It is generally recognised that animals perform best at certain temperatures, so-called optimal temperatures. To understand how measures of performance, such as growth or running ability, will change with changing temperatures we need to understand the physiological processes limiting performance.
One compelling theory, known as the oxygen and capacity-limited thermal tolerance (OCLTT), suggests that a reduction in oxygen availability limits performance.
As temperature shifts away from optimal, the demand for oxygen in tissues is thought to outpace the rate of supply. This means a shift to anaerobic metabolism, a process far less efficient than aerobic metabolism, causing a reduction in performance.
Emily Lombardi, Candice Bywater and Craig White wanted to test the theory and used the speckled cockroach as their experimental animal because they are easy to breed and keep in the lab. Unlike other studies testing the OCLTT hypothesis, their interest was in the less extreme ends of the temperature range, which the OCLTT hypothesis specifically addresses.
First, Emily and her colleagues calculated the temperature that optimised growth by allowing juvenile cockroaches to develop for 35 days at 8 temperatures ranging between 10 and 36 °C. They also determined the temperatures (both above and below the optimal temperature) where growth was reduced by 32%. These 3 temperatures (optimal, lower and upper developmental temperatures) were then used in oxygen manipulation experiments.
Next, they ran an experiment including the three temperatures plus three oxygen concentrations; atmospheric (21%), hypoxic (10%) and hyperoxic (40%). Cockroaches were assigned to one of the 9 treatments and growth rate was measured repeatedly over 5 weeks. Running performance – time on a treadmill – was measured at the end of the 5 weeks at the three temperatures. Tracheal morphology was also quantified because, in some species, changes to the oxygen delivery system can alleviate the demand for increased oxygen supply.
If the OCLTT theory held, then the team expected to see a difference in performance at the different oxygen concentrations at each temperature. But, instead, they found increasing oxygen concentrations did not mitigate the effects of sub-optimal temperatures. There was also no evidence that tracheal morphology changed as a result of the developmental temperature or oxygen environment. The team concluded that oxygen supply was not the main determinant of temperature-related performance limitations for the speckled cockroach.
It seems, the OCLTT may not provide the unifying theory its proponents hoped, but instead, a species’ thermal tolerance is likely shaped by a range of factors.