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Ecology

Good companions

Population density influences life span of termites

RICARDO SOLAR / UFVIdeal density benefits societyRICARDO SOLAR / UFV

During fieldwork in the 1990’s, Og de Souza needed to make a somewhat unusual purchase in São Gotardo, in Minas Gerais State. Hastily entering one of the few stores in this city of 30,000 residents he asked for one thousand rolls of toilet paper, but not just any toilet paper. He needed paper that had not been bleached or perfumed, “the one where you can still see the little letters of the newspaper from which it was made,”  he recalls. The store assistant was surprised by the order, but Souza chose to leave him in the dark. “If I’d explained this,” he says, “he would never have believed me.”

Indeed, it is difficult to imagine that this paper would have a scientific use: bait to catch termites. At the cost of a lot of toilet paper and painstaking work – it is often necessary to analyze the jaw or the shape of the digestive tract of insects to differentiate one species from another – Souza has been helping clarify the function of termites in recycling carbon and the factors that lead these wood-eating insects to choose their food sources in nature. “Termites are a key species for the functioning of the ecosystem where they live and they can generate benefits for agriculture that more than offset the damage they cause,”  said Souza, currently a researcher at the Federal University of Viçosa (UFV) in Minas Gerais.

Souza’s work also provides a possible answer to the so-called Darwin dilemma, a paradox the English naturalist came up against in the nineteenth century while preparing his book, On the Origin of Species. At that time, it was difficult to explain how an apparently disadvantageous situation could be favored by natural selection. The reason that underlies many of the studies on animals that live in large complex societies, such as ants and bees, this dilemma, in the case of the almost 2,800 known species of termites, can be defined as follows: what would be the benefit of life in society if most of the individuals do not reproduce, since in each nest only the king and queen procreate and the workers and soldiers are sterile?

In a partnership with the physicist Octávio Miramontes from the National Autonomous University of Mexico, Souza seems to have found an answer: life in society, for some reason not yet well understood, increases the life expectancy of insects. Individually, sterile termites seem to lose out because they do not transmit their genetic characteristics directly to future generations. However, they reproduce indirectly when they help their parents produce fertile siblings, with whom they share part of the genome – a gain that is leveraged further when they have a long life expectancy.

In the UFV Termitology Laboratory, Souza’s team conducted a series of experiments based on the assumption that there is a density of individuals that facilitates interaction between them and favors life in society. Souza uses the example of a passenger on a bus. If the bus is empty, the passenger has no one to talk to. When the vehicle is crowded, the person cannot interact with all the others and smaller and fragmented relationship groups emerge. Finally, in the so-called ideal situation, there is a proportion of passengers that allows each one of them to exchange information with all the others. “Our suspicion was that life in society, in groups where there is an appropriate density, would somehow allow the termites to live longer,” explains Souza, who calculates the ideal density by dividing the area that an individual occupies in a particular location by the total area available. In the case of termites, this ratio is between 0.12 and 0.2: together, all the termites occupy at most 20% of the area.

In one of their first experiments, the group from Minas Gerais put the termites in test tubes with no access to food or water. When the termites were isolated (one insect in each tube), they survived for 100 hours at most. In groups, however, each one could live up to 250 hours. During the tests, the researchers observed no cannibalism, which could explain the longer life. “At the time, we still didn’t know what the ideal density was,” he says. “We adjusted this variable and considered the overpopulation scenarios of termites in the subsequent study, which confirmed the advantages of having an appropriate density [a suitable number of individuals in a group].”

In another test, the researchers applied a drop of insecticide to each termite and separated the insects into three groups: one with few individuals, one with an ideal number, and a third that was overpopulated. In the first and third groups, the maximum lifetime was 38 hours. With a better sociable scenario, the termites survived for 46 hours – 8 hours more than in the other groups.

Souza’s team is now investigating the factors that influence longevity. “There are indications that social contact triggers enzymatic processes that, for a certain time, reduce the toxic potential of the venom, thus neutralizing the effect of the insecticide,” he says. He believes that the influence of ideal sociability is long-lasting and relevant, since it has been observed in situations of extreme stress, such as starvation and poisoning.

RICARDO SOLAR / UFVAnts are the main predators of termitesRICARDO SOLAR / UFV

Natural contact
Although there are no known techniques for defining optimal density in termite mounds, Souza argues that it must exist in nature. “There are indications that this grouping pattern occurs in almost everything, from robots to ants.” The researcher from UFV also believes that in natural colonies the density oscillates around the optimum scenario the whole time.

In a termite nest, a queen lays approximately 80,000 eggs per day, almost one egg per second. In these circumstances, workers that live eight hours longer would indirectly contribute to the reproductive success of the colony: they would alleviate the work of the queen, which in this time could produce another 30,000 eggs. At the same time, longevity is also advantageous for the sterile workers: the more termites there are, the greater the chance of siblings emerging which become kings and queens and pass on the genes of the sterile termites, thus resolving Darwin’s dilemma.

