Dominance Hierarchy - Regulatory Mechanisms in Eusocial Organisms

Regulatory Mechanisms in Eusocial Organisms

The suppression of reproduction by dominant individuals is the most common mechanism that maintains the hierarchy. In eusocial mammals this is mainly achieved by aggressive interactions between the potential reproductive females. In eusocial insects, aggressive interactions between sexuals are common determinants of reproductive status, such as in the bumblebee Bombus bifarius, the paper wasp Polistes annularis and in the ants Dinoponera australis and D. quadriceps. In general aggressive interactions are ritualistic and involve antennation (drumming), abdomen curling and very rarely mandible bouts and stinging. The winner of the interaction may walk over the subordinated, that in turn assumes a prostrated posture on the substrate. This stereotyped behavior is common in social insects, especially within ponerine ants, and was also observed in interactions of naked mole rats.

In order to be effective, these regulatory mechanisms must include traits that make an individual rank position readily recognizable by its nestmates. The composition of the lipid layer present on the cuticle of social insects is usually the clue used by nestmates to recognize each other in the colony and to access information regarding the reproductive status of each one (and therefore its rank in the hierarchy). However, another recent finding suggest that visual cues may also transmit the same information. The paper wasp Polistes dominulus have individual facial marks ("facial badges") that permit them to recognize each other and to identify the hierarchy/reproductive status of each individual. Individuals that have their badges modified by painting were aggressively treated by their nestmates, suggesting that advertising a false ranking status is a costly behavior and it is suppressed in these wasps.

In addition, other behaviors have been demonstrated to be involved in the maintenance of reproductive status in social insects. The removal of a thoracic sclerite in Diacamma ants inhibit ovary development and the only reproductive individual of this naturally queenless genus is the one that retains its sclerite intact. This individual is called gamergate and is responsible for mutilating all the newly emerged females, to maintain its social status. Gamergates of Harpegnathos saltator arise from aggressive interactions, forming a hierarchy of potential reproductives.

In the honey bee Apis mellifera, pheromone produced by the queen mandibular glands is responsible for inhibiting ovary development in the worker caste. “Worker policing” is an additional mechanism that prevents reproduction by workers, found in bees and ants. Policing may involve oophagy and immobilization of egg-layers from the worker caste. In some ant species such as the carpenter ant Camponotus floridanus, eggs from queens have a peculiar chemical profile that workers can distinguish from worker laid eggs. When worker-laid eggs are found, they are eaten. In some species, such as Pachycondyla obscuricornis, workers may try to escape policing by shuffling their eggs within the egg pile laid by the queen.