A herd is a social group of certain animals of the same species, either wild or domestic . The form of collective animal behavior associated with this is called herding . These animals are known as gregarious animals.
75-412: The term herd is generally applied to mammals, and most particularly to the grazing ungulates that classically display this behaviour. Different terms are used for similar groupings in other species; in the case of birds, for example, the word is flocking , but flock may also be used for mammals, particularly sheep or goats . Large groups of carnivores are usually called packs , and in nature
150-681: 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
225-483: A better chance of not being closest to, and thus vulnerable to attack by, the water snake if he was between other frogs. As a result, modeled frogs jumped to smaller gaps between neighboring frogs. This simple example was based on what Hamilton identified as each frog's domain of danger, the area of ground in which any point was nearer to that individual than it was to any other individual. The model assumed that frogs were attacked from random points and that if an attack
300-552: A clade that is closely related to the perissodactyls. The desmostylians were large amphibious quadrupeds with massive limbs and a short tail. They grew to 1.8 metres (6 ft) in length and were thought to have weighed more than 200 kilograms (440 lb). Their fossils were known from the northern Pacific Rim , from southern Japan through Russia , the Aleutian Islands and the Pacific coast of North America to
375-406: A coordinated group where individuals have distinct roles. Many human groupings, such as army detachments or sports teams, show such coordination and differentiation of roles, but so do some animal groupings such as those of eusocial insects, which are coordinated through pheromones and other forms of animal communication. A herd is, by definition, relatively unstructured. However, there may be two or
450-408: A few animals which tend to be imitated by the bulk of the herd more than others. An animal in this role is called a "control animal", since its behaviour will predict that of the herd as a whole. It cannot be assumed, however, that the control animal is deliberately taking a leadership role; control animals are not necessarily socially dominant in conflict situations, though they often are. Group size
525-463: A group of Spinner dolphins is able to corral fish into a smaller volume, which makes catching them easier, as there is less opportunity for the fish to escape. Furthermore, large groups are able to monopolize resources and defend them against solitary animals or smaller groups of the same or different species. It has been shown that larger groups of lions tend to be more successful in protecting prey from hyenas than smaller ones. Being able to communicate
600-682: A herd is classically subject to predation from pack hunters . Special collective nouns may be used for particular taxa (for example a flock of geese, if not in flight, is sometimes called a gaggle ) but for theoretical discussions of behavioural ecology , the generic term herd can be used for all such kinds of assemblage. The word herd , as a noun, can also refer to one who controls, possesses and has care for such groups of animals when they are domesticated. Examples of herds in this sense include shepherds (who tend to sheep), goatherds (who tend to goats), and cowherds (who tend to cattle). When an association of animals (or, by extension, people)
675-452: A higher survival than those who did not. Furthermore, individuals must have benefited from such movements more often than they were harmed (i.e. forced onto the periphery and attacked). This idea has, in fact, gained support. A study conducted by Reluga and Viscido found that natural selection of localized movement rules of members within a population could, in fact, promote the evolution of the selfish herd. Further, it has been shown that how
750-423: A plausible movement rule should be statistically likely to benefit its followers, should be likely to fit the capabilities of an animal, and should result in a compact aggregation with desired central movement. Identified movement rules include: Research has revealed a variety of factors that may influence chosen movement rules. These factors include initial spatial position, population density, attack strategy of
825-418: A population must have been selected. Because such rules are often complex, it is unlikely that they would have evolved in a single step. Rather, simple rules that considered solely the nearest neighbor in guiding movement may have given rise to the evolution of more complicated rules. This proposed succession would only occur if individuals who moved toward their nearest neighbor in the face of predation showed
SECTION 10
