Convergent complexities

The behaviour of complex social insect societies, and specifically the army ants, is more than a digression. This is because the emergence of such biological complexity may be much more constrained than is sometimes imagined.51 At the least it is a reminder that carnivorous species can be more successful, at least in terms of colony size, than the farming ants (and, as we shall see below, termites). Success, as even the most jaded (if not jealous) scientist well knows, is relative. Size, we are reassured from the most reliable sources, is not all. In their relative adaptive contexts social wasps and bees have taken over their respective worlds. So, too, the attine ants have evolved a remarkable system of fungal cultivation. In each of these examples we have evidence of extensive appropriation of resources, capable of maintaining large and complex societies. Life beyond these 'cities' and 'armies' remains, of course, diverse and marvellous: the biological world is by no means reduced to a monochrome. Nevertheless, the repeated rise of such societies, and at least evidence of the displacement and ultimate extinction of less successful equivalents, suggests that such an arrangement is a biological inevitability.

So, too, in any specific case if a group 'decides' to adopt a particular strategy, say agriculture, then the routes to success will be very limited. It is time, therefore, to return to the parallels between ant and human farming, and thereby consider some implications of such a commonality. As in our mushroom farms, the activities are carried out underground, since, unlike plant crops, the fungi have no need for sunlight. The farms are located in elaborate nests, equipped with ventilation shafts52 and in certain species also dump-pits - some large enough to house a man53 - which are used for waste disposal (Fig. 8.3). Just as in human societies, control of the waste is very important for the health of both the colony and the fungal colonies, and it now appears that the risky business of waste management54 may be 'allocated' to the older workers, nearing the ends of their lives and less valuable to the colony. It seems that once assigned to dump management55

Fungus Fruiting Chamber

figure 8.3 The fungus farm of the attine ants, with large central chamber containing pendant clumps of the fungus and one worker (!) Note the tunnel system leading to the outside world, and the capacious dump-chambers. (Reproduced from fig. 29 of Jonkman (1980; citation is in note 53), with permission of Blackwell Wissenschafts Verlag GmbH and the Jonkman family.)

figure 8.3 The fungus farm of the attine ants, with large central chamber containing pendant clumps of the fungus and one worker (!) Note the tunnel system leading to the outside world, and the capacious dump-chambers. (Reproduced from fig. 29 of Jonkman (1980; citation is in note 53), with permission of Blackwell Wissenschafts Verlag GmbH and the Jonkman family.)

the workers seldom leave, not least because of an aggressive response by other occupants of the nest.

Human farming has evolved independently a number of times, and so, too, has agriculture among the attine ants. Present evidence indicates that the domestication of fungus was first achieved approximately 50 Ma ago, but there have been repeated episodes (at least five) of domestication of various groups of fungi as distinct cultivars.56 What originally might have triggered this innovation? Here, too, there could be parallels to the invention of human agriculture, where it has been suggested that in some instances, as in ancient Mesoamerica, its development was spurred by significant climatic changes.57 So, too, a comparable set of upheavals, perhaps even the ecological mayhem that succeeded the K/T impact event, could have been the crucial factor in shifting these ants to agriculture.58 This ability to cultivate fungus is more than just an evolutionary curiosity. Not only is it strongly convergent on our agricultural activities, but as Neal Weber has remarked, 'the evolution of a unique skill in fungus culturing has freed these ants from the usual food limitations and has enabled them to expand in colony size. In the process a few species have developed some of the largest ant colonies known,'59 with populations estimated to reach at least seven million,60 and typically exceeded only by the aggressive army ants (p. 201). Gigantic populations dependent on highly organized societies: does it sound familiar? And, as with the eusocial bees, there may even be lessons for us. The fungal gardens are usually monocultures. But as humans know (or should know), monocultures have their risks, being notoriously liable to invasion by viruses, fungi, and other pests. The use of the streptomycene antibiotics (note 22) has, of course, a direct parallel in human medicine. What seems quite remarkable is that the resistance conferred by the antibiotics seems to have been maintained by these ants for literally millions of years, in comparison to our experience where in only a few decades we have seen the seemingly inexorable rise of 'superbugs', resistant to nearly all treatments. In reality the system of ant farming and its crop protection is presumably much more dynamic. It appears that the more virulent strains of pathogenic fungi occur in the more advanced species of attine ants, which in turn are more reliant on monocultures. It is hardly surprising that there is evidence for exchange (and stealing) of fungal cultures between colonies.61

