There is no attempt to be comprehensive: the focus is on two species of army ants, the swarm raider Eciton burchelli and the column raider Eciton hamatum, with references to others. And there are bits of first-person "discovery" narrative, with a focus on a specific research station in Costa Rica: an encounter with an army ant raid, a last minute plane flight to observe a colony fission, and so forth. There are also amazing colour photographs, mostly illustrating details of behaviour, with stand-alone captions that allow easy browsing.
"The army ant Eciton burchelli and the leaf-cutting ant Atta cephalotes are arguably the two superstars of Neotropical myrmecology. Where the army ants' raiding or emigration columns collide with the foraging activity of the leaf-cutting ants, small scuffles can arise. This image shows two Eciton burchelli workers taking on a huge Atta cephalotes soldier, grabbing her by the antennae."
On the other hand, there's no attempt to provide any general biology background. Kronauer assumes a basic understanding of ant anatomy and life-cycle, along with some evolutionary biology and ecology. His focus is on the distinctive features of army ant colonies, how they behave, how they interact with other organisms, and how they evolved.
Kronauer begins with a brief history of the legends surrounding army ants and of their early scientific study. What is now Eciton Burchelli was originally Eciton hamatum, with some confusion over type specimens and the common problem of lone males being named separately to colonies.
The three key features of army ants are spontaneous mass raiding, nomadism, and reproduction by colony fission. This "army ant adaptive syndrome" evolved twice in the Dorylinae subfamily, once in the New World and once in the Old, and once in the ponerine genus Leptogenys, but individual army ant traits appear elsewhere. (The most technical material in the book is a four page table "Known life history characteristics of select species in the subfamily Dorylinae other than the army ants".)
Large colony sizes and spontaneous mass raiding (rather than scout-initiated group raiding) enable unusual diets. Most army ants prey exclusively or almost exclusively on other ants: Eciton hamatum raids in columns and targets the nests of both ants and termites. Swarm raiders in contrast target a broad range of prey, though few take vertebrate prey of any size; Eciton burchelli swarms can be 20 metres across and 2 metres deep. Kronauer explores the low level mechanics of how raids are initiated and organised.
"workers of Aphaenogaster araneoides rush out of the nest carrying brood at the slightest disturbance, as if they were only waiting right at the nest entrance for army ants to attack. In several ant species, workers leave the nest as soon as the army ant approach is detected, carrying pupae and larvae in a partial evacuation of the colony. However, at least some of the brood is usually still pillaged."
In Eciton burchelli the colony remains stationary for around three weeks, while the queen's gaster swells up, eggs are laid and larvae pupate. Then the eggs begin to hatch and callow workers begin to eclose from the pupae, and the colony becomes nomadic, moving most nights for around two weeks, with the larvae and any remaining eggs and pupae being transported. Kronauer considers the proximate and ultimate causes of this behaviour, as well as its mechanics.
"Eciton burchelli emigrations often last between six and eight hours, but the precise duration depends on a lot of factors, such as the colony size, the distance covered, and whether the process gets disrupted by adverse weather conditions. While some emigrations are completed by midnight, others last until the early morning hours. In the vast majority of cases, however, the colony has concluded its relocation before dawn, and at daybreak, no visible signs of the nightly procession remain."
Army ant colonies reproduce by fission (as in honey bees) rather than through foundation by solitary winged queens. "When an Eciton burchelli colony approaches fission it raises a brood of up to six young queens and about 2,500 to 4,000 males, but no workers."
"An important corollary of colony fission and wingless queens is that army ant colonies disperse rather poorly. Although their nomadic lifestyle can lead to genetic mixing across contiguous habitats, colonizing suitable terrain across any substantial body of water is essentially impossible. Army ant males, on the other hand, disperse on the wing, and Eciton burchelli males can probably fly for several kilometres."
One result of army ant colonies being persistent, without a founding-queen bottleneck, is that they offer a permanent ecological niche for other organisms. A final chapter "The Travelling Circus" looks at the array of "myrmecophile" and "inquiline" (sharing living space) mites and beetles and blind snakes and others, at how they avoid attack by the ants and at their ecological and evolutionary interactions with both the ants and one another.
"While either pretending to be one of the ants or being well protected against attacks certainly helps when crashing the party, army ant nest intruders face a challenge that the guests of other ants are hardly confronted with: joining the caravan when the circus hits the road. To mites, a group that is ripe with phoretic species and ectoparasites that attach to their host, this usually comes naturally. Social parasites that are derived from free-living ancestors, on the other hand, require novel adaptations to accomplish the feat."
For me, Army Ants finds a happy place alongside Hölldobler and Wilson's The Leafcutter Ants, in between the detail of something like Hölldobler and Wilson's classic The Ants and more popular works such as Erich Hoyt's The Earth Dwellers.
February 2021
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