Venom is a secretion containing one or more toxins produced by an animal.[1] Venom has evolved in a wide variety of animals, both predators and prey, and both vertebrates and invertebrates.

Venoms kill through the action of at least four major classes of toxin, namely necrotoxins and cytotoxins, which kill cells; neurotoxins, which affect nervous systems; and myotoxins, which damage muscles. Biologically, venom is distinguished from poison in that poisons are ingested, while venom is delivered in a bite, sting, or similar action. Venomous animals cause tens of thousands of human deaths per year. However, the toxins in many venoms have potential to treat a wide range of diseases.

The use of venom across a wide variety of taxa is an example of convergent evolution. It is difficult to conclude exactly how this trait came to be so intensely widespread and diversified. The multigene families that encode the toxins of venomous animals are actively selected, creating more diverse toxins with specific functions. Venoms adapt to their environment and victims and accordingly evolve to become maximally efficient on a predator's particular prey (particularly the precise ion channels within the prey). Consequently, venoms become specialized to an animal's standard diet.[2]

Venoms cause their biological effects via the toxins that they contain; some venoms are complex mixtures of toxins of differing types. Among the major classes of toxin in venoms are:[3]

Venom is widely distributed taxonomically, being found in both invertebrates and vertebrates; in aquatic and terrestrial animals; and among both predators and prey. The major groups of venomous animals are described below.

Venomous arthropods include spiders, which use fangs — part of their chelicerae — to inject venom; and centipedes, which use forcipules — modified legs — to deliver venom; along with scorpions and stinging insects, which inject venom with a sting.

In insects such as bees and wasps, the stinger is a modified egg-laying device — the ovipositor. In Polistes fuscatus, the female continuously releases a venom that contains a sex pheromone that induces copulatory behavior in males.[12] In Polistes exclamans, venom is used as an alarm pheromone, coordinating a response with from the nest and attracting nearby wasps to attack the predator.[13] In Dolichovespula arenaria, the observed spraying of venom out of their sting has been seen from workers in large colonies.[14] In other cases like Parischnogaster striatula, the venom is applied all over their body as an antimicrobial protection.[15] The venom from Agelaia pallipes has inhibitory effects on processes like chemotaxis and hemolysis which can lead to organ failure.[16]

Many caterpillars have defensive venom glands associated with specialized bristles on the body, known as urticating hairs, which can be lethal to humans (e.g., that of the Lonomia moth), although the venom's strength varies depending on the species.[17]

Bees synthesize and employ an acidic venom (apitoxin) to cause pain in those that they sting to defend their hives and food stores, whereas wasps use a chemically different alkaline venom designed to paralyze prey, so it can be stored alive in the food chambers of their young. The use of venom is much more widespread than just these examples. Other insects, such as true bugs and many ants, also produce venom.[18] At least one ant species (Polyrhachis dives) has been shown to use venom topically for the sterilisation of pathogens.[19]

There are venomous invertebrates in several phyla, including jellyfish such as the dangerous box jellyfish[20] and sea anemones among the Cnidaria,[21] sea urchins among the Echinodermata,[22] and cone snails[23] and cephalopods including octopuses among the Molluscs.[24]

Venom is found in some 200 cartilaginous fishes, including stingrays, sharks, and chimaeras; the catfishes (about 1000 venomous species); and 11 clades of spiny-rayed fishes (Acanthomorpha), containing the scorpionfishes (over 300 species), stonefishes (over 80 species), gurnard perches, blennies, rabbitfishes, surgeonfishes, some velvetfishes, some toadfishes, coral crouchers, red velvetfishes, scats, rockfishes, deepwater scorpionfishes, waspfishes, weevers, and stargazers.[25]

Among amphibians, some salamanders can extrude sharp venom-tipped ribs.[26][27]

Some 450 species of snake are venomous.[25] Snake venom is produced by glands below the eye (the mandibular gland) and delivered to the victim through tubular or channeled fangs. Snake venoms contain a variety of peptide toxins, including proteases, which hydrolyze protein peptide bonds, nucleases, which hydrolyze the phosphodiester bonds of DNA, and neurotoxins, which disable signalling in the nervous system.[28] Snake venom causes symptoms including pain, swelling, tissue necrosis, low blood pressure, convulsions, hemorrhage (varying by species of snake), respiratory paralysis, kidney failure, coma and death.[29] Snake venom may have originated with duplication of genes that had been expressed in the salivary glands of ancestors.[30][31]

