When we talk about fruits, we generally think of the fleshy, juicy structures frugivorous birds, bats or primates feed upon, mostly in tropical ecosystems. Indeed, the fruit is the reward the plant offers to the animal that disperses its seeds. But, if we move back to temperate ecosystems, the structures that contain the seeds are generally dry, inconspicuous fruits like achenes or capsules. In his well-known paper, D.H. Janzen (1984) proposed that the reward for the large generalist herbivore dispersers in this case was actually the nutrient-rich foliage surrounding the dry fruits; the “vegetative portions […] function ecologically as an attractive fruit”. This led to the “Foliage is the Fruit” hypothesis.
To revisit this hypothesis and its associated predictions, we present recent results obtained from our long-term research program on long-distance seed dispersal by large herbivores (greenhouse germination tests, feeding experiments …) and from data collected in the literature and compiled in two recent meta-analyses (Albert et al. 2015a, b). We developed an approach based on the traits of the plants being dispersed, the sympatric herbivorous ungulates (Cervus elaphus, Capreolus capreolus and Sus scrofa) studied, and their interactions.
We addressed and confirmed 8 out of the 10 initial predictions proposed by Janzen. As expected, we found that large herbivorous ungulates select the most nitrophilous plants. One other key factor we investigated is the position of the fruit on the plant (“seed releasing height” in trait databases) in relation to animal shoulder height. We also found that the fruits and associated seeds dispersed by the three ungulates mainly belonged to plants from open habitats (grasslands, scrublands …) and that their light, rounded seeds persisted in the long-term in the soil seed bank. The plants consumed mainly corresponded to plants present in the preferred feeding habitats of the three ungulates. Most of the fruits and seeds that clung to animal fur either came from open habitats or from nitrophilous plants; this pattern confirms that nutrient-rich foliage leads to a higher encounter rate between plant reproductive parts and the animal body. We suggest considering this latter assertion as an extension of the “Foliage is the Fruit” hypothesis.