Within the animal kingdom there are numerous examples where many individuals of the same species aggregate. Fish school to avoid predation and to minimise cost of transport, penguins huddle to keep warm in the harsh Antarctic winter, and snakes aggregate to save heat and ensure reproduction in early spring. Many insect species also aggregate and the reasons seem to be diverse. A recent study looked further into one such example and found that the provisionary shield bug (Parastrachia japonensis) relaxes when it gets in touch with its own kind.
The provisionary shield bug feeds only on the fruit of the plant Shoepfia jasminodora, consequently experiencing prolonged periods without food. When starving, the bugs enter diapause and form aggregations that hang suspended from the leaves and branches of a variety of evergreen plants. Aggregating behaviour has been shown to conserve water and energy in other insect species. To investigate whether this is also the case for the provisionary shield bug, Sumio Tojo and colleagues measured the bug's oxygen consumption, under a variety of experimental conditions, including altered humidity, where they manipulated the number of individuals in the respirometry chamber.
The team's study revealed that the metabolic rate of diapausing bugs halves when they are allowed to aggregate. This effect seems to be attributed to a reduction in physical activity, as the bugs calm down when they touch each other. By contrast, the reduced metabolism did not seem to be related to water conservation, as the metabolic rate of aggregations remained low irrespective of the relative humidity in the respiratory chamber. When the researchers looked further into the cues for the soothing effect of company, they found that physical contact was needed. Thus, if five bugs were in the same respirometry chamber but were prevented from direct physical contact, they did not reduce their metabolic rate. If a bug was allowed to aggregate with dead specimens of its own species, only some of the metabolic reduction occurred,but if these dead specimens were washed in an organic solvent or replaced by specimens of a different species, the effect was totally obliterated. Thus,physical stimulation of a kindred species is needed for the bugs to relax.
The metabolic reduction associated with aggregation may be of great importance for the provisionary shield bug, as it must survive nine months of the year without an appropriate food source. Indeed, Tojo and colleagues showed that the reduction in metabolism was primarily associated with diapause and that the low metabolic rate of aggregating bugs persisted throughout the diapause season. This low metabolic rate allowed the bugs to survive more than 20 weeks at 25°C with access only to water, and more than 80% survived for a whole year.
Besides being an interesting study of the ecology of the provisionary shield bug, the present study underlines the importance of the conditions under which test animals are investigated. Measurements of resting metabolic rate must be conducted in an optimal environment, even for a simple bug. Curiously, this particular bug species might not be that simple, as it seems to be highly socially evolved in other respects; the females are known for their provisionary behaviour, transporting fresh deliveries of food to nests containing newly hatched juveniles, suggesting that provisionary shield bugs really do seem to get in touch quite often.
