The dispersal mode of any plant species is probably the result of many different pressures and constraints. Phylogenetic constraints are responsible for the fact that entire families or genera usually exhibit only slight variations on a single mode of dispersal. Nonetheless, large variation in some families or genera (e.g., Acacia) is evidence that these constraints are not universal. Other limits to the evolution of fruits and seeds also emerge from the many, sometimes conflicting, selection pressures impinging on different traits (size, shape, pulp chemical composition, seed coat thickness, etc.). Fruit consumption
by vertebrate dispersers, in particular, has selected for fruit traits that enhance their detectability; these fruits, thus, tend to have a conspicuous coloration, distinctive odor, or a combination of both. A common pattern found both in the tropics and in the temperate zones is that bird-dispersed plants usually have red- or black-colored fruits. In some species, a bicolored fruit advertisement, contrasting the ripe fruits with the surrounding foliage, is what presumably gives visual conspicuousness (what has been termed the 'foliar flag' hypothesis) (Figure 4). Also, some ripe fruits reflect ultraviolet (UV) light which enhances the detectability by birds, as their color vision extends to the near-UV. The fruits dispersed by vertebrates also tend to have a pulp rich in water and carbohydrates while being poor in protein and lipids; however, there is much interspecific variability in nutrient composition, and fruit pulp quality does not show to be a trait reflecting plants' adaptations to dispersers. Fruit pulp also usually contains secondary metabolites (phenolics, alkaloids, etc.), sometimes to the point of being lethal to animals, which require an adaptive explanation not yet found. One possibility is that such compounds serve as defense against microbial pathogens and invertebrate pests that preclude the consumption of the fruits by legitimate dispersers.
Regarding animal adaptations, frugivores do not require important morphological and physiological adaptations, especially those that are only occasional frugivores, although the 'strict frugivores' can have the following distinctive traits: such birds tend to have shorter, broader, and flatter bills, and wider gapes than those not consuming fruits; some birds also have smaller and less muscular gizzards, larger livers, and shorter intestines; frugivorous bats have shorter canines and broader palates than insectivorous ones; frugivorous lizards have longer intestines than those consuming mostly animal material. Besides coping with the nutritional imbalance and the secondary compounds of the pulp, these frugivores need to cope with the spatial and temporal unpredictability of fruits. This is probably the reason why there are not frugivores specializing on only one or a few plant species.
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