Seed dispersal of Zoysia japonica by sika deer: An example of the “foliage is the fruit” hypothesis

Zoysia japonica, a low growing grass, is tolerant to grazing and trampling. Kinkazan Island in northern Japan is inhabited by sika deer (Cervus nippon). The deer population increased in the 1970s at a shrine garden in the western part of the island, leading to expansion of Zoysia swards around the shrine garden and their colonization of remote open patches. The expansion around the garden is due to elongation of the rhizome, but expansion to remote places may be due to endozoochory by deer. This appears to be a good example of the “foliage is the fruit” hypothesis (the FF hypothesis) proposed by Janzen (1984; American Naturalist 123:338–353). To demonstrate this, we confirmed the expansion of the Zoysia swards and tested the traits of Zoysia by field surveys and indoor experiments. The Zoysia peduncles stood among the leaves, and sika deer fed on both the seeds and leaves. One deer fecal pellet contained about 20 seeds at its peak in June. In the feeding experiment, the survival rate through digestion was 38%. In the greenhouse experiment, the germination rate was 72%. An outdoor experiment showed that germination rates of the ingested seeds were 5% in a dark habitat and 58% in a bright habitat. Zoysia exhibited many of the traits presented by the FF hypothesis, and sika deer functioned as seed dispersal agents. Therefore, the Zoysia—sika deer relationship seems to support the FF hypothesis.