FIGS & SEXUALITY: Of the c. 150 Bornean fig species 47 species are monoecious (both the tree and each fig fruit on that tree is bisexual) and 103 species are dioecious (unisexual) i.e. each plant is either male or female and produces only male or female figs.
SEX CHANGE: From DNA studies it is thought that all figs were originally dioecious and monoecious figs were a later development in fig evolution. A counter argument is that (morphologically) all dioecious figs retain the ability to become monoecious and in fact this reversal has happened at least once in Ficus evolution with Ficus racemosa. Ficus racemosa is closely related to other dioecious figs but is monoecious. Another possibility is that some dioecious fig trees may retain the ability to temporarily produce figs of the opposite sex but this hypothesis has yet to be systematically investigated.
WORLDWIDE the proportion of monoecious to dioecious fig species is around 50:50. With figs dioecy is best developed and most common in the wet evergreen rainforests of Borneo. However this is generally true of all rainforest plants. Dioecy in plants is most common in the windless understorey of tropical rainforest worldwide.
THE DISADVANTAGES OF DIOECY: In Borneo dioecious figs are usually locally common shrubs and lianas of the sub-canopy whilst monoecious figs are relatively scarce tall trees or stranglers which produce fig fruits (and fig wasps) at canopy level. Research has shown that these canopy level fig wasps are dispersed long distances by wind currents. However dioecious fig wasps are rarely found in canopy level insect traps and are assumed to fly from one fig tree to another in the forest understorey, where wind is almost non-existent.
EXTERNAL CLUES to the sexuality of dioecious figs are most obvious with Sections Sycidium and Eriosycea (all members of which are dioecious). Male figs are nearly always larger and paler than female figs. The fact that some dioecious male and female fig fruits could be distinguished by sight was first pointed out by Corner (1939) in relation to Ficus punctata herbarium collections. Recognition of sex differences of fig fruits in the field was first pointed out by Hill (1967) working in Hong Kong.
Lambert (1992) Fig Dimorphism in Bird-Dispersed Gynodioecious Ficus was the first to describe external morphological sexual differences in Malesian figs.
“A comparison of the figs from three male Ficus species F. heteropleura, F. obscura and F. parietalis at Kuala Lompat, West Malaysia showed that there were distinctive, consistent differences in colour, texture and morphology between figs on male and female plants. Male figs are always paler in coloration than female figs and do not exhibit the colour changes associated with ripening. The male figs of F. parietalis are pale orange at all stages whilst female trees produced orange seed figs which ripen dark red.” Adapted from Lambert (1992).
Ficus parietalis male figs at Mulu. Note that the ostiole is open indicating that male wasps have bored a hole through the bracts surrounding the ostiole allowing the pregnant female wasps to escape and carry male pollen to a nearby receptive female fig.
For an example of a fruiting female Ficus parietalis see this link