#1732776191677900-620: A semi-technical usage in behavioral finance to describe the largest group of market investors or market speculators who tend to "move with the market", or "follow the general market trend". This is at least a plausible example of genuine herding, though according to some researchers it results from rational decisions through processes such as information cascade and rational expectations . Other researchers, however, ascribe it to non-rational process such as mimicry , fear and greed contagion . "Contrarians" or contrarian investors are those who deliberately choose to invest or speculate counter to
975-680: A series of convex polygons surrounding each individual in which all points within the polygon are closer to that individual than to any other. Movements toward the center of an aggregation are based upon a variety of movement rules that range in complexity. Identifying these rules has been considered the "dilemma of the selfish herd". The main issue is that movement rules that are easy to follow are often unsuccessful in forming compact aggregations, and those that do form such aggregations are often considered too complex to be biologically relevant. Viscido, Miller, and Wethey identified three factors that govern good movement rules. According to such factors,
1050-438: A wide variety of taxa and thus, has likely evolved independently on several occasions. Dilution of predation risk is one of many proposed benefits that have facilitated the selection of such behavior. Much research has been devoted to understanding the possible evolution of the selfish herd and thus, the plausibility of the theory. In order for the selfish herd to have evolved, movement rules that decreased domains of danger within
1125-420: Is (scramble or direct) competition with other group members. In general, it is clear that the amount of resources available for each individual decreases with group size. If the resource availability is critical, competition within the group may get so intense, that animals no longer experience benefits from living in groups. However, only the relative importance of within- and between-group competition determines
1200-458: Is a cladistic (evolution-based) group, or merely a phenetic group ( form taxon ) or folk taxon (similar, but not necessarily related). Some studies have indeed found the mesaxonian ungulates and paraxonian ungulates to form a monophyletic lineage, closely related to either the Ferae (the carnivorans and the pangolins ) in the clade Fereuungulata or to the bats . Other studies found
1275-401: Is an important characteristic of the social environment of gregarious species. The reason why animals form herds can not always be stated easily, since the underlying mechanisms are diverse and complex. Understanding the social behaviour of animals and the formation of groups has been a fundamental goal in the field of sociobiology and behavioural ecology . Theoretical framework is focused on
1350-424: Is beneficial. Whenever groups, such as shoals of fish, synchronize their movements, it becomes harder for predators to focus on particular individuals. However, animals that are weak and slower or on the periphery are preferred by predators, so that certain positions within the group are better than others (see selfish herd theory ). For fit animals, being in a group with such vulnerable individuals may thus decrease
1425-435: Is described as a herd, the implication is that the group tends to act together (for example, all moving in the same direction at a given time), but that this does not occur as a result of planning or coordination. Rather, each individual is choosing behaviour in correspondence with most other members, possibly through imitation or possibly because all are responding to the same external circumstances. A herd can be contrasted with
1500-444: Is harsh and cold: By staying close together animals experience better thermoregulation, because their overall surface to volume ratio is reduced. Consequently, maintaining adequate body temperatures becomes less energetically costly. The collective force of a group mobbing predators can reduce risk of predation significantly. Flocks of raven are able to actively defend themselves against eagles and baboons collectively mob lions, which
1575-539: Is impossible for individuals alone. This behaviour may be based on reciprocal altruism, meaning animals are more likely to help each other if their conspecifics did so earlier. Animals living in groups are more likely to find mates than those living in solitary and are also able to compare potential partners in order to optimize genetic quality for their offspring. Domestic animal herds are assembled by humans for practicality in raising them and controlling them. Their behaviour may be quite different from that of wild herds of
SECTION 20