The agricultural activities of the attine ants are surely one acme of arthropod organization, and further parallels with human agriculture may well emerge. As a leading worker, Ulrich Mueller, has remarked: 'The more I study [the fungal gardens] the more analogies I find between human agriculture and ant agriculture.'62 Given the nutritional value of fungi and the ease of propagation, it is not so surprising that other insects have also learnt the art of agriculture. Thus the ambrosia beetles, a type of weevil, have evolved a mutualistic relationship with a group of ascomycetes known as the ophiostomatoid fungi.63 The general arrangement is certainly rather different from that of the attine ants because these beetles bore into tree trunks, so producing a complex series of galleries. The beetles are responsible for introducing the fungi. These are carried in containers (mycangia)64 that consist of cuticular pockets with associated secretory glands that evidently 'control the growth and form' of the fungi.65 The mycangia share, therefore, a basic function, but their form is extremely varied, and in different species they occur on many parts of the carapace. As ever in evolution, 'needs must': who cares whether the container (in this case the fungal-ferrying mycangia) comes from, so long as it works? So far as beetle and fungus are concerned, both sides are winners. Evidently the advantage to the fungi is to be smuggled into new tree hosts, incidentally making these beetles a serious economic pest.66 The beetles benefit because the fungi help to block plant defences, such as the secretion of resins,67 and also provide a source of food. Yet in other respects this fungal agriculture is reminiscent of that pursued by the ants. For example, it has become an obligate association, shows vertical transmission from generation to generation, and provides the main food source, and interestingly in these beetles appears to have 'evolved at least seven times'.68

It is also possible that a convergent situation has arisen in the termites, which although often referred to as 'white ants', are actually relatives of the cockroaches, whereas ants are closer to the wasps. In one group, the macrotermitinids, there is a striking and close symbiotic union between fungus and termite.69 In contrast with the at-tine ants, which are restricted to the New World, this association is an Old World one, probably originating in Africa and then spreading to the Middle East and southern Asia. Termite nests are among the more spectacular of animal constructions, with an elaborate system of chambers and tunnels.70 The tunnels are also used for foraging runs outwards from the nest, for up to 50 m; one calculation gave for a single nest a total tunnel length of almost 6 km. The foraging tunnels repeatedly branch but crossroads are very rare. The walls of tunnels are smoothly plastered and sharp bends are cambered. These tunnels enable excursions to be made in relative safety; the termites ascend to the surface by short access tunnels that slope upwards from the main tunnels. These access tunnels are less well built, and may be sealed when not in use. Their diameter is such that they can also be blocked by the heads of the large soldier termites. The nest itself is engineered to allow both the circulation of air and the venting of waste gases. Also present are storage areas and, at least in experimental set-ups, latrines. As is also the case in other colonies of social insects (and their analogues), there are other inhabitants, some beneficial, some not. Some termite colonies, for example, play host to a group of flies known as the phorids, and here, too, convergences emerge.71

Growing within the termite nest are spectacular combs of the fungus, aptly named Termitomyces. It belongs to a group known as the basidomycetes72 and thus is related to the fungi cultivated by the attine ants, as well as to the more familiar mushrooms and toadstools.

On occasion, Termitomyces emerges from the nest to form on its outside a mushroom-like fruiting body (which is edible to humans), but evidently this fungus cannot survive outside the termite colony. Thus, if the termites are removed the combs are soon invaded by other sorts of fungi and bacteria. Maintenance of the combs may involve salivary secretions and weeding, yet earlier workers such as Roger Heim73 were dismissive of the term 'cultivation'.74 Since then views have changed, and as Johanna Darlington briskly notes: 'Heim's views of the adaptable fungus forcing itself upon the reluctant termite is at odds with more recent experience of Termitomyces as a delicate and finnicky organism requiring care and cosseting. It seems best merely to take note of Heim's arguments and pass on.'75 Yet, while the association seems to be agricultural, the precise nature of this symbiosis is still somewhat elusive. Again to quote Darlington,

The fungus is a passive partner, absolutely dependent on the termites for its survival and success. The termites, on the other hand, invest a lot of time and energy in caring for the fungus. They gain substantially from the relationship, although it is proving difficult to discover exactly how. The evidence favors a nutritional role, with the fungus comb acting as an external stomach,76 breaking down less digestible components of the forage and so making them available to the termites ... An advantage of the system is that the digestive work of the symbiont is carried on outside the termites' bodies, unlike the gut symbionts that do a similar job for the lower termites. Lower termites have to carry fermentation tanks around with them, while the Macrotermitinae leave them parked at home.77

Evidently this agriculture is different from that of the attine ants, just as human agricultures vary.

