Polyergus breviceps
Encyclopedia
Polyergus breviceps is a species
of ant
which is endemic to the United States
. It is a social parasite
of other ants, namely of Formica gnava but also of Formica occulta and Formica argentea. Polyergus is an inquiline
parasite, having lost its ability to take care of its young and themselves. "The workers do not forage for food, feed the young or the queen, or even clean up their own nest". In order to survive, Polyergus workers raid Formica nests in order to steal the pupa
e which once hatched, become workers of the mixed nest. This sort of relationship is not unique, of the approximately 8,800 species of ants, at least 200 have evolved some form of symbiotic
relationship with one another. What makes Polyergus special is the way a newly mated queen can, all by herself, take over a Formica nest and start a new colony.
(a mixed nest where Formica
workers are already enslaved) and forage
for a suitable raid target. If one is found, the scouts
will return to the nest, rally the other Polyergus, and head out in a raiding column. If it was a Formica nest, the Polyergus incite a "panic-flee" response from the Formica by releasing formic acid
and take the Formica pupae back to their nest to be slaves. Occasionally, a new queen will embark on a raiding column with the other Polyergus. She will then, after mating, leave the column and forage for a suitable Formica nest
. Howard Topoff did a considerable amount of work in evaluating how the queen then takes over the colony. After finding a Formica nest, she finds an entrance and is immediately attacked by Formica worker. The queen responds by biting with her sharp mandible
s and releasing a pheromone
from her enlarged Dufour’s gland that, unlike many other parasitic ants, has a pacifying effect. The queen quickly searches for and locates the Formica queen and, with her adapted mandibles, proceeds to bite and lick various parts the Formica queen for an average of 25 minutes. "Within seconds of the host queens death, the nest undergoes a most remarkable transformation". The Formica workers cease to be aggressive to the intruder and start to groom the Polyergus queen as if it were their own. The takeover now complete, the Polyergus queen gains not only a nest, but a worker caste
as well. She then lays her eggs and the cycle continues.
Specific to Polyergus, when the queen first enters a Formica nest she releases a pheromone from her enlarged Dufour’s gland. Topoff did experiments to show that this pheromone has an important facilitative effect in colony usurpation; it reduces the aggression of the defending Formica workers. The researchers took the Dufour’s, pygidial, and poison glands from freshly mated Polyergus queens, using water as a negative control. These were crushed in distilled water to make a solution containing their extracts. Because it was impossible to prevent a Polyergus queen from secreting their own pheromones during a live encounter, they used the harvester ant Pogonomyrmex occidentalis
(naturally attacked by Formica) as the subject. These ants were dipped in the above solutions, one ant/solution/test at a time, and placed in a petri dish
with three Formica occulta "attackers". They were observed for three minutes to see for how long the Pogonomyrmex was attacked.. The results were as follows: "the mean duration of aggression by the Dufour’s gland treatment was 53.3 seconds...The mean duration of aggression for the water, pygidial gland, and poison gland controls were: 143.5 seconds, and 137.2 seconds and 132.2 seconds respectively". Apparently, at some time, Polyergus queens evolved the capacity to passively facilitate colony takeovers.
Topoff and Ellen Zimmerli also did experiments to prove that the Polyergus queen "tricks" the Formica colony by obtaining chemicals from the Formica queen in the process of killing her. In one test, the Formica queen was killed (by flash freezing then thawing) prior to Polyergus contact. Even though it was already dead, the Polyergus queen bit, stabbed, and licked the queen just as if it were alive: and the Polyergus was consequently accepted by the colony. Another test showed that if no Formica queen was present, then Polyergus had little chance of a successful takeover. Clearly, the Formica queen is providing some sort of chemical(s) to the Polyergus queen, however unintentionally. That takeover can occur even if the host queen is dead, but not if she is not present, proves that chemical absorption is important. The Polyergus queen needs only kill one host queen to be accepted. If the colony was polygynistic
, the Polyergus queen can take her time finding the other queens. "Hour by hour, day by day, she methodically locates and kills every Formica queen, sometimes taking several weeks to clear out all remnants of opposition".
of ant dulosis and almost all of it has at least some empirical evidence to defend its positions. Notably, that nearly all raiding ant species have is an adaptation of their mandibles that make them big or piercing or both. It makes sense that the very thing that enables an ant to be a better raider causes that same ant to become dependent on others; the parasite loses the use of its mandibles for actual work. On this and other evidence, it has been proposed that predation
is the precursor to slavery. It has also been suggested that colony multiplication by adoption and budding followed by temporary parasitism can lead to dulosis. While observations indicate that colony multiplication can lead to temporary parasitism, it is apparently very questionable that temporary parasitism leads to dulosis.
