Hot hearts can beat faster when meat is on the menu

As our planet continues to warm, biologists are sweating over how animals will cope with the changing climate. For cold-blooded animals (ectotherms), high environmental temperatures pose a particular problem because their body temperature closely matches that of their surroundings. Imagine being unable to turn down your house's thermostat when outside temperatures get blisteringly hot – yikes! To make matters worse, the heat accelerates many vital physiological processes in ectotherms, including heart rate. Because hearts can only beat so fast while maintaining a proper rhythm, there are limits to the temperature that ectothermic animals can tolerate. But, if these animals can find ways to increase their maximum heart rate, allowing their hearts to keep pumping at temperatures that would otherwise prove fatal, they may gain an advantage in a warming world. In a fascinating new study led by Emily Hardison from the University of California, Santa Barbara, USA, a team of researchers tested whether diet can help ectothermic opaleye (Girella nigricans) regulate their hearts better at high temperatures.Opaleye are small fish commonly found in the rocky shallows along the west coast of North America, and their generalist diet allows them to choose between eating plants and/or animals to meet their nutritional requirements: a herbivorous diet is rich in antioxidants and minerals, while a carnivorous diet is packed with proteins and fats. To understand how these different diets might impact opaleye heart function, the researchers first maintained fish at 12°C and fed them a carnivorous (brine shrimp), herbivorous (algae) or omnivorous (algae and brine shrimp) diet for 2 weeks. Next, they exposed the fish to temperatures ranging from approximately 12°C to 34°C and monitored how fast their hearts were beating at each 1°C increment. To make sure the fish were giving it their all, the researchers gave them a little boost with two different drugs to encourage their hearts to beat as fast as possible.The team found that maximum heart rate was similar between all fish, despite being fed different diets. The fish's heart rates increased with rising temperature, reaching a peak of 150 beats min−1 at approximately 28°C. So, the researchers decided to take their study one step further and see whether the fish could modify how their hearts work after prolonged exposure to a high temperature. This time the researchers warmed the fish up to 20°C for 2 weeks while feeding them the same carnivorous, herbivorous or omnivorous diets. After the hot tub-like experience, the team measured each fish's maximum heart rate across the same range of temperatures. Interestingly, the meat-eating fish (carnivorous and omnivorous diets) now had a higher maximum heart rate at 28°C than they did before. Herbivorous fish, in contrast, had the exact same maximum heart rate as before, indicating that they were not able to adjust to the warming climate. It turns out that a little bit of meat on the menu allowed the fish's hearts to beat faster in the heat – an adjustment that might make them more heat tolerant overall.Hardison and her colleagues then pondered why the opaleye on a meat-based diet could handle the heat better. Maybe it's what's on the menu that counts? After all, meaty meals are usually high in fatty acids, which serve as tiny building blocks for many cellular structures in the body. The researchers analyzed the fatty acid composition of the fish's hearts and, as expected, the meat-eaters had a fatty acid composition better suited to high temperatures. Now, this doesn't mean that people should start devouring sirloin steak in the name of thermal tolerance, but it does suggest that a balanced diet might just give animals an edge against the elements in our rapidly warming world.

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