Tonic immobility

Tonic immobility, unlearned, reflexive, and reversible behavioral state in animals characterized by extreme motor inhibition (an almost complete suspension of immediate and automatic actions in response to a stimulus), which may make an animal appear lifeless. Animals may enter this state under different situations, but it is most frequently seen in the context of predator-prey interactions. An animal displaying tonic immobility has an altered heart rate and stiff legs—making it look dead—and may remain motionless for hours. In this state the animal remains largely unresponsive to external stimuli. Tonic immobility has been observed in a wide range of species—from insects to mammals—and it manifests differently in each species.

Several other terms have been used to describe this condition, such as thanatosis (Greek: thanatos, meaning “death”), playing dead, and death feigning (a term first used by English naturalist Charles Darwin). Some experts consider death feigning to involve a more complex set of actions that end in tonic immobility. Death feigning is also thought to be a conscious choice made by an animal. Research, however, has yet to confirm whether all animals entering tonic immobility do so on purpose. It is also not clear whether a predator, for instance, believes its prey to be dead on observing such behavior. Therefore, most experts prefer the term tonic immobility to describe the behavioral state that makes animals temporarily immobile and unresponsive. The term is neutral and does not make assumptions about an animal’s intentions when displaying the behavior or its effects on an observer’s perception and response.

Characteristics

A rigid posture, tense muscles, and a lack of movement are the hallmarks of tonic immobility. The condition, however, may involve additional physical characteristics that vary across species. In invertebrates such as insects, abdominal pumping, which helps circulate air through the tracheal respiratory system, is significantly decreased during this state. Insect heart rate, however, appears to be considerably increased. In vertebrates the heart and breathing rates decrease during tonic immobility. Vertebrates may also salivate, defecate, urinate, and bleed upon entering this state. Some animals, such as the Virginia opossum (Didelphis virginiana) and the Eastern hognose snake (Heterodon platirhinos), produce foul-smelling secretions and stick out their tongues. Catalepsy—in which the limbs remain in whatever position they have been moved—is common to both vertebrate and invertebrate animals displaying tonic immobility. Although animals do not respond to external stimuli in this state, they remain aware of their surroundings.

Control

In a situation involving predation or similar threats to survival, tonic immobility is induced when an animal is caught or held by an aggressor, such as a predator. In some cases it may be triggered simply by the predator being nearby. Tonic immobility starts as a reflex motor response, but ending it involves complex neurological processes to assess whether the threat is gone. The structures and chemical messengers of the nervous system involved in initiating, maintaining, and terminating the response vary across taxa. For example, activities of the flexor and extensor motor neurons, which are specialized nerve cells that control limb movement, are responsible for the stiff posture of insects entering tonic immobility. Other excitatory and inhibitory neurons and the head ganglia (groups of nerve cell bodies) also play a role in initiating and controlling the response in insects. In vertebrates components of the parasympathetic nervous system control most of the responses displayed. Neurochemicals such as dopamine, serotonin, octopamine, and corticotropin-releasing factor play important roles in controlling the duration of the immobile state in vertebrates and invertebrates.

Several factors influence the responses seen in tonic immobility. These include age, body size, mating status, level of starvation, and biological rhythm of the animal, as well as the ambient temperature, season, and the activity in which the animal was engaged immediately before entering tonic immobility. The behavior may also be under partial genetic control and shows wide variations within and between populations of the same species.

Reasons

Animals have been documented displaying tonic immobility in more than one biological context. In all instances the behavior offers some advantage to the animal.