We've all heard the boiling frog cautionary tale: place a frog in hot water and it will immediately jump out. Gradually heat the water and the frog will bask in the warmth until it's too late. This story is used to advise people against the dangers of ignoring small, but consistent changes that can lead to disastrous outcomes. The parallels with climate change are obvious. Ironically, many experiments investigating the effects of warming on organisms use short-term exposures to high temperatures, akin to dropping the frog into a boiling bath. Understanding the consequences of long-term heating due to climate change requires experiments more like the long exposure, slowly boiled frog scenario. But, finding a study site that's experienced artificially warmed conditions over decades is not trivial. Luckily, Erik Sandblom and Fredrik Jutfelt from the University of Gothenburg knew of one such spot in their native Sweden and assembled a team to test whether slow-cooked fish really are worse off.
A region of water on the east coast of Sweden, known as the Biotest enclosure, receives heated water from a nearby power plant, which keeps the temperature 5–10°C warmer on average than the surrounding Baltic Sea. Fish have lived in these warmed waters for the past 30 years, enough time for individuals to acclimate and, potentially, for generations to adapt. The team wanted to know whether this life-long warming produces more heat-tolerant fish. They first compared the lower and upper cardiorespiratory performance (metabolic rate, heart rate and cardiac output), or physiological ‘floors’ and ‘ceilings’, of European perch (Perca fluviatilis) collected from the warmed Biotest waters and the cooler Baltic Sea, as well as Baltic Sea fish acutely warmed to Biotest temperatures for 24 h. They then calculated each group's thermal tolerance, essentially performing the ‘frog in the slowly heated water’ experiment, by gradually increasing the water temperature in the aquaria, measuring how hot it got before each fish lost its balance, and then subtracting each individual's environmental temperature from the temperature at which they toppled over.
The team found that lower cardiorespiratory performance ‘floors’ had adapted over generations to accommodate life in hot water: the Biotest fish, which had experienced long-term warming, showed lower resting metabolic rate and heart rate compared with the Baltic Sea perch that had been acutely warmed to Biotest temperatures for 24 h. However, there were only limited adjustments to the fish's upper cardiorespiratory performance ‘ceilings’ as the Biotest perch and the acutely heated fish had similar maximum metabolic rate, cardiac output and heart rate. The Biotest perch did manage to keep their balance in slightly higher temperatures than the Baltic Sea perch, finally toppling over in water that was about 2.2°C warmer than that for the Baltic Sea group. However, this probably doesn't give them an edge compared with their cool-water neighbors when it comes to surviving temperature extremes. As they live so close to their maximum temperature limit, the Biotest perch had a significantly reduced warming tolerance, likely making them more susceptible to extreme heat waves than Baltic Sea perch despite years of acclimation to warmer waters.
Like the proverbial frog in boiling water, gradual temperature changes are easy to ignore, and some individuals may even adjust to a certain extent. But, Sandblom, Jutfelt and their colleagues have provided a curt reminder that complacency towards our changing climate comes with consequences. Just when the next heat wave will hit is anyone's guess. The trouble is, most organisms can't simply jump out of the water when it gets too hot.
