Helmeted Hornbill Buceros vigil feeding on ripe Ficus dubia figs next to the Belalong Canopy Walkway in Brunei. Photo by Hans Hazebroek
Ficus dubia produces the largest fig fruit of any strangling fig in Borneo. This is an evolutionary strategy by the fig tree (1) To ensure that the fig is swallowed only by the largest frugivorous birds i.e. hornbills and (2) To avoid seed predation of the fig seeds by green pigeons.
Because they have no teeth, green pigeons have to swallow figs whole when they are ripe but still firm. Like chickens and pheasants, green pigeons have small stones (grit) in their gizzards which are used to grind up fig seeds.
Ficus dubia figs are so large that only the Large Green Pigeon (only one out of five species in Borneo) can swallow Ficus dubia figs whole.