Turtles are remarkable creatures that have been around for over 200 million years. One of the most fascinating facts about turtles is that their heart never stops beating, not even when they hibernate during the winter months.
If you’re short on time, here’s a quick answer to your question: A turtle’s heart never stops beating due to evolutionary adaptations that allow its heart and other organs to function at very low rates to conserve energy.
In this approximately 3000 word article, we’ll explore the incredible anatomy and physiology that allows turtles to continuously circulate blood and oxygen, even in extreme environments and when resources are scarce.
The Turtle Heart and Circulation
Basic Anatomy of the Turtle Heart
The turtle heart has three chambers – two atria and one ventricle. This three-chambered heart circulates blood in only one direction, unlike the four-chambered hearts of mammals. The ventricle chamber pumps blood to the lungs and body simultaneously through adaptations like the anatomical “divider” within it.
Turtles, with their slow metabolisms and low energy lifestyles, require less circulatory support than fast-moving mammals, so their cardiovascular systems can get by with simpler structures.
Unidirectional Blood Flow
The turtle’s ventricle ejects mixed oxygenated and deoxygenated blood in only one direction through the large systemic arch. Valves prevent any backward flow into the heart chambers. This unidirectional flow means no separation of pulmonary and systemic circuits as in mammalian circulation.
Though less efficient at oxygen delivery than a double-circuit system, the turtle cardiovascular anatomy meets the reptile’s metabolic needs.
Supporting Metabolic Processes at Low Levels
With extremely slow metabolisms and heart rates averaging under 50 beats per minute ( unlike the 60-100 beats per minute of humans), turtles require much less circulatory system support than mammals. Their internally cool bodies (often below 25°C/ 77°F) have similarly cool energy needs.
So a three-chambered linear heart works well enough, supplying steady nutrients and oxygen at rest or for the turtle’s very slow movements.
Surviving Without Oxygen
Turtles Can Function Anaerobically
Turtles have amazing adaptations that allow them to survive for extended periods without oxygen. Their slow metabolism and ability to function anaerobically enables them to endure lack of oxygen that would be fatal to most animals (1).
When oxygen is limited, turtles shift to glycolysis, breaking down glucose for energy without requiring oxygen. This produces lactic acid, which builds up in muscles during anaerobic exertion. While this can be painful and damaging for us, turtles have special mechanisms to handle lactic acid accumulation.
Lactic Acid Buffering and Control
Turtles have high concentrations of carbonates and bicarbonates in their blood and tissues that chemically buffer lactic acid (2). This prevents acidosis that would otherwise result from lactic acid buildup.
Turtles can also convert the lactic acid into glucose through gluconeogenesis when oxygen becomes available again. This recycles the lactic acid back into fuel instead of allowing it to reach toxic levels (3). Pretty nifty, huh?
In addition, studies have found signals that indicate turtle muscles themselves may help control lactic acid production (4). By regulating the amount of lactic acid generated during anaerobic metabolism, turtles prevent excessive amounts from accumulating.
This self-monitoring system is evidence of the turtle’s superb adaptation for surviving without oxygen.
Hibernation Adaptations
Turtles take anaerobic endurance to the extreme when they hibernate over the winter (or aestivate during hot, dry periods). While hibernating, a turtle’s metabolism slows down so drastically that it can get by on a tiny fraction of its normal oxygen needs for months without taking a breath (5).
Its heart rate declines from around 20-40 beats per minute (bpm) to just 1-2 bpm, but never actually stops entirely. This allows vital organs like the brain and heart to subsist on limited energy reserves until spring brings renewal.
Turtle heart rate awake | 20-40 beats per minute |
Turtle heart rate when hibernating | 1-2 beats per minute 😴 |
Some studies even found turtles surviving without oxygen for 4-5 months at near freezing temperatures (6)! Thanks to their exceptional anaerobic adaptations, turtles can hibernate in underwater mud without any source of oxygen until conditions improve. That’s why a turtle’s heart never stops beating!
Incredible! No wonder turtles have been around for over 200 million years.
Extreme Temperature and Pressure Tolerance
Maintaining Function in the Cold
Turtles have evolved remarkable physiological adaptations that enable them to withstand and function in extreme cold. Their key strategy is to slow down their metabolism to require less oxygen and energy.
When cold-blooded turtles are exposed to frigid temperatures, their heart and respiration rates drop dramatically, a condition known as brumation. For example, a turtle’s heart rate may decrease from around 50 beats per minute to just 6 or 7 beats when its body temperature drops below 50°F.
Some turtles can even survive temperatures below freezing if insulated by water or mud.
Turtles also have high concentrations of glycogen and glucose in their organs, which act as natural antifreeze to prevent freezing damage. Their blood contains anti-clotting adaptations to avoid dangerous clotting when blood flow slows.
Furthermore, many turtles can uptake oxygen through their skin and cloacal lining when underwater, supplementing their minimal lung respiration.
Some turtle species, like Blanding’s turtles and box turtles, may also migrate short distances to hibernate in the muddy bottoms of lakes and ponds. The mud insulates against cold air temperatures while the surrounding water resists freezing.
Turtles can overwinter this way for months while remaining alive but dormant.
Membrane Adaptations For Pressure Changes
Turtles have also evolved adaptations to withstand significant pressure changes. Aquatic turtles that regularly dive endure rapid pressure increases, while eggs incubating underground endure pressure from overlying soil.
Turtles have membranes in their shell joints and flexible shells that compress to equalize pressure differentials.
For example, leatherback sea turtles have collagenous connective tissue between their bony scutes. This allows compression of up to 2000 pounds per square inch as they dive over 1000 meters deep. Similarly, turtle egg shells have minuscule pores that allow just enough gas exchange for embryonic development while preventing fatal crushing.
Additionally, the turtle respiratory system is resistant to pressure changes. Lungs are attached to the inside of the shell rather than the shoulders, preventing lung squeeze. Turtle blood has adaptations to optimize oxygen delivery despite pressure shifts, including increased hemoglobin oxygen affinity, Bohr Shift reductions, and centrifugal augmentation of diffusion.
Conclusion
In summary, turtles possess a robust cardiovascular system capable of maintaining circulation in a wide range of extreme environments, especially at very low metabolic rates.
This allows them to survive months without oxygen when hibernating in ice-covered ponds or to withstand pressure changes when diving hundreds of feet underwater. Their incredible evolutionary adaptations are why turtles continue to thrive after hundreds of millions of years on Earth.