Book review: “The guests of ants: How myrmecophiles interact with their hosts”

Bert Hölldobler and Christina L. Kwapich, 2022. The Guests of Ants: How Myrmecophiles Interact with Their Hosts. Harvard University Press, Cambridge. 566 pp. EUR. 72,00

A Book Review by Christoph von Beeren

“Oh no!”

This was my first thought when opening the book “The guests of ants – how myrmecophiles interact with their hosts” by Bert Hölldobler and Christina Kwapich. I had agreed to review it for Myrmecological News but did not expect to find a monumental piece of work packed with over 550 pages of information about ant guests. I needed to somehow squeeze it in between work and family. However, this turned out to be easier than I initially thought because once I had started reading, it was impossible to put the book down.

The book starts with the personal notes of Bert Hölldobler, describing how his interest in ant guests arose. As a child, he regularly accompanied his father to an abandoned limestone quarry in Bavaria where they found a high diversity of ants and their guests, building the foundation for his lifelong fascination with these creatures. After the publication of his Pulitzer Prize-winning book “The Ants” in 1990, his co-author and friend E.O. Wilson had apparently kept pushing him to write an additional book about ant guests. Luckily, Wilson succeeded. Bert Hölldobler then met and teamed up with Christina Kwapich to write this remarkable overview about ant-associated organisms.

Before diving into the world of ant guests, the authors provide the necessary background information about the biology of ants. They introduce terms relevant for the discussion of the exploitation mechanisms used by ant guests. This brings on board all readers, including those that are not familiar with the biology of ants. In the glossary “ant guests” or “myrmecophiles”, are defined as organisms that spend at least part of their life cycle with ant colonies. This includes a broad taxonomic spectrum, including, for example, microbes, worms, fungi, arthropods, and vertebrates. The book covers this diversity by presenting a selection of well-studied and/or fascinating cases of ant-myrmecophile interactions. Listing them here would fill many pages, and in the following, I thus picked out a few remarkable examples that enthused me, and that highlight the marvelous adaptations of these creatures.

Workers of the Neotropical turtle ant Cephalotes atratus are entirely black. However, occasionally specimens were collected with a bright reddish gaster. The reason for this color variant is the nematode Myrmeconema neotropicum. The gasters of infected ants are filled with nematode eggs, causing the abnormal color that closely resembles berries growing nearby. The parasite not only changes the ants’ coloration but also their behavior. Infected ants hold their red gaster upright and show hardly any escape or defense response. You might have guessed it already; the nematode’s next victim is likely a frugivorous bird. The parasite’s life cycle is completed by re-infecting the host ants, because they feed, to a large part, on bird droppings.

While bacteria, worms and fungi provide stunning examples of ant-symbiont interactions, most examples given in the book introduce arthropod myrmecophiles, reflecting their high species diversity. A whole chapter is dedicated to lycaenid caterpillars due to their diverse mechanisms used to appease the ants, sometimes even inducing their own adoption into the heart of the colony – the inner nest chambers. Another chapter is dedicated to ant crickets, a group of myrmecophiles both authors have studied. Many of these agile guests have cracked the feeding code of ants and induce mouth-to-mouth exchange of liquid food (trophallaxis). Drawing from examples of different aleocharine staphylinid beetles, the authors illustrate the varying ecological niches occupied by myrmecophiles and depict a possible evolutionary scenario for the transition from loose ant associations as predators at foraging paths to highly specialized and host-specific ones as species that have achieved social integration. One example for the latter is the rove beetle Lomechusa pubicollis, a guest whose larvae as well as adults live inside the nest where they are fed by the ants. The intruders are treated amicably as if they were members of the ant society. Partly, this is mediated by a multitude of exocrine glands that protrude to hair tufts (so called trichomes), mostly on the upper side of the beetle’s abdomen. The trichomes’ chemical exudates attract and appease the ants, inducing them to pick up and carry the adult beetles into the nest. Trichomes are found in a wide variety of beetles and unveiling the chemical compounds altering the ants’ behavior is certainly a playground for future research.

The multitudes of strategies and mechanisms that enable ant guests to associate with their hosts are a focus of the book. These include chemical repellency and trickery, chemical and vibro-acoustical mimicry, as well as morphological and behavioral adaptations to myrmecophily. Notably, the authors do not only introduce relevant studies but also critically discuss them, sometimes even suggesting additional experiments. To me, this is a major strength of the book, ideally inspiring careful experimental verification in the future. The book closes with the diverse assembly of ant-associated vertebrates. These include ant birds feeding on the prey flushed out by army ants, birds taking “baths” in ants (anting) to deter parasites, tadpoles developing in leaf-cutter ant nests, as well as snakes, whose eyes are protected from ant attacks by translucent scales.

After having read all these astonishing examples of ant-myrmecophile interactions, the epilogue unveils that the abandoned limestone quarry – the habitat for all the marvelous ant guests Bert Hölldobler encountered as a boy – had been destroyed. To me, this personal anecdote mirrors the severe biodiversity crisis we are currently facing, strengthening the feeling that, besides doing entomological research, each of us should also invest more resources into conservation to protect ecologically valuable habitats. The book was a superb read and the many stunning pictures a pure enjoyment. The book will certainly serve as one of the main references for myrmecophile researchers, and hopefully inspire a new generation to dive into the fascinating world of ant guests.