Fruit hardness, size, and weight for wild fruits in Uganda and Madagascar

Fleshy fruits have evolved to be attractive to frugivorous seed
dispersers. As a result, many fruit traits like size, color, scent and
nutritional content are assumed to be the result of selective pressures
exerted by frugivores. At the same time, fruit traits are also subjected
to a set of other selective pressures and constraints. One such trait is
fruit hardness. On one hand, haptic cues have been suggested to play a
role in frugivore behavior, potentially driving selection on fruit skin
traits, including hardness. On the other hand, fruit skin traits also
interact with antagonists, e.g., seed predators and microbes, which can
also impose selective pressures, particularly with respect to fruit
hardness.  Yet skin hardness may also be driven not by adaptive
response to biotic pressures, but rather a mechanical constraint: fruits
are packed with water-rich pulp and seeds which exert mechanical pressure
on the skin. Skin hardness may simply result from the need for fruits to
withhold internal pressure to maintain fruit integrity. We test the fruit
mechanical constraint hypothesis on a set of 95 native Malagasy plant
species. We used a theoretical structural formula to model internal
pressure experienced by fruit skins to test whether variation in pressure
predicts fruit hardness. We find that, while positively correlated,
internal stress explains very little of the variance in fruit hardness.
This suggests that mechanical constraints play a minor role in driving
fruit hardness, and that biotic factors may therefore contribute
substantially to variation in fruit hardness.