frugivores, fruits tend to be bigger [84]. This could have a strong effect on the
potential of effective chemical signaling in fruits, as all else being equal, larger fruits
would emit stronger scents. In cases where the olfactory signal is selected to allow
animals to identify that an individual fruit is ripe [46, 78], an individual fruit needs to
emit an amount of scent strong enough to be detected. As a consequence, the costs of
olfactory signaling in small fruits would be much higher and might drive them not to
signal ripeness through scent.
4.2.2 Phylogenetic Constraints
Phylogenetic constraints are hard to detect since they are a function of the time since
speciation, selection pressures, and other constraints. Yet a common method to
approximate them is to observe to what extent closely related taxa are similar.
Although fruit scent is harder to compare between species due to its multidimensionality (a scent bouquet is composed of dozens, if not hundreds,
of VOCs), a few studies addressed the question whether ripe fruits of closely related
taxa tend to emit similar scents.
In a community of 30 species from Madagascar, Nevo et al. [51] found no effect
of phylogeny on ripe fruit scent. In a cluster analysis, closely related taxa were found
to be more similar to other species than to congeneric or confamilial species. These
results have been replicated on a sample of 49 species from Uganda (Nevo et al.,
unpublished data) and South Germany [156]. Within the fig clade (Ficus spp.),
a similar trend was found as on a global scale, as far-related taxa with similar
ecology (bat dispersal) were found to be more similar to one another than they are
to more closely related bird-dispersed species [38]. However, at a more local level,
closely related taxa that are dispersed by bats did show clustering, indicating some
phylogenetic conservatism in fruit scent [38].
An important point is that all these studies dealt with the different VOCs in fruit
scent profiles as independent variables. In other words, they assume that a switch
from compound A to B is equally likely to a switch from A to C. This assumption is
problematic because some compounds are synthesized through the same pathways,
and thus switches between them are more likely [153]. A recent study offers
a method to address this issue by integrating the biochemical pathways to statistical
analyses [153]. However, this approach is not easily implemented in large datasets in
which many compounds are not fully identified, and hence not all biochemical
pathways are known.
5
Conclusions and Future Directions
The understanding of the evolution and ecological functions of fruit scent has
evolved tremendously in the past decade. The VOC profiles of ripe, and sometimes
unripe, fruits of dozens of species from the neotropics, continental Africa,
Madagascar, and Southeast Asia have been published and used to address various
evolutionary and ecological questions. In combination with some behavioral studies,
they have shown that fruits emit a tremendous diversity of scents which are used by
17 Fruit Scent: Biochemistry, Ecological Function, and Evolution
417
potential of effective chemical signaling in fruits, as all else being equal, larger fruits
would emit stronger scents. In cases where the olfactory signal is selected to allow
animals to identify that an individual fruit is ripe [46, 78], an individual fruit needs to
emit an amount of scent strong enough to be detected. As a consequence, the costs of
olfactory signaling in small fruits would be much higher and might drive them not to
signal ripeness through scent.
4.2.2 Phylogenetic Constraints
Phylogenetic constraints are hard to detect since they are a function of the time since
speciation, selection pressures, and other constraints. Yet a common method to
approximate them is to observe to what extent closely related taxa are similar.
Although fruit scent is harder to compare between species due to its multidimensionality (a scent bouquet is composed of dozens, if not hundreds,
of VOCs), a few studies addressed the question whether ripe fruits of closely related
taxa tend to emit similar scents.
In a community of 30 species from Madagascar, Nevo et al. [51] found no effect
of phylogeny on ripe fruit scent. In a cluster analysis, closely related taxa were found
to be more similar to other species than to congeneric or confamilial species. These
results have been replicated on a sample of 49 species from Uganda (Nevo et al.,
unpublished data) and South Germany [156]. Within the fig clade (Ficus spp.),
a similar trend was found as on a global scale, as far-related taxa with similar
ecology (bat dispersal) were found to be more similar to one another than they are
to more closely related bird-dispersed species [38]. However, at a more local level,
closely related taxa that are dispersed by bats did show clustering, indicating some
phylogenetic conservatism in fruit scent [38].
An important point is that all these studies dealt with the different VOCs in fruit
scent profiles as independent variables. In other words, they assume that a switch
from compound A to B is equally likely to a switch from A to C. This assumption is
problematic because some compounds are synthesized through the same pathways,
and thus switches between them are more likely [153]. A recent study offers
a method to address this issue by integrating the biochemical pathways to statistical
analyses [153]. However, this approach is not easily implemented in large datasets in
which many compounds are not fully identified, and hence not all biochemical
pathways are known.
5
Conclusions and Future Directions
The understanding of the evolution and ecological functions of fruit scent has
evolved tremendously in the past decade. The VOC profiles of ripe, and sometimes
unripe, fruits of dozens of species from the neotropics, continental Africa,
Madagascar, and Southeast Asia have been published and used to address various
evolutionary and ecological questions. In combination with some behavioral studies,
they have shown that fruits emit a tremendous diversity of scents which are used by
17 Fruit Scent: Biochemistry, Ecological Function, and Evolution
417