In this article we illustrate the very different seeds of 3 Bornean figs and compare the very different dispersal ecology of both fig fruits (primary dispersal) and the fig seeds (secondary dispersal). Seed photos by Dr Zainal Zahari Zainuddin the BORA site manager of the fig orchard at the Sabah Ficus Germplasm Project.
All Borneo’s figs produce very small seeds of very varied shapes and surfaces but relatively similar sizes – between a maximum of 2 mm long and a minimum of 0. 4 mm wide.
In contrast the size of the fig fruits that contain the seeds varies hugely between 4 mm in diameter e.g. F. excavata, F. spiralis and F. lanata up to 15 cm in diameter e.g. F. punctata and F. densechini. There is no relationship between the size of the fig fruit and the size of the seeds. Because fig fruits vary in size much more than fig seeds, large fig fruits tend to contain many more seeds (100,000+ with Ficus punctata) than small figs (e.g. often only 1 or 2 seeds in Ficus deltoidea).
The reason that fig fruits vary so much in size and colour is obvious. Figs fruits have evolved to attract specific targeted disperses. Thus giant orange Ficus punctata figs are targeted at large primates such as gibbons and orangutans with colour vision, big appetites and strong teeth, whilst most small figs that ripen bright red such as F. scaberrima are targeted at small birds with good colour vision that can swallow these figs whole. Birds have no teeth ! Medium sized figs that ripen green are targeted at bat dispersal. They are just the right size to be carried by a bat and they normally grow in locations where they can be snatched easily by a bat in flight. Fruit bats have a very good sense of smell but no colour vision.
So what explains the universally small size of fig seeds and their very varied shapes and surfaces. The advantages of small size are;
(1) Tiny fig seeds are too small for most mammals and birds to eat so they escape predation except by tiny tree mice and beetles.
(2) Small seeds spread the risk of failure. It has been calculated that some large fig trees produce well over a billion seeds in a lifetime. Only one survivor is required for the genes to survive. In addition;
(3) The very varied shapes and surfaces of the tiny seeds are believed to have evolved to enable ants to easily collect, carry and hoard the seeds. Some ants predate the seeds but other ant species just collect the seeds to feed on the dried pectin pockets that surround the seed. Once the ants have collected and stored the seeds they may be lost and forgotten allowing them to survive and germinate.
Note that no detailed study has yet been carried out on the relationship between fig seed morphology and secondary dispersal by ants so these comments should be treated as an untested hypothesis.
Ripe female fig Ficus fistulosa fig. The figs which ripen green are dispersed by small Cynopterus fruit bats. Although the bats can and do disperse some fig seeds the majority are either defecated or dropped below a feeding perch, where they accumulate in small mounds. Thus the very rough and knobbly seeds appear to have evolved to make the seeds easy for ants to carry away from the feeding perch.
Ficus fistulosa figs hang in bunches from the trunk of of the tree making it easy for fruit bats to snatch them in flight and feed on them elsewhere.
Ficus sagittata figs grow high up in the canopy and provide food for small birds which presumably defecate the seeds from a local perch.
Ficus sagittata fig seeds have a clearly defined “wing” around the edge of the seed which presumably evolved so that ants could carry the seeds back to a local storage site.
In comparison to the other fig seeds illustrated above the seeds of Ficus scaberrima are larger than average and their smooth shape and would make it very difficult for them to be carried by an ant. Ficus scaberrima is a small ” strangler” which normally establishes low down in the understorey of a host tree. The seeds are wedge shaped so they easily get jammed in cracks in the bark . It is curious that F. scaberrima fig seeds do not seem to be secondarily dispersed by ants because F. scaberrima has a very strong relationship with ants and even provides specialized housing to attract ants.
Ripe Ficus scaberrima figs are dispersed by small understorey birds particularly bulbuls and flowerpeckers. The seeds are larger than most fig seeds and have no carrying handles for ants.
Ficus scaberrima. The intricate relationships between the figs of Borneo and the ants of Borneo provide many opportunities for further research.