Most of us don't particularly enjoy getting cold, but we can keep going. Insects, however, simply keel over when temperatures fall too low: ‘they enter a coma-like state,’ explains Anders Findsen and colleagues from Aarhus University, Denmark. However, the mechanisms behind this loss of control are not clear. The team explains that insects were thought to enter a chill coma when extracellular potassium levels rise in muscle, causing the muscle to depolarize and become incapacitated. So the team decided to measure the responses of locusts to cooling, the impact of chilling on the extracellular potassium levels in the insect's tibial muscles and the effect of these changes on the muscles' force production to learn more about how low temperatures send insects into a coma (p. 1297).
Cooling the animals gradually or rapidly until they stopped moving but did not freeze, the team found that most of the insects entered a chill coma at temperatures around freezing. However, instead of disrupting the insects' delicate ionic balance, the low temperatures barely disturbed the balance of salts in their bodies. Next, the team measured the force produced by the insect's tibial muscle as they electrically stimulated it to contract while maintaining normal levels of extracellular potassium, and measured a colossal 80% fall in force production at low temperatures. And when the team raised the extracellular potassium levels from 10 to 30 mmol l−1, the force fell by 40%. ‘Combining these two stressors almost abolished force production’, the team says. So ionic disturbance and low temperature disrupt muscle function in cold insects, and the team suggests that both factors could also contribute to insects' recovery from chill coma.