Also from this work come possible practical applications, such as defining the most appropriate time for destroying the termites that infest the closets of homes and apartments. “If there is a greater tolerance to insecticides in groups with an ideal number of individuals, the most efficient thing would be to apply control measures when the population of the nest is above or below this density,”  says Souza.

More recently, the researcher from Viçosa managed to answer another question that had intrigued him since he started studying termites in 1985: in the natural environment, why do these insects infest some dead tree trunks and preserve others”  In search of an explanation, Souza started another experiment in 2004, on a piece of land next to his countryside property in Coimbra, southeast of Belo Horizonte. He and biologist Ana Paula Albano Araújo, at the time his PhD student, spread rolls of toilet paper – pure cellulose, the main carbohydrate of plants and the source of energy for termites – on the ground. On some of these rolls, they recorded the presence of ants, which are natural predators of termites. The objective was to check if the presence of these threatening insects would have an influence on termites’ search for food.

In autumn, the beginning of the season of milder temperatures, the termites took on average 107 days to spread out to the larger rolls and 68 days to colonize the smaller rolls, when the paper contained ants. Generally, the termites occupied rolls that were free of danger (no ants) faster and advanced more eagerly onto the larger rolls, which were taken over in just 42 days, as compared to 66 days to occupy the smaller ones.

In the summer, the experiment produced the same pattern. The termites took 352 days to invade the smaller rolls that were also populated by ants and 224 days to occupy the larger ones. Without ants close by, the termites acted faster. Once again, they occupied the bigger rolls in 140 days, long before entirely taking over the smaller ones, which took 220 days.

In Souza’s assessment, termites seem to assess the cost benefit ratio. “The limited availability of food is not worth the effort of colonizing a small area,”  he explains. They proved to be on the alert for predators, preferring lower risk areas.

Prospecting – Termites literally use their heads when it comes to choosing the wood they will consume. They beat their heads against the block of wood and assess the vibrations produced, showed Theodore Evans, of the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia, in his 2005 study in PNAS. “Their heads function as a kind of sonar,” explains Souza.

In another study, conducted in an area of Atlantic Forest in Conceição da Barra, in Espírito Santo, Souza and the biologist, Fernanda Sguizzatto de Araújo saw that, in addition to liking big trees, termites prefer dead ones. According to the work, published this year in Sociobiology, the likelihood of there being termite galleries in dead trees with trunks 40 cm or more in diameter exceeded 50%. The same percentage of infestation was only observed in live trees with trunks whose diameter was at least 80 cm. Trees that have already died can be completely eaten, whereas in living trees the termites only manage to attack the bark. “Once again the availability of food seems to influence the choice,”  adds Souza.

In developing this work, Souza was able to explain in more detail the role of termites in the carbon cycle, which enters into the composition of atmospheric gases and the organic matter that constitutes plants and animals. Once dead, decaying trunks mix with the soil to form humus, which is digested by bacteria. It is a slow process (it is calculated that a large trunk takes 50 to 100 years to be consumed only by these microorganisms), but termites can speed this up. They consume the cellulose available in the soil, digest it and then release carbon into the atmosphere as carbon dioxide (CO2), according to a 2009 article in the Bulletin of Entomological Research.

The action of termites in the degradation of cellulose can be easily observed by the color of the soil. It is lighter when the colonies are efficient in processing the humus and dark brown in termite-free areas. As the humus contains water and other nutrients besides cellulose, it is interesting that termites do not consume it completely. At the same time, processing by termites allows the carbon to return to the plants as CO2, as Souza explains. This balance is important for agriculture. “The Kayapó Indians know this very well and use pieces of termite mound as fertilizer in the trenches in which they plant yams and sweet potatoes,” says the researcher.

More recently, he has been focusing on analyzing another curious feature of termite colonies: the harmonious coexistence of many different species of these insects. There are records of more than 1,500 species of insects living in termite nests. There are also seven different genera of termites inhabiting the same nest, apparently without conflict. The secret of peaceful contact seems to be determined by the stomach. “The species have varied diets and do not compete for food,” says Souza. He also suspects that individuals of species different from the one that colonized the termite nest often manage to go unnoticed, thanks to a chemical camouflage: a compound released by the cuticle (the layer covering the skeleton of insects), which is capable of deceiving the owners of the condominium. In this year’s article in PLoS Biology, researchers from Harvard University, in the United States, showed that the diversity of species exists not only in the tunnels of the termite mounds: where there are termites, there is also greater plant and animal diversity. “Termites,” Souza states, “are extremely important for maintaining biodiversity and the ecological balance of a particular region.”

Scientific Articles
ARAÚJO, F. S. et al. Bottom-up effects on selection of trees by termites (Insecta: Isoptera). Sociobiology. v. 55, n. 3, p. 725-34. 2010.
DE SOUZA, O. et al. Trophic controls delaying foraging by termites: reasons for the ground being brownBulletin of Entomological Research. v. 99, p. 603-09. 2009.

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