Venomous mammals are mammals 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 four orders of mammals: Eulipotyphla, Monotremata, Primates, and Chiroptera. 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. Extratarsal spurs observed in extinct Mesozoic mammals are homologous with those seen in extant monotremes, and this feature was widespread. Whether or not these extratarsal spurs served as a venom delivery system is up for debate. While species such as Gobiconodon and Zhangeotherium possessed an extratarsal spur, they are hypothesized to be nonvenomous due to the lack of the spur being hollow or channeled, as seen in modern venomous Ornithorhynchus. Whether or not the lack of venom is due to the loss of a basal trait is difficult to determine with current fossil records. In 2005, Fox and Scott presented evidence of a venom delivery system in late Paleocene fossils found in Alberta. Bisonalveus browni was a small eutherian in the order Cimolesta. Fox and Scott argue that the anterior grooves of the upper canines in B. browni serve as venom channels. They also present a group of isolated lower mammalian teeth of unknown species that also feature these grooves. These specimens were reexamined and rejected on the basis that the evidence provided may not be enough to prove the presence of venom in these species. Fox and Scott found no fossa that would be indicative of the presence of a venom gland. Also, grooved teeth are a feature of many species of nonvenomous mammals, so the presence of grooved teeth alone is not indicative of venom. There is no phylogenetic support for the presence of venom in B. browni. The genus Beremendia may have featured individuals with a venom delivery system. The first lower incisor of several species exhibited a groove that may have served as a canal for venom to travel from a venom gland to the tip of the tooth. Support for a venom gland is seen in the fossa in the mandibular symphysis. Also, Beremendia is phylogenetically close to Solenodon, which has extant members that are venomous. Euchambersia is an example of a fossil therapsid (a relative of mammals) with grooved teeth, and maxillary fossa to match, indicating the presence of venom glands. Euchambersia differs in that the upper canines are the teeth that are grooved, and the grooves are almost enclosed, forming a tube. More recently, nonhomologous species in the genus Nesophontes were hypothesized to be venomous. Species within Nesophontes exhibit grooved upper canines, similar to other species suspected of venom delivery. Where Nesophontes differ, however, is in the presence of grooves on the first and second upper premolars. The grooves are similar in morphology to helodermatid lizards, which feature a deep anterior groove and shallow posterior groove, as well as grooves on other teeth, which suggests that Nesophontes grooved premolars also aided in delivery of venom in addition to the upper canines. Nesophontes mandibular morphology is similar to that of existing venomous Eulipotyphlans, which further supports the hypothesis that they possessed venom.
Examples
Eulipotyphla (previously known as insectivores) Vampire bats and 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.
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