Shaking, or a tremor, is a rhythmic, involuntary oscillation of a body part resulting from alternating or synchronized contractions of opposing muscle groups. This behavior is a universal physiological response observed across a vast range of animal species, from mammals to insects. Understanding why an animal shakes requires distinguishing between four distinct biological and behavioral categories.
Shivering for Temperature Regulation
The most common reason for an animal to shake is shivering thermogenesis, a mechanism used by endotherms, including mammals and birds, to generate heat. This involuntary action is initiated when the core body temperature drops below its set point, triggering a reflex in the posterior hypothalamus. Rapid, unsynchronized contractions of skeletal muscles occur without producing mechanical work, generating a significant amount of heat as a byproduct of the metabolic process.
This heat production defends against hypothermia and is a primary way to maintain thermal homeostasis. Shivering can increase the body’s metabolic heat production up to four times the resting level. Small mammals, such as mice or shrews, and many birds rely heavily on this mechanism because their high surface area-to-volume ratio causes them to lose heat quickly to the environment.
Unlike non-shivering thermogenesis, which uses brown fat, shivering uses the large mass of skeletal muscle tissue. This muscle activity is under the rapid control of the nervous system, allowing the animal to quickly adjust heat production to match changes in environmental temperature.
Tremors Caused by Stress and Fear
Shaking can also be a direct result of an animal’s emotional state, particularly fear or high arousal. This tremor is a neurological response mediated by the sympathetic nervous system, known as the “fight-or-flight” response. When an animal perceives a threat, the body releases hormones like adrenaline and cortisol, preparing the muscles for immediate action.
The resulting muscle tension and increased neurological activity manifest as visible trembling, even when the animal is warm. This is often seen in domesticated animals, such as a dog trembling after a loud noise or frightening experience. This tremor also serves an adaptive function by helping to discharge the excess energy and hormones built up during the stress response.
Allowing the body to tremor after a stressful event helps the nervous system return to a state of balance. Stress tremors are a response to neurological overload rather than a drop in temperature.
Shaking as a Symptom of Illness
When shaking is not attributable to cold or fear, it may signal underlying physiological distress or disease. A common example is a fever, where the immune system raises the body’s temperature set point to fight infection. The animal shakes to increase its body temperature to meet this new, higher set point, similar to cold shivering.
Metabolic conditions can also induce tremors by disrupting the nervous system. Low blood sugar (hypoglycemia) or severe electrolyte imbalances can lead to generalized shaking as the nervous system becomes unstable. Toxins, such as certain pesticides or heavy metals, may also cause involuntary, rhythmic movements by interfering with normal nerve function.
Pathological tremors are often associated with neurological disorders, such as diseases affecting the cerebellum, which controls coordination. For instance, Generalized Tremor Syndrome in dogs is an immune-mediated condition that causes full-body shaking, particularly in small breeds. These tremors are abnormal movements that typically worsen with excitement or exercise.
Functional and Intentional Shaking Behaviors
Shaking can be a deliberate or functional behavior that is neither a reflex to a threat nor a sign of illness. The most recognized instance is the “wet dog shake,” a vigorous, full-body rotation used by many furry mammals, including mice, cats, and dogs, to quickly remove water. This action is efficient, allowing a dog to shed over 70% of the water from its coat in just a few seconds.
This functional shaking conserves body heat and energy that would otherwise be lost to evaporation. The reflex is triggered by specialized touch receptors around hair follicles, sensing moisture or irritants. Shaking can also serve as a social signal or a marker of transition between activities in certain animals.
Some insects engage in intentional shaking for biological purposes, such as vibrating their wings to warm up muscles before flight. This pre-flight warm-up is a form of functional thermogenesis that prepares the flight muscles to operate optimally.