GOD SAVE THE QUEENBiology 342 Fall 2011Physiological & Environmental Effects on Hierarchical Rank in Ants. |
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Who’s Running the Show?There’s a reason that ant colonies are a favorite example of complex systems theorists. Complex systems can be roughly defined as a network of heterogenous parts that interact in relatively simple ways with each other on the local level, but as a result of this mass activity more variable and self-regulating global behaviors emerge spontaneously. The functional units respond only to local information, but each individual action transmits further information throughout the system so that the whole can optimize resource and task allocation. Ants embody this notion, as simple individuals occupy distinct roles in an overall division of labor that allows the whole colony to behave cohesively and intelligently. The key to this in ant colonies is dynamic task allocation. Most models and empirical research indicate that the overall behavioral output of an individual ant depends on an interaction between fixed physiological and morphological attributes of the ant (which depends on caste-based ontogenetic differences) and transient environmental cues (often in the form of explicit chemical signals from fellow ants). Factors Intrinsic to the Individual -Size: In many species there is pronounced size polymorphism between castes. Different tasks may be better suited by a certain overall size. This also supports the Spatial Fidelity Zone model (SFZ) of social insect organization. The SFZ model states that there is a correlation between ant size (or, for the politically correct among you, “ant corpulence”) and an ant’s pace. The differentially paced ants will have different positions in space relative to the colony center, and these different positions in space will dictate which task the ant will perform. The idea is that intrinsic morphological differences, such as size, cause a natural sort of sifting that allows for differentiation.
Structural or Stigmergic Factors-Chemical gradients: Another important component of the SFZ model is the proposed layout of chemical gradients throughout the colony. These gradients supposedly radiate out from the center, where the queen is, and so at any given moment that ant can tell where it is in the colony via pheromone detection. These gradients give important information about which task an individual should be performing in a certain region of space. It is likely that the gradients do not simply radiate concentrically from the center, but from multiple sites of high activity, such as the colony entrance. Multiple pheromones, communicating different information, may exist in spatially distributed gradients, often overlapping to create a combinatorial vernacular for potential task allocation.
For the colony to operate efficiently, the number of individuals carrying out a task has to adjust to the colony’s overall needs. There is evidence that exactly how an individual decides which task to take up is largely based off of environmental cues. The Changing Environment of the ColonyAs the colony interacts with other components of the ecosystem, the overall needs can vary dramatically. Ants have been shown to efficiently deal with these incoming demands so that a relatively invariant setpoint is maintained, much like homeostasis. If a particularly abundant new source of food is found, then there is an increase in foraging ants. Conversely, if a flood washes a bunch of debris into the nest, than more maintenance ants are required. The ants become “aware” of these global needs through local cues from other ants, mainly through chemosensory/olfactory or tactile communication.
Cuticular HydrocarbonsWhen two ants encounter one another, they run their respective antennas over each other. They are actively picking up different signals that may be emitted by the other ant. One of the most informative signals is to be found in a dollop of mixed hydrocarbons located on the ants’ cuticle. These cuticular hydrocarbons (CHC) provide unique chemical label that contains information of the ant’s identity, such as its caste and where it’s been recently. Ants can recognize whether another is a foreigner or a nestmate based off of differences in the CHC composition. This exhibits a high degree of sensitivity and discrimination on the part of the ant olfaction system. But it is even more subtle yet! Ants can recognize what task a nestmate has been performing. So, an individual can glean important social information that contributes to the ongoing neural computations that constantly must decide which behavior to engage in. This last bit is absolutely integral to optimization within the colony. Interestingly, the caste differences in CHC label composition often arise because of purely environmental reasons. For example, the CHC of foragers is subject to hotter, drier conditions than those castes that stay primarily in the colony, so there are a different ratio of n-alkanes to n-alkenes (extremely simple and common components of the CHC label) due to all the bond breaking that results from frequent exposure. In terms of another ant’s sensory detection, this results in a phenomenally distinct label that the receiving ant processes in such a way as to bias them towards adopting foraging behavior. Greene and Gordon performed a simple experiment that is highly illustrative of the role of this recognition process in task allocation. In red harvester ants (Pogonomyrmex barbatus), patrollers leave the nest in the morning to survey the foraging area. If the patrollers return, then the foragers are signaled to leave and find food. If the patrollers do not return, the foragers remain near the nest entrance without leaving. The researchers captured the patrollers as they were out and dropped plastic beads coated with appropriate quantities of the relevant CHC label into the nest entrance, in a patroller-mimic assay. They found that this did incite foragers to emerge from the nest, while the no-bead control condition did not. Pheromones Cuticular hydrocarbons contain a lot of situational information that may influence the subsequent behavior of a fellow ant. But CHCs are just the tip of the iceberg when it comes to chemical communication in ants. A complex array of messages are conveyed via the ant exocrine system, through the medium ofpheromones. Across species, there are the metapleural glands, the Dufour glands, the mandibular glands, the venom glands, the hindgut, the postpharyngeal glands, the pyrigidal glnd, Pavan’s glands, the rectal glands and the Tibial glands. Each of these glands produces a wealth of substances, depending on the species. This allows for a species- and colony-specific chemical vernacular, meaning that communication lines are somewhat protected and a given chemical cue can have an arbitrary degree of specificity. This has important implications for communication and social organization. A given gland may produce several pheromones at one time and several glands may be used in conjunction, allowing for combinatorial complexity in potential messages. This is a notion that has only recently been acknowledged, so the depth and complexity of social communication between ants (and indeed, any social insect species) is poorly understood. RecruitmentThe recruitment hypothesis asserts that there is a degree of plasticity in the social role of any ant, such that most individuals can be recruited to a new task. The mechanism of this recruitment is hypothesized to be the various signal streams available to an individual at any point in time. Recruitment abounds throughout the social structure of colony, though it is not a totally arbitrary network in which all points are exactly equal. That being said, the most transparent subsystem through which to explore recruitment is that of foraging. Since foraging provides the energy input to the entire colony, it is afforded a high degree of salience. There are several different kinds of recruitment in foraging. -Tandem running: a patroller returns with food, regurgitates a sample for other foragers, then positions itself so that its venom gland is exposed and produces a drop of pheromone that we’ll call “foraging pheromone” (the literal chemical identity doesn’t matter so much, since this changes across species). A nearby ant will extend its antennae to the region, in an exchange of tactile and olfactory communication. The patroller ant will then return to the food source, with the second ant trailing behind. Ant Chemosensation The vaporization mentioned above, while overall dissipating the signal, is integral to the ants’ ability to use the pheromone for navigation. As it vaporizes, it creates a sort of “vapor tunnel” that the ant is constantly picking up on by waving its antennae around in the air, in a directed fashion. Its movements are guided by something called osmotrpotaxis, which is essentially the detection of concentration levels. To begin to explain this, the anatomy of ant olfaction must be introduced. Whole is More than the Sum of its Parts
The functioning of the colony as a whole depends on the social divisions between subpopulations within the colony. The overall behavioral output of an individual, and therefore what tasks it is engaged in, depends on constant synthesizing of different kinds of cues that orient and dispose it towards certain patterns of behavior. These cues are overlapping systems of overt communication from other ants, whether it is direct and instructional pheromone emission or static cuticular hydrocarbon information stores, as well as incidental information about the colony’s environment. Ants are fantastic information processing organisms, but in the end incapable of much on its own. However, when such complex systems are linked in ways that give rise to so much opportunity for feedback, then fine-tuned regulation is possible. This is how an ant colony can achieve such incredible degrees of organization.
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