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.