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Reflex bleeding and shedding of limbs

Those of you who read last week's article dealing with the various bizarre methods used by certain animals to hunt down prey, and also protect themselves from predators, would no doubt have found it hard to believe that animals could be so deceptive. Today, we examine methods other than hypnosis, the 'dance of death' and 'playing dead' or thanatotis used by the animals for survival purposes.

Releasing vile-tasting froth and foul-smelling fluids or even blood, are some unusual defence ploys used by some creatures. Perhaps, the most dramatic of these is autohaemorrhaging, also known as 'reflex bleeding'.

Animals that resort to this technique either secrete blood and poisonous fluids, or even squirt it, to ward off would-be attackers. Three species of North American desert-dwelling lizards, Phrynosoma cornutum, P. solare and P. coronatum which are sometimes commonly known as horned toads are famous for autohaemorrhaging. When threatened with danger, these lizards are known to use various methods of defence.

Initially they would attempt to intimidate their would-be attackers by inflating their bodies, to give an impression that they are much bigger than what they look like. However, if this fails, the lizards would try to ward off predators by hissing and jumping at them. And what do you think they do when this too produces no results? Resort to squirting blood at the attackers.

How do these lizards execute this rather grotesque (distorted) defence mechanism, you may wonder. They do so by using a series of thin-walled, blood-filled spaces known as sinuses within their eye sockets.

When the lizards rapidly increase the blood pressure within these sinuses, it causes the sinuses to break suddenly. The blood is then forced out in jet-like squirts of crimson droplets.

The force in which this blood, which may also contain a distasteful chemical, is squirted is so powerful, that it can shoot up to distances of 1.2m (four feet). The squirting can be repeated several times, if necessary, in order to frighten the predator. In fact, the horned toads are among the few creatures that can actually squirt blood to ward off would-be predators.

Apart from these lizards, a number of insects too use this autohaemorrhaging technique to deter predators. Among the insects that resort to this startling behaviour, the most dramatic are grasshoppers of the genus Dictyophorus. When threatened, the hydrostatic pressure within the grasshopper's body increases. This forces blood plasma out of the weak pores in the body's cuticle.

What is bizarre is that, when the blood plasma mixes with air, it converts into a disgusting froth that covers the entire body surface of the insect. As the froth contains a repellent so noxious (harmful), any creature that attempts to eat the insects, drops it and makes a hasty retreat.

The grasshopper then reabsorbs much of its blood by reducing its body's internal hydrostatic pressure. Even ladybirds have a reflex bleeding mechanism, but they secrete (produce and discharge) blood from their limp joints. Female fireflies of the genus Photuris also release droplets of blood from their thorax that contain distasteful substances known as lucibufagins (LBG).

Other animals that autohaemorrhage belong to various insect families within the beetle order Coleoptera. The bloody-nosed beetle (Timarcha spp) releases bright-red blood around its mouth. However, the Lytta vesicatoria, which belongs to the oil beetle family, and is known as the blister beetle, does not release blood, but a liquid called cantharidin, which when in contact with human skin, causes it to blister.

It has been found that the cantharidin secreted by the blister beetle has valuable medical properties, and is often used to treat skin problems such as warts. Relatives of the blister beetle known as oil beetles too, secrete a greasy substance which has a strong, unpleasant taste, to deter predators.

Not only insects, but also certain snakes resort to autohaemorrhaging. The West Indian wood snake is famous for this. It uses a series of dramatic acts to ward off enemies and hungry predators. Initially it coils into a tight ball and secretes a foul-smelling fluid. At the same time it turns its eyes bright red by releasing blood into them. However, it does not squirt it out in the way that the horned toad lizards do.

Instead, it increases the blood pressure within small capillaries just inside its mouth, making them to grow larger, until they burst, releasing scarlet blood down from the snake's mouth. This effect, coupled with the strong odour of the secreted fluid, gives the impression that the snake is not only dead and decomposing, but also damaged in some way or even diseased, making it unsuitable to eat.

Another extraordinary and dramatic method animals use to avoid being captured, is the shedding of limbs. The ability to shed tails and even limbs is called autotomy by scientists.

Of the 16 classified families of lizards, species of no less than 11 are known to resort to the clever means of getting away by shedding their tails. Not only can these lizards survive it, but they can also grow a new body part to replace the one lost!

The secret of this amazing process lies in the structure of a typical lizard's tail. Each of its caudal vertebrae from the sixth onward contains a weak horizontal 'break' or fracture plane which is made of cartilage, instead of bone. That is why it can snap easily if it's held by a predator.

As the blood vessels and nerves are constricted (compressed or made narrow) within each vertebra's fracture plane, the bleeding is minimal when the tail does snap off. If you have ever seen a lizard or a house gecko shed its tail, when a cat or some other animal attempts to catch it, and wondered why the tail continues to thrash violently for a short time even after it has broken, it is because the tail contains a functioning neuromuscular system.

The thrashing tail usually distracts the predator, giving the lizard or gecko a chance to escape. However, there are certain disadvantages in tail shedding. Even though lizards can grow another tail after shedding its original one, the new one is not as flexible as the previous one.

In older lizards, the new tail may be short and not as perfect as the first one. Another significant fact is that a lizard can shed its tail only once. The new tail it grows does not have separate vertebrae and its internal skeleton is nothing more than a cartilaginous rod. It can snap off only at its base, and not at any point along its length, like in the original tail. If a lizard that has a new tail attempts to escape a predator by shedding its new tail, it would lose the entire tail, the second time around.

Then, the other disadvantage of tail shedding is the possibility of the regeneration process going wrong. The lizard could end up with a new tail which is forked, or even end up with two to three tails. This type of deformed tail could prove to be a hindrance to the lizard, making it less mobile. It could also attract more attention from predators.

The most significant disadvantage of tail shedding is the loss of social standing, the tail shedder has to face. In some invertebrates, autotomy can involve the loss of one or more legs. Crabs are known to sacrifice one or more of their limbs if attacked by a predator.

Though the limbs will regrow, they are poor imitations of their originals, like the new grown tails of lizards. Another creature adept at autotomy are the starfishes which have spiny skeletons, and five (or more) arms or limbs in a star-like arrangement.

In the case of starfishes however, the regeneration is more dramatic than in crabs or lizards, because as long as the shed limb is not eaten by the predator, there is a possibility of it growing into a complete starfish. However, this process can take place only if the broken limb still contains a section of the central body disc of the starfish that shed the limb. Now isn't that really amazing?

Find out more about the attack and defence techniques used by the inhabitants of the animal kingdom.

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