AppearanceThe nominate subspecies (R. m. marginata) has been studied extensively in India, though little is known of its biology elsewhere within its range, or the biology of any of the other subspecies. In India, it has an aseasonal, indeterminate and perennial colony cycle, which means that nest initiation occurs round the year, and nests are active throughout the year.
These wasps make gymnodomous nests (open, not covered by an envelope) with one or more petioles, which they coat with ant-repellant chemicals. The nests are usually found in closed spaces with small openings, inside bushes and within various man-made structures like electric poles, broken pillars, crevices of buildings, electric cable boxes, switch boards, tube light holders, the bottom of park benches, and even from within dustbins and letter boxes. Nests that can be accessed only through very small openings are well protected from the hornets Vespa tropica, which are the prime predators of these wasps. Nest sizes range from 0 to 722 cells (mean ± s.d.: 133.7 ± 119.2), with 1 to 200 females (mean ± s.d.: 21.9 ± 22.3) and 0 to 33 males (mean ± s.d.: 2.4 ± 5.4).
Males are produced aseasonally, and are thus found throughout the year in a subset of nests in the population. There is only a single queen in any nest of R. marginata, and she is not morphologically distinguishable from the workers. The nests are made of paper, which is produced by masticating cellulose (collected usually from plant sources) and mixing it with saliva.
The queen lays a single egg per cell, and the larvae grow inside the cells, being fed by the workers. The largest larvae spin a cap of silk on their cells and pupate inside. R. marginata nests can be founded either by solitary or multiple foundresses, and nest usurpation, adoption and joining are also quite commonly observed.
Colonies of R. marginata consist of a queen and many non-egg laying workers. The number of workers can range from 0, solitary foundress nest, to over 100. The average period of development from egg to adult is 62 days. However, a female only has a .12 probability of surviving the full 62 days. Therefore, if she is solitary nesting, her expected reproductive success is low. Gadagkar estimated that a female increases her expected reproductive success by 3.6-fold as a result of group nesting.
There are three possible paths available to a female. She can leave the nest to form a new nest by herself or with others, she can remain in the natal nest as an altruistic worker for the remainder of her life, or she can work for her natal nest for a while and then drive away the queen to become the new replacement queen.
Gadagkar devised a unified model that makes predictions about what proportion of the population of R. marginata “should opt for a selfish solitary nesting strategy and what proportion should opt for an altruistic worker strategy” (853). From this, he was able to predict that 5% should opt for the selfish solitary nesting strategy while 95% should opt for the altruistic worker strategy.
Gadagkar et al. genotyped R. marginata mothers and daughters at a “few non-specific esterase loci” to infer the genotypes of the haploid fathers or estimate the number of fathers needed to produce the daughters observed (850). The researchers ultimately found, “R. marginata queens mate with 1–3 different males and the average relatedness among their daughters thus drops from the theoretically expected 0.75 to about 0.50, thus entirely negating the advantage of haplodiploidy for social evolution, as predicted by Hamilton” (851). Gadagkar “found no evidence for intra-colony kin recognition” (851).
Primitively eusocial societies are typically headed by behaviourally aggressive queens, who use aggression to suppress worker reproduction. However, the queen in R. marginata is a "docile sitter" who does not use physical aggression to maintain her reproductive monopoly in the colony. Instead she uses a non-volatile pheromone to regulate the reproduction of the workers. She uses this pheromone to signal her presence and fecundity to her workers, who perceive this signal and refrain from reproducing.
The queen interacts very rarely with her workers, and direct or indirect physical interactions are not used by the workers to perceive their queen. The queen probably uses abdomen-rubbing behaviour to apply her pheromone on the nest material, through which the workers perceive her presence in the colony.
When the queen is removed from the colony, the pheromone decays, and eventually the workers no longer perceive the queen signal. One of the workers then increases her aggression drastically; this individual, the "potential queen" (PQ), develops her ovaries within a few days and assumes the role of the queen.
Primitive wasp societies are known to have distinguishable succession hierarchies, i.e., the loss of the queen results in her successor becoming the next egg-layer. Typically, such hierarchies are based on dominance rank, age or in some cases, body size.
However, in R. marginata, the potential queen, or the individual who steps up her aggression immediately after queen removal and eventually becomes the queen, seems to be an unspecialized individual in the presence of the queen. She is not unique in her dominance rank, behavioural repertoire, age, body size or ovarian condition. However, within minutes of queen removal, the potential queen becomes obvious to an observer due to her heightened aggression. Interestingly, the potential queen maintains this high aggression for only a few days, and gradually reduces the levels of aggression over a week or so, while she develops her ovaries.
Contrary to popular belief among scientists, the potential queen seems to require this heightened aggression, not to suppress the ovarian development in her nestmates, but to boost her own development.
Typically, this heightened aggression is one-way, and shown by a single individual. She hardly even receives any aggression from the others in the colony. It has now been established that though the identity of the potential queen is cryptic to the observer in the presence of the queen, the wasps "know" who their successor is, and hence she does not face any challenge from her nestmates.
The workers aid the queen in foraging, nest-building, and brood care. Gadagkar calculated that the genetic relatedness between the worker and the brood ranged from .0165 to .75.
When the queen is lost or removed, one of the non-egg laying workers becomes extremely aggressive, then gradually becomes docile, develops ovaries, and begins egg laying. Gadagkar et al. observed that workers will continue working for the replacement queen. Bang and Gadagkar demonstrated that there is not just one designated worker to take over reproduction, but a long queue of potential queens that are not only daughters, but also sisters, nieces, and cousins. They found that age is a substantial predictor, though not a perfect one.
Males do not assist in any of the colony maintenance activities while they reside in the nest. However, they do consume the food and liquid brought to the nest by females. The behaviors they do perform include, “soliciting food, antennating nest, antennating another wasp, feeding self, snatching things, fanning wings, body jerks, wing jerks and dominance–subordinate behaviors” (963-4).Due to this behavior, females show physical aggression towards males in the nest.
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