The Chimera Tree: Symbiosis to an Extreme

The Chimera Tree

The chimera tree is a biological marvel, in that it is a partnership of two species from different kingdoms of life who operate as a single organism, neither being able to exist on its own. In the case of the chimera tree, the partners are a species of eusocial ant and a broad-leafed sunflower tree, both found in warm equatorial regions which experience alternating hot, dry seasons and cool rainy ones. The ants in these relationships occur in several widely disparate castes, the most obvious of which are small warriors with immense cutting jaws, whose sole purpose in life is to guard the trunk and branches of their host tree from its enemies, both leaf-eating insects and animals and the creeping growth of neighboring trees threatening to shade their own. The warriors rarely enter the tree at all, living almost entirely on the bark and have such large jaws they cannot feed themselves and so are fed by an even smaller, less conspicuous but even more numerous caste, the workers, which maintain the nest complex inside the tree and never leave its refuge. The workers live in specialized hollow channels just under the tree's bark, and there are fed upon specialized nectaries running along the inside of the tunnels, which trickle a high-energy sap that supplies all of their dietary needs. The worker ants actively supplement their tree with the bodies of the insects killed by the warriors and the trimmings of adjacent branches they have clipped, which they take from the warriors at the entrances to the nest in exchange for mouthfuls of nutritious sap. All of this organic matter is then dragged deep into the nest and deposited into large bolls inside the stems where compost piles are maintained. The worker ants delicately dismember the large insects and break up the leaves and stems into a finely shredded pulp, which is then kept just moist enough with their own droppings and saliva, and warm enough by the constant swarm of maintaining ants, to provide a perfect environment for decomposition. If the compost becomes too dry, the worker ants will carry back pulped leaves soaked in water to moisten it, and if it becomes too wet, their response is to add a healthy portion of dry shredded bark, or even a little sand. As the compost breaks down, small root hairs emerge from the inside of the chamber to absorb the nourishment, thereby feeding the tree.

Somewhere deep inside the tree exists the queen, many times larger than her children, and a smaller but still quite sizeable king, who are also fed and protected by the tireless workers. As in her ancestors going back hundreds of millions of years, the queen's sole purpose is to lay eggs and perpetuate her colony, but unlike other ants she also maintains a pair bond to a single male who regularly mates with her in her chambers. Though the vast majority of her offspring will be sterile workers and warriors, once or twice per year - after the heavy rains - she produces a different type of egg, which when raised by the colony will mature into much larger winged adults that - unlike their sisters - are fully capable of reproduction. Though the typical worker ant in the chimera tree colony is just a fifth of of an inch long, and the warriors just a centimeter or so in size, the young kings are a full inch in length and the queens more than one and half inches long, both with sturdy and powerful wasp-like wings. Their jaws are especially powerful and unlike the nearly-blind workers (but like the warriors), they have very large eyes. Because they cannot move through the small channels provided by the tree which are used by the workers, they are raised not lower down in the hardwood of the tree but in special nurseries carved out inside the young stems at the very top of the tree, which are soft and easily molded by gnawing into a structure large enough to accommodate them. As larvae both males and females are fed especially well, but even so males in a given colony mature as much as three weeks before their sisters, which limits the chances of inbreeding. While the queens are still pupating, the males emerge in mass, using their large jaws to chew their way out of their nurseries and climbing out onto the branches to dry their wings.

It is at this point that things turn back to the tree. It has already been established how the ants nourish and protect it in exchange for a place to live, but even more remarkable is that the ants are also responsible for the dispersal of the tree, the planting of its seedling, and the cultivation of the next generation's host. As each young king ant leaves its nest, it first crawls along the stem of the tree in search of provisions to last it on its voyage to independence. The emergence of these young male ants has been timed flawlessly with the production of the chimera tree's mature seeds. While the ancestors of the chimera tree's ants were already planting their seeds, in this species the seeds sprout while still on the parent plant, producing a healthy seedling with the beginnings of roots and several green leaves attached to the seed, which still contains a large store of food. In earlier forms this food store was vital to provide the seedling with nourishment, but now the seedling germinates while still tethered to its parent which still provides it with all it needs to grow. The energy stored in the seed now exists entirely to feed the ant and encourage it to pluck the seedling from its parent and carry it away to a new site where it can be deposited, take root, and begin to grow.