#17327761916771650-427: Is nearer to a particular individual than to any other individual. Such antipredator behavior inevitably results in aggregations. The theory was proposed by W. D. Hamilton in 1971 to explain the gregarious behavior of a variety of animals. It contrasted the popular hypothesis that evolution of such social behavior was based on mutual benefits to the population . The basic principle governing selfish herd theory
1725-931: Is positively correlated to their fitness. If their energy requirement is fixed and additional energy is not increasing fitness, they will use as little time for foraging as possible (time minimizers). If on the other hand time allocated to foraging is fixed, an animal's gain in fitness is related to the quantity and quality of resources it feeds on (Energy maximizers). Since foraging may be energetically costly (searching, hunting, handling, etc.) and may induce risk of predation , animals in groups may have an advantage, since their combined effort in locating and handling food will reduce time needed to forage sufficiently. Thus, animals in groups may have shorter searching and handling times as well as an increased chance of finding (or monopolizing) highly profitable food, which makes foraging in groups beneficial for time minimizers and energy maximizers alike. The obvious disadvantage of foraging in groups
1800-411: Is primarily a direct effect of group size instead of the position within a herd. Greater group sizes result in higher visibility and detection rates for predators, but this relation is not directly proportional and saturates at some point, while the risk of being attacked for an individual is directly proportional to group size. Thus, the net effect for an individual in a group concerning its predation risk
1875-466: Is still unclear. However, some animals have found countermeasures such as propolis in beehives or grooming in social animals. Staying together in groups often brings energetic advantages. Birds flying together in a flock use aerodynamic effects to reduce energetic costs, e.g. by positioning themselves in a V-shaped formation. A similar effect can be observed when fish swim together in fixed formations. Another benefit of group living occurs when climate
1950-528: Is that in aggregations, predation risk is greatest on the periphery and decreases toward the center. More dominant animals within the population are proposed to obtain low-risk central positions, whereas subordinate animals are forced into higher risk positions. The hypothesis has been used to explain why populations at higher predation risk often form larger, more compact groups. It may also explain why these aggregations are often sorted by phenotypic characteristics such as strength. W. D. Hamilton proposed
2025-563: Is the fiddler crab . When exposed to a predator, fiddler crabs move in ways that are consistent with the selfish herd theory. Dispersed groups are more likely to form an aggregate when subjected to danger and crabs attempt to run toward the center of a forming group. Selfish herd behavior is seen also in: Although the selfish herd theory is widely accepted, it has been deemed implausible in certain situations. It may not fully account for aggregations in 3-dimensional space, in which predatory attacks may come from above or below. This means that
2100-624: The Great American Interchange . Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 Mya) saw a major change: the artiodactyl species with their more complex stomachs were better able to adapt to a coarse, low-nutrition diet, and soon rose to prominence. Nevertheless, many perissodactyl species survived and prospered until
2175-519: The mesonychians and the artiodactyls. The first artiodactyls looked like today's chevrotains or pigs: small, short-legged creatures that ate leaves and the soft parts of plants . By the Late Eocene (46 million years ago), the three modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); and Ruminantia (the goat and cattle group). Nevertheless, artiodactyls were far from dominant at that time:
2250-438: The "herd". Ungulate Ungulates ( / ˈ ʌ ŋ ɡ j ʊ l eɪ t s , - ɡ j ə -, - l ɪ t s , - l ə t s / UNG -gyuu-layts, -gyə-, -lits, -ləts ) are members of the diverse clade Euungulata ("true ungulates"), 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
2325-513: The anthracotheres, except that which evolved into Hippopotamidae , became extinct during the Pliocene without leaving any descendants. The family Raoellidae is said to be the closest artiodactyl family to the cetaceans. Consequentially, new theories in cetacean evolution hypothesize that whales and their ancestors escaped predation, not competition, by slowly adapting to the ocean. Mesonychians were depicted as "wolves on hooves" and were
Herd - Misplaced Pages Continue
