Is endozoochoric seed dispersal by large herbivores an evolutionary adaptation? Revisiting the Janzen's ‘Foliage is the fruit’ hypothesis

Many plant species develop fruits to attract animals that will eat them and then disperse the seeds. However, there are many plant species, whose seeds are dispersed endozoochorically, but their fruits are not particularly attractive to animals. The “Foliage is the fruit” (FF) hypothesis proposes that entire biomass of the plant exists to encourage herbivores to eat it, in order to enhance seed intake and dispersal (Janzen, 1984).

We tested the FF hypothesis by combining the results from the greenhouse seedling emergence method (GR) and DNA-metabarcoding of plant remnants in faeces of European moose (Alces alces L.). We processed 665 samples by the GR and 429 by the MB method, hypothesizing that if the safe passage of seeds through the gut of a large herbivore is the result of an evolutionary adaptation to endozoochoric dispersal, then the species composition of plants revealed by the two methods should largely overlap and the abundance of seedlings revealed by the GR method should be positively correlated in time with the read abundance of DNA of the same species. The large discrepancy between the lists of species detected by DNA metabarcoding and the GR method argues against the FF hypothesis. However, in the case of Urtica dioica, Lysimachia vulgaris and Lythrum salicaria, some clues of evolutionary adaptation to endozoochoric dispersal were revealed for: 1) their foliage is attractive to herbivores; 2) seeds are small, rounded in shape, yielded in large numbers and pass safely through the herbivore's gut; 3) the abundance of seeds (seedlings) was significantly and strongly influenced by the abundance of the plant biomass (DNA reads) in dung samples; 4) peaks in seed abundance and biomass consumption coincided in time. However, it should be considered that moose's diet is mostly composed of woody browse, which makes this animal not an optimal model for testing the Janzen's hypothesis and studies on typical grazers are needed in this respect.