After plucking one of the large seeds with its attached sprout, the young kings take flight with its secured beneath their body, in search of a fertile patch of soft soil to deposit it, whilst munching upon the stored food in the seed all the while (to no harm to the seedling itself, which is untouched and is able to survive up to a week at this time in a state of near dormancy.) After anywhere from a few minutes to a few days, the ants drop to the ground near a bare patch of moist, fertile soil near the edge of a forest or riverbank where he instinctively knows his seedling will stand a good chance at survival. Here, he now begins a more strenuous job than is undertaken by any other male ant species in ensuring the survival of the next generation. He at once begins to burrow, digging a small chamber in the dirt at the entrance to which he carefully maneuvers the seedling into position, covering its roots with dirt and cleaning any debris from its leaves. If it is dry, he may even bring it several mouthfuls of water to help it establish in its first few days. Once his burrow is constructed, however, his goal is to attract a queen. He sits outside his burrow, vibrating his wings in such a way as to produce an audible buzz to call the females down to him, and he waits. Eventually, within a week or two, all of the new females in the forest will be emerging. Having to take no part in the energy-intensive process of constructing a nest, they don't require a food source right away and indeed, there are usually no seedlings left for them to collect. Indeed, their job right now is very easy - the males, who are produced in greater numbers to compensate for their higher mortality, outnumber them three to one, and they need only follow the sounds of their suitors to the ground and pick out a mate. They land, scurrying along between the nests of different males and investigating their burrows, finally settling on one that they find satisfactory. The ants then mate, and within just a few days the female begins to deposit eggs inside the burrow.

For the first generation, the king and queen ant have to provide for their young ones entirely on their own in their burrow underground. They exhibit a strong pair bond unusual among insects and alternate leaving the nest and taking flight in search of food, which at this stage in their life is entirely composed of other insects which they dispatch and bring back to the den to feed their larvae. As the first generation of larvae hatch, they develop into another unique caste; infertile but still winged females, who now replace their parents in the search for food to feed their younger siblings. By this time, the tree seedling which has been planted along the top of the burrow should have established and begun to send up new growth, fed by the waste products and carcass remnants in the burrow over which it was planted. Within three months in fertile conditions, just as the dry season begins to return, it can reach a height of two feet and has produced its first boll - a hollow pocket on its stem, which the king and queen ant immediately adopt as their new quarters, climbing up the stem and chewing a small opening to allow their access. At this time the female ceases to produce flying workers and switches entirely to the smaller, nearly blind caste which will maintain the nest from here on out, as well as smaller numbers of also infertile warriors, and the flying workers die off. The underground portion of the nest is abandoned, and the young tree begins producing nutritious sap inside the boll which the ants can feed upon. As soon as the sap begins to flow, the king and queen cease feeding on meat and undergo a drastic physical change, shedding their wings and in the case of the female developing a greatly distended abdomen that may eventually reach three inches in length, in which she produces enormous quantities of eggs.

The tree now goes dormant for the dry season, and the female produces only a few workers. In the fall, however, when the rains return, it jumps back into growth quickly,sporting large heart-shaped leaves up to a foot in length, and as growth resumes the queen ant kicks up her egg prooduction. As the tree increases in height, the growing numbers of worker ants carry their king and queen into ever higher chambers where they are safer, gnawing out naturally-occurring small channels in the stem to fit their greater girth and over months and years producing a large vertical highway up through the stem through which distinct nest segments can be accessed. The first compost heaps are constructed in the abandoned queen's quarters low down on the tree's trunk, but as the colony and tree are both maturing and the king and queen are moved less frequently and have been moved nearer to the top of the tree, the workers will carve out special cavities in the tree specifically for composting purposes. After a year, the tree can reach a height of ten feet; by five years, it can be thirty feet tall and very wide with many branches. Like many sunflower trees, its grows extremely quickly in its early years, slowing down significantly past ten or fifteen years of age but living for a relatively long time.

As soon as the ants have moved into the trunk of their host tree, the colony begins to produce fertile females at the start of every rainy season. Males, though, will not be born until the tree is old enough to produce its own offspring, sometime around its tenth year. This will be the first year that new kings will emerge, first only in small numbers matching the initially small seed crop of a young tree but eventually by the thousands. The queen ant of the chimera tree is the longest-lived of all insects, and if her host tree is healthy she may survive for up to fifty years, tucked away deep inside its trunk. Her king, however, has a maximum lifespan of much less, no more than five years. The first king thus almost never lives to produce sons, but will produce thousands of daughters.

The death of a king is an obvious problem for the chimera tree colony. Though this ant species is able to produce female workers to maintain their colony through parthenogenisis indefinitely, to produce a seasonal flush of fertile male ants to spread her genes she requires a male partner, and so as soon as one king dies and moves on to the compost heap her workers must immediately go search for another. To do so, the queen produces another small flush of rare flighted workers in the next breeding season after her mate's death. These workers are much larger than their flightless siblings and physically identical to the queen, but unable to mate. Their only role in life is to capture a new male to ensure the colony's long-term survival.

