Reptilian Hearts: The Ultimate Survival Machines That Can Keep Beating Without Oxygen, Brain, or Body

Reptilian Hearts: The Ultimate Survival Machines That Can Keep Beating Without Oxygen, Brain, or Body

Turtles are fascinating creatures that can live for a long time, sometimes even longer than humans. But what happens when they die? Do they stop moving immediately, or do they keep twitching for a while? And what about their hearts? Do they stop beating as soon as the turtle is declared dead, or do they keep pumping blood for hours, days, or even weeks?

In this article, we will explore the amazing phenomenon of reptilian hearts that can keep beating after death, even when separated from the body. We will also look at the reasons behind this phenomenon, such as the slow metabolism, high concentration of ions, and special cardiac tissue of reptiles. We will focus on turtles as an example, but also mention other reptiles that share this trait.

Part 1: How to Tell If a Turtle Is Dead or Alive

Before we dive into the topic of post-mortem heartbeats, we need to address a more basic question: how can we tell if a turtle is dead or alive? This may seem like an easy question, but it is actually quite tricky. Turtles are notoriously difficult to diagnose for death, because they have several adaptations that allow them to survive in harsh conditions and appear lifeless.

One of these adaptations is hibernation. Turtles can lower their body temperature and metabolic rate to conserve energy and survive cold winters. During hibernation, they may bury themselves in mud or sand, or hide under rocks or logs. Their breathing and heartbeat become very slow and faint, and they may not respond to external stimuli. To an inexperienced observer, they may look dead, but they are actually alive and waiting for warmer weather.

Another adaptation is anoxia tolerance. Turtles can survive without oxygen for a long time, thanks to their ability to store glycogen in their liver and muscles, and use anaerobic metabolism to produce energy. They can also reduce their oxygen demand by shutting down non-essential organs and functions. This allows them to stay underwater for hours or even days without coming up for air. Some turtles can even breathe through their cloaca (the opening for excretion and reproduction) or their skin, by absorbing dissolved oxygen from the water. When turtles are exposed to anoxia (lack of oxygen), they may enter a state of torpor (reduced activity and responsiveness), which can also make them look dead.

A third adaptation is decapitation survival. Turtles can survive having their heads cut off for a short period of time, because their brains are not essential for controlling their basic bodily functions. Their spinal cord and peripheral nerves can still send signals to their muscles and organs, allowing them to move and breathe. Their hearts can also keep beating without any input from the brain, thanks to their special cardiac tissue that we will discuss later.

These adaptations make it hard to determine if a turtle is dead or alive by just looking at it. Some signs that may indicate death are:

• Rigor mortis (stiffening of the muscles)
• Putrefaction (decomposition of the body)
• Livor mortis (pooling of blood in the lower parts of the body)
• Algor mortis (cooling of the body)
• Clouding of the eyes
• Absence of reflexes
• Absence of heartbeat

However, these signs are not always reliable or conclusive. Rigor mortis may not occur in cold-blooded animals like turtles, or may be delayed by low temperatures. Putrefaction may also be slowed down by low temperatures or dry environments. Livor mortis may not be visible in turtles with dark shells or skin. Algor mortis may not be significant in ectothermic animals like turtles, whose body temperature depends on the environment. Clouding of the eyes may be caused by dehydration or injury, not necessarily death. Absence of reflexes may be due to hibernation or torpor, not necessarily death. And absence of heartbeat may be due to faintness or irregularity, not necessarily death.

Therefore, to confirm if a turtle is dead or alive, it is necessary to perform a thorough physical examination and use diagnostic tools such as stethoscope, electrocardiogram (ECG), ultrasound, or necropsy (post-mortem examination). However, even these methods may not be definitive, as we will see in the next part.

 

Part 2: How the Reptilian Heart Keeps Beating After Death

One of the most remarkable features of reptilian hearts is their ability to keep beating after death, even when separated from the body. This phenomenon has been observed in various reptile species, such as turtles, snakes, lizards, and crocodiles. But how is this possible? What makes the reptilian heart so resilient and independent?

