Weapon (biology)

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The impala uses horns to compete for resources. Male impala profile.jpg
The impala uses horns to compete for resources.

In biology, a weapon is a specialized physical trait that is used by animals to compete with other individuals for resources. [1] Most commonly, the term refers to structures that males use to fight other males off for access to mates. [2] 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. [3]

Contents

Many weapons evolve through sexual selection, as they are most often used to fight off competitors for access to mates. [3] 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. [4] More broadly, weaponry in animals may consist of any specialized morphology that is present within an organism to aid in its advantage against rivals. [1] 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. [5]  

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. [1] 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. [2]        

Occurrence

The horn-like projections of a chameleon are used to fight other males on tree trunks. Male Long-nosed Chameleon (Calumma gallus), Vohimana reserve, Madagascar.jpg
The horn-like projections of a chameleon are used to fight other males on tree trunks.

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. [5]

In mammals, weapons are common and take a number of diverse forms. They are most common among the ungulates. [5] Antlers, complex and unique weapons that are an extension of an animal's skull, are found only among male deer, ungulates who are even-toed ruminants. [6] Rhinoceroses are even-toed ungulates which have horns made of keratin; both sexes use these horns in contests with other individuals of the same species. [7] 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. [5]

The horns of a rhinoceros beetle are an example of one of the many weapons insects possess. Kabutomushi-JapaneseBeetle-July2004.jpg
The horns of a rhinoceros beetle are an example of one of the many weapons insects possess.

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. [8] 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. [5] 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. [5] [9] [10]

Weapons are very widespread among insects, having been observed in nearly every major taxonomic group. [5] 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. [11] [12] 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. [13] 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. [2] [14] 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. [15] They are also, though somewhat less commonly, used as a weapon to directly attack other males. [16] [5]

Evolution

The horns that give Triceratops its name were used as weapons. Triceratops Hendrickx2.jpg
The horns that give Triceratops its name were used as weapons.

Animal weaponry is capable of drastic and rapid diversification in form, with closely related species, even within the same clade, having distinctly different weaponry. [5] One hypothesized mechanism for this is the gradual change in weaponry function from purely physical agonistic behavior, to a stronger emphasis on display, resulting in potentially more elaborate weapons. For instance, many dinosaurs may have had structures that were once weapons but were later used as ornaments to attract mates. [5] Another possible mechanism is the presence of male-male combat, with diverging lineages of animals expanding into different habitats and then fighting under different conditions. Changes in the physical contest between species in different environments may potentially drive the evolution in modified weaponry. Sites that have localized, defendable resources, like a single food or water source, or a breeding burrow, often are where species with weapons are found. [5]

Sexual selection has been a main focus on weaponry and antagonist interactions between animals, with males that present the largest weapons having the highest probability of winning. [17] Sexual selection has been credited by previous researchers as the main influence of nature's extravagant weaponry for the purpose of attractive females. [5] During agonistic interactions, there is the potential risk of weaponry lost resulting in a possible decrease in an individual's fighting ability and overall fitness. [17]

A trilobite of the genus Ceratarges, showing large spines probably used as weapons. WLA hmns Trilobite Ceratarges sp.jpg
A trilobite of the genus Ceratarges , showing large spines probably used as weapons.

Most animal weapons probably emerged independently. The weapons of ungulates, for instance, are believed to have evolved independently during the Age of Mammals, and are not from an earlier, basal clade of mammals. One group that had weapons early in the history of multicellular life were the trilobites, one of the earliest arthropods, and a dominant life form in the Paleozoic. [18] [5] Many species had horns or spikes which are theorized to have been used in intraspecific combat. [5] Many dinosaurs also had weapons such as spines, spikes, and plates, although the exact use of these is not known for all species—some may have been used for mating displays more than as weapons. [5]

An exception to the general independence of weapon evolution is found in stag beetles, whose common ancestor is believed to have had mandibles, much like modern stag beetles. However, that species likely had little or no sexual dimorphism in the mandible phenotype, unlike modern stag beetles, in which males usually have markedly larger mandibles than females. [19] [5] Dung beetles, on the other hand, have evolved and lost their weapons many, many times over their history, [5] with the single genus Onthophagus , whose ancestor likely had horns, undergoing at least ten evolutionary events where horns were gained or lost. [20]

Usage & Characteristics

Agonistic behaviors amongst animals for resources have been studied by many researchers, and specifically the interaction of weapons during these exchanges of behavior. [1] Not only the presence of weaponry, but also specific characteristics of the weaponry itself can have an effect on the outcome of competition in determining the winners and losers from an intraspecies competition. [5] The display of weaponry has been found to be favored in animals that frequently engage in contest as a mechanism to decrease the costs of aggression. [5]

A fiddler crab uses its large claw as both a display ornament for females, and a weapon to directly compete with other males. Uca leptodactyla in Margarita Island.jpg
A fiddler crab uses its large claw as both a display ornament for females, and a weapon to directly compete with other males.

