Life History

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Revision as of 07:02, 14 April 2019 by CPeeters (talk | contribs) (TWO CONTRASTING STRATEGIES OF COLONY FOUNDATION)
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Life history describes the birth, growth, reproduction and death of an organism. Life history theory attempts to explain how such features have evolved and influence various aspects of a species' biology, e.g., morphology, behavior, ecological interactions and population dynamics. In ants, life history details are complicated by sociality. Biologists are interested in understanding the life histories in order to better understand individual species, ants as a whole and, more generally, the evolution of sociality.

The Antwiki webpage The Ant Life Cycle provides a general introduction to the life cycle of an ant colony.

TWO CONTRASTING STRATEGIES OF COLONY FOUNDATION

The ability to disperse and establish new colonies is crucial to the ecological success of ants. However, founding queens are often alone, which means that they face the same dangers as solitary insects. Colony-founding behaviours have diversified tremendously across the ants, and underlie their capacity to colonize almost all terrestrial habitats. Unlike social wasps and bees, many ant species evolved (1) queens that can store large amounts of metabolic reserves, (2) workers that are much smaller than their queens. Both these adaptations make it possible to raise the first workers without outside food (claustral foundation), and this is arguably central to the tremendous success of ants belonging in subfamilies Dolichoderinae, Formicinae and Myrmicinae.

During independent colony founding (ICF), lone queens need to raise the first generation of offspring without the help of nestmates. Following aerial dispersal and mating with a foreign male, they locate a suitable nesting site and break off their wings. Foundresses excavate or take over an existing shelter, and lay a first batch of eggs. During the next few weeks (sometimes months), they must guard and feed their offspring. ICF is a risky strategy, especially when queens need to forage outside the nest (non-claustral foundation). ICF necessitates the annual production of a large number of sexuals, and this ability is affected by colony size. In many phylogenetically independent lineages, ICF has been replaced by a strikingly different founding mode, whereby queens are not alone. In such dependent colony founding (DCF) species, existing colonies divide into two or more daughter groups which soon become autonomous. Thus the queens disperse on foot to a new nesting site together with nestmate workers, and the latter feed and protect the brood.

A unique feature of ants is the evolution of permanently wingless queens in many species exhibiting DCF. Many social wasps and bees start new colonies by DCF, but there is universal retention of flying workers and queens. This flight constraint is absent in ants: wingless workers are the major players during the division of existing colonies, and since dispersal occurs exclusively on foot, the production of winged queens is selected against. Accordingly, ergatoid (no wings) queens (see list of species) and brachypterous (short-winged) queens (list of species) evolved convergently in species belonging to over 50 genera (Peeters 2012).

MUTUALISM WITH FUNGI OR SAP-FEEDING INSECTS. In a restricted number of genera, claustral ICF is possible due to trophic interactions with a mutualist, hence foundresses lack large metabolic reserves. During the mating flight of Acropyga and Tetraponera, foundresses carry a gravid pseudococcid (scale insect) to the stem cavity where they settle - being clonal, the sap-suckers multiply and supply sufficient honeydew to feed the first worker brood. Similarly in Azteca, dispersing queens carry an ascomycete fungus in their infrabuccal pocket; after settling in a domatium, this fungus is used to process parenchyma tissue scraped away from the plant. This is then used to feed the first larvae, meaning that the foundress does not need to forage outside. Acquisition of appropriate scale insects is an important step in the successful establishment of a new colony on a host plant. In contrast to the above examples of vertical transmission of the mutualistic partner, foundresses in other species need to find scale insects or fungi near the newly established nest (horizontal transmission). Cladomyrma lives together with mealybugs (also a scale insect), and first instars ('crawlers') disperse to settle into incipient nests of the ants. Melissotarsus and Rhopalomastix live in a mutualism with diaspidid scale insects inside living trees, and it is likely that crawlers locate tunnels newly chewed by founding queens.

PARASITISM IS ALSO A STRATEGY OF INDEPENDENT FOUNDING In several lineages, newly mated queens attempt to enter existing colonies of their own or different species, following which they exploit local resources to raise their offspring. As discussed by Peeters & Molet (2010), social parasitism is a form of ICF because queens are not helped by nestmate workers. They disperse alone and take considerable risks when trying to enter the host colony. Host colonies are nothing more than a resource of the environment to be exploited, similar to insect prey. Many authors have considered parasitism to be DCF because queens ‘depend’ on their host colonies, however claustral queens also 'depend' on their metabolic reserves, and non-claustral queens 'depend' on the food they gather outside. What is crucial in DCF species is that queens depend on closely related nestmate workers, i.e. there is convergence of genetic interests.

The founding strategies of a majority of ant species remain unstudied. Indeed, there are no published data about colony foundation for many large genera. This scarcity of information stems from the necessity of studying colony foundation in the field. DCF is especially difficult because it can occur unpredictably over the year, and it needs to be distinguished from simple nest emigrations (Cronin et al. 2013). In species where the colony founding strategy is unknown, data on queen morphology (alive or dead) can generate testable hypotheses. The thorax architecture of queens and workers reflects the size and geometry of head muscles (Keller et al. 2014). Among species with flying queens, some have strong neck muscles (similar to those of workers) because they need to forage outside the incipient nests (non-claustral ICF), and their prothorax is larger than in claustral species. In non-flying queens, the absence of wing muscles is associated with a simplified thorax (i.e. fusion of sclerites), and such queens generally perform DCF.

Survey of the various founding strategies documented across 49 ant genera. By Christian Peeters


MATING STRATEGIES ARE DISTINCT FROM FOUNDING STRATEGIES

Mating usually precedes colony founding, but these are two distinct events affected by very different selective pressures (Peeters & Molet 2010). ICF queens in some species mate close to their natal colony (“female-calling”) before dispersing, while in other species they mate far away (“male-aggregation”). DCF queens mate near their natal colony, or even inside it. This seems true even in DCF species with queens that are capable of flying: since newly mated queens stay in their natal nest, they cannot fly to male-aggregations that are far away. Female-calling is associated with lower mortality since flight means increased risks of predation or getting lost.