2400-1153: The case with the relationship between the pecoran families and the baleen whale families. See each family for the relationships of the species as well as the controversies in their respective articles. Below is the general consensus of the phylogeny of the ungulate families. Equidae [REDACTED] Tapiridae [REDACTED] Rhinocerotidae [REDACTED] Camelidae [REDACTED] Tayassuidae [REDACTED] Suidae [REDACTED] Tragulidae [REDACTED] Antilocapridae [REDACTED] Giraffidae [REDACTED] Cervidae [REDACTED] Moschidae [REDACTED] Bovidae [REDACTED] Hippopotamidae [REDACTED] Balaenidae [REDACTED] Cetotheriidae [REDACTED] Balaenopteridae [REDACTED] Physeteridae [REDACTED] Kogiidae [REDACTED] Platanistidae [REDACTED] Ziphiidae [REDACTED] † Lipotidae [REDACTED] Pontoporiidae [REDACTED] Iniidae [REDACTED] Delphinidae [REDACTED] Phocoenidae [REDACTED] Monodontidae [REDACTED] Perissodactyla and Artiodactyla include
2475-456: The chance of being preyed upon even further. The effect of collective vigilance in social groups has been widely studied within the framework of optimal foraging theory and animal decision making. While animals under the risk of predation are feeding or resting, they have to stay vigilant and watch for predators. It could be shown in many studies (especially for birds) that with increase in group size individual animals are less attentive, while
2550-683: The close of the Eocene epoch, with only one genus, Mongolestes , surviving into the Early Oligocene epoch, as the climate changed and fierce competition arose from the better adapted creodonts . Ungulates were in high diversity in response to sexual selection and ecological events; most ungulates lack a collar bone . Terrestrial ungulates were for the most part herbivores, with some of them being grazers . However, there were exceptions to this as pigs, peccaries, hippos and duikers were known to have an omnivorous diet. Some cetaceans were
2625-423: The competition for food can lead to the misuse of warning calls, as was observed for great tits: If food is scarce or monopolized by dominant birds, other birds (mainly subordinates) use antipredatory warning calls to induce an interruption of feeding and gain access to resources. Another study concerning a flock of geese suggested that the benefits of lower vigilance concerned only those in central positions, due to
2700-439: The costs and benefits associated with living in groups in terms of the fitness of each individual compared to living solitarily. Living in groups evolved independently multiple times in various taxa and can only occur if its benefits outweigh the costs within an evolutionary timescale. Thus, animals form groups whenever this increases their fitness compared to living in solitary. The following includes an outline about some of
2775-579: The early Eocene. They appeared very similar to modern forms, but were about half the size, and lacked the proboscis. The first true tapirs appeared in the Oligocene . By the Miocene , such genera as Miotapirus were almost indistinguishable from the extant species. Asian and American tapirs were believed to have diverged around 20 to 30 million years ago; and tapirs migrated from North America to South America around 3 million years ago, as part of
2850-588: The early Eocene. Fossils of Hyrachyus eximus found in North America date to this period. This small hornless ancestor resembled a tapir or small horse more than a rhino. Three families, sometimes grouped together as the superfamily Rhinocerotoidea, evolved in the late Eocene: Hyracodontidae , Amynodontidae and Rhinocerotidae , thus creating an explosion of diversity unmatched for a while until environmental changes drastically eliminated several species. The first tapirids, such as Heptodon , appeared in
2925-437: The enormous brontotheres and the bizarre chalicotheres . The largest perissodactyl, an Asian rhinoceros called Paraceratherium , reached 15 tonnes (17 tons), more than twice the weight of an elephant . It has been found in a cladistic study that the anthracobunids and the desmostylians – two lineages that have been previously classified as Afrotherians (more specifically closer to elephants) – have been classified as
3000-642: The extinct mesonychians are carnivorous. Ungulate is from the Late Latin adjective ungulatus ' hoofed ' . Ungulatus is a diminutive form of Latin unguis ' nail ' (finger nail; toe nail). Euungulata is a clade (or in some taxonomies, a grand order) of mammals. The two extant orders of ungulates are the Perissodactyla (odd-toed ungulates) and Artiodactyla (even-toed ungulates) . Hyracoidea (hyraxes) , Sirenia (sea cows, dugongs and manatees) and Proboscidea (elephants) were in
3075-442: The fact that the possibly more vulnerable individuals in the flock's periphery have a greater need to stay attentive. This implies that the decrease in overall vigilance arises simply because the geese on the edge of the flock comprise a smaller group when groups get large. A special case of collective vigilance in groups is that of sentinels. Individuals take turn in keeping guard, while all others participate in other activities. Thus,
