Authors: Colin Tudge
In the Smithsonian’s Panama studies, when more than one kind of wasp colonizes the same syconia, both wasps may serve as perfectly good pollinators. But this is not always so. A study in Africa has shown that at least in one case, one of the cryptic wasps that colonizes one particular species of fig tree behaves simply as a parasite. It lays its eggs in the flowers and so feeds its young, but it does no pollinating. It is a cheat: an archetypal freeloader. Game theory predicts that freeloader fig wasps might exist—it’s a possible niche—and so they do. Again we see that the simple one-to-one relationship between figs and wasps, evolved over millions of years into perfect mutualism, is not quite so cozy as it has seemed. The relationship, like all mutualisms, is dynamic. It is always prone to decay.
Often, too, any given fruit may be colonized by more than one foundress from the same species. This raises another set of complications—complications that again have been predicted by modern evolutionary theory, and that again (very satisfyingly) have now turned out to be what actually happens.
Let me refer you back (as Perry Mason would say) to an earlier comment: that the proportion of young male wasps born within a given syconium varies from around 5 percent (one in twenty) to around 50 percent. The preliminary point is that many creatures can adjust the sex ratio of their offspring. Humans cannot do this, but we do not shine at everything. Such adjustment is especially easy for wasps (and bees and ants) because in these insects, the females all develop from fertilized eggs, while the males develop parthenogenetically, from unfertilized eggs. The mother wasp (or bee or ant) keeps the sperm separate from her eggs until the time comes for laying, so she can decide in the light of circumstance whether or not to fertilize them before laying. Again the mechanism seems so subtle that it beggars belief, and yet it is the case.
Theory predicts that when only one foundress colonizes a particular fruit, the ratio of males should be low: one in twenty rather than one in two. But the more foundresses there are, the more we would expect the ratio of males to increase. The point is that each female strives to pass on as high a proportion of her own genes as possible. If all the larvae within any particular fruit are her own offspring, then all her daughters are bound to be mated by her own sons (if they are mated at all). The shortcomings of incest and inbreeding are apparently outweighed by the advantage that the mother thereby passes on her genes both via her sons and via her daughters. So she needs only enough sons to ensure that her daughters are all fertilized—and one son per twenty daughters seems enough. It is good, of course, to focus on daughters, because they are the ones that lay the eggs that supply the grandchildren generation.
But if there is more than one foundress per fruit, then the young males find themselves with rivals who are not simply their brothers (who would be genetically very similar) but come from a different lineage (albeit of the same species). In such circumstances, we might suppose that it could pay a female to produce sons exclusively—provided those sons are big and tough enough to mate all the daughters of all the other foundresses. The theory shows, however, that it never pays to produce more sons than daughters. A 50:50 ratio of sons and daughters is the maximum. Again, natural history supports the theory. Dr. Herre and his colleagues have shown that as the number of foundresses per fruit rises to about six, so the proportion of males rises to around 50 percent.
But here comes another twist. The males have one function only: to mate with the females. Apart from that, they are a dead loss—both from the wasp’s point of view and from the fig’s. After all, the fig has to sacrifice a seed for every young wasp that is born. The youngsters that matter to the fig are the females, which fly off to pollinate other figs. As far as the fig is concerned, the fewer males, the better. This, in turn, implies that figs should encourage wasps to enter their syconia one at a time: that they should evolve some limitation on access (and there are many comparable examples in nature). As things are, small syconia are much less likely to attract multiple foundresses than are large syconia—so we would expect natural selection to favor small syconia. In reality, while some fig species do have small syconia, others have larger ones. So why does natural selection ever favor big syconia? This question will be raised twice more as this narrative unfolds, in two quite different contexts. Whichever way you look at it, big syconia seem like bad news. Yet there is an answer, which will be provided later in this chapter. Patience, gentle reader.
As if the game between figs and wasps were not convoluted enough, there enters now a third set of players: parasitic nematode worms.
