The survival contest of endozoochory: Conflicting interests in a frugivorous avian–plant mutualism

Abstract

Abstract The nutritious pulp of fleshy fruits facilitates seed dispersal via endozoochory. Frugivores are considered legitimate seed dispersers when they digest only the pulp but do not damage the seeds. Nevertheless, fruit pulp, in contrast to seeds, is low in nitrogen, thus potentially stimulating seed digestion in the fruit consumers, therefore raising a potential conflict of interest between the plant and the fruit consumer. Pycnonotus xanthopygos is an important distributor of seeds of the desert shrub, Ochradenus baccatus. Our study demonstrated that P. xanthopygos consumed fewer O. baccatus fruits compared with Musa acuminata, due to the presence of bioactive substances O. baccatus pulp juice. The passage of the O. baccatus seeds through the digestive system of P. xanthopygos resulted in damage of up to 80% of the seeds and 20% decrease in the weight of the defecated seeds. Despite the damage to the seeds, passage through the digestive system of P. xanthopygos significantly increased the germination of the surviving seeds. Pulp juice and/or drying of the fruits (on the shrub or in the laboratory) reduced seed germination, and separation of seeds from the pulp (manually or by the bird's digestive system) significantly increased germination. Our study demonstrated variable reduced digestibility of seeds and pulp when P. xanthopygos consumed O. baccatus fruits compared with M. acuminata. This effect was correlated with inhibition of digestive enzymes in O. baccatus fruits. Synthesis. Our results show that there is a complex interaction between Ochradenus baccatus and Pycnonotus xanthopygos where consumption of the fruits is both beneficial and harmful to the plant, due to seed damage on the one hand, yet promotion of seed germination on the other. This suggests that frugivory in general may benefit seed distributors and seed predators simultaneously, yet also highlights a potential conflict of interest inherent in endozoochory.