Army Ants: Nature’s Ultimate Social Hunters
Kronauer D. 2020 Army Ants: Nature’s Ultimate Social Hunters. – Harvard University Press, Cambridge, Massachusetts, USA, 384 pp; ISBN 978-0674241558, EU € 58.50
A Book review by Ajay Narendra
When we think of animals that hunt socially, we may immediately visualise a pack of wolves or a pride of lions chasing down their prey. Our thoughts then must come to the swarm of ants, the legionary army ants, that rule the floors and the tree trunks of the tropical rain forests. Which other beast attacks relentlessly for several hours to capture anything that moves along its path, a path that spans over several hundreds of meters?
In Army Ants, Daniel Kronauer, a master storyteller, unwraps the bivouac of the army ants to take us along the journey of their raiding swarms, the intricate bridges they construct, and the enigmatic arthropod fauna that live alongside these formidable hunters. The book showcases how the joys of watching animals in the wild can lead to exciting discoveries. The society of the army ants offers the breadth to address Tinbergen’s four questions to understand behaviour: mechanism, ontogeny, adaptive value, and phylogeny. These form the essence of the six chapters, each of which are complemented with more than 150 action-packed photographs.
Army Ants begins with the discovery and the confusion that reigned early on in the naming of the two champions of the book, Eciton hamatum and Eciton burchellii. This chapter immerses us into the world of early explorers and taxonomists where Kronauer identifies ant species from paintings and descriptions from the early 1700s. He carefully traces research by Johan Christian Fabricius, Pierre André Latreille and Auguste Forel during which the swarm raider E. burchellii was called different names and finally to Carlo Emery who resolved the nomenclature. This chapter highlights the importance of type specimens and the need for developing identification keys to the elusive and wasp-like male ants. Even though this chapter is historical in nature, each section includes the latest on army ant behaviour and biology.
Next, in the ancestry chapter, we head to the mid-Cretaceous period where ants first evolved from their wasp-like ancestors. Following a concise summary of how and what information is required to determine the origin of ants and the relationships between different ant subfamilies, Kronauer presents exhaustive information of various life history characteristics of army ants and also non-Doryline ants that exhibit army-ant like behaviour. This is a precious resource as he identifies the gaps in our current knowledge to guide future research. To a layman, some terminology such as dichthadiigyne and subdichthadiiform may seem daunting at first, but clear descriptions are provided to help the reader. While the book does not have a full list of all army ant species or a key to the army ant genera, there are appropriate and sufficient references to guide the astute reader.
The next three chapters, mass raiding, nomadism, and colony fission, include fascinating stories and reasons to study army ants. Kronauer’s first-hand experience provides us with a window into the raiding swarm of Eciton as they pillage through the tropical forest, invading colonies of other ants and feasting on arthropods, without discriminating between walking, flying, or jumping animals. We learn how these raids are organised; that there are ‘pushing parties’ that initiate raids when trail pheromones are absent; ants that encounter gaps form living bridges for nestmates to walk to optimise traffic flow; and how information is transferred between nestmates. We learn the triggers for the development of such a bridge and when and why such bridges are abandoned. Most impressive is the vast array of prey captured by army ants which informs us of the extent of specialisation exhibited by each species. While the focus of the book is on two species of army ants, Eciton hamatum and Eciton burchellii, a reference to other army ants (Aenictus, Dorylus, Labidus, Neivamyrmex) or those that exhibit army-ant like behaviour (Leptogenys, Ooceraea) are made where possible. A distribution map of the army ants at the genus level would have revealed the extent of overlap, but this is accessible from the provided references (e.g., Borowiec 2016). A significant section is devoted to the art of building a bivouac, with clear information on the discrete nomadic and statuary phases, the factors that trigger these and what occurs during these phases. We are then treated to a relatively rare phenomenon, the formation of a new army ant colony, which involves splitting of an existing colony into two. This is riveting stuff – a non-fiction thriller!
For all the aggression that the army ants display, in the last chapter Travelling Circus, Kronauer makes a compelling argument why they are the perfect hosts and provide a highly conducive environment to host myrmecophiles. Here we are introduced to a dazzling array of arthropods that conceal their identity, some that live on specific parts of ants, others that live within the nest of the army ants, and a few that in fact join the swarm as the ants travel to new bivouac sites. The most spectacular among them all is a clown beetle that passively transports itself, often undetected, by holding on to the ant, Eciton mexicanum, between its petiole and postpetiole (von Beeren and Tishechkin 2017).
Army Ants will undoubtedly be adored by all myrmecophiles. The book is visually captivating with stunning photography that Kronauer uses to deftly introduce fundamental concepts of eusociality, diversity, social parasitism, mimicry, and collective behaviour which will appeal to a non-specialist audience too. The book is likely to excite those interested in social robotics, networking, and communication who draw inspiration from these highly specialised mass swarming ants to understand emergent behaviour.
Three decades ago, a monumental treatise and Pulitzer Prize winner, The Ants (Hölldobler and Wilson 1990), inspired an entire generation of students to take up myrmecology. Army Ants is set to unfurl the next swarm of myrmecologists.
Borowiec ML 2016: Generic revision of the ant subfamily Dorylinae (Hymenoptera, Formicidae). – Zookeys 608: 1-280.
Hölldobler B & Wilson EO. 1990: The Ants. – Belknap, Harvard University Press, Cambridge, MA.
Kronauer D. 2020: Army Ants: Nature’s Ultimate Social Hunters. – Harvard University Press, Cambridge, Massachusetts. 384pp
von Beeren C & Tishechkin AK. 2017: Nymphister kronaueri von Beeren & Tishechkin sp. nov., an army ant-associated beetle species (Coleoptera: Histeridae: Haeteriinae) with an exceptional mechanism of phoresy. – BMC Zoology 2: 3