The Alhambra in Granada, Spain.

I recently traveled to Andalusia, in the southern part of the Iberian Peninsula, to meet fellow myrmecologists Christian Peeters, from the Université Pierre et Marie Curie, and Alberto Tinaut, from Universidad de Granada. The reason for my trip was that I am fortunately enough to have been invited to collaborate in one of their ongoing projects studying the native ant species Monomorium algiricum. We set out to collect some colonies of this species as well as some others in the genus.

This was quite a pleasant trip. Collecting was a success, our host treated us well, and the scenery was great. The historic center of Granada is remarkable beautiful and, to my surprise, the locals speak a language very much like Spanish, for which I am accidentally fluent.

Sierra Nevada, southern face. The collecting site for M. algiricum is at the center of the picture on the mountain slope far back.

The species we are studying is also quite charming. Monomorium algiricum was originally described in the 1950s from a population in northern Africa, but the species happens also to be well established on the European side of the Mediterranean sea at mid-elevation in the Sierra Nevada, near Granada. Peculiar about this species is that the queens never develop wings, having a rather reduced thorax that very much resembles the worker’s. In general, such reproductive individuals are termed ergatoid queens, that is, worker-like queens (for ergatos = gk. worker).

The wingless thorax in the ergatoid queen of Monomorium algiricum.

Flightless queens pose a tremendous change in strategy when it comes to how the colony reproduces. The textbook version of an ant colony’s life cycle entails a newly emerged winged queen leaving the maternal nest and flying away by herself to find a suitable bachelor (or a suitable party of them). After mating, the queen sheds her wings and burrows into the ground to start a new colony by raising the first generation of workers, also by herself, without ever leaving the nest. The queen is able to perform this “claustral” colony founding by burning up her fat bodies together with the huge wing muscles to produce the energy necessary to lay eggs and feed the growing larvae. Most species of the  ~400 known in the genus Monomorium more or less display this mode of colony reproduction. By contrast, reproduction in species with ergatoid queens entails a process of colony budding: the newly mated flightless queen goes out of the maternal nest for a casual walk with some of the workers and never comes back, settling nerby to start a new independent colony. Together with the loss of wings, ergatoid queens lack the flight musculature normally used as body reserves, so they depend on the help provided by the accompanying workers to procure food and raise further generations of workers.

Collecting Monomorium subopacum in the Mediterreanan coast. Christian Peeters (standing) and Alberto Tinaut.

Christian Peeters has devoted more than two decades studying the various aspects of these “alternative” modes of colony reproduction that correlate with different queen morphologies, for which ergatoid queens are just one type among many. A major result of his extensive work is that the textbook caricature of colony life cycle I just described¹ is anything but the norm. Moreover, during their evolutionary diversification ants have deviated from the “normal” mode of reproduction multiple times independently in an astonishing high degree. Within Monomorium alone, the evolutionary switch from winged queens to ergatoid ones has probably occurred multiple times. The biology of ergatoid queens in Monomorium is better known for the North American species after the pioneering work done by William Morton Wheeler in the early 1900s, therefore the interest in the Mediterranean M. algiricum.

Collecting complete live colonies of Monomorium (Note: the hole was NOT done with the aspirator).

But, what’s in all these for me, a person more interested in the homology of obscure body structures than in colony reproduction dynamics and adaptation scenarios (hell, adaptation is not even in my vocabulary)? Well, it is exactly the fact that changes in colony reproduction strategy in ants is intimately linked to changed in the morphology of the reproductive individuals. As in the case of ergatoid queens, these morphological changes can be dramatic. These adult forms are not like workers, nor like the usual winged queens. These forms correspond to completely different castes and come with their unique anatomical specializations.

Colonies will normally have multiple queens. We want to know exactly how many there are per nest.

Just within Monomorium, the morphology of ergatoid queens is quite variable. For example, in the the North American wingless forms it is possible to homologise most thoracic plates with the ones in the thorax of a flying individual, even though the plates are fused together. But in M. algiricum the sutures between the much reduced skeletal parts are completely gone, and one has to guess what corresponds to what by careful examination of the faint depressions on the cuticle and the internal attachment of some of the muscles left.

Tinaut's lab in Granada's University. Peeters performs dissections to check how many queens have been inseminated and if their ovaries are active.

What we hope to do is to assemble a detailed picture of the life history of M. algiricum from many different angles and so understand how does this species fits into the larger pattern of the evolution of ergatoids in the genus.

Notes