The flighted workers produced after the death of a queen are identical to the very first offspring she produced when establishing her colony, and despite their lack of sex organs - their development having been stunted by the queen's hormones - are nonetheless clones of her own genes and can thus be produced without mating. They have powerful jaws, which they will use to apprehend their new king. To find one, all that they must do is hone in on the wing-buzzing calls that young males produce to attract a fertile queen to their burrow and trick the resident male into believing they are a fertile, receptive young queen. Acting very interested in the naive young king's nest, she allows him to lead her into the chamber. Instead of mating with him here, however, she then proceeds to remove first his wings, then his large mandibles, with her own jaws. The male is large, but not so much as she is, and rarely escapes her attack. Having prevented him from getting away or fighting back, she drags him from his burrow and takes off with him back to her colony's nest tree, where she hands him off to the smaller workers. The incapacitated male is then pulled deep into the colony toward the queen's quarters. The journey takes several days, as the tunnel to the queen must be manually enlarged inch by inch by their gnawing jaws to accommodate something the size of a fertile king ant, but eventually they reach the queen and deposit the male. If she considers him suitable, she accepts him and they will begin mating immediately. His lack of wings and jaws are of no problem in his new home, where he is now fed and attended by the workers as one of the colony. Often, however, multiple flying workers will succeed in capturing a king and deliver them all in tandem to the nest. The queen will permit only a single partner in her quarters at a time, and usually favors the first, killing and eating any latecomers. So while through the abduction by flighted worker ants even some young males who would otherwise not find mates are still able to reproduce, the majority of these excess males end up becoming nothing more than food. Life, for most young male chimera ants, is a short and bleak one of much hard work and little reward.

The chances of the tree seedlings left abandoned by their gardeners when males are either abducted or die off without attracting a queen are not good. Though once planted the individual trees can survive on their own, they are very vulnerable to being destroyed by nearby trees with active colonies, both of their own species and others, for they lack any defenses on their own. If they do survive they will still produce small bolls and hollow channels in their stems without an active ant colony, but as the ants normally chewed and expanded these to their lacking, a chimera tree without an ant population develops a much smoother appearance, with few outwardly visible protuberances and lumps as occur in inhabited trees, and is often visibly stunted as a result of receiving less nutrition in the form of the ants' internal compost heaps. Because it is the instinct of emerging males to plant their own seeds, not to search out established trees, even later-hatching pairs rarely make use of them and thus most often these trees are eventually colonized by different, less specialized symbiotic ant species. Though this has the effect of providing the tree some protection from competitors and browsers, when it comes time to flower and produce young it will be at a great disadvantage, for these other species will not disperse their seedlings. Most of the tree's young will thus wither on its stems, and the few which break free will only fall directly beneath the parent tree where they rarely survive. Chimera trees that go through life without the chimera tree ant do not reach comparable sizes to their counterparts with active ant colonies, they have shorter lifespans, and they almost never successfully reproduce. The situation is similar with young pairs of chimera ants whose planted seedlings do not survive. Though the young king and queen can survive for a few months in a below ground burrow, they will never produce fertile young in this environment without the stimulus of the flowering host tree. The ants also rely on the sap of their host tree to provide moisture during the seasonal drought, and without one may struggle to find water. If the seedling they had cultivated over their initial nest fails, some pairs and their flighted workers will search out another unoccupied host and colonize it, but if the spare tree has already been taken by another ant species they will be driven off. If they are unable to find a host within a few months, the young colony will usually die out at the onset of the dry season without ever producing fertile offspring.

Over as long as fifty years, a single colony of ants can hold onto their tree as it grows to heights of seventy feet and develops a trunk, interwoven with tunnels and chambers, of twenty feet wide. The eventual scale of the colony is astoundingly large, especially considering only a single queen exists in each colony, with a mature tree housing as many as half a billion active workers at any given time, all of which are siblings.

After several decades, though, eventually the sole queen of an old chimera tree will die, and this her colony cannot survive. With no one available to produce new members of the flying castes, her latest king trapped deep inside the tree, and her offspring all infertile, the colony dies out in a matter of one to two years. As its numbers drop, other ant species often invade, slaughtering the last survivors and taking up the enormous nests as their own. Often many different species take over different levels of the tree, eventually coming together and fighting deep inside it where their territories converge. Due to the sheer size of a chimera tree and the extent of tunnels and chambers throughout it, it is impossible for any one new colony to claim the entire thing as the chimera ants did so gradually over a period of many decades as the tree was growing, though new pairs of chimera ants will often attempt to colonize portions of the tree's uppermost reaches near the tree's seedlings develop. As long as at least a small population of the chimera ant persists in the treetops the tree itself stands a chance at distributing its seeds, but these secondary colonies rarely last for more than a few seasons before being driven out by the growing colonies of more aggressive species that moved into the tree's lower sections. When all of its symbiotes are gone, the chimera tree's days are numbered. Though the individual may stand another twenty or thirty years, its seedlings are no longer distributed and reproduction is halted. After millions of years of close co-evolution with its insect partners, it cannot effectively distribute its young on its own any longer, and is effectively castrated when the last chimera ants vacate its trunk.