Mechanism
​Most strongly believed is that tonic immobility is highly correlated with hormone levels (Hoagard 1928). Experiments including adrenaline show that animals enter this state in situations that have equal opportunity for a typical "fight or flight" reaction, instead they choose tonic immobility. These stressful situations show increased levels of adrenaline, and when animals already in tonic immobility are injected with adrenaline, they remain in the state longer.​
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While hormones, specifically adrenaline, have been linked to tonic immobility, scientists are still perplexed at how quickly an animal can enter the state. In mammals and sharks, suddenly inverting them into a dorsal position has been shown to induce the behavior (Davie et al. 1993). It is hypothesized that adrenal glands in these animals produce very intense and immediate reactions to solicit tonic immobility (Hoagard 1928).​
Figure 5. Plot showing the distribution of points for 5 Anoles (lizard species) according the the Arhenius equation, a formula for the dependence on temperature of reactions rates. Periods refers to periods of tonic immobility.
Exactly why an animal might induce tonic immobility, or fake death, varies under the circumstances for each species.
The causation of tonic immobility has been researched since the early 1900s. Possible causes of tonic immobility include predatory responses, mating reactions, hormone levels, and even external factors like temperature have all been considered possible factors. However, scientists have determined that tonic immobility is not reliant on any neuron center in particular (Davie et al 1993 &Hoagard 1928).​
While in the state of tonic immobility, animals experience decreased blood pressure and heart rates (Davie et al. 1993). Yet they still receive signals through their ventilatory systems and are able to exhibit physical responses, such as continuing to breathe, digestion, spinal reflexes, and other bodily functions. Darwin found that animals in the state remain still and in a position that is not quite identical to ones that their species might be in for natural death (Hoagard 1928). But the stillness, rather than the realistic positioning, might be the biggest benefit, since many predators, like lizards, have retinas that are most sensitive to movement.
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In regards to the neurological and physiological mechanisms behind tonic immobility, there "virtually nothing is known...in any taxa" (Brooks et al. 2011). Even getting preliminary data on the role of adrenaline in tonic immobility is difficult, since little hormonal research is done on sharks or snakes.​