arboreal mammals, also the Japanese giant flying squirrel as the case study, in
their seasonal environments. This flying squirrel has a peculiar manner to eat only
the central part of a single leaf. The measurements of the microscale distributions
of chemicals within the single leaf and the seasonal changes in leaf chemicals of
available trees in their habitat suggest that sugar concentration is an important
factor affecting which species of trees they eat, and the total phenolic concentration affects which parts of the single leaf they eat.
Keywords
Glucose · Flying squirrels · Food choice · Quercus trees · Secondary metabolites ·
Total phenolics
1
Introduction
Mammalian herbivores eat plant materials and are forced to make optimal choices
regarding their daily diets by selecting plants with higher nutritional value and by
avoiding those with possible toxins to maintain and develop their bodies [1–3]. The
main nutritional resources from plants are sugars, proteins, and minerals; some
species have higher concentrations of these than others [3]. However, plants usually
include fibers (e.g., cellulose, hemicellulose, and lignin), which are poor energy
sources, and also develop secondary metabolites, such as toxins (e.g., phenolics and
alkaloids) and digestion inhibitors (condensed tannins) [1–5]. Food selection by
mammalian herbivores based on the abundance of valuable nutrients and plant
defensive chemicals has been documented and, in most cases, they select food
materials with substantial amounts of preferable nutrients over those with other
components [2, 6–11].
Many mammals are considered to have four primary taste modalities (sweet,
salty, bitter, and sour), with umami, a savory taste associated with specific amino
acids, being a fifth modality, as in humans [12, 13]. Sweet and umami promote
feeding behavior, whereas bitter and sour tastes reduce food intake, and the
perceptions of sweetness may contribute to preferences for fruits, flowers, and
other foods high in soluble sugars [12]. Clear preferences for sweetness and/or
soluble sugar have been documented in animals ranging from relatively small
mammals, such as frugivorous bats [14, 15], to large mammals, such as roe deer
and primates [8, 9, 16]. In contrast, a significant number of plant secondary
metabolites are bitter or otherwise unpleasant in taste [4]. The taste of bitterness
may predict the deterrent effects of toxins and digestibility-reducing tannin compounds in fruits and leaves [17] and may be a principal cause of food rejection [18,
19]. Therefore, herbivorous mammals often show behavioral avoidance and/or
tolerance of plant defensive secondary metabolites, such as high concentrations of
tannins and related polyphenolics [1, 20–23], although several phenolics such as
flavonoids and tannins also function as antioxidants and these compounds contribute exclusively to the browning reactions observed in injured or pathogen-invaded
plant tissues [24, 25].
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M. Ito and F. Hayashi
their seasonal environments. This flying squirrel has a peculiar manner to eat only
the central part of a single leaf. The measurements of the microscale distributions
of chemicals within the single leaf and the seasonal changes in leaf chemicals of
available trees in their habitat suggest that sugar concentration is an important
factor affecting which species of trees they eat, and the total phenolic concentration affects which parts of the single leaf they eat.
Keywords
Glucose · Flying squirrels · Food choice · Quercus trees · Secondary metabolites ·
Total phenolics
1
Introduction
Mammalian herbivores eat plant materials and are forced to make optimal choices
regarding their daily diets by selecting plants with higher nutritional value and by
avoiding those with possible toxins to maintain and develop their bodies [1–3]. The
main nutritional resources from plants are sugars, proteins, and minerals; some
species have higher concentrations of these than others [3]. However, plants usually
include fibers (e.g., cellulose, hemicellulose, and lignin), which are poor energy
sources, and also develop secondary metabolites, such as toxins (e.g., phenolics and
alkaloids) and digestion inhibitors (condensed tannins) [1–5]. Food selection by
mammalian herbivores based on the abundance of valuable nutrients and plant
defensive chemicals has been documented and, in most cases, they select food
materials with substantial amounts of preferable nutrients over those with other
components [2, 6–11].
Many mammals are considered to have four primary taste modalities (sweet,
salty, bitter, and sour), with umami, a savory taste associated with specific amino
acids, being a fifth modality, as in humans [12, 13]. Sweet and umami promote
feeding behavior, whereas bitter and sour tastes reduce food intake, and the
perceptions of sweetness may contribute to preferences for fruits, flowers, and
other foods high in soluble sugars [12]. Clear preferences for sweetness and/or
soluble sugar have been documented in animals ranging from relatively small
mammals, such as frugivorous bats [14, 15], to large mammals, such as roe deer
and primates [8, 9, 16]. In contrast, a significant number of plant secondary
metabolites are bitter or otherwise unpleasant in taste [4]. The taste of bitterness
may predict the deterrent effects of toxins and digestibility-reducing tannin compounds in fruits and leaves [17] and may be a principal cause of food rejection [18,
19]. Therefore, herbivorous mammals often show behavioral avoidance and/or
tolerance of plant defensive secondary metabolites, such as high concentrations of
tannins and related polyphenolics [1, 20–23], although several phenolics such as
flavonoids and tannins also function as antioxidants and these compounds contribute exclusively to the browning reactions observed in injured or pathogen-invaded
plant tissues [24, 25].
346
M. Ito and F. Hayashi