As animals heat up, their metabolism accelerates. This is because biochemical reactions speed up, leading to an increased demand for oxygen in cells and tissues. At high temperatures, the ventilatory system of some marine animals is unable to supply mitochondria with enough oxygen to continue producing ATP and there is a harmful switch to anaerobic metabolism, which ultimately leads to death. Essentially, the supply of oxygen by the animal's gas exchange system cannot meet the increased metabolic demand from the cells as the temperature rises, meaning that oxygen limitation is probably the main cause of heat death in organisms. However, insects have a far more efficient and highly developed gas exchange system than marine animals, with tracheae penetrating directly into the flight muscle in some species. Therefore, is oxygen limitation, or hypoxia, the cause of heat death in insects?
John Lighton from the University of Nevada, Las Vegas designed an experiment to test this idea using Drosophila melanogaster. He used a technique developed previously with Robbin Turner that allows the simultaneous measurement of the flies' gas exchange and activity under ever increasing temperatures, known as thermolimit respirometry. This technique allows scientists to accurately determine the temperature at which an individual insect suffers what is known as heat death, or critical thermal limit. This is usually associated with the onset of muscle spasms. Because animals can no longer fly, mate or escape predation, it is also considered an ecologically relevant measure of the temperatures that may affect their survival in the wild. Lighton measured individual flies under a variety of different oxygen concentrations ranging from 2.5 to 21%. Using this method, he reasoned that he could observe exactly which oxygen level reduced the animals' critical thermal limit, if such a reduction took place at all.
Under normal conditions, 21% oxygen, the flies could tolerate a toasty 39°C. Lighton found that below 10% oxygen, their thermal tolerance reduced significantly - to 36°C at 2.5% oxygen. The flies had lost 3°C of their heat tolerance, which supported the oxygen limitation hypothesis. Could such low oxygen values have significant effects on Drosophila thermal tolerance in the wild? Oxygen levels as low as 5% are only found at very high altitudes such as the peak of Mount Everest, and the flies obviously do not live at such altitudes or low oxygen levels under normal conditions in nature. But Lighton notes that, particularly during high activity levels such as flight, when metabolic rates are higher, oxygen levels in the tracheae may drop below 10% and could limit performance. So when flies are buzzing around frantically, oxygen limitation may become important, leaving them in danger if it's also very hot.
Because researchers have investigated how oxygen availability limits thermal tolerance in so few insect species, Lighton is careful to conclude that while some insects may not become oxygen limited, it is clear that Drosophila melanogaster is affected to some degree. Scientists now need to unravel whether or not different insect species have evolved alternative ways of dealing with oxygen limitation during extreme temperature exposure, which will help them understand how hypoxia might set insect temperature tolerance.
