Venomous mammals are animals of the class Mammalia that produce venom, which they use to kill or disable prey, to defend themselves from predators or conspecifics or in agonistic encounters. Mammalian venoms form a heterogeneous group with different compositions and modes of action, from three orders of mammals: Eulipotyphla, Monotremata, and Chiroptera. It has been proposed that some members of a fourth order, Primates, are venomous. To explain the rarity of venom delivery in Mammalia, Mark Dufton of the University of Strathclyde has suggested that modern mammalian predators do not need venom because they are able to kill quickly with their teeth or claws, whereas venom, no matter how sophisticated, requires time to disable prey.
In spite of the rarity of venom among extant mammals, venom may be an ancestral feature among mammals, as venomous spurs akin to those of the modern platypus are found in most non-therian Mammaliaformes groups.
Venom is much more common among other vertebrates; there are many more species of venomous reptiles (e.g. venomous snakes) and fish (e.g. stonefish). Some birds are poisonous to eat or touch (e.g. hooded pitohui) though no bird species is known to be venomous. There are only a few species of venomous amphibians; certain salamandrid salamanders can extrude sharp venom-tipped ribs.
Definitions
Several definitions of venomous animals have been proposed.
Bücherl states that venomous animals must possess at least one venom gland, a mechanism for excretion or extrusion of the venom, and apparatus with which to inflict wounds.
Mebs writes that venomous animals produce venom in a group of cells or gland, and have a tool, the venom apparatus, which delivers the venom by injection during a bite or sting. The venom apparatus in this definition encompasses both the gland and the injection device, which must be directly connected.
Fry et al. found that a venom is a secretion produced in a specialized gland in one animal and delivered to a target animal through the infliction of a wound. This secretion must contain molecules that disrupt normal physiological processes so as to facilitate feeding or defense by the producing animal. Additionally, the feeding secretion of hematophagous specialists (e.g. vampire bats) may be regarded as a specialized subtype of venom.
Evolutionary history and paleontology
Venomous mammals may have been more common in the past. Most non-therian mammals possess tarsal spurs akin to those of the modern platypus, suggesting that this feature was very widespread, with gobiconodontids and Zhangheotherium being among the clearer examples. The absence of venom spurs in non-Mammaliaformes cynodonts suggests that venom was an ancient mammalian synapomorphy and ancestral characteristic.
Canine teeth dated at 60 million years old from two extinct species, the shrew-like Bisonalveus browni and another unidentified mammal, show grooves that some palaeontologists have argued are indicative of a venomous bite. However, other scientists have questioned this conclusion given that many living non-venomous mammals also have deep grooves down the length of their canines (e.g., many primates, coatis and fruit bats), suggesting that this feature does not always reflect an adaptation to venom delivery.
Examples
Eulipotyphla (previously known as insectivores) With the exception of vampire bats, the Eulipotyphla are the only mammals so far observed to produce toxic saliva. These species have significantly enlarged and granular submaxillary salivary glands from which the toxic saliva is produced.
The Cuban solenodon (Atopogale cubana) and Hispaniolan solenodon (Solenodon paradoxus) look similar to large shrews. They both have venomous bites; the venom is delivered from modified salivary glands via grooves in their second lower incisors. Recent study has identified the gene regulatory network responsible for the development of venom delivery systems in these small mammals. Due to the overexpression of kallikreins in their saliva, solenodon bites cause vasodilation and may result in circulatory shock. It was reported that death was frequent among Hispaniolan solenodons kept together in the same enclosure, with bite marks on their feet being the only observable cause. Such use in competition may be a secondary aspect of the insectivore venom.