Other convergences also emerge. Communication among ants is famous for its pheromone-based system, but in the termites seismic communication is achieved in the soldier caste by drumming the head against the substrate,78 a method also used by subterranean mammals (Chapter 6, p. 141). Seismic communication is also used by the leaf-cutter ants.79 In these ants, although the sound is produced as a stridulation, it is transmitted through the ground. Typically stridulations are produced only when the individual attine ant is immobilized, such as occurs by collapse of part of the earthworks. These cries of distress elicit rapid digging by other workers, who can detect sounds through 5 cm of earth and will begin rescue work if the thickness is no more than 3 cm. In the termites this alarm reaction can evidently be triggered either by air currents or vibrations. The more advanced fungus-growing termites are particularly sensitive in this respect. Rather remarkably, the signal can be propagated by chains of soldiers, each re-amplifying the signal in a way analogous to the now-redundant methods of human communication using smoke and drums,80 as well as the periodic re-amplifications observed in the transmission of signals along the nervous system.

Although they are in certain respects less sophisticated, it is appropriate to mention in passing the so-called ant-gardens, which are arboreal earthen structures known as 'cartons'. In this arrangement certain species of epiphytic plant are encouraged to grow on the nests, from seeds that are collected by the ants and planted in the nest wall. In due course these plants germinate, grow (and thereby provide a source of extra-floral nectar), extend roots that probably help to strengthen the nest, eventually flower, and so produce seeds, the fruit of which is eaten before they are planted ... and so the cycle continues.81 In a few instances fungi are employed in the 'cartons', probably to help bind the structure and possibly also to release antibacterial chemicals. Interestingly, in some cases the fungus is effectively a monoculture, apparently maintained by weeding and feeding. So far as can be told, however, the fungal products themselves are not directly cropped.82 Other activities of these garden ants include the pruning of surrounding vegetation, probably to create a 'fire zone' to reduce the risk of invasion by other ants,83 the collection of vertebrate faeces, presumably as fertilizer, and perhaps the choice of plants that, like those cartons with fungi, release chemicals that help to keep pathogens at bay.84

This section on the ants (and termites) has included more than its fair share of digressions, but I hope that the common thread of exploring convergences still runs through the narrative. Consider again the army ants: living in the midst of their mobile and aggressive column has its self-evident risks, yet various species have managed to insinuate themselves into the ants' social system. Most striking in this instance are probably the various staphylinid beetles. These have effectively transformed their bodies into an ant shape that readily deceives the actual ants.85 Hiding yourself in an aggressive raiding column may yield various benefits, not least an uncontested share of the

figure 8.4 A staphylinid beetle in association with the army ant Eciton. (Photograph courtesy of David Kistner, California State University, Chico.)

booty and also some protection from predators. Typically the beetles travel in the centre of the column, and are not found in the vanguard. In times of excitement, such as during an episode of emigration, the beetles may ride on the ants themselves and despite continuous an-tennal interrogation never seem to be recognized as interlopers. The way in which these beetles have transformed themselves into ant-mimics is very striking (Fig. 8.4), and the selective pressure to survive in a jostling mass of aggressive workers can explain how such a transformation in the staphylinids has occurred independently on multiple occasions.86 In at least one instance the convergence extends to a colour matching, whereby a geographical variation in the ant coloration is matched by the beetle.87 As David Kistner (Fig. 8.5) points out, given that the ants are effectively blind, this mimicry supports the idea that it serves to dupe 'educable predators which sit by the raiding columns to pick up insects which are stirred up by the raid'.88 Nor is this the only example of a convergence between a mimic and an army ant. A common association is the attachment of mites to the exterior of insects (and other animals such as birds and mammals, clinging respectively to feathers and hair; pass me the nit comb), but in the case of the mite Planodiscus 'the sculpture of the mite and

212 alien convergences?