ENTER THE NEMATODES
It has been suggested that every species of creature on earth above a certain size has its own specialized nematode parasite; if this were so, it would mean that the total number of species on earth is equal to the number of non-nematodes times two. Whether this is so or not, it does seem that every species of pollinator wasp
does
have its very own species of nematode parasite. All the nematode parasites of Panamanian fig wasps belong to the same genus:
Parasitodiplogaster.
Since STRI, where Dr. Herre works, is based in Panama, this is the genus he and his colleagues have studied most.
The life cycle of
Parasitodiplogaster
nematodes is superimposed on that of the wasps they attack. Not all figs are infested with nematodes, but in those that are, the nematodes will have reached the immature, dispersal stage by the time the young female wasps are emerging from their flowers. The worms then enter the wasp’s body cavity and begin to consume it from within. Their efforts are not immediately fatal, however, and so their host carries them on to another syconium. When the infested wasp finally dies, generally after laying her eggs in the next syconium, up to twenty or even more adult nematodes crawl from her body, mate, and lay their eggs. The young nematodes hatch before the young wasps emerge—and so they are ready to invade the young wasps and begin the cycle afresh.
In general, the relationship between parasite and host is as delicate as that between partners in a mutualistic relationship. The aim of the parasite is to grow and reproduce, and for this it must feed upon its host. If it feeds too vigorously, it is liable to kill the host. If it is too decorous in its approach, it loses out to rival parasites who are more vigorous and so breed more quickly. In general, then, it pays parasites to be as vigorous—“virulent”—as possible, but without overdoing it.
Now a further twist. Theory predicts that if nematodes infest wasps that occupy fruits on their own—one wasp per syconium—they should be less virulent. After all, if they are too virulent, and kill their host wasps, they have no chance at all of being transmitted to a new fruit to lay eggs of their own. But if the nematodes attack wasps that invade fruits more than one at a time, they can afford to be more virulent. It doesn’t matter too much if some of the young host wasps are killed off, since there are liable to be others that are not killed, and will carry the nematodes to pastures new. The Smithsonian scientists found that this prediction stands up. Wasps that invade fruits singly generally manage to fly off to new fruits even when they are infested with nematodes. But in fruits that entertain more than one foundress, a proportion of infested foundresses perish before they leave the syconium of their birth.
Nematodes are clearly bad news for the wasps; and particularly virulent nematodes are bad for the figs, too. After all, the fig has to sacrifice one of its would-be seeds for every wasp that is produced, and the sacrifice is wasted if the wasp then dies from nematode attack. Again, it seems that figs would be better off producing syconia that attract only one foundress. Again, small-sized syconia seem advantageous—because, in general, the bigger the syconium, the more foundresses it is liable to attract. So the question is prompted again: why do some figs continue to produce large syconia?
There is more.
COOL FIGS AND HOT FIGS
Although figs surely have no love for gall wasps, they do go to great lengths to protect the vital pollinator wasps. In particular—as again revealed by the Smithsonian studies—they maintain a temperature within the syconia that allows the young wasps within to develop.
As a preliminary observation, the scientists showed that when the temperature is only 5° to 10°C (9° to 18°F) higher than the ambient temperature at midday, the pollinator wasps of Panama (or at least two species of them) are incapacitated or die. But, say Dr. Herre and his colleagues in a paper published in 1994, “Such lethal temperatures would be expected in objects exposed to full sunlight.”
1
And such objects include the syconia of figs, hanging on their trees. So the researchers measured the temperature inside syconia—and found that they stayed more or less near the ambient temperature: still comfortable for the young wasps developing within them. Even on the fiercest days, the temperature within small syconia never rose above 32°C (90°F), which wasps find perfectly acceptable.
Yet there was a greater oddity. For although the physical theory is complicated, it suggests that small fruits should find it easier to stay cool than large fruits do. In fact, the larger fruits were often even cooler than the small ones. So how do figs in general stay cool? And how is it that the large ones—apparently in defiance of physics—tend to be the coolest of all?