Venom is found in a few other reptiles such as the Mexican beaded lizard,[32] the gila monster,[33] and some monitor lizards including the Komodo dragon.[34] Mass spectrometry showed that the mixture of proteins present in their venom is as complex as the mixture of proteins found in snake venom.[34][35] Some lizards possess a venom gland; they form a hypothetical clade, Toxicofera, containing the suborders Serpentes and Iguania and the families Varanidae, Anguidae, and Helodermatidae.[36]

Euchambersia, an extinct genus of therocephalians, is hypothesized to have had venom glands attached to its canine teeth.[37]

A few species of living mammals are venomous, including solenodons, shrews, vampire bats, the male platypus and the slow loris.[25][38] Shrews are known to have venomous saliva and most likely evolved their trait similarly to snakes.[39] The presence of tarsal spurs akin to those of the platypus in many non-therian Mammaliaformes groups suggests that venom was an ancestral characteristic among mammals.[40]

Extensive research on platypuses shows that their toxin was initially formed from gene duplication, but data provides evidence that the further evolution of platypus venom does not rely as much on gene duplication as once was thought.[41] Modified sweat glands are what evolved into platypus venom glands. Although it is proven that reptile and platypus venom have independently evolved, it is thought that there are certain protein structures that are favored to evolve into toxic molecules. This provides more evidence as to why venom has become a homoplastic trait and why very different animals have convergently evolved.[42]

Venomous animals resulted in 57,000 human deaths in 2013, down from 76,000 deaths in 1990.[43]

Venoms, found in over 173,000 species, have potential to treat a wide range of diseases, explored in over 5,000 scientific papers.[33] Snake venoms contain proteins which can be used to treat conditions including thrombosis, arthritis, and some cancers.[44][45] Gila monster venom contains exenatide, used to treat type 2 diabetes.[33]

Solenopsins extracted from fire ant venom has demonstrated biomedical applications, ranging from cancer treatment to psoriasis.[46][47]

Venom is utilized as a trophic weapon by multiple predator species. The coevolution between predators and prey is a driving force of venom resistance, which has evolved multiple times throughout the animal kingdom. Repeated interactions between two species can generate coevolution.[48] The coevolution between venomous predators and venom resistant prey is best described as a chemical arms race.[49] Predator and prey pairs are expected to associate with one another for stable periods of time.[50] Venom is used as a chemical weapon by predator species. As the predator capitalizes on susceptible individuals, the surviving individuals are limited to those who are able to evade predation.[51] Resistance phenotypes typically increase over time as the predator becomes increasingly unable to subdue prey that have developed this new resistance phenotype.[52]

The cost of developing a venom resistance is high, for predator and prey.[53] Developing an entire physiological resistance is extremely costly, however it maximizes chances of survival for prey species and allows predator species to expand into underutilized trophic niches. If it is possible for an animal to evade predation through something less costly like a behavioral modification, the development of a physiological modification becomes unnecessary.[54]

Pen-tailed treeshrews are the only known mammals that consume alcohol every night, other than humans. According to a study of treeshrews in Malaysia, they spend several hours per night consuming the equivalent of 10 to 12 glasses of wine with an alcohol content up to 3.8% drinking naturally fermented nectar of the bertam palm. This nectar contains one of the highest alcohol concentrations of all natural foods. Pen-tailed treeshrews frequently consume large amounts of this nectar while showing no signs of intoxication. Measurements of a biomarker of ethanol breakdown suggest that they may be metabolizing it by a pathway that is not used as heavily by humans. Their ability to ingest high amounts of alcohol is hypothesized to have been an evolutionary adaptation in the phylogenic tree. However, how pen-tailed treeshrews benefit from this alcohol ingestion or what consequences of consistent high blood alcohol content might factor into their physiology is unclear.[55]

One of the most heavily researched cases of venom resistance is the California Ground Squirrel, which is resistant to the venom of the Northern Pacific Rattlesnake. The predator-prey pair have coexisted for generations. The repeated interactions fostered the development of an anti-snake venom defense in the California ground squirrels. Researchers found evidence supporting the theory that venom resistance is driven by coevolution among these populations of California ground squirrels.[56] They use toxin scavenging to negate the effects of the haemolytic toxins of their rattlesnake predators, demonstrating a physiological resistance to rattlesnake venom. Resistance in these ground squirrels is population dependent. In areas where rattlesnake populations are very dense, there is a significant increase in resistance in the squirrels compared to populations where rattlesnakes are rare.[57] Rattlesnakes demonstrated local adaptions in the effectiveness of their venom in order to overcome the venom resistant squirrels.[58]

