In parentheses, one should note that the army-ant syndrome has evolved independently several times.27 This particular example of convergence is important for two reasons. The first is simply connected to the intrinsic fascination of a sophisticated biological organization, replete with militaristic metaphors and an associated folklore of marauding columns of invincible ants sweeping all before them. In fact, in some parts of the world an invasion of a house is welcomed as it sweeps out pests and other unwelcome visitors, and for the most part the army ants offer no danger to humans.28 Arguably of greater importance is the emergence of a complex system that encapsulates a remarkable social system and thereby defines a different sort of collective intelligence.29 As such the army ant adaptive syndrome has several noteworthy and recurrent features. The most celebrated aspect is the relentless mobile columns, capable among other things of building bridges constructed of their own bodies (see also note 32), and self-organized into an advancing front that can span as much as 20 metres of tropical forest floor. Animals flee,30 but those either trapped or overwhelmed are rapidly dismembered, the pieces being despatched back to the nest by exceptionally efficient teamwork.31 Such transport of the dismembered pieces of prey is referred to as super-efficient, inasmuch as the total transported to the nest typically involves cooperative efforts and exceeds in bulk what the individual ants could achieve, however the piece was divided. Such super-efficient transport is one of the convergent features of army-ant behaviour. It is also important to note that recruitment to the team is highly dynamic, with individuals 'choosing' involvement. The nest itself provides protection for the young, yet it is built of the living bodies of the ants.32 Interestingly, in these bivouacs (Fig. 8.2), which are temporary because the colony is highly nomadic, there is a careful thermoregulation of nest temperature.33
In the context of this book, however, the principal topic is not convergence per se, but the attempt to establish the likelihood of the repeated emergence of complex biological systems. Thus it comes as little surprise to learn that population sizes of some leaf-cutter ant colonies are enormous, with the inference that such numbers can be sustained only by the ants' careful system of fungal agriculture. Yet, these 'cities' are overtopped in numbers by some species of army ant, notably Dorylus, where the population of a single colony can reach 20 million workers. Such enormous numbers seem to be a result of both growth and aggression, and as such emerge as a result of an arms race.34 Even though genuine arms races in natural biological systems are probably rather uncommon,35 such examples as the army ants awake uneasy echoes of human organization.
202 alien convergences?
Not only is this true for the army ants, but study of other social insects may reveal other interesting evolutionary parallels. Consider, for example, the advanced eusocial bees,36 with their familiar division of labour, notably the queen and pollen-gathering workers. These animals have fascinated humans for millennia. In England it is still the custom, when the beekeeper has died, for a friend to go to the hive and tell the news to his or her bees. In a different way, in past centuries the industry and organization of the hive invited obvious political analogies. Now this fascination extends to evolution with the realization that bees have cognitive capabilities and a plasticity of behaviours that otherwise are known only in the vertebrates,37 and, no less surprisingly, other features such as exhibiting sleep-like states.38 Or is it surprising? Sleep may have several functions, but one widely agreed purpose is the need to consolidate memories.39
In any event, the complexity of bees not only has some intriguing parallels to vertebrate mental processes, but has been arrived at by an independent evolutionary trajectory that may have an unnerving parallel in our hominid history. This is because there is evidence that a once-wide diversity of eusocial bee groups has been whittled down through competition to only two groups (the apinid honey40 and meliponinid stingless bees41) over geological time.42 Interestingly, although the evolution of advanced eusociality in these two groups of bees has generally been regarded as stemming from a common ancestor, recent work suggests that they arrived at this arrangement by convergence.43 Moreover, Michael Engel remarks that it appears that during the geological expanses of time, aggressively foraging advanced-eusocial species had a negative impact, not only on non-social species44 but also and especially on other advanced eusocial lineages. A similar situation is reported from another highly competitive, social species (albeit not eusocial) for which the fossil record indicates the aggressive exclusion of related social genera and species that may have caused their extinction - namely Homo sapiens ... the only survivor of a prior hominid radiation.45
'Big, fierce societies'46 may lead not only to the emergence of collective intelligences but to ones that are, from our point of view, so alien as to be useful in envisaging extraterrestrial societies. In this context some remarks by Nigel Franks, a leading specialist on army ants, are intriguing, if also in places self-confessedly speculative. In reviewing the extraordinary social structure of these insects he observes how it is among the army ant colonies 'we see the emergence of flexible problem solving far exceeding the capacity of the individual', a capacity that finds no rival in other animals, apart from humans. This, as Franks stresses, presupposes effective communication, so that 'intelligence, natural or artificial, is an emergent property of collective communication ... This is exactly what happens when army ants pass information from individual to individual through the "writing" and "reading" of symbols, often in the form of chemical messengers or trail pheromones, which act as stimuli for changing behavior patterns.' So it is that we can see a transformation of a system from being hard-wired to flexible via 'increasingly sophisticated patterns of communication'.47 Such a pheromone-based society48 may be one of the few genuine alternatives to the emergence of human-like intelligence, which as discussed in the next chapter is itself almost certainly convergent. In genuinely speculative mode Franks draws attention first to eye reduction (each eye to a single facet in Eciton), which poses the question as to how the colony can navigate, given the vestigial eyesight of each worker. He continues,
In my wildest dreams, I imagine that the whole swarm behaves like a huge compound eye, with each of the ants in the raid zone front contributing two lenses to a 10- or 20 m wide "eye" with hundreds of thousands of facets. Each ant has possibly only the slightest directional preference, but through tactile signals and trail pheromones these preferences might be collated and amplified across the swarm. In this way the army ants could comprise a parallel-processing computer of intriguing yet awesome simplicity.49
Since writing this, Franks has continued his exploration of the convergent organization of army ant societies, and has also presented an absorbing analysis of the self-organization of the swarm raids. As he points out, the rules for a raiding front can in themselves be quite simple, even though the net result is uncannily 'intelligent' with an 'active architecture'. His comments on biological self-organization are of particular interest. It is, he explains, an assumption that such self-organization is 'order for free ... By this they [other authors] seem to suggest that self-organizing biological patterns occur so automatically that natural selection has had little influence over the evolution of their structure. I believe that such a viewpoint is extremely misleading'. So when we consider how army ants organize their swarm raids Franks argues that the processes of natural selection remains in force, for instance, by rather precisely [tuning] both the behavioural rules of thumb of the ants and the chemical properties (e.g. volatility) of their pheromones ... Self-organization theory does not suggest that natural selection has had no role in the creation of certain patterns in biology - rather it suggests that natural selection has rather less to do than one might expect given the complexity of the global structure. As illustrated by the swarm raids of Old World and New
World army ants, natural selection may have had to select for a surprisingly small and simple set of rules to generate swarm-raiding patterns. Thus self-organization theory may help to explain why we observe such a high level of convergent evolution in certain biological structures.50
Was this article helpful?