Why Some Animals Fake Death — and What it Means

The motivation behind everything that animals do is to stay alive, at least until they have produced offspring. Any species that cannot do this is not going to last for long. But are there occasions when being dead could be useful? Or rather, giving the appearance of death. Faking your own death is a strategy adopted by several animals – in science, it is called ‘apparent death’ and the state is called ‘tonic immobility’ or thanatosis.

Tonic immobility is a reflex immobility response – the animal’s body does it automatically. Also, they cannot stop it from happening, and they cannot rouse themselves from it. It is triggered by the inhibition of motoneurons (nerve cells), which causes a motor block thanks to a cascade of neurochemicals and hormones. However, the condition is transient and, when whatever triggered it has gone, the animal will recover. Let’s look at some animals that use it to great effect and examine the reasons why they do it.

Animals Faking Their Own Death as a Defense Mechanism

When animals use thanatosis as a defense mechanism, it is different from simply freezing from fear. Many animals freeze when they spot a predator, but crucially, the predator has not seen them. Animals freeze to prevent detection and to aid their camouflage – they stay very still and hope that they will not be spotted. In tonic immobility, the predator has already seen the animal, possibly made contact with it, and the behavior is used to stop a further attack and to stop themselves from being eaten.

Animals in tonic immobility usually alter their posture (fall on the ground) and become rigid. Vertebrates in this state have a slower heart and breathing rate and may even protrude their tongue. It is observed in invertebrates, fish, amphibians and reptiles, birds, and mammals. Here are a few animals that use tonic immobility as a defense mechanism.

Opossum

Perhaps the most famous user of tonic immobility is the opossum. When they are attacked by a predator, they collapse onto their sides with rigid muscles. Their mouth hangs open and froths with saliva whilst they discharge both urine and feces. Meanwhile, their eyes are wide open, staring into space. No wonder predators are put off!

Sharks

Several sharks exhibit tonic immobility when they are inverted. Their dorsal fin straightens, and their breathing and muscle contractions become steadier. It lasts for around 15 minutes, but it may put stress on the shark’s body. It can be fatal in some cases. Orcas use it to their advantage by flipping sharks over so that they can attack them unhindered.

Chickens and Ducks

Both chickens and ducks have shown tonic immobility when faced with predators. Humans can induce it in chickens by restraining them on their side, stomach, or back. However, the response varies by species and breed.

Other Animals That Exhibit Defensive Tonic Immobility

Tonic immobility is seen in both domestic and wild rabbits. The North American hog-nose snake is perhaps the most well-known example of it happening in reptiles, but it has also been recorded in grass snakes and iguanas.

In the sea, it is seen in sea turtles and many types of fish, including goldfish, trout, and medaka. The brown widow spider enters this state when it is shaken from its web and it is seen in other spider species. In some wasps, it can be triggered by tapping their antennae, and it is used by young fire ants to avoid confrontation with other ants.

Tonic Immobility as a Hunting Strategy

Scavengers are animals that feed on the dead bodies of other creatures. The sleeper cichlid fish that lives in Lake Malawi in East Africa, along with some other cichlids that live in other parts of Africa and Central America, use this to their advantage. They fake their own death by lying on the lake bottom and changing their coloration to a blotchy pattern so they look dead. But it’s a trap. When a hungry scavenger arrives to take a bite, the cichlid springs back to life and eats them!

Faking Death in the Context of Mating

Predators are not the only threats that some species face. Some animals display a behavior called ‘explosive mating’ where several (sometimes many) males try to mate with a single female at the same time. The males resort to harassment, forced copulation, and intimidation to get a turn at mating with their chosen female. Several males may cling to a female at the same time, and this can result in her getting injured or even in her death.

There is very little that a female can do about this, but faking her own death is an option of last resort! It inevitably puts the males off. The whole point of mating for them is to produce the next generation and pass on their genes. A deceased sexual partner is not going to be able to do that. So, it would be fair to say that his ardor is cooled somewhat. Here are some animals where this has been observed.

European Common Frog (Rana temporaria)

Mating can be a hazardous time for female common frogs because multiple males will pile onto a single female, forming a “mating ball” within which she can get injured. When the female has had enough, she communicates this in a few ways. She rotates around her own body axis to try and remove the males and gives a ‘release call’ which sounds like a mixture of a grunt and a squeak. If this does not work, she uses tonic immobility. In this state, her body stiffens, and her arms and legs are outstretched from her body. It can happen in water or on land. In laboratory observations, tonic immobility was seen more frequently in smaller and younger females. The state can last for two hours.

Sharp-Ribbed Newt (Pleurodeles walt)

This species of highly aquatic newt lives in Portugal, Spain, and northern Morocco. They are a large newt with a flattened head. Mating takes place in water and requires the male to hold onto the female with his front legs around hers – this can last for hours. Once things get to this stage, the female has little choice but to participate, but can end the relationship by faking her own death. In observational studies, females have been observed avoiding mating using tonic immobility. This made it impossible for the male to apply pheromones to the female’s nose, which is a required prerequisite to mating. In all of the observed cases, it resulted in the male giving up.

Nursery Web Spider (Pisaura mirabilis)

These long-legged spiders with a slender abdomen can be found all over Europe and the UK, as well as the Canary Islands, Madeira, and some parts of Russia, China, and North Africa. They are commonly found in grassland, scrub, long grass, and other dense vegetation. Their mating ritual is both fascinating and bizarre and involves gift-giving and death feigning. In contrast to the amphibians we have considered so far, it is the male spider that fakes his own death.

The mating ritual begins with the male offering his nuptial gift (prey wrapped in silk) to the female. Mating is a high-risk activity for these males because females can attack and eat them. However, thanatosis both keeps him alive and increases his chances of successfully mating. Studies have shown that death-feigning males were more successful in obtaining the chance to copulate with females. So, they seem to be using it to overcome female resistance to copulation. Also, some males use it more than others.

Moorland Hawker Dragonflies (Aeshna juncea)

The moorland hawker (also called the common hawker or sedge darner) is one of the larger hawker dragonflies and is found across Europe, Asia, and northern North America. They like habitats near water. Mating takes place when males seize females in the air, often without consent. The males can be aggressive, and their approaches can result in females getting hurt or even drowning. So, to avoid the males’ attentions, the females use death feigning. They freeze in mid-air and crash to the ground, which is risky but effective.

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