The platypus

Platypus

The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

 is one of the few mammals to produce venom

Venom

Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite or a sting...

. Males have a pair of spurs on their hind limbs. The male's pair of spurs spits out a cocktail of poisons that, while excruciatingly painful, is not lethal to most animals.

Spur and crural gland

Venom is produced in the crural glands of the male, which are kidney-shaped alveolar gland

Alveolar gland

In contrast to tubular glands, in the second main variety of gland, the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar glands ....

s located in the upper thigh connected by a thin-walled duct to a calcaneus spur on each hind limb. Female platypuses, in common with echidnas, have rudimentary spur buds which do not develop (dropping off before the end of their first year) and lack functional crural glands. The spur is attached to a small bone which allows articulation; the spur can move at a right angle to the limb allowing a greater range of attack than a fixed spur would allow. The spur normally lies flat against the limb but is raised when required.

Venom

The crural gland produces a venom secretion containing at least nineteen different peptide

Peptide

Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...

s, in addition to non-protein components. Those peptides which have been sequenced and identified fall into three categories: defensin

Defensin

Defensins are small cysteine-rich cationic proteins found in both vertebrates and invertebrates. They have also been reported in plants. They are, and function as, host defense peptides. They are active against bacteria, fungi and many enveloped and nonenveloped viruses. They consist of 18-45 amino...

-like peptides (OvDLPs), C-type natriuretic peptides (OvCNPs), and nerve growth factor (OvNGF). The different chemicals in the venom have a range of effects from lowering blood pressure to causing pain and increasing blood flow around the wound. Coagulating effects have been seen during experiments on laboratory animals, but this has not been observed consistently. Unlike snake venom

Snake venom

Snake venom is highly modified saliva that is produced by special glands of certain species of snakes. The glands which secrete the zootoxin are a modification of the parotid salivary gland of other vertebrates, and are usually situated on each side of the head below and behind the eye,...

, there appears to be no necrotic

Necrosis

Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...

 component in the Platypus' venom - although some muscle wastage has been observed in cases of envenomation in humans, it is likely that this is due to the inability to use the limb while the effects of the venom persist. It is unknown whether the pain caused is a result of the associated oedema around the wound or whether the venom has a component which acts directly on the pain receptors.

Although platypus venom has a broadly similar range of effects and is known to consist of a similar selection of substances to reptilian venom, it appears to have a different function from those poisons produced by non-mammalian species: its effects are not life threatening but nevertheless powerful enough to seriously impair the victim. It is not used as a method of disabling or killing prey, and although it acts as a defensive mechanism, only males produce venom. Since production rises during the breeding season it is theorized that it is used as an offensive weapon to assert dominance and control territory during this period. Additionally, the gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s involved in venom production in the two are, whilst related, distinct. Hence, this appears to be an example of convergent evolution

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

.

A unique feature of the venom is the presence of a D-amino acid. This is the only known such example in mammalian systems.

Effect on humans

Although powerful enough to kill smaller animals, the venom is not lethal to humans. However, it produces excruciating pain which may be intense enough to incapacitate the victim. Swelling rapidly develops around the entry wound and gradually spreads outwards. Information obtained from case studies

Case study

A case study is an intensive analysis of an individual unit stressing developmental factors in relation to context. The case study is common in social sciences and life sciences. Case studies may be descriptive or explanatory. The latter type is used to explore causation in order to find...

 shows that the pain develops into a long-lasting hyperalgesia

Hyperalgesia

Hyperalgesia is an increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.-Types:...

 that can persist for months but usually lasts from a few days to a few weeks. A clinical report from 1992 showed that the severe pain was persistent and did not respond to morphine

Morphine

Morphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...

. There have been no reported human fatalities.