Most weapons that are studied are utilize to injure other individuals. These include the most famous weapons, such as antlers, horns, and mandibles; conversely, some animals have specialized "weapons" that actually fulfill a defensive role, such as the horned weevil, which have sheaths that can neutralize the horns of other beetles. [2] [21] Some of these weapons can be used to inject chemicals into enemies; tentacles and stingers are examples of this phenomenon. [2] Other weapons are used to displace their opponents, [1] usually to block burrows or entrances from other individuals. Some termites, for instance, use their heads as plugs to physically impede other termites from invading through the colony entrance. [2] These termites also use their mandibles to displace enemies. [22] Finally, many weapons are used for displays, with males using their large physical features to attract females to mate with. [2] [5] Additionally, most organisms display their weapons before fighting as a threat to other individuals. [1] In general, displays are considered any behavior that is used to show a species' fighting ability without any physical contact, and the term applies to signalling both potential mates and potential opponents. This behavior allows for rivals to have the chance to assess the weaponry that is present to determine whether to engage in physical agonistic behavior or not. [1]

Female Weaponry

Since the publication of The Descent of Man and The Origin of Species, research has been heavily focused on weaponry in male animals, potentially leaving out females and their possible mechanisms behind present weaponry. [23] Presence of weapons in females, and female-female competition has been seen in many species for better fitness of both sexual and social selection. [24] [23] Social selection, first hypothesized by Mary Jane West-Eberhard, is a broader term that includes both sexual and non sexual behaviors to increase an individual's fitness. [23] For example, research has suggested that horns present in female bovids, may have evolved from competition for resources such as food. [24]  

 


Related Research Articles

<span class="mw-page-title-main">Sexual selection</span> Mode of natural selection involving the choosing of and competition for mates

Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with, and compete with members of the same sex for access to members of the opposite sex. These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.

<span class="mw-page-title-main">Ungulate</span> Group of animals that walk on the tips of their toes or hooves

Ungulates are members of the diverse clade Euungulata, which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.

<span class="mw-page-title-main">Artiodactyl</span> Order of mammals

Artiodactyls are placental mammals belonging to the order Artiodactyla. Typically, they are ungulates which bear weight equally on two of their five toes: the third and fourth, often in the form of a hoof. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, most perissodactyls bear weight on an odd number of the five toes. Another difference between the two is that many artiodactyls digest plant cellulose in one or more stomach chambers rather than in their intestine as perissodactyls do. The advent of molecular biology, along with new fossil discoveries, found that cetaceans fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla to this group, while others opt to include cetaceans within the existing name of Artiodactyla. Some researchers use "even-toed ungulates" to exclude cetaceans and only include terrestrial artiodactyls, making the term paraphyletic in nature.

<span class="mw-page-title-main">Antler</span> Extensions of the skull found in animals of the family Cervidae (deer)

Antlers are extensions of an animal's skull found in members of the Cervidae (deer) family. Antlers are a single structure composed of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They are generally found only on males, with the exception of reindeer/caribou. Antlers are shed and regrown each year and function primarily as objects of sexual attraction and as weapons.

<span class="mw-page-title-main">Stag beetle</span> Family of insects

Stag beetles are a family of about 1,200 species of beetles in the family Lucanidae, currently classified in four subfamilies. Some species grow to over 12 centimetres, but most to about 5 cm (2 in).

<span class="mw-page-title-main">Sexual dimorphism</span> Condition where males and females exhibit different characteristics

Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consist of most animals and some plants. Differences may include secondary sex characteristics, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection. These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.

<span class="mw-page-title-main">Bovidae</span> Family of mammals belonging to even-toed ungulates

The Bovidae comprise the biological family of cloven-hoofed, ruminant mammals that includes cattle, yaks, bison, buffalo, antelopes, sheep and goats. A member of this family is called a bovid. With 143 extant species and 300 known extinct species, the family Bovidae consists of 11 major subfamilies and thirteen major tribes. The family evolved 20 million years ago, in the early Miocene.

<span class="mw-page-title-main">Lek mating</span> Type of animal mating behaviour

A lek is an aggregation of male animals gathered to engage in competitive displays and courtship rituals, known as lekking, to entice visiting females which are surveying prospective partners with which to mate. It can also refer to a space used by displaying males to defend their own share of territory for the breeding season. A lekking species is characterised by male displays, strong female mate choice, and the conferring of indirect benefits to males and reduced costs to females. Although most prevalent among birds such as black grouse, lekking is also found in a wide range of vertebrates including some bony fish, amphibians, reptiles, mammals, and arthropods including crustaceans and insects.

<span class="mw-page-title-main">Horn (anatomy)</span> Animal anatomy of hornlike growths

A horn is a permanent pointed projection on the head of various animals that consists of a covering of keratin and other proteins surrounding a core of live bone. Horns are distinct from antlers, which are not permanent. In mammals, true horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae (pronghorn) and Bovidae. Cattle horns arise from subcutaneous connective tissue and later fuse to the underlying frontal bone.

