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