Herd - Misplaced Pages Continue
3150-565: The first major mammalian predators, appearing in the Paleocene. Early mesonychians had five digits on their feet, which probably rested flat on the ground during walking ( plantigrade locomotion), but later mesonychians had four digits that ended in tiny hooves on all of their toes and were increasingly well adapted to running. Like running members of the even-toed ungulates, mesonychians ( Pachyaena , for example) walked on their digits ( digitigrade locomotion). Mesonychians fared very poorly at
3225-409: The group escape of prey in which the safest position, relative to predation risk, is not the central position, but rather the front of the herd. The theory may be useful in explaining the escape strategy chosen by a herd leader. Members at the back of the herd have the greatest domain of danger and suffer the highest predation risk. These slow members must choose whether to stay in the herd, and thus be
3300-407: The herd, he may endanger himself—causing dissipation of his protective buffer. Five types of herd leadership have been proposed based on the decisions of the leader: Although some types of escape are seemingly altruistic, they promote the stability of the herd, and thus decrease the predation risk of the leader. This choice is often affected by the terrain of the area. Gregarious behavior occurs in
3375-413: The hind feet, and four on the front feet. They were herbivorous browsers on relatively soft plants, and were already adapted for running. The complexity of their brains suggest that they already were alert and intelligent animals. Later species reduced the number of toes, and developed teeth more suited for grinding up grass and other tough plant food. Rhinocerotoids diverged from other perissodactyls by
3450-464: 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. The term means, roughly, "being hoofed" or "hoofed animal". As a descriptive term, "ungulate" normally excludes cetaceans as they do not possess most of
3525-869: The late Pleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change. The artiodactyls were thought to have evolved from a small group of condylarths, Arctocyonidae , which were unspecialized, superficially raccoon-like to bear-like omnivores from the Early Paleocene (about 65 to 60 million years ago). They had relatively short limbs lacking specializations associated with their relatives (e.g. reduced side digits, fused bones, and hooves), and long, heavy tails. Their primitive anatomy makes it unlikely that they were able to run down prey, but with their powerful proportions, claws, and long canines, they may have been able to overpower smaller animals in surprise attacks. Evidently these mammals soon evolved into two separate lineages:
3600-402: The location and type of food to other group members may increase the chance for each individual to find profitable food sources, a mechanism which is known to be used by both bees (via a Waggle dance ) and several species of birds (using specific vocalisations to indicate food). In terms of Optimal foraging theory , animals always try to maximize their net energy gain when feeding, because this
3675-425: The major effects determining the trade-offs for living in groups. Perhaps the most studied effect of herds is the so-called dilution effect. The key argument is that the risk of being preyed upon for any particular individual is smaller within a larger group, strictly due to the fact that a predator has to decide which individual to attack. Although the dilution effect is influenced by so-called selfish herding , it
3750-400: The majority of large land mammals. These two groups first appeared during the late Paleocene , rapidly spreading to a wide variety of species on numerous continents, and have developed in parallel since that time. Some scientists believed that modern ungulates were descended from an evolutionary grade of mammals known as the condylarths . The earliest known member of this group may have been
3825-400: The most likely targets, or whether to desert the herd, and signal their vulnerability. The latter may entice the pursuit of the predator to this sole individual. In light of this, the decision of the escape route by the front members of the herd may be greatly affected by actions of the slowest members. If the leader chooses an escape strategy that promotes the dispersal of the slowest member of
SECTION 50
#17327761916773900-591: The most recent study recovers them as within the true ungulate assemblage, closest to Carodnia . In Australia, the recently-extinct marsupial Chaeropus ("pig-footed bandicoot") also developed hooves similar to those of artiodactyls, an example of convergent evolution . Perissodactyls were thought to have evolved from the Phenacodontidae , small, sheep-sized animals that were already showing signs of anatomical features that their descendants would inherit (the reduction of digit I and V for example). By