There is an ongoing evolutionary contest between toxicity and resistance. The resistance of eels to sea snake venom is a good example of coevolution between predator-prey pairs. Sea snake venom is composed of complex mixtures of neurotoxins, myotoxins, nephrotoxins, and other nontoxic substances.[59] The composition of the sea snake venom is species specific. The biggest piece of evidence for this as a case of coevolution is that eels that are favored by sea snakes as prey have unusually high tolerances to the venom of the sea snake.[60] Studies have analyzed the resistance of four species of eels to two different sea snakes: one is a dietary generalist and the other is an eel specialist.[61] The eels were more resistant to the venom of the eel specialist sea snake. Non-prey fishes exhibited very low levels of resistance to the sea snake venom, further supporting coevolution.[62] The genetic mechanisms allowing clownfish to interact with sea anemones is still somewhat unclear.[63] Only 10 known species of anemones are hosts to clownfish and only certain pairs of anemones and clownfish are compatible with one another.[64][65]

All sea anemones produce venoms delivered through discharging nematocysts and mucous secretions. The toxins are composed of peptides and proteins. They are used for prey acquisition and to deter predators by causing pain, loss of muscular coordination and tissue damage. Clownfish have a protective mucous that acts as a chemical camouflage or macromolecular mimicry preventing “not self” recognition by the sea anemone and nematocyst discharge.[66] The sea anemone perceives the fish as its “self” most likely by the same mechanism which prevents the discharge of nematocysts when its tentacles come into contact with each other.[67] Clownfish exhibit either strict host specificity or are environmental niche generalists and can associate with a variety of sea anemone species.[68] In some species, the mucus of clownfish was found to change during acclimation to resemble that of the specific species of sea anemone.[68] For the Clownfish, a relationship with the sea anemone is an obligatory one. In some cases, it is an obligatory relationship for the anemone as well. In all cases, the interaction between the two is mutually beneficial. Clownfish and sea anemones are one of the most compelling cases of symbiosis. The relationship provides mutual protection from predators and the exchange of nutrients. Divergent natural selection drives adaptive diversification through ecological speciation. The ability of clownfish to use sea anemones as hosts has evolved 4 independent times.  The obligate relationship between the clownfish and the sea anemone has allowed the radiation of clownfishes.[69]

Inhabiting the Americas from southeastern Canada to southern Ecuador, Kingsnakes, of genus Lampropeltis, are constrictors that prey on many venomous snakes.[70] In order to prey upon the venomous snakes, Kingsnakes have actually evolved a resistance rather than incrementally increasing their resistance to a point of immunity, like many species have. Kingsnakes resistance levels are currently known to be fixed for the duration of its life and have not been found to change with age or exposure. It is thought, that Kingsnakes have developed this evolutionary adaptation through a process called a co-evolutionary arms race with natural selection at the forefront.[71] Kingsnake predators that were slightly better able to tolerate the effects of the venom were more likely to survive with resulting genocide of the Kingsnakes that were inherently not sufficiently resistant. While simultaneously, the venomous snakes with more potent venom were more likely to survive the predatory nature of the Kingsnakes, thus escalating the arms race.

However, the nature of the arms race has put a stipulation onto the Kingsnakes. Kingsnakes have evolved resistance only to the venom of snakes that are in their immediate environment, like copperheads, cottonmouths, and North American rattlesnakes, but not to the venom of, for example, king cobras or black mambas. Even through geographical boundaries, Kingsnake venom resistance has varied between species. They found that blood from Eastern Kingsnakes (Lampropeltis getula) had the widest spectrum of protection against the venoms tested and was the most effective at neutralizing many rattlesnake venoms, but the least effective against copperhead venom. Blood from kingsnakes from Florida & the Gulf Coast was the most effective at neutralizing the venom of copperheads & cottonmouths. Mole Kingsnake (Lampropeltis calligaster) blood is about 75% as effective at neutralizing Mojave Rattlesnake (Crotalus scutulatus) venom as the blood of Eastern Kingsnakes. Gray-banded Kingsnakes (L. alterna) have moderate neutralization potential against Western Diamondback (C. atrox) venom, but none against Eastern Diamondback (C. adamanteus) venom.[72]