<span class="mw-page-title-main">Hercules beetle</span> Species of beetle

The Hercules beetle is a species of rhinoceros beetle native to the rainforests of southern Mexico, Central America, South America, and the Lesser Antilles. It is the longest extant species of beetle in the world, and is also one of the largest flying insects in the world.

<span class="mw-page-title-main">Japanese rhinoceros beetle</span> Species of beetle

Allomyrina dichotoma, also known as the Japanese rhinoceros beetle or Japanese horned beetle, is a species of rhinoceros beetle. They are commonly found in continental Asia in countries such as China, the Korean peninsula, Japan, and Taiwan. In these areas, this species of beetle is often found in broad-leaved forests with tropical or sub-tropical climates. This beetle is well known for the prominent cephalic horn found on males. Male Japanese rhinoceros beetles will use this horn to fight other males for territory and access to female mating partners. Upon contact, males will attempt to flip each other onto their backs or off of their feeding tree. In response to selective pressures, smaller male A. dichotoma have adapted a "sneak-like behavior". These smaller beetles will attempt to avoid physical confrontation with larger males and try to mate with females.

Monogamous pairing in animals refers to the natural history of mating systems in which species pair bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.

<i>Cervus</i> Genus of deer and elk

Cervus is a genus of deer that primarily are native to Eurasia, although one species occurs in northern Africa and another in North America. In addition to the species presently placed in this genus, it has included a whole range of other species now commonly placed in other genera. Additionally, the species-level taxonomy is in a state of flux.

Sex-limited genes are genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and have no penetrance, or are simply 'turned off' in the other. In other words, sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. This term is restricted to autosomal traits, and should not be confused with sex-linked characteristics, which have to do with genetic differences on the sex chromosomes. Sex-limited genes are also distinguished from sex-influenced genes, where the same gene will show differential expression in each sex. Sex-influenced genes commonly show a dominant/recessive relationship, where the same gene will have a dominant effect in one sex and a recessive effect in the other. However, the resulting phenotypes caused by sex-limited genes are present in only one sex and can be seen prominently in various species that typically show high sexual dimorphism.

<span class="mw-page-title-main">Courtship display</span> Communication to start a relationship with someone or to get sexual contact

A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), vocalizations, mechanical sound production, or displays of beauty, strength, or agonistic ability.

A biological ornament is a characteristic of an animal that appears to serve a decorative function rather than a utilitarian function. Many are secondary sexual characteristics, and others appear on young birds during the period when they are dependent on being fed by their parents. Ornaments are used in displays to attract mates, which may lead to the evolutionary process known as sexual selection. An animal may shake, lengthen, or spread out its ornament in order to get the attention of the opposite sex, which will in turn choose the most attractive one with which to mate. Ornaments are most often observed in males, and choosing an extravagantly ornamented male benefits females as the genes that produce the ornament will be passed on to her offspring, increasing their own reproductive fitness. As Ronald Fisher noted, the male offspring will inherit the ornament while the female offspring will inherit the preference for said ornament, which can lead to a positive feedback loop known as a Fisherian runaway. These structures serve as cues to animal sexual behaviour, that is, they are sensory signals that affect mating responses. Therefore, ornamental traits are often selected by mate choice.

<i>Onthophagus taurus</i> Species of beetle

Onthophagus taurus, the taurus scarab, is a species of dung beetle in the genus Onthophagus and the family Scarabaeidae. Also known as the bull-headed dung beetle, it is a species that specializes in cattle dung and is widely utilized to maintain clean pastures, making it agriculturally valuable. These beetles are typically 8–10 millimetres (0.31–0.39 in) in size. The males of this species exhibit distinct characteristics: large “major” males possess long, sweeping, curved horns resembling those of a longhorn bull, while small “minor” males have tiny horns that project upward from the back of their heads. Females, on the other hand, lack horns. These small beetles are oval shaped, the color is usually black or reddish brown. Sometimes the pronotum has a weak metallic sheen.

<span class="mw-page-title-main">Sexual selection in mammals</span> Mode of natural selection

Sexual selection in mammals is a process the study of which started with Charles Darwin's observations concerning sexual selection, including sexual selection in humans, and in other mammals, consisting of male–male competition and mate choice that mold the development of future phenotypes in a population for a given species.

<span class="mw-page-title-main">Sexual selection in insects</span>

Sexual selection in insects is about how sexual selection functions in insects. The males of some species have evolved exaggerated adornments and mechanisms for self-defense. These traits play a role in increasing male reproductive expectations by triggering male-male competition or influencing the female mate choice, and can be thought of as functioning on three different levels: individuals, colonies, and populations within an area.

<i>Gnatocerus cornutus</i> Species of beetle

Gnatocerus cornutus, or the broadhorned flour beetle, is a common species of beetle in the Tenebrionidae family. It is distributed worldwide and is commonly found in warm, tropical regions, preferring grains, yeasts, and flours as its main diet. The total development time is about 47 days. G. cornutus is predominately found in mills as a storage pest and has a wide distribution.

References

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