Why do understorey Licuala palm fruits turn from red to white and then
black when ripe?
Abstract
Licuala ferruginea Becc., a tropical forest understorey palm, is
observed to have fruits that appear red in colour when unripe, turning
pink, then white, purple and finally black in colour as they ripen. We
monitored 13 fruiting palms in rainforest fragments and recorded the
consumption of fruits by animals via camera traps. We also documented
the fruiting phenology of two palms in the nursery. In the rainforest
fragments, a Cream-vented Bulbul (Pycnonotus simplex) was observed
plucking a mature purple fruit from a L. ferruginea palm, before flying
away with the fruit in its beak. This was the only bird that was
observed feeding on the mature fruit. A range of mammals, dominated by
edge species such as the Long-tailed Macaque and Wild Boar, were
observed to consume L. ferruginea fruits indiscriminately across all
five colour stages, thereby limiting the dispersal of the fruits. Forest
bulbul gape sizes also matched the fruit size, suggesting that forest
bulbuls are the likely dispersers of the palm in the original forest
where edge species are not in high densities. We further posit that the
initial phase of red fruits, with high contrasting red reflectance
against a green foliage background, might be a form of early
advertisement to birds. The fruit then turns pink and white, which have
high green reflectance and is less contrasting, thereby reducing the
conspicuity of the fruit. This allows the fruit to ripen with high
fructose and glucose content, and turn purple and black, which are known
visual cues for birds. This study provides indicative support for the
dispersal syndrome hypothesis and highlights the potential effects of
forest fragmentation on plant-frugivore interactions. Keywords: Licuala,
Arecaceae, dispersal syndrome, fruit, bulbul, reflectance