Ants on Brazilian island have up to 12 queens per colony

The ant species Odontomachus hastatus establishes a hierarchy among its queens, adapting to local ecological conditions.

By Elton Alisson

Agência FAPESP – The popular belief that an ant colony is made up of a single queen that reigns pacifically over an army of worker ants is far from the reality of the colonies of the species Odontomachus hastatus.

Studies conducted by researchers from the Biology Institute at Universidade Estadual de Campinas (IB- Unicamp) have shown that the colonies of this ant species can have more than one queen. Given the scarcity of appropriate places for founding colonies, the queens form alliances to guarantee survival and form the largest possible army of worker ants. After consolidating the colony, however, they establish a hierarchy through ritualized fights in which one establishes herself as the alpha queen and assumes the task of laying the largest quantity of eggs.

The other queens become subordinates and, although impregnated, are obligated to lay fewer eggs than the alpha queen and play the role of worker ants.

“When we think of a colony of social insects, such as ants that live in nests, it is common to believe that it is a harmonious organization in which there is well-divided labor and a single queen is responsible for reproduction. It is counter-intuitive to imagine that there are alliances following conflicts in which a queen ant attacks another in a ritual manner and inhibits the other from reproducing to become the alpha queen, as we have observed in a series of studies,” comments Paulo Sérgio Oliveira, an IB professor and coordinator of the study.

Oliveira and his group have studied several colonies of Odontomachus hastatus in São Paulo State Park, on Ilha do Cardoso on the state’s coast.

One of the findings in samples collected from the field for the FAPESP-funded master’s research was that the majority of the colonies of ants in this species found and analyzed in the Ilha do Cardoso State Park have between 2 and 12 queens capable of laying eggs. The discovery was published in 2011 in an article in the magazine Insectes Sociaux.

In analyzing studies of the same ant species conducted in 2007 at the Natural Reserve of Nourage in French Guiana by researchers at the University of Toulouse’s Institute of Biological Sciences in France, the Brazilian researchers observed that their French colleagues did not find any insect colony with more than one queen in the South American country.

In addition to attempting to understand why the behavior and colony formation of the same species varied between different locations from 2011 to 2012, the researchers from the two countries conducted a comparative study under the auspices of an agreement between FAPESP and the National Center for Scientific Research (CNRS) in France.

The study, which was also part of a Regular Research Grant at FAPESP, revealed that Odontomachus hastatus adopted behaviors and social strategies associated with different ecological circumstances to guarantee their survival.

“It is rare to observe a social insect utilizing strategies of varied behaviors in two places with ecologically and geographically different circumstances, as was possible to verify through the project,” explains Oliveira.

Social strategies

One of the hypotheses raised by researchers to explain the difference is the possible availability of places for insects to build nests. The Restinga Forest found in the Ilha do Cardoso State Park grows on sand and frequently floods because the area has more rainfall than the Amazon region. Accordingly, the pregnant queens make their nests in the roots of bromeliads attached to tree trunks. The bromeliad most frequently used for nests has a tuft of roots with diameters of more than 30 centimeters. Roots of this size are much rarer on Ilha do Cardoso than elsewhere.

The equatorial Amazon forest of the Natural Reserve of Nourage, however, offers many locations for ants to build their colonies – in palm trees and in other trees, in dry trunks and even on the ground.

As the number of bromeliads of suitable size at the Ilha de Cardoso State Park is limited, the queen ants develop social strategies – such as alliances for joint nests – to ensure survival of their colony.

The researchers propose that Odontomachus hastatus on Ilha do Cardoso could adopt two strategies for their colonies.

In the first strategy, queens impregnated during the nuptial flight of summer – when queen ants and virgin males are released from their nests to copulate – meet and simultaneously occupy the same bromeliad to begin to lay eggs, living together in the same colony and cooperating in the production of worker ants.

In the second strategy, the daughter queens return after impregnation during the nuptial flight to the colonies of their birth, where they are readopted and begin to live under the same roof with a new family structure.

Both cases of alliances create a family arrangement called polygyny, in which a colony of ants has more than one pregnant queen that is laying eggs.

“Because finding an unoccupied bromeliad of an appropriate size to form nests is difficult on Ilha do Cardoso, there must be pressure for ants to form joint colonies or, in the case of daughter queens, to return home after being impregnated,” observes Oliveira. “However, we still do not know if the Odontomachus hastatus ants adopted only one or both hypothetical strategies to form their colonies in bromeliads on Ilha do Cardoso.”

The researchers marked 170 colonies on Ilha do Cardoso and in French Guiana. Through genetic analyses currently being conducted in France of pregnant queens in approximately 40 marked colonies, the researchers intend to verify the existence of a familial relationship among the queens.

“If the results of the tests are positive, this could indicate that the pregnant daughter queens are readopted by their queen mothers. If not, the most likely hypothesis is that several queens join to found a colony without necessarily having a familial relationship but only form a temporary alliance,” says Oliveira.

The advantages of association

According to the IB-Unicamp professor, one of the advantages of joining forces for queen ants of Odontomachus hastatus is that they rapidly form large armies to protect them and guarantee the survival of the anthill because there are more queens laying eggs.

In the case of daughter queens, it is much less risky to return home after impregnation than attempting to found a new colony. Furthermore, in the two situations, the subordinate queens that are in the line of succession for the kingdom can occupy the throne in the event of the death of the alpha queen.

“Remaining in a nest where there are several other queens gives a pregnant queen a chance both to eventually lay eggs and, in the future, to reach the top of the hierarchy and become the alpha queen,” explains Oliveira.

To win the post of alpha queen, however, the candidate must secure her position through force. Through ritual fights in which the queens use their antennas and jaws to duel, the pregnant queens establish a hierarchy in which the winner takes the top spot and becomes largely responsible for placing eggs in the colony. The queens that are defeated and lose the fights have their ovarian development inhibited, lay fewer eggs and ultimately serve as worker ants.

“It is similar to what happens in a band of gorillas, where they take the role of alpha male through ritual fights and impede members who are lower in the hierarchy from impregnating females,” comments Oliveira.

According to the researcher, the colonies of Odontomachus hastatus can contain up to 12 queens and approximately 600 worker ants 1.5 centimeters in length. The larger the tuft of bromeliad roots, the greater the size of the colony.

In addition to Odontomachus hastatus ants, other species of insects can have colonies with more than one queen. Some of these species also found joint colonies. However, instead of establishing a hierarchy before developing an a colony, one female reaches alpha queen status and eliminates the competitors, affirms the researcher.

“We tend to look at nature and simplify the organization of social insects such as ants, which can have several forms of behavior and social strategies for adaptation and survival. By studying these patterns of behavior, we begin to understand how the social organization of insects that are forced to live together works and the various characteristics that we don’t yet know,” says Oliveira.