ABOVE: An immature Yellow-rumped Flowerpecker Prionochilus xanthopygius  feeding on ripe Ficus uniglandulosa figs at the Belalong Field Center in Temburong, Brunei Darussalam, Borneo.

Note the numerous black ants on the twig that appear to be collecting the flaking bark from the twig. The top blue arrow points to a waxy gland at the base of the leaf which provides ant food. The bottom blue arrow points to the base of an ant house (domatia)  which appears to be under construction.

In this article we hypothesize that the  flaking bark characteristic of Ficus uniglandulosa twigs has evolved to provide building materials for ants to construct domatia on the underside of the leaves.   The purpose behind providing housing and food for ants is  to “pay” the mercenary army of ants to stay on the plant to provide a defense against leaf eating insects.

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The orange arrow points to an ant house that is either under construction or  has been attacked  and damaged by an ant predator- probably a bird.
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The orange arrow points to completed ant domatia (housing) on the underside of a Ficus uniglandulosa leaf.
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Herbarium  collections of Ficus glandulosa  are easy to  distinguish from other fig species  because of the characteristic  flaking upper layer (periderm)  of the bark. This  photograph is a close up of the TYPE collection from Penang.

A closely related  Bornean fig Ficus scaberrrima  is also known to have a mutualistic relationship with ants by developing slits  for ants to enter the hollow twigs which can then be used by ants for housing,

According to Maschwitz  (1994)  who made a study 37 different Bornean fig species with hollow twigs,  in a Ficus uniglandulosa twig she examined,  out of 7 internodes  of a slightly swollen hollow branch one had a closed slit similar to the slits she found in Ficus scaberrima. 

Maschwitz et al (1994) Ficus obscura a non specific ant plant,