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Army ants take the stage

2021; Elsevier BV; Volume: 31; Issue: 4 Linguagem: Inglês

10.1016/j.cub.2021.01.024

1879-0445

John T. Longino,

Insect and Arachnid Ecology and Behavior

Daniel Kronauer sits on a folding stool in the dawn rainforest of La Selva Biological Station in Costa Rica. He is watching a pulsing mass of army ants, a ‘bivouac’, as the first rays of sunlight strike its surface. As he describes it, a shiver of activity spreads through the bivouac: the beast is about to rise, and a new swarm raid is imminent. This passage is in his new book, Army Ants: Nature’s Ultimate Social Hunters, and it transported me immediately to the sights and sounds of the La Selva forest. Kronauer not only brings us up to date on one of the all-time greatest hits of biological exuberance and adaptation, he does so in a spellbinding way that keeps us turning the pages. Anyone who has spent time in tropical forests is familiar with army ants. Massive sheets of ants ooze through the forest, the slowly advancing edge creating a pandemonium of fleeing arthropods. It is one of the true spectacles of tropical life, routine for indigenous peoples but definitely not an accustomed sight during Europeans’ first tropical encounters. Maria Sibylla Merian, a naturalist and artist who traveled to the Dutch colony of Suriname in the early 1700s, was probably the first to introduce army ants to the European world, and since then they have figured prominently in both the popular imagination and scientific inquiry. The scientific study of army ants took a great leap forward with the work of T.C. Schneirla, who spent decades following army ants in the rainforest of Panama, unlocking the secrets of their foraging and reproductive rhythms. His work culminated in the 1971 book, Army Ants: A Study in Social Organization. Subsequent major reviews and updates include the chapter on army ants in Hölldobler and Wilson’s 1990 book, The Ants, and Gotwald’s 1995 book, Army Ants: The Biology of Social Predation. With the emergence of new field observations and advances in molecular techniques, in particular, it is well past time for the next installment. Kronauer’s book begins with a prologue that sets the stage, emphasizing how the terror of an army ant raid has inspired cultural fascination. I knew about the famous short story Leiningen Versus the Ants but was delighted to be reminded that the last Aureliano Buendía in García Márquez’s One Hundred Years of Solitude was devoured by army ants. In the prologue, Kronauer explains that the book has two main protagonists that are prominent and iconic Neotropical army ant species, and that the stage for his protagonists is predominantly La Selva Biological Station, where Kronauer and colleagues have conducted much of their research. Chapter One delves into the early history of discovery and naming, including the first observations of the large wingless queens of army ants, and the long-delayed identification of peculiar flying insects as the males. Chapter Two covers army ant ancestry, a topic that has perhaps changed the most since Gotwald’s book. The latest molecular phylogeny is integrated with a comparative look at the hallmarks of the army ant lifestyle: mass raiding, nomadism, and colony fission. Chapter Three is all about mass raiding: how raids are organized, what the ants feed on, raid followers (birds, butterflies, and parasitic flies), and the ecological impact on prey communities. This chapter showcases some of Kronauer’s own studies of raid organization in Ooceraea biroi, a “proto-army ant” that he has developed as a model organism that can be reared and studied in the lab. It also highlights the recent work of Philipp Hönle and colleagues, who used DNA barcoding to provide the first real measurements of army ant prey specialization. Chapter Four concerns nomadism, the army ant habit of regularly moving camp. The iconic and best studied species exhibit ‘phasic behavior’, in which colonies alternate between nomadic and statary phases (the latter being a non-nomadic phase), and reproductive cycles are synchronized and coordinated with phases in a neatly adaptive manner. Chapter Five focuses on colony fission, which covers the intricate and unusual sex lives of army ants and their mode of reproduction. Chapter Six, aptly named ‘The Traveling Circus’, jumps to a fulsome treatment of myrmecophiles, the many arthropod species that have become specialized associates of army ants, insinuating themselves into army ant society. They are a true rogues’ gallery of beetles, flies, mites, and myriad other creatures, ranging from commensals to parasites of the traveling ecosystem that is an army ant colony. An epilogue wraps it up, followed by a glossary, a list of the literature cited, and an index. By design, the book is engagingly written and short, yet it will also be a valuable addition to the specialist’s library. The text is jargon-free, with frequent expositions of relevant general concepts, such as phylogeny, levels of selection, and inclusive fitness. The book can be easily read and enjoyed by a non-specialist, but at the same time it provides a guide to the technical literature, with in-text citations to over 600 references. Gems of writing are sprinkled throughout, such as alliterative “pot-hole plugs” (single workers spanning gaps) and “attraction to action” (worker response to moving prey). Workers schlep prey, stop at roadside restaurants, and exhibit road rage. A particularly flashy myrmecophile becomes a “glitter bullet” and is likened to a human cannonball in a sparkly sequined costume. A new concept is introduced with this passage: “Who determines that the conditions are right for a colony to divide? Is it the queen who decides to lay an extraordinary batch of eggs, or is it the workers who decide to raise an extraordinary cohort of larvae?” Such an elegant and simple way to state a basic question. When pondering the impact of myrmecophiles on army ants, Kronauer contrasts the number of ant workers per myrmecophile (5,000) with the number of humans per rat in New York City (4), citing a study titled ‘Does New York City really have as many rats as people?’ Brilliant. The text swings easily between general ideas and natural history details, and between existing knowledge and the author’s own field observations. We can learn about the general distinction of migratory versus nomadic behavior, and also that army ants carry their larvae slung beneath them, head forward, and mouth up (who knew!?). The illustrations, making up nearly half of the book, are mostly large color photos taken by Kronauer, and as I read the book I found myself eagerly seeking out the images that accompanied textual explanations. I know he must have been particularly proud of the single photo that captured interspecific interactions, age polyethism, and myrmecophily all in one go. Now for my Rodney Dangerfield moment (taxonomists don’t get no respect). Something missing from this book, and from any of the previous syntheses of army ant biology, is a section on the species-level taxonomy of army ants. I had hopes when I read Chapter One, which has a lovely mystery story involving the arcane rules of zoological nomenclature and taxonomic priority. But there it ended, with a mention of Max Winston’s studies of species boundaries in Eciton burchellii (the main protagonist) appearing only late in the book. Admittedly, taxonomy is not in Kronauer’s wheelhouse. But a fascinating feature of the army ant story is how parallel taxonomies have arisen for workers and males because they are so rarely found together. Males come to light, sporting a morphological candy box of elaborate genitalia, which have been used as the basis for many species names. Workers gathered from foraging columns form their own morphological clusters, resulting in a separate set of species names. Matching of males and workers has come slowly, yet we now have the molecular tools to readily do so. It is remarkable and somewhat mysterious to me that, with DNA barcoding, we now know more about the species boundaries of myrmecophiles than the army ants themselves! Let us hope that army ants will continue to course through tropical forests and that naturalists will continue to follow them, learning their ways. Kronauer, for whom critical instrumentation includes both PCR machines and folding stools, is an inspiration. His book is not a culmination but rather a progress report. Like the irresistible urge to follow an army ant column through the dense underbrush of a tropical rainforest, I will continue to follow Kronauer’s work as he weaves back and forth between field observations and controlled laboratory experiments. I eagerly await the next installment.