3975-437: The notoungulates were closely related to the perissodactyls. The oldest known fossils assigned to Equidae date from the early Eocene , 54 million years ago. They had been assigned to the genus Hyracotherium , but the type species of that genus is now considered not a member of this family, but the other species have been split off into different genera. These early Equidae were fox-sized animals with three toes on
4050-403: The only modern ungulates that were carnivores; baleen whales consume significantly smaller animals in relation to their body size, such as small species of fish and krill ; toothed whales, depending on the species, can consume a wide range of species: squid , fish, sharks , and other species of mammals such as seals and other whales. In terms of ecosystem ungulates have colonized all corners of
4125-466: The optimal group size and ultimately the decision of each individual whether or not to stay in the group. Since animals in groups stay near each other and interact frequently, infectious diseases and parasites spread much easier between them compared to solitary animals. Studies have shown a positive correlation between herd size and intensity of infections, but the extent to which this sometimes drastic reduction in fitness governs group size and structure
4200-409: The overall vigilance suffers little (many eyes effect). This means food intake and other activities related to fitness are optimized in terms of time allocation when animals stay in groups. However, some details about this concepts remain unclear. Being the first to detect predators and react accordingly can be advantageous, implying individuals may not fully be able to rely only on the group. Moreover,
4275-589: The past grouped within the clade "Ungulata", later found to be a polyphyletic and now invalid clade. The three orders of Paenungulata are now considered a clade and grouped in the Afrotheria clade, while Euungulata is now grouped under the Laurasiatheria clade. In 2009, morphological and molecular work found that aardvarks, hyraxes, sea cows, and elephants were more closely related to each other and to sengis , tenrecs , and golden moles than to
4350-413: The perissodactyls and artiodactyls, and form the clade Afrotheria . Elephants, sea cows, and hyraxes were grouped together in the clade Paenungulata , while the aardvark has been considered as either a close relative to them or a close relative to sengis in the clade Afroinsectiphilia . This is a striking example of convergent evolution . There is now some dispute as to whether this smaller Euungulata
4425-437: The perissodactyls were much more successful and far more numerous. Artiodactyls survived in niche roles, usually occupying marginal habitats , and it is presumably at that time that they developed their complex digestive systems , which allowed them to survive on lower-grade food. While most artiodactyls were taking over the niches left behind by several extinct perissodactyls, one lineage of artiodactyls began to venture out into
4500-426: The planet, from mountains to the ocean depths ; grasslands to deserts and some have been domesticated by humans . Selfish herd theory The selfish herd theory states that individuals within a population attempt to reduce their predation risk by putting other conspecifics between themselves and predators. A key element in the theory is the domain of danger , the area of ground in which every point
4575-491: The predator attacks plays a crucial role in whether or not selfish herd behavior can evolve. Although the selfish herd promotes decreased predation risk to many of its members, a variety of risks have been associated with such aggregations. Groupings may make prey more conspicuous to predators and may increase intraspecific competition. Furthermore, individuals in the desired central positions may have lower feeding rates and may be less vigilant. An extensively studied example
SECTION 60
#17327761916774650-510: The predator, and vigilance. Individuals holding initially central positions are more likely to be successful at remaining in the center. Simpler movement strategies may be sufficient for low density populations and fast-acting predators, but at higher densities and with slower predators, more complex strategies may be needed. Lastly, less vigilant members of a herd are often less likely to obtain smaller domains of danger as they begin movement later. The selfish herd theory may also be applied to
4725-419: The proto-whale Pakicetus and other early cetacean ancestors collectively known as Archaeoceti , which eventually underwent aquatic adaptation into the completely aquatic cetaceans . The other branch became the anthracotheres , a large family of four-legged beasts, the earliest of whom in the late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of
