You might have heard that the hummingbird is the fastest animal with its incredibly fast heart rate, but did you know that the pigmy shrew has the smallest heart in the world? With a heart only the size of a grapefruit, it has to eat three times its weight per day to keep up with its metabolism.
Thirteen-lined ground squirrels, also known as striped gophers, spend their winter in torpor (a state of inactivity) by sleeping in their burrows for weeks at a time. This conserves energy so they can search for food. While in torpor, their heart and breathing slow down and they can go for days without needing a drink.
Scientists are trying to understand how this happens. In a study, they tracked the animals’ respiration, heart rate and body temperature during hibernation. The scientists found that the 13-lined ground squirrels ate a lot before hibernating, depositing fat tissue in their hearts and switching to a metabolic state called ketosis that burns the stored fat for fuel instead of carbohydrates. This protects the animals from oxidative damage to the heart.
During hibernation, the shrews’ heart rates depress and they consume one-fifth of their normal amount of oxygen. To protect the heart, the shrews switch off genes that drive their standard carbohydrate-burning metabolism, and they store up energy in a type of brown adipose tissue called triglycerides. The triglycerides provide a steady source of fuel and they release less damaging free radicals than glucose. The research suggests that the metabolic shift may be a key factor in why hibernating animals experience fewer heart problems than nonhibernators.