Book review – Ant Encounters: Interaction Networks and Colony Behavior

9-minute read
keywords: entomology, ethology

This is the second of a trio of reviews in which I take a brief detour into ants and collective behaviour more generally. I previously reviewed The Ant Collective, a graphical introduction to ant behaviour, and am here turning to entomologist Deborah M. Gordon’s 2010 book Ant Encounters before finishing with her recent book The Ecology of Collective Behavior. The core question driving this book is how ant colonies get anything done given that no one is in charge. Her contention, supported by a wide-ranging survey of examples, is that ant colonies function through numerous ants interacting to form a dynamic network. Stated this pithily, I admit it might not sound like much of an answer but rather a rephrasing of the question using fancy words. What do you mean, “interaction network”? If so, read on: this primer is full of fascinating biological examples and interesting insights that will hopefully clarify the above, providing you with a bigger picture of how and why ants behave the way they do.

Ant Encounters: Interaction Networks and Colony Behavior, written by Deborah M. Gordon, published by Princeton University Press in April 2010 (paperback, 167 pages)

A brief tangent to get started. Ant Encounters was the first in the then-new series Primers in Complex Systems, produced by the Santa Fe Institute in New Mexico. This is a think tank that studies complex systems in, amongst others, biology, physics, sociology, and computer sciences. Next to a small cadre of resident researchers, they host a rotating cast of visiting academics and provide an environment for cross-disciplinary research, discussion, and exchange of ideas. Some other books I have reviewed here have been the product of, or at least been helped along by, time spent here. Seven books have been published so far in this series, though there have been no new additions since 2019. If this first book sets the tone, these are very interesting and accessible little books that are worth checking out and I have listed them below. Anyway, back to Gordon and ants.

This book follows her popular book Ants at Work published another decade earlier in 1999. At the time of writing Ant Encounters, Gordon had already been studying ants for some three decades, since the early 1980s, mostly the desert-dwelling red harvester ant Pogonomyrmex barbatus. Her work and ideas formed in response to the then-prevailing view that ants are effectively genetically programmed to perform particular tasks. Her early work instead found that what ants do depends on interactions with other ants, which in turn can modify their environment, which in turn can feed back on subsequent interactions, etc. In other words, the behaviour of colonies arises from dynamical, constantly shifting networks of interactions. (The Ant Collective neatly showed in pictures how this works in the waxing and waning of a foraging trail.) After introducing this concept in the first chapter, the remaining six chapters flesh it out with numerous examples at different levels of biological organization.

A logical first question to ask is what ants actually do when they meet. A lot of feeling each other up is what. Communication in ants is firstly tactile, with ants using their antennae to touch each other (entomologists speak of antennation). One of Gordon’s key points is that “the pattern of interaction itself, rather than any signal transferred, acts as the message” (pp. 47–48). Experiments in harvester ants showed that what stimulated foragers to leave the nest was the encounter rate with patrollers returning in the morning. One question for Gordon I was left with was whether smell acts as a message. After all, a second important channel is chemical, with ants, like many other insects, carrying hydrocarbons (so-called cuticular hydrocarbons) on their exterior. These serve as an identity badge but also change during tasks: Gordon’s work on harvester ants showed how ant odour changes as they go outside e.g. to forage. Interestingly, there is individual variation in how active ants are but this is not fixed. Remove particularly active ants and others will take up their workload; there are no “forager heroines” (p. 66).

“A logical first question to ask is what ants actually do when they meet. A lot of feeling each other up is what. […] One of Gordon’s key points is that “the pattern of interaction itself […] acts as the message“”

Scaling up a little to the level of local interactions, Gordon objects to the idea of “division of labour” introduced by famed entomological titan E.O. Wilson. He happened to study some of the few ant species where workers have different body sizes and tried to demonstrate that there was a caste system with size-based task specialisation. Though partially successful, even here individual behaviour changes as needed, with e.g. removal of minors (small ants) causing larger majors to switch to brood care. More importantly, Gordon adds, in most ant species workers are of similar size. Whereas “division of labour” implies static procedures and permanently assigned roles, her preferred term “task allocation” highlights the dynamic, flexible nature of ant behaviour. A related idea that is scrutinized is age polyethism: tasks changing as a function of age. I encountered this in Tschinkel’s book Ant Architecture and though young ants are indeed born in the depths of the nest, the idea that they move up with age and get “promoted” to brood care, nest construction, and finally foraging is more often proposed than backed up with data in the published literature. Experiments on carpenter ants showed that if one age group was removed, others would take over their tasks, suggesting that colony needs override ant age.

