Stand beneath a fig tree, look up and scan for flowers. None!
This is because hundreds of minuscule flowers line the inner surface of each fig, never to see sunlight. Pollination of these hidden flowers is a complicated story.
A normal flower rests on the slightly swollen end of its stalk, called the receptacle. In all species of figs, flowers still rest on the receptacle. But the receptacle has extended outward, upward and over to create a hollow sphere, with the tiny, densely packed flowers lining the inner surface of this sphere.
The ostiole is located at the opposite end of the fig to the stalk. It is a tiny, tight entrance into the fig, the only access for the tree's specific pollinator fig to reach the flowers. The ostiole is designed as a one-way passage. No exit through the ostiole is possible!
The image below shows a syconium, or fig, of a Ficus sycomorus, cut in half, showing its tiny flowers, and the ostiole at the bottom.
Pollination of female flowers sealed inside a fig is achieved in a mutual relationship between the tree and tiny wasps. These wasps are about one to two millimetres in length.
Each fig species has its own pollinating wasp species. There are about 755 Ficus species in the world, and a like number of pollinating wasp species – one unique species of wasp for each species of Ficus, with minor exceptions. Hybridisation between species is thus blocked.
The Fig tree cannot reproduce without the wasp, and the wasp can only breed inside the figs. If either the fig or wasp species dies out, the inevitable consequence is extinction for the other.
But the story is not perfect. Some species of wasps, entirely different from the pollinating wasps, use figs purely for their own reproduction. They pierce the fig from the outside with their unusually long ovipositors, shown in the image below. They lay their eggs inside the fig.
The grubs that emerge from these alien eggs will kill developing fig wasps and take over their galls.
These parasitic wasps are obstacles in nature's remarkable process of fig and pollinating wasp reproduction. Despite this, the fig tree and it's wasps always overcome alien hurdles and both have been reproducing successfully over millions of years.
Fig trees that are ready for their female flowers to be pollinated emit a pheromone, a chemical scent, or volatile, that is unique to that species of fig. The scent is called a floral volatile organic compound. Only the correct wasp species is attracted to that scent.
Once a female wasp follows the scent to a fig, she fights her way in through the ostiole. Her body and wedge-shaped head are designed to assist her passage, and the ostiole with its bracts is specifically designed to allow only her species to enter.
The female wasp usually loses her wings and antennae as she drags herself through the tight-fitting ostiole. This does not matter, for she will die in the fig.
Once inside the fig, the female wasp, typically with pollen sacs full, lays her own eggs, one per flower, by inserting her ovipositor down the styles into the ovules. In doing so, she leaves pollen from her natal fig on the stigmas of the tiny female flowers.
At some point, prudently, the ostiole seals over, limiting the number of wasps in the fig all seeking to lay eggs in the florets.
But here lies an obvious reality. Not all flowers can be given to wasp reproduction. Some must be pollinated and produce seeds for the tree.
To solve this problem, the tree produces flower styles of different style lengths. Some are too long for the wasp’s ovipositor to reach the ovary. The wasp does not deposit an egg unless her ovipositor reaches the ovary. Finding a flower with a style matched to its ovipositor length, the wasp will lay an egg.
The term monoecious means that each plant bears both male and female flowers. Most Ficus species in Africa are monoecious, with each fig containing both female and male flowers.
Female pollinator wasps cannot distinguish between male and female flowers, but male flowers usually have style lengths that match the wasp’s ovipositor length, and are therefore more likely to receive wasp eggs.
Those flowers that contain wasp eggs develop galls, or growths, in the ovaries. The galls are an abundant food source for the wasp larvae when the eggs hatch.
After several weeks, the well-fed larvae pupate, and adult wasps emerge, males before females. The male wasps have no wings, buy they do have powerful jaws. They are born in the fig, and they will die in the fig or, if they exit the fig through the tunnels they have dug for the females, they will likely be eaten by ants on the fig's outer surface.
Male wasps are borne with only two missions to deliver in their short lives. The first is to mate with the slowly emerging female wasps, which they do with intense determination while the females are still in their galls. The male anatomy is perfectly adapted so to do.
The second is to chew tunnels through the wall of the fig, using those specifically-designed jaws, for the now-fertilised female wasps to depart, and fly away.
Beyond enabling female wasps to leave, those new tunnels dug by the male wasps have another function. They allow oxygen to enter the fig.
Aroused by a little fresh air seeping into the fig through these new tunnels, the females are genetically programmed to fill their sacs with pollen before heading for an exit.
Newly-inseminated females fly away in search of other fig trees of the same species which are emitting pheromone-like scents. Their flowers are ready for pollination. That remarkable, unique scent emitted from the fig tree signals to the fig wasps to come, enter figs and fulfil their mission.
Oxygen entering the fig is the catalyst for the females deliberately to fill their sacs with pollen and head for an exit created by the males.
Oxygen also triggers the ripening of the fig, which takes about five days.
Thus, only after the female wasps have left the fig, and the tree’s fertilised female flowers have developed seeds, do the figs respond to oxygen and become edible and appetising. Then, the banquet starts for a wide range of birds, mammals, reptiles and others, each feasting, but also carrying away loads of fig seeds.
No one eats live wasps!
New-born females emerge from the tiny tunnels and fly in an instant to avoid predatory ants and others, for wasps can be a nourishing meal. Many wasps perish, but a far larger number fly free in search of figs containing flowers ready for pollination.
The bodies of both the male wasps born in the fig, and the females who entered through the ostiole, will be digested by enzymes in the fig.
Female fig wasps are short-lived, and they need fruiting figs to maintain the cycle of life. Fig trees may produce more than one crop of figs through the year, and different trees in the same species do not synchronise their fruiting.
Remarkably, every species of fig tree, possibly with members widely distributed, enables its wasps to survive continually.
Wind or air drift are great transporters of tiny wasps, allowing this vital insect to cover territory, testing the air for those remarkable signalling scents.
Ripe figs are eaten by birds, bats, animals and humans, and all these are agents to disperse the tree’s seeds.
To the human taste, figs of some bushveld species are bitter, due to the gall flowers within the fig. Human taste buds, however, does not deter many animals and birds from devouring such figs and spreading seeds far and wide.
Interdependency is the key! It allows the fig tree and its wasps each to reproduce. Evidence from fossilised figs indicates that this highly specialised symbiotic relationship may reach back about 60 million years.
The details of this reproductive story read like fiction. But it is fact!
big landscapes!
A Ficus glumosa, or the Hairy Rock Fig, stands on the right, with Mangake mountain in the distance. This landscape lies between Berg-en-Dal and Pretoriuskop.