The answer lies in the structure and function of the reptilian cardiac tissue. Unlike mammalian hearts, which have a specialized pacemaker region called the sinoatrial node (SA node) that initiates and regulates the heartbeat, reptilian hearts have multiple pacemaker regions distributed throughout the atria and ventricles. These regions are composed of nodal cells, which are modified cardiac muscle cells that can generate spontaneous electrical impulses without any external stimulation. These impulses travel through specialized conducting fibers called nodal tissue, which connect the different pacemaker regions and coordinate their activity.

The main pacemaker region in reptiles is located in the sinus venosus (SV), a thin-walled chamber that receives blood from the body and empties into the right atrium. The SV generates impulses at a regular rate and sends them to the atria and ventricles through the nodal tissue. However, if the SV is damaged or removed, other pacemaker regions can take over and maintain the heartbeat. The most important of these backup pacemakers are located in the atrioventricular node (AV node), which is situated at the junction of the atria and ventricles, and in the ventricular apex (VA), which is located at the tip of the ventricle.

The AV node and VA can generate impulses independently of the SV, but at a slower rate. The AV node also acts as a gatekeeper that regulates the transmission of impulses from the atria to the ventricles, preventing them from contracting too fast or too slow. The VA can also influence the contraction of the ventricle by sending retrograde impulses (backward impulses) to the rest of the cardiac tissue.

The presence of multiple pacemaker regions and conducting fibers in reptilian hearts gives them a high degree of autonomy and adaptability. They can adjust their rate and rhythm according to various factors, such as temperature, oxygen level, hormonal status, and neural input. They can also survive injury or ischemia (lack of blood supply) better than mammalian hearts, because they have more backup systems and alternative pathways for electrical conduction.

This also explains why reptilian hearts can keep beating after death, even when separated from the body. As long as they have enough oxygen and nutrients, they can generate their own impulses and contract without any input from the brain or nervous system. They can also resist decay longer than other organs, because they have a low metabolic rate and a high concentration of ions that prevent bacterial growth.

However, this does not mean that reptilian hearts are immortal or invincible. They still depend on external factors for optimal functioning, such as temperature regulation, hormonal balance, and neural modulation. They also have limits to their endurance and resilience, especially when exposed to extreme conditions or prolonged stress. Therefore, it is important to monitor and evaluate their health and performance using appropriate diagnostic tools and methods.

 

Conclusion:

The reptilian heart exhibits remarkable post-mortem contractility, even when isolated from the body. This phenomenon is attributed to the distinctive structure and function of the reptilian cardiac tissue, which has multiple pacemaker regions and conducting fibers that can generate and coordinate electrical impulses autonomously of the brain or nervous system. The reptilian heart also possesses adaptations that enable it to withstand harsh conditions and exhibit apparent lifelessness, such as hibernation, anoxia tolerance, and decapitation survival. However, these adaptations also pose challenges to the diagnosis of death in reptiles, and necessitate a comprehensive physical examination and diagnostic tools such as stethoscope, electrocardiogram, ultrasound, or necropsy to assess their cardiac status. Reptile cardiology is an emerging field of veterinary medicine that requires further research and application to improve the health and welfare of captive reptiles.

Sources:

 

Kik MJL, Mitchell MA. Reptile Cardiology: A Review of Anatomy and Physiology, Diagnostic Approaches, and Clinical Disease. Seminars in Avian and Exotic Pet Medicine. 2005;14(1):52-60. https://vetmed.illinois.edu/mmitch/pdf/reptilecardiology.pdf

 

Crossley DA II. Reptilian cardiovascular anatomy and physiology: evaluation and monitoring (Proceedings). DVM360. 2009. https://www.dvm360.com/view/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings

 

Rupprecht C, Kühn C, Witten PE, et al. Reptilian heart development and the molecular basis of cardiac chamber evolution. Nature. 2009;461(7260):95-98. https://www.nature.com/articles/nature08324

 

Johnson A, Clinton J, Stevens R. Turtle heart beats five days after death. Amer Biol Teacher. 1957;19(6):176-177. https://online.ucpress.edu/abt/article/19/6/176/4675/Turtle-Heart-Beats-Five-Days-after-Death

Here are some photos that were taken during a necropsy we did on a turtle with some of our Vet Program students:

 

 

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