Speaking of age, colonies are not static entities and, though there are very few studies on this, colony growth influences behaviour. Particularly important are ant-plant interactions with ants defending host plants from herbivores, thus stimulating plant growth, which in turn begets a larger colony (a nice example of niche construction if ever there was one). In many such mutualisms, there is a third party such as scale insects or aphids that feed by sucking a plant’s sap but are vulnerable to predators. Ants protect them in exchange for the sugary honeydew these sap-suckers excrete from their anuses. The plants tolerate the sap-suckers because the ants thus attracted protect against more harmful herbivores. Ecologist Robert May is quoted as describing such three-way interactions as “an orgy of mutual benefaction” (p. 124).

Scaling up further brings Gordon to the level of inter-colony interactions, both conspecific (between colonies of the same species) and heterospecific (between colonies of different species). Interactions are both direct, e.g. encounter rate with neighbours indicating the size of their colony, and indirect, e.g. competition for food being a zero-sum game: what one colony eats is not available to the other. Bar some spectacular exceptions, fighting between conspecifics is usually avoided. In response to neighbourly interactions, harvester ant patrollers will redirect the next day’s foraging trails. When it comes to heterospecific interactions, conflict has been better studied and Gordon highlights the rise of invasive ant species in the last three decades as another opportunity to do so. In general, numerical advantage trumps body size, with small-bodied species capable of simply swarming larger-bodied ones. A further, delightful level of interaction is between ants and the organisms living in their midst (so-called myrmecophiles) that I discussed at length when reviewing The Guests of Ants.

“In many ant-plant interactions, there is a third party involved, such as scale insects or aphids, in what has been described as “an orgy of mutual benefaction“”

The final level Gordon considers is that of evolution. How did colony organization evolve from ancestors that did not live in colonies? In particular, as workers are unable to reproduce, how did worker sterility evolve? W.D. Hamilton linked it to the haplodiploid mating system of ants where males hatch from unfertilized eggs and carry a single set of chromosomes (haploidy) while females hatch from fertilized eggs and carry the regular two sets of chromosomes (diploidy). One quirk of this system is that, if a queen mates with only a single male, females are more related to their sisters than to their daughters, with kin selection maintaining worker sterility once it somehow arises. The fly in the ointment that Gordon points out is that many queens mate with several males, so Hamilton’s maths quickly unravels, leaving the mystery of worker sterility intact. (In Endless Forms, Sumner explores this story more in-depth, showing there are other reasons why Hamilton’s explanation does not work). What we do know is that ants evolved from vespoid wasps, themselves a case study in the evolution of sociality. Interestingly, worker sterility has evolved many times in wasps and shows more flexibility, with wasps able to become egg layers when needed, arguing against a sudden mutation being responsible for worker sterility in ants. The matter remains unresolved, but Gordon does think that describing social structure in terms of who lays the eggs is “misleading language [that] equates the allocation of egg-laying with social organization and directs attention away from everything else that makes up the diverse and complex social organization of ant colonies” (pp. 130–131).

If all of the above left your head spinning somewhat, then you are not alone. There are some really interesting challenges to established ideas in this slim book, but it took some effort to distil these key points. Much as was the case for The Guests of Ants, this book presents a bewildering variety of examples from many different species, reflecting the state of play in myrmecology. A recurrent theme is that we have extremely limited data. Of the approximately 11,000 ant species known to exist, only 50-odd have been studied in any detail and even then only in fragmentary fashion. “What we have so far are only some of the pieces of many different puzzles; fragments of the picture for army ants, other pieces for harvester ants, others still for fire ants, and so on” (p. 144). Even in this limited sample, exceptions and diversity abound. A brief final chapter considers what it would take to construct models of ant behaviour. This would come to occupy Gordon for the next decade, resulting in the recent publication of The Ecology of Collective Behavior to which I will turn next.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

Ant Encounters