The question of whether a squirrel can suffer a heart attack often arises from observing their sudden, erratic movements or unexpected collapse. While these small mammals possess a cardiovascular system similar to humans, the way they avoid a classic heart attack is profoundly different. This resistance is due to deep-seated physiological and genetic adaptations that protect their circulatory health.
How a Heart Attack Happens
A heart attack, medically termed a myocardial infarction (MI), occurs when blood flow to a section of the heart muscle is severely reduced or entirely cut off. This cessation of blood supply deprives the heart tissue of oxygen, leading to cell death. In the human model, this event is overwhelmingly caused by a pre-existing condition called atherosclerosis.
Atherosclerosis involves the progressive buildup of fatty deposits, cholesterol, and cellular waste, known as plaque, within the walls of the arteries. This plaque hardens and narrows the blood vessels, particularly the coronary arteries that feed the heart itself. A heart attack is typically triggered when one of these unstable plaques ruptures.
When the plaque interior is exposed to the bloodstream, it activates the body’s clotting system. This instantly forms a blood clot (thrombus) over the rupture site. If the resulting clot completely blocks the coronary artery, the downstream heart muscle experiences acute ischemia and begins to die.
The Squirrel’s Resistance to Atherosclerosis
True myocardial infarction, caused by atherosclerotic plaque rupture, is exceedingly rare in wild squirrels, especially in tree squirrels and ground squirrels. This resistance is rooted in their genetics, metabolism, and natural environment, as these animals do not develop the widespread plaque buildup seen in humans.
Their natural diet of nuts, seeds, and fungi, combined with a consistently high activity level, helps maintain cardiovascular health. A key protection is found in the physiology of hibernating species, such as the 13-lined ground squirrel, which can tolerate conditions instantly fatal to humans.
During winter hibernation, ground squirrels undergo cycles of torpor. Their heart rate drops from hundreds of beats per minute to as few as five, and their body temperature approaches freezing. Every few weeks, they rapidly rewarm during an interbout arousal, increasing their metabolism over 200-fold in hours. This cycle causes repeated periods of temporary ischemia and reperfusion, mimicking numerous small heart attacks and strokes.
The squirrel’s heart is genetically adapted to survive these dramatic fluctuations without sustaining permanent damage or tissue scarring. They maintain a naturally antithrombotic state, meaning their blood is less likely to form dangerous clots during minimal blood flow. This mechanism allows their heart tissue to be highly resistant to cell death even when oxygen supply is low, effectively preventing the classic MI.
Other Causes of Sudden Collapse in Squirrels
When a squirrel suddenly collapses, the cause is usually not an atherosclerotic heart attack but an alternative form of sudden death or severe trauma.
Stress Cardiomyopathy
One common cause is stress cardiomyopathy, also known as capture myopathy in wildlife. This occurs when an animal experiences extreme fright or physical exertion, such as being chased by a predator or trapped. The overwhelming surge of stress hormones (catecholamines) floods the system, causing acute damage to the heart muscle and other organs. This intense, adrenaline-driven event leads to sudden cardiac failure, causing the animal to collapse and die rapidly, even without underlying coronary disease.
Trauma
Trauma is a leading cause of sudden collapse, particularly in urban environments where hazards are human-made. Vehicle strikes are a frequent cause of fatal injury, often resulting in massive internal trauma or severe neurological damage that leads to immediate death. Falls from trees, whether due to misjudgment or a fight, can also result in fatal head or spinal injuries.
Poisoning
Poisoning from secondary exposure to common rodenticides is an equally sudden threat. Anticoagulant poisons interfere with the blood’s clotting ability, causing fatal internal hemorrhaging that may not manifest for several days. Other toxins, like certain non-anticoagulant rodenticides, can cause rapid neurological symptoms, including severe seizures and respiratory paralysis, leading to a sudden, visible collapse.
Infectious Diseases
Infectious diseases also account for unexplained deaths, sometimes causing rapid systemic failure. For instance, the squirrelpox virus is highly lethal to European red squirrels and can lead to a quick decline. Similarly, certain parasitic and viral infections, such as Adenovirus, can cause rapid illness and death in wild populations, making a sudden collapse a final symptom of an underlying systemic issue.