In addition, about 100 years ago, Carlo Emery
observed that "the slave-making temporarily and permanently parasitic ants originate from closely related forms which serve them as host". Jurgen Heinze did a series of experiments that prove this observation, known today as Emery's Rule
. Rather than just relying solely on morphological cues, he used enzyme
gel electrophoresis
assays to create a detailed phylogenetic
picture of host and parasite connections. His experimental results support a loose version of Emery’s rule. On one end of the spectrum, they saw near identical electromorphs
between species in most host and parasite pairs studied. However, it also appears that at least one example does not support Emery’s rule in that (Leptothorax paraxenus) differed from its host in several electromorphs. The selective forces involved in the evolution of social parasitism are not fully understood, and it is unlikely that one model will fit all relationships.
"Any hypothesis
explaining the evolution of slavery in Polyergus must account both for the origin of group raiding with brood capture, and for non-independent colony foundation by queens". Topoff gives an adequate hypothesis to the evolution of Polyergus by integrating three processes: queen takeover, olfactory imprinting, and territorial fighting. Here are his main points. A free-living ancestor of Polyergus, that is a scavenger
, would band together with others for intraspecific contest. Occasionally a queen would invade colonies of Formica. Initially the queen would drive off the Formica queen and workers and appropriated the Formica pupae (he gives as example the queens of Formica wheeleri that do this now). The killing of the Formica queen and adoption by the workers would evolve from this. His previous work (which was presented earlier) shows what would have evolved for adaptation to occur: "(1) sharp mandibles for killing the Formica queen, (2) a pheromone that reduces aggression from resident workers, (3) a tendency to hold onto the dead Formica queen long enough to absorb her odors...". The next step would be olfactory
imprinting
between the two species, a consequence of living one’s life from start to finish in the presence of both species. Polyergus’ ancestor then forages and encounters a colony of the same species. Again, they recruit nestmates to a territorial raid. In a brilliant insight, Topoff suggest this trait explains the raiding behavior. The Polyergus worker, raised by Formica as conspecifics, "identifies individual of Formica as belonging to its own species, another territorial raid is incited. Thus from the standpoint of Polyergus... slave raids are equivalent to territorial raids". The captured brood is imprinted as well and they become workers in the original nest. As time went on "Our ancestral Polyergus could easily slide in the direction of facultative parasitism". Eventually, Polyergus ancestors lost the ability to take care of themselves and become the inquiline Polyergus we see today.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of ant
Ant
Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...
which is endemic to the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. It is a social parasite
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
of other ants, namely of Formica gnava but also of Formica occulta and Formica argentea. Polyergus is an inquiline
Inquiline
In zoology, an inquiline is an animal that lives commensally in the nest, burrow, or dwelling place of an animal of another species. For example, some organisms such as insects may live in the homes of gophers and feed on debris, fungi, roots, etc...
parasite, having lost its ability to take care of its young and themselves. "The workers do not forage for food, feed the young or the queen, or even clean up their own nest". In order to survive, Polyergus workers raid Formica nests in order to steal the pupa
Pupa
A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...
e which once hatched, become workers of the mixed nest. This sort of relationship is not unique, of the approximately 8,800 species of ants, at least 200 have evolved some form of symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
relationship with one another. What makes Polyergus special is the way a newly mated queen can, all by herself, take over a Formica nest and start a new colony.
Slave raids and colony takeovers
Polyergus workers will emerge from their nestAnt colony
An ant colony is an underground lair where ants live, eat and mate. Colonies consist of a series of underground chambers, connected to each other and the surface of the earth by small tunnels. There are rooms for nurseries, food storage, and mating...
