3.3
Multimodality: Color and Scent
Animals rarely rely on a single sensory modality, and it would therefore be naive to
think that any of the animals discussed above relies solely on scent to find fruits.
Using different senses, animals respond to a combination of signals and cues which
can be either complementary or redundant [20, 149]. In the former, different cues
provide different information, while in the latter the information is the same and the
function of the redundant signals is either to provide backup or to ensure that a wide
range of frugivores, which emphasize different senses and receive the message.
In fruit foraging and selection, olfaction is often used along with vision or
echolocation in bats. Thies et al. [101] found that olfaction and echolocation play
complementary roles, as the former is used for longer-distance detection and the
latter for more fine-scale final localization of fruits. In contrast, in primates, olfaction
probably plays an important role in close-range selection within a patch [46, 51, 78],
while visual cues have been hypothesized to be most relevant for identification of
fruit patches over longer distances [111]. However, olfactory and visual cues can
also be redundant. This can be demonstrated in polymorphous primate species,
in which some individuals possess full trichromatic vision while the rest are dichromats, i.e., red-green color blind [109, 110]. While feeding in the same patch,
dichromatic white-faced capuchin monkeys (Cebus capucinus imitator) sniff fruits
significantly more than trichromatic group members [117, 150]. This indicates that
dichromatic individuals compensate for the lesser access to visual cues by acquiring
more information through olfaction. Interestingly, ripe fruit ingestion rates are higher
in trichromats [113], possibly indicating that either visual cues are more accurate or,
more realistically, that their acquisition is more time-efficient.
4
Other Factors Affecting Fruit Scent Evolution
While plants are under selection to advertise ripeness through fruit scent, other
factors are likely to drive the evolution of fruit scent. These include other adaptive
functions scent may fulfill, trade-offs, and various constraints.
4.1
Fruit Defense
Like other plant parts, ripe fleshy fruits are subjected to attack by vertebrate,
invertebrate, and microbial antagonists [16]. VOCs are routinely involved in leaf
defense [17] but are rarely considered in fruit defense. Nonetheless, since a major
fraction of fruit VOCs such as terpenes, green leaf volatiles, and phenolics play some
defensive role in other plant parts, they are likely to be involved in fruit defense too
[16]. Thus, plants may be selected to alter the emission rates of individual or multiple
VOCs, and hence change their scent, in response to antagonists. At the same time,
even compounds which are defensive in other tissues or play a defensive role in
fruits may primarily be selected due to their secondary function in attracting seed
17 Fruit Scent: Biochemistry, Ecological Function, and Evolution
415
Multimodality: Color and Scent
Animals rarely rely on a single sensory modality, and it would therefore be naive to
think that any of the animals discussed above relies solely on scent to find fruits.
Using different senses, animals respond to a combination of signals and cues which
can be either complementary or redundant [20, 149]. In the former, different cues
provide different information, while in the latter the information is the same and the
function of the redundant signals is either to provide backup or to ensure that a wide
range of frugivores, which emphasize different senses and receive the message.
In fruit foraging and selection, olfaction is often used along with vision or
echolocation in bats. Thies et al. [101] found that olfaction and echolocation play
complementary roles, as the former is used for longer-distance detection and the
latter for more fine-scale final localization of fruits. In contrast, in primates, olfaction
probably plays an important role in close-range selection within a patch [46, 51, 78],
while visual cues have been hypothesized to be most relevant for identification of
fruit patches over longer distances [111]. However, olfactory and visual cues can
also be redundant. This can be demonstrated in polymorphous primate species,
in which some individuals possess full trichromatic vision while the rest are dichromats, i.e., red-green color blind [109, 110]. While feeding in the same patch,
dichromatic white-faced capuchin monkeys (Cebus capucinus imitator) sniff fruits
significantly more than trichromatic group members [117, 150]. This indicates that
dichromatic individuals compensate for the lesser access to visual cues by acquiring
more information through olfaction. Interestingly, ripe fruit ingestion rates are higher
in trichromats [113], possibly indicating that either visual cues are more accurate or,
more realistically, that their acquisition is more time-efficient.
4
Other Factors Affecting Fruit Scent Evolution
While plants are under selection to advertise ripeness through fruit scent, other
factors are likely to drive the evolution of fruit scent. These include other adaptive
functions scent may fulfill, trade-offs, and various constraints.
4.1
Fruit Defense
Like other plant parts, ripe fleshy fruits are subjected to attack by vertebrate,
invertebrate, and microbial antagonists [16]. VOCs are routinely involved in leaf
defense [17] but are rarely considered in fruit defense. Nonetheless, since a major
fraction of fruit VOCs such as terpenes, green leaf volatiles, and phenolics play some
defensive role in other plant parts, they are likely to be involved in fruit defense too
[16]. Thus, plants may be selected to alter the emission rates of individual or multiple
VOCs, and hence change their scent, in response to antagonists. At the same time,
even compounds which are defensive in other tissues or play a defensive role in
fruits may primarily be selected due to their secondary function in attracting seed
17 Fruit Scent: Biochemistry, Ecological Function, and Evolution
415