Perhaps, the scientists surmised, the large fruits cooled themselves by evaporation, as leaves do, or as mammals do when they sweat. Evaporation would be effected, as in leaves, via holes (stomata) in the syconium surface. To test this idea, the scientists simply covered the figs in grease, to block the stomata. Sure enough, the temperature inside the big syconia then rose by about 8°C (14°F). When the outside temperature was at 29°C (84°F, which is common enough), the temperature inside the big greasy fruits rose to around 37°C—hot enough to kill the wasps inside within about two hours. Small fruits do not need such refinements. They have no stomata, or very few.
So the big fruits can keep themselves cool—but only at a cost. They have to waste a considerable amount of water to do so. We have already seen two reasons why small syconia seem preferable to large ones. There are more male wasps in the big syconia (because there are more foundresses), which is wasteful. The nematodes are more virulent in the big fruits (because there are more foundresses), which is wasteful again. Now, to cap it all, the big fruits have to waste water, a precious commodity, just to keep themselves cool. So why do any figs have big fruits? How could natural selection have favored such an apparent absurdity?
I must delay the answer still further. It lies under the heading of seed dispersal, the generalities of which we should look at first.
SCATTERING OF SEED
Many plants, temperate and tropical, rely on animals to disperse their seeds. As with the pollinators, the relationship is mutualistic, with give and take on both sides. The tree gets its seeds dispersed, to be sure. But animals cannot afford to run charities, and they must have their quid pro quo. Sometimes they expect to eat a proportion of the seeds, and so squirrels typically consume at least as many acorns as they scatter. When trees produce fleshy fruits, animals may simply eat the pulp and then either spit out the seeds (as monkeys may often be seen to do with machine-gun efficiency) or else allow the seeds to pass through their guts (whereupon they are deposited with their own consignment of fertilizer).
Always, though, and inevitably, there is tension. If a particular tree evolves to become dependent on a particular disperser, and the disperser disappears, then the tree might disappear with it. Thus many a seed seems simply to languish in tropical forests—though perhaps in the past dispersed by long-gone dinosaurs or some extinct giant mammal. On the other hand, if the dispersers become too common then they may eat too many of the seeds, and then the tree is also liable to die out. Balance is all. Many thousands of examples could be cited, but a couple must suffice.
The first is the almendro tree,
Dipteryx panamensis,
from the Fabaceae family, which grows on Barro Colorado Island, in the heart of Panama, and for many years has been studied by scientists of the Smithsonian Tropical Research Institute. Egbert Leigh, who has worked on the island for the past thirty years, introduced me to the almendro one very rainy morning. It is indeed lovely, with bark the color of pale pink salmon and a trunk that forks and forks again to produce, says Dr. Leigh, “a graceful, somewhat hemispherical crown of compound leaves spiralled around its twigs.”
There is only about one almendro per 2.5 acres on Barro Colorado, but that is a fairly typical number for a tropical-forest tree. Come June and July, it produces fine bunches of pink flowers at the ends of its twigs. These are apparently triggered by the onset of the rains, in late April and early May. Certainly if the start of the rainy season is not clearly marked—if, for example, the previous dry season is not as dry as it should be—then the almendro produces far fewer flowers, and so far fewer fruits. This is bad news for Barro Colorado’s animals, for the almendro is a serious food tree. Thus small quirks of weather can have far-reaching effects. As global warming continues to bite, we can expect the weather to become quirkier and quirkier.
The fruits, as befits a legume, are produced in pods: a hard wooden pod covered in a thin layer of sweet green pulp, with a single big seed inside; there are twenty or more fruits per square meter of crown in a good year. This is a prodigious crop and, says Dr. Leigh, “swarms of animals flock to the feast.” Some take the fruit directly from the trees. These include some carnivores, like the kinkajou and coati (many carnivores are omnivorous—notably bears), and also monkeys, bats, and squirrels. Some take the fruits from the ground, including agoutis and pacas, which are big relatives of the guinea pig (and resemble small antelope or deer), spiny rats (also related to guinea pigs, rather than to rats), peccaries (New World pigs), and the occasional tapir. Many of these feasters simply eat the sweet pulp around the wooden pods. But some—notably peccaries, squirrels, spiny rats, and agoutis—gnaw through the hard casing as well, to the bean inside.