All ant colonies have at least one egg-laying queen. She is an essential member of the family. Without her there would be no colony. Some species of ant have only one queen in each nest. Others may have several, even numbering in the hundreds. Colonies/species that have only one queen are known as monogynous. Those with more than one egg-laying queen are called polygynous. This article will discuss polygyny in ant colonies. The information within this article was taken from the book The Behavioural Ecology of Ants (Tertiary Biology Level), by John H Sudd and Nigel R Franks, 1987.
The Evolution of Polygyny
The worker ants within an ant colony are sterile females, descending from one of the queens present in the nest. An exception would be those brought into the nest as pupae from other nests by raiding parties. It can be said that the workers in a single queen colony are closely related sisters. The workers share the same mother, but perhaps not the same father, as some queens mate with more than one male during the annual mating flights. However, workers within colonies with multiple queens may not share the same mother and/or father. Referring back to Buschinger, 1974, Sudd and Franks state that about half of all European species of ant are polygynous.
Though ant colonies/species can be divided into two main classes; monogynous and polygynous, it is worthy of note that polygyny is not simply the presence of more than one queen. There are different types of polygyny in ant colonies; primary and secondary, with some variations in each.
Primary Polygyny
There are two different methods in which polygyny can occur within an ant colony. There are those that start off with a single queen but later accept additional queens. This process is known as haplometrosis. Then there is pleometrosis, in which several queens come together to start a new colony. The latter normally results in monogyny, that is though the queens come together initially. However, once workers are produced, they may kill all but one of the queens.
Alternatively, the excess queens may be driven out of the nest, rather than killed. Some of these queens may take a portion of the worker force with them to start a new colony. It is rare for colonies using the pleometrosis to remain polygynous once a worker force is established. It is interesting to note that the workers who kill the surplus queens in pleometrosis initiated colonies, could well be killing their own mother. There is no evidence that shows that an individual worker ant recognises its own mother in such a situation.
In a study of polygyny in ant colonies, it was suggested by Sudd and Franks that a reason why pleometrosis rarely result in permanent polygynous colonies, is because the founding queens may be unrelated, or distantly related. This would result in competing family factions.
Pleometrosis
Lasius niger queens can often come together to initiate a new colony using the pleometrosis method. However, only one queen will be tolerated once the worker force hatches. All but one of the queens will eventually be killed. Lasius flavus, on the other hand, can come together in pleometrosis. When the worker force is produced, the queens split up, taking part of the worker force with her. I have seen colonies of Lasius flavus where this happens. Rather than the queens leaving to form their own colonies, they remove themselves to separate parts of the same nest. They each act as if they are the only queen, with their own workers, yet part of one larger nest.
Secondary Polygyny
Also, known as permanent polygyny, this method of colony establishment occurs when an established colony adopts new fertilised queens. Such a colony may be started by a single queen using the budding method. Here the queen departs her parent nest with a portion of the brood and worker force. New queens are produced by this single queen, who then leave the nest to mate, and return to become one of a number of queens.
Studies have been conducted into the beneficial and detrimental effects of multi-queened colonies amongst ants. It might seem that having a polygynous colony is a very beneficial position to be in. After all, doesn’t that mean more eggs are being laid, and that means the colony is eternal, as new queens can replace those that die? Well, yes, that can be a factor, at least in theory. However, such studies have suggested that multiple queens can be a drain on colony resources. A consideration is that if there are too many egg-laying queens, there may be more eggs present than the colony actually requires to maintain a healthy turnover of workers. Too many queens could, in fact, present itself as a resource drain on the colony.
Genetics among workers
Other studies have also considered the family relationship between the queen ants within a polygynous nest. By this I mean, are the queens actually related to each other, such as being sisters or nieces. A colony consisting of a single queen, who has mated with just one male, will have workers that share genes. A single queen colony who queen has mated with more than one male, will have workers that have the same maternal genes, but differing paternal genes. However, a colony that has multiple queens, mated by different males, will have a much higher incidence of workers who do not share the same maternal and paternal genes. Sudd and Franks have concluded, following the various studies conducted by themselves and others, that the relatedness of the queens within polygynous nests is, it seems, often fairly low.
The Behaviour of Queens within Polygynous Colonies.
How do queens within polygynous nests get along with each other? Are they hostile, or friendly? Are they selfish, or do they cooperate? Is there a hierarchy among them? Again, many studies have been conducted into the research of this aspect of polygynous behaviour among queens.
It is understood that queens in polygynous colonies recognise their own eggs among those laid by the other queens. They may eat the eggs of the other queens, ensuring a dominant worker force related genetically to one particular queen. There can also appear to be a hierarchy among polygynous queens. Evesham (1984) conducted studies into this behaviour between queens in colonies of the British ant, Myrmica rubra. He found that there was a marked difference in the behaviour of queens to one another. Some were fairly placid towards the other queens and tended to remain in one particular part of the nest. Whereas other subordinate queens, would roam throughout the nest acting aggressively towards one another.
It would seem that within polygynous colonies, particularly, those of the species studied, there is a hierarchy among the queens. Colonies that contain multiple queens who act aggressively towards one another could be described as being oligogyny.
Oligogyny in Polygynous Colonies.
Hölldobler and Wilson (1977), stated that there may be reproductive competition between polygynous queens. Yet they generally are tolerant of each other, and indeed may cluster together within the nest. However, as introduced above, this peaceful cooperation between queens is not always the case. Hölldobler and Carlin (1985), studied polygyny in ant colonies among the Australian ant Iridomyrmex pupureus, also known as the Australian meat ant. Hölldobler and Carlin found that when several queens were forced to cohabit together within a small nest, aggression is present between them. They display bouts of “boxing” with their antennae, in which the queens can assert their dominance over other queens.
The dominant queen tends to produce a higher number of eggs when compared to the subordinate queens. They found that as the nest grows, these queens tend to separate into different parts of the nest. This results in the dominance hierarchy between them breaking down. They live as if they were monogyne queens in a multi-queened colony, producing approximately an equal number of eggs. However, they did find that in some circumstances there would remain a dominate queen. She would exert an influence over the number of eggs laid by the subordinate queens. It seems she is, in some unknown way, able to inhibit their reproduction.
As you can hopefully gain from this article, that polygyny in ant colonies is not as simple as just have more than one egg-laying queen present. There are many fascinating factors and influences at play here.