Dominance Hierarchy - Dominance Hierarchies in Eusocial Insects

Dominance Hierarchies in Eusocial Insects

In insect societies, only one to few individuals members of a colony can reproduce, whereas the other colony members have their reproductive capabilities suppressed. This conflict over reproduction in some cases result in a dominance hierarchy. Dominant individuals in this case are known as queens and have the obvious advantage of performing reproduction and benefiting from all the tasks performed by their subordinates, the worker caste (foraging, nest maintenance, nest defense, brood care and thermal regulation). Accordingly to Hamilton’s rule, the reproduction costs of the worker caste are compensated by the contribution of workers to the queen’s reproductive success, with which they share genes. This is true not only to the popular social insects (ants, termites, some bees and wasps), but also for the naked mole rat Heterocephalus glaber. In a laboratory experiment, Clarke and Faulkes (1997) demonstrated that reproductive status in a colony of H. glaber was correlated with the individual’s ranking position within a dominance hierarchy, but aggression between potential reproductives only started after the queen was removed.

Social insects above mentioned, excluding termites, are haplodiploidy. Queen and workers are diploid, but males develop from haploid genotypes. In some species, suppression of ovary development is not totally achieved in the worker caste, which opens the possibility of reproduction by workers. Since nuptial flights are seasonal and workers are wingless, workers are usually virgin and only able to lay eggs that are not inseminated. These eggs are in general viable, developing into males. A worker that perform reproduction is considered a 'cheater' within the colony, because its success in leaving descendants becomes disproportionally larger, compared to its sisters and mother. The advantage of remaining functionally sterile is only accomplished if every worker assume this 'compromise'. When one or more workers start reproducing, the 'social contract' is destroyed and the colony cohesion is dissolved. Aggressive behavior derived from this conflict may result in the formation of hierarchies, and attempts of reproduction by workers are actively suppressed.

In some species, especially in ants, more than one queen can be found in the same colony, a condition called Polygyny. In this case, another advantage of maintaining a hierarchy is to prolong the colony lifespan. The top ranked individuals may die or lose fertility and "extra queens" may benefit of starting a colony in the same site or nest. This advantage is critical in some ecological contexts, such as in situations where nesting sites are limited or dispersal of individuals is risky due to high rates of predation.