A preliminary test of a prediction from the rafting hypothesis for the presence of non-flying mammals on islands
Non-flying mammals are assumed to have reached oceanic islands by raft from islands of water-edge vegetation. From this hypothesis we can infer that oceanic islands should contain a greater proportion of water-edge species than do continental islands. Without a good sample of mammalian fauna on oceanic islands, we test an altered version of this prediction. At the height of the last ice age, sea levels dropped by 120m. Therefore, immigrants to islands separated by water depths of 120m or more (deep-water islands) should have arrived more often over-water than did immigrants separated by seas of less than 120m depth (shallow-water islands), which immigrants could have reached overland. By comparison to shallow-water islands, deep-water islands should be dominiated by water-edge species. We used a multivariate binomial logit generalized linear model accounting for area of island, median body mass of species, predominant habitat of islands, and island region to compare the numbers of water-edge and total species on deep-water islands to the numbers on nearby shallow-water islands (N = 65 species in 42 genera on 16 deep-water islands and 10 shallow-water islands in three regions of Sunda namely Mentawai off the coast of Sumatra, and Palawan and Sulu, north-east of Borneo). The results contradict the rafting hypothesis: if there was a difference between the deep- and shallow-water islands, water-edge species were significantly less common on the deep-water islands instead of more common. We suggest accidental and deliberate transport by humans as a likely means of cross-sea distribution of terrestrial mammals in the Sunda region.