(a mixed nest where Formica
Formica
Formica is a genus of ants of the family Formicidae, commonly known as wood ants, mound ants, or field ants. Formica is the type genus of the Formicidae, and of the subfamily Formicinae...
workers are already enslaved) and forage
Foraging
- Definitions and significance of foraging behavior :Foraging is the act of searching for and exploiting food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce...
for a suitable raid target. If one is found, the scouts
Reconnaissance
Reconnaissance is the military term for exploring beyond the area occupied by friendly forces to gain information about enemy forces or features of the environment....
will return to the nest, rally the other Polyergus, and head out in a raiding column. If it was a Formica nest, the Polyergus incite a "panic-flee" response from the Formica by releasing formic acid
Formic acid
Formic acid is the simplest carboxylic acid. Its chemical formula is HCOOH or HCO2H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings. In fact, its name comes from the Latin word for ant, formica, referring to its early...
and take the Formica pupae back to their nest to be slaves. Occasionally, a new queen will embark on a raiding column with the other Polyergus. She will then, after mating, leave the column and forage for a suitable Formica nest
Nest
A nest is a place of refuge to hold an animal's eggs or provide a place to live or raise offspring. They are usually made of some organic material such as twigs, grass, and leaves; or may simply be a depression in the ground, or a hole in a tree, rock or building...
. Howard Topoff did a considerable amount of work in evaluating how the queen then takes over the colony. After finding a Formica nest, she finds an entrance and is immediately attacked by Formica worker. The queen responds by biting with her sharp mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
s and releasing a pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
from her enlarged Dufour’s gland that, unlike many other parasitic ants, has a pacifying effect. The queen quickly searches for and locates the Formica queen and, with her adapted mandibles, proceeds to bite and lick various parts the Formica queen for an average of 25 minutes. "Within seconds of the host queens death, the nest undergoes a most remarkable transformation". The Formica workers cease to be aggressive to the intruder and start to groom the Polyergus queen as if it were their own. The takeover now complete, the Polyergus queen gains not only a nest, but a worker caste
Caste
Caste is an elaborate and complex social system that combines elements of endogamy, occupation, culture, social class, tribal affiliation and political power. It should not be confused with race or social class, e.g. members of different castes in one society may belong to the same race, as in India...
as well. She then lays her eggs and the cycle continues.
Mechanisms
Nearly all slave making ants have mandibular adaptations that help them attack others; Polyergus is no different.Specific to Polyergus, when the queen first enters a Formica nest she releases a pheromone from her enlarged Dufour’s gland. Topoff did experiments to show that this pheromone has an important facilitative effect in colony usurpation; it reduces the aggression of the defending Formica workers. The researchers took the Dufour’s, pygidial, and poison glands from freshly mated Polyergus queens, using water as a negative control. These were crushed in distilled water to make a solution containing their extracts. Because it was impossible to prevent a Polyergus queen from secreting their own pheromones during a live encounter, they used the harvester ant Pogonomyrmex occidentalis
Pogonomyrmex occidentalis
Pogonomyrmex occidentalis is an ant species.-Phylogeny:The ant genus Pogonomyrmex consists of 16 species Pogonomyrmex occidentalis is an ant species.-Phylogeny:The ant genus Pogonomyrmex consists of 16 species Pogonomyrmex occidentalis is an ant species.-Phylogeny:The ant genus Pogonomyrmex...
(naturally attacked by Formica) as the subject. These ants were dipped in the above solutions, one ant/solution/test at a time, and placed in a petri dish
Petri dish
A Petri dish is a shallow glass or plastic cylindrical lidded dish that biologists use to culture cells or small moss plants. It was named after German bacteriologist Julius Richard Petri, who invented it when working as an assistant to Robert Koch...
with three Formica occulta "attackers". They were observed for three minutes to see for how long the Pogonomyrmex was attacked.. The results were as follows: "the mean duration of aggression by the Dufour’s gland treatment was 53.3 seconds...The mean duration of aggression for the water, pygidial gland, and poison gland controls were: 143.5 seconds, and 137.2 seconds and 132.2 seconds respectively". Apparently, at some time, Polyergus queens evolved the capacity to passively facilitate colony takeovers.
Topoff and Ellen Zimmerli also did experiments to prove that the Polyergus queen "tricks" the Formica colony by obtaining chemicals from the Formica queen in the process of killing her. In one test, the Formica queen was killed (by flash freezing then thawing) prior to Polyergus contact. Even though it was already dead, the Polyergus queen bit, stabbed, and licked the queen just as if it were alive: and the Polyergus was consequently accepted by the colony. Another test showed that if no Formica queen was present, then Polyergus had little chance of a successful takeover. Clearly, the Formica queen is providing some sort of chemical(s) to the Polyergus queen, however unintentionally. That takeover can occur even if the host queen is dead, but not if she is not present, proves that chemical absorption is important. The Polyergus queen needs only kill one host queen to be accepted. If the colony was polygynistic
Polygenism
Polygenism is a theory of human origins positing that the human races are of different lineages . This is opposite to the idea of monogenism, which posits a single origin of humanity.- Origins :...
