Table 1). Water contents
may be one of the indices of leaf softness, so that herbivores may prefer to eat leaves
including more water if they avoid harder leaves [47, 48, 71]. The south Indian leaf
monkey prefers young leaves containing more water to mature leaves. In this case, it
is suggested that leaves with more water may include more water-soluble materials
such as sugars and minerals [46]. The functions of leaf water in leaf selectivity will
be carefully treated depending on the situation.
Ash concentrations affect food selection positively in colobus monkeys and
chimpanzees, but do not in other herbivores (Table 1). Elemental analysis of leaves
is unclear in its effect on leaf selectivity (Table 1).
2.2
Which Leaves They Eat Within the Same Species
Comparisons of the effects of leaf chemicals among different tree species in the
previous Sect. 2.1 include many complicated factors for analyses. Leaf chemicals
often co-vary and multispecies comparisons sometimes lead misunderstanding by
ignoring phylogenetic constraints [72]. To be clear the effects of leaf chemicals on
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M. Ito and F. Hayashi
42) examinations show negative effects, but most (33 of 42) show no effects and
some (4 of 42) show positive effects on food selection (Table 1). Clear negative
effects are shown particularly in food selection by arboreal marsupials among
conspecific trees variable greatly in their leaf phenolic contents (also see Sect. 2.2).
Thus, interspecific comparisons do not support the general prediction of the
negative effects of the secondary metabolites on their food choice. This is probably
because correlations between secondary metabolite and other leaf chemical concentrations mask the effects of each factor on herbivore’s food selection. For example,
there is a positive correlation between sugar and phenolic contents between Quercus
tree species (see Sect. 4). If the herbivores select leaves with higher sugar concentrations, they also select leaves with higher phenolic concentrations, resulting positive selection by phenolics in Table 1 [39]. Besides such methodological difficulties,
this is partly caused by herbivore’s feeding behavior that they often consume a small
amount of leaves from a variety of taxonomically distinct tree species. This type of
feeding may minimize a risk of toxic damages by the leaves with unknown toxic
substrates and/or have a role for continuous sampling to learn their toxicity [1, 67,
68]. Learning is emphasized to be important for herbivorous mammals to select
appropriate food in the field because they live long and establish their home ranges in
the spatially structured forest [5, 69, 70]. Repeated experiences enable them to learn
appropriate feeding sites, leaf selection, and feeding manners. On the other hand,
some mammals can produce salivary proteins as a defense against dietary tannins
[64]. In black howler monkeys, they always secrete tannin-binding salivary proteins
and therefore they obtain nutrients from leaves even containing high levels of
tannins [51].
Leaves containing more water are preferable by giant flying squirrels, but this
factor is not seemed to affect food selection of primates (Table 1). Water contents
may be one of the indices of leaf softness, so that herbivores may prefer to eat leaves
including more water if they avoid harder leaves [47, 48, 71]. The south Indian leaf
monkey prefers young leaves containing more water to mature leaves. In this case, it
is suggested that leaves with more water may include more water-soluble materials
such as sugars and minerals [46]. The functions of leaf water in leaf selectivity will
be carefully treated depending on the situation.
Ash concentrations affect food selection positively in colobus monkeys and
chimpanzees, but do not in other herbivores (Table 1). Elemental analysis of leaves
is unclear in its effect on leaf selectivity (Table 1).
2.2
Which Leaves They Eat Within the Same Species
Comparisons of the effects of leaf chemicals among different tree species in the
previous Sect. 2.1 include many complicated factors for analyses. Leaf chemicals
often co-vary and multispecies comparisons sometimes lead misunderstanding by
ignoring phylogenetic constraints [72]. To be clear the effects of leaf chemicals on
356
M. Ito and F. Hayashi