figure 8.5 David Kistner and his wife in the field in Ecuador, collecting staphylinid beetles and army ants. (Photograph courtesy of David Kistner, California State University, Chico.)

the ant's leg is nearly identical. Also the arrangement and number of setae on the mite approximates the arrangement and number of setae on the leg. Thus when the ant grooms its leg, the tactile stimulation [as it passes over the mite] will be similar to that of the leg itself.'89

In Chapter 6 (pp. 116-117)1 briefly introduced the tent-building capacities of the aptly named weaver ants. Here, too, there are some striking cases of mimicry, which here involve weaver ants and a crab spider, Amyciaea forticeps.90 As it happens, the weaver ant in question, found in India and known as the Indian Red Ant (Oecophylla smaragdina), is also associated with another spider (Myrmarachne plataleoides) that in turn is 'a perfect mimic of the red ant; so perfect is this mimicry that even experienced biologists may pass it by as an ant, in the field.'91 Despite its close association with the Indian Red Ants, which can be very aggressive, M. plataleoides takes good care to avoid the ants. Possibly it employs its mimicry to fool other ants from whom it steals their young, as well as to avoid predation by being mistaken for its genuinely aggressive counterpart. The other spider, Amyciaea, has only a generalized similarity to the Indian Red Ant, but this is compensated for by a rather remarkable behavioural convergence (see also p. 285, Chapter 10) which seems to entail its pretending to resemble an ant in distress. Thus, when a nearby ant adopts an alarm attitude, thereby disturbing the other ants. This then gives the spider an opportunity to attack. Once seized, the meal is concluded as the spider suspends itself from a silk thread, safe from the other ants.92

The convergence of mimicry of insects and spiders to an ant morphology has 'evolved at least 70 times',93 and as such gives a series of fascinating insights into both the malleability of biological systems and the likelihood of establishing adaptive explanations. The transformation of staphylinid beetles to an ant-like form has already been noted, and given that ants are also insects it is relatively easy to see how other insects might come to mimic them. Even so, there is a subtlety inasmuch as the closeness of mimicry might be compromised as the insect changes in size and shape. The solution, of course, is for the mimic to resemble successively, in a series of moults, two or more species of ant.94 The case of the spiders, the other principal ant mimics, is more remarkable, given the more obvious differences in body plan, and here, too, illusions connected to segmentation, colour, appendages, eyes, and surface texture have all been arrived at by a series of ingenious modifications. Not surprisingly there are degrees of mimesis, but in at least some instances there is evidence that the phy-logenetically most derived are also the most ant-like,95 and as such will define evolutionary trends. As already noted for one spider, these may be supplemented (or complemented) by various behavioural modifications. Not all insects need look like ants to insinuate themselves into ant colonies, and many closely associated species evidently rely on compatible and thereby convergent chemistries or textures.

In passing one should note that the vast area of biological mimicry, where one species comes to resemble another, is a perfectly good example of convergence. Its classic manifestation is in Batesian mimicry, where an animal, usually an insect, comes to resemble closely an unrelated but noxious species and thereby escapes predation. Mullerian mimicry, on the other hand, is where two species, both unpalatable, converge to resemble each other. So well known are these mimicries and their various manifestations,96 that for the purposes of this book they can, I hope, largely be taken for granted.97 The complexity of both ant and termite colonies, not to mention their various guests and symbionts, also provide a rich field within the topic of convergence, and one that can be analysed at several levels. Passing comment has already been made on such built structures as trails and tunnels, farms, and cartons, and it is clear that much remains to be discovered about these various constructions in an adaptive context.98

These insects, moreover, are by no means the only arthropods to have evolved constructional abilities. Take, for example, the beach-dwelling orypodid crabs. These crustaceans are highly territorial and protect their domains from intrusion by a variety of complex behaviours that include the building of mud fences and also the plugging of neighbouring burrows. Sometimes the occupant is evicted, but in other cases it is entombed. As might be expected, at least some of these behavioural repertoires are convergently acquired within this group.99 Nor is this the only example of such sophistication of behaviour. The fiddler crab is famous for its signalling, achieved by movement of its hypertrophied claw. It now transpires that the signalling for mates, in a process known as 'lekking' in which numerous associated males signal simultaneously to choosy females (a well-known characteristic of some birds), also occurs in these crabs.100

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