, the Polyergus queen can take her time finding the other queens. "Hour by hour, day by day, she methodically locates and kills every Formica queen, sometimes taking several weeks to clear out all remnants of opposition".
Evolution
Considerable work has been published on the evolutionEvolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of ant dulosis and almost all of it has at least some empirical evidence to defend its positions. Notably, that nearly all raiding ant species have is an adaptation of their mandibles that make them big or piercing or both. It makes sense that the very thing that enables an ant to be a better raider causes that same ant to become dependent on others; the parasite loses the use of its mandibles for actual work. On this and other evidence, it has been proposed that predation
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
is the precursor to slavery. It has also been suggested that colony multiplication by adoption and budding followed by temporary parasitism can lead to dulosis. While observations indicate that colony multiplication can lead to temporary parasitism, it is apparently very questionable that temporary parasitism leads to dulosis.
In addition, about 100 years ago, Carlo Emery
Carlo Emery
Carlo Emery was an Italian entomologist.Born in Naples, Carlo Emery was professor of Zoology at the University of Bologna. He later worked in Geneva...
observed that "the slave-making temporarily and permanently parasitic ants originate from closely related forms which serve them as host". Jurgen Heinze did a series of experiments that prove this observation, known today as Emery's Rule
Emery's Rule
In 1909, the entomologist Carlo Emery noted that social parasites among insects tend to be parasites of species or genera to which they are closely related. Over the years, this pattern has been recognized in many additional cases, and generalized to what is now known as Emery's Rule...
. Rather than just relying solely on morphological cues, he used enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
gel electrophoresis
Electrophoresis
Electrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
assays to create a detailed phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
picture of host and parasite connections. His experimental results support a loose version of Emery’s rule. On one end of the spectrum, they saw near identical electromorphs
Morphism
In mathematics, a morphism is an abstraction derived from structure-preserving mappings between two mathematical structures. The notion of morphism recurs in much of contemporary mathematics...
between species in most host and parasite pairs studied. However, it also appears that at least one example does not support Emery’s rule in that (Leptothorax paraxenus) differed from its host in several electromorphs. The selective forces involved in the evolution of social parasitism are not fully understood, and it is unlikely that one model will fit all relationships.
"Any hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
explaining the evolution of slavery in Polyergus must account both for the origin of group raiding with brood capture, and for non-independent colony foundation by queens". Topoff gives an adequate hypothesis to the evolution of Polyergus by integrating three processes: queen takeover, olfactory imprinting, and territorial fighting. Here are his main points. A free-living ancestor of Polyergus, that is a scavenger
Scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...
, would band together with others for intraspecific contest. Occasionally a queen would invade colonies of Formica. Initially the queen would drive off the Formica queen and workers and appropriated the Formica pupae (he gives as example the queens of Formica wheeleri that do this now). The killing of the Formica queen and adoption by the workers would evolve from this. His previous work (which was presented earlier) shows what would have evolved for adaptation to occur: "(1) sharp mandibles for killing the Formica queen, (2) a pheromone that reduces aggression from resident workers, (3) a tendency to hold onto the dead Formica queen long enough to absorb her odors...". The next step would be olfactory
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
imprinting
Imprinting (psychology)
Imprinting is the term used in psychology and ethology to describe any kind of phase-sensitive learning that is rapid and apparently independent of the consequences of behavior...
between the two species, a consequence of living one’s life from start to finish in the presence of both species. Polyergus’ ancestor then forages and encounters a colony of the same species. Again, they recruit nestmates to a territorial raid. In a brilliant insight, Topoff suggest this trait explains the raiding behavior. The Polyergus worker, raised by Formica as conspecifics, "identifies individual of Formica as belonging to its own species, another territorial raid is incited. Thus from the standpoint of Polyergus... slave raids are equivalent to territorial raids". The captured brood is imprinted as well and they become workers in the original nest. As time went on "Our ancestral Polyergus could easily slide in the direction of facultative parasitism". Eventually, Polyergus ancestors lost the ability to take care of themselves and become the inquiline Polyergus we see today.