Animal weapon

In biology, a weapon is a specialized physical trait that is used by animals to compete with other individuals for resources. Most commonly, the term refers to structures that males use to fight other males off for access to mates. They can also be used to defend resources in intraspecific competition, or to ward off predators. Examples of weapons include horns and antlers, both among the most recognizable weapons, though even within those categories, the structure of the specific weaponry is often unique to the species, with a wide variety of designs observed across many genera.

Many weapons evolve through sexual selection, as they are most often used to fight off competitors for access to mates. A mate is won in battle either by a male chasing off a fellow competitor or killing it, usually leaving the victor as the only option for the female to reproduce with, favoring males with particularly effective weaponry. More broadly, weaponry in animals may consist of any specialized morphology that is present within an organism to aid in its advantage against rivals. Many hypotheses have been produced by researchers to possibly explain the mechanisms behind the evolution of weapons, with studies detailing the intensity, duration, and conclusion of intraspecific combat, as well as analyzing the rapid diversification within species. 

Since Darwin's publication The Descent of Man, extensive research has been done on the presence of agonistic behavior and the usage of animal weaponry by different species. Weaponry displays in animals have been found to increase their likelihood of survival in different ways, such as when interacting with other individuals or trying to find another mate, or to defend against predators.        

Weapons are common among many genera of animals. Among vertebrates, they are most often found in mammals and fish, and are also known to occur in reptiles, though far less commonly in that class. Many species of dinosaur, an extinct clade of vertebrates, also possessed weapons. Arthropods, such as arachnids and crabs, also have species that wield weapons, and they are extraordinarily widespread among insects.

In mammals, weapons are common and take a number of diverse forms. They are most common among the ungulates. Antlers, complex and unique weapons that are an extension of an animal's skull, are found among male deers, as well as female reindeers, ungulates who are even-toed ruminants. Horns, permanent pointed projections consisting of a covering of keratin and other proteins surrounding a core of live bone, are found commonly among bovids, as well as the pronghorns, which are ruminant artiodactyls. Rhinoceroses are odd-toed ungulates which have horns made of keratin; both sexes use these horns in contests with other individuals of the same species. Walruses and elephants, non-ungulates, both possess elongated tusks. Apart from mammals, the only other group of land vertebrates that shows widespread adoption of weapons are the chameleons, who possess horn-like structures for fighting over access to mates.

Some species of fish have weapons, though these traits are not as widespread as in mammals. Sawfish are named for their long rostrum, which can be used to inflict damage on other fish. Unicornfish may also use their strange horn-like forehead protection as a weapon, although the general use of this and many similar structures in fish are still somewhat enigmatic. Male salmon notably feature intraspecific competition for mates, and they use their elongated and toothy jaw to fight other males, both over access to females and over access to breeding sites.

Weapons are very widespread among insects, having been observed in nearly every major taxonomic group. Insects possess a huge variety of weapons, often with entirely different uses and modes of action. Rhinoceros beetles, like the mammal they are named for, have large horns which they famously use to fight for mates. Stag beetles have enlarged mandibles, which resemble the antlers of stags. Male stag beetles use these weapons to wrestle each other for favoured mating sites in a way that parallels the way stags fight over females. Fights may also be over food, such as tree sap and decaying fruits. Harlequin beetles have legs that are longer than their entire body, using which they protect a suitable spot for a female to lay eggs. They also possess strong mandibles to bite other males, which can include removing their legs. Camel crickets use spines on their tibias for two reasons; (1) to fight other males for access to females, and (2) to pin mates as a form of coercion. Among arthropods that are not insects, male fiddler crabs have large claws that comprise half their body mass and are used to attract mates elaborate waving displays with the claw. They are also, though somewhat less commonly, used as a weapon to directly attack other males.

Among various birds, spurs are a common weapon for defending against threats and fighting others of the same species. Spurs also occur in other animals, such as platypuses, which is also venomous.