Why do understorey Licuala palm fruits turn from red to white and then black when ripe?

TL;DR

This study explores how the changing colors of Licuala palm fruits influence animal interactions and seed dispersal in tropical forests.

Contribution

The study provides new insights into the dispersal syndrome hypothesis through color and sugar changes in Licuala palm fruits.

Findings

Birds like forest bulbuls are likely effective dispersers of Licuala palm fruits.

Mammals consume the fruits but do not disperse them effectively.

Color changes in fruits may signal ripeness and attract specific frugivores.

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.

We record the changes in colour, sugar content, reflectance and predation of the fruits of a tropical understorey palm, Licuala ferruginea, as they ripen. Based on these signatures, we posit that the fruits are likely dispersed by birds such as forest bulbuls, and are predated upon but ineffectually dispersed by edge forest mammal species such as wild boar and long‐tailed macaques. This studdy provides indicative support for the dispersal syndrome hypothesis and highlights the potential effects of forest fragmentation on plant–frugivore interactions.

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