As all organisms warm, performance slowly increases until they reach an optimum (where performance is greatest), but then with further warming performance quickly declines. The rapid decline above optimum temperatures means overheating can be dangerous, risking physiological failure or even death. Unfortunately temperatures are rising.
www.tcd.ie/e3/news/what-goes-up-must-come-down–scientists-unearth-universal-thermal-performance-curve-that-shackles-evolution-/