4800-454: The same or related species, since both their composition (in terms of the distribution of age and sex within the herd) and their history (in terms of when and how the individuals joined the herd) are likely to be very different. The term herd is also applied metaphorically to human beings in social psychology , with the concept of herd behaviour . However both the term and concepts that underlie its use are controversial. The term has acquired
4875-587: The seas. The traditional theory of cetacean evolution was that cetaceans were related to the mesonychian . These animals had unusual triangular teeth very similar to those of primitive cetaceans. This is why scientists long believed that cetaceans evolved from a form of mesonychian. Today, many scientists believe cetaceans evolved from the same stock that gave rise to hippopotamuses. This hypothesized ancestral group likely split into two branches around 54 million years ago . One branch would evolve into cetaceans , possibly beginning about 52 million years ago with
4950-625: The somewhat tapir-like pyrotheres and astrapotheres , the mesaxonic litopterns and the diverse notoungulates . As a whole, meridiungulates were said to have evolved from animals like Hyopsodus . For a while their relationships with other ungulates were a mystery. Some paleontologists have even challenged the monophyly of Meridiungulata by suggesting that the pyrotheres may be more closely related to other mammals, such as Embrithopoda (an African order that were related to elephants ) than to other South American ungulates. A recent study based on bone collagen has found that at least litopterns and
5025-469: The southern tip of Baja California . Their dental and skeletal form suggests desmostylians were aquatic herbivores dependent on littoral habitats. Their name refers to their highly distinctive molars, in which each cusp was modified into hollow columns, so that a typical molar would have resembled a cluster of pipes, or in the case of worn molars, volcanoes. They were the only marine mammals to have gone extinct. The South American meridiungulates contain
5100-645: The start of the Eocene , 55 million years ago (Mya), they had diversified and spread out to occupy several continents. Horses and tapirs both evolved in North America; rhinoceroses appear to have developed in Asia from tapir-like animals and then colonised the Americas during the middle Eocene (about 45 Mya). Of the approximately 15 families, only three survive (McKenna and Bell, 1997; Hooker, 2005). These families were very diverse in form and size; they included
5175-439: The strength of social bonds and trust within these groups have to be much higher than in the former cases. Hunting together enables group-living predators, such as wolves and wild dogs, to catch large prey, which they are unable to achieve when hunting alone. Working together significantly improves foraging efficiency, meaning the net energy gain of each individual is increased when animals are feeding collectively. As an example,
5250-496: The theory in an article titled "Geometry for the Selfish Herd". To date, this article has been cited in over 2000 sources. To illustrate his theory, Hamilton asked readers to imagine a circular lily pond which sheltered a population of frogs and a water snake . Upon seeing the water snake, the frogs scatter to the rim of the pond, and the water snake attacks the nearest one. Hamilton proposed that in this model, each frog had
5325-417: The theory of marginal predation. This theory states that predators attack the closest prey, who are typically on the outside of an aggregation. From this, Hamilton suggested that in the face of predation, there should be a strong movement of individuals toward the center of an aggregation. A domain of danger may be measured by constructing a Voronoi diagram around the group members. Such construction forms
5400-447: The tiny Protungulatum , a mammal that co-existed with the last of non-avian dinosaurs 66 million years ago. However, many authorities do not consider it a true placental, let alone an ungulate. The enigmatic dinoceratans were among the first large herbivorous mammals, although their exact relationship with other mammals is still debated with one of the theories being that they might just be distant relatives to living ungulates;
5475-472: The two orders not that closely related, as some place the perissodactyls as close relatives to bats and Ferae in Pegasoferae and others place the artiodactyls as close relatives to bats. Below is a simplified taxonomy (assuming that ungulates do indeed form a natural grouping) with the extant families, in order of the relationships. Keep in mind that there were still some grey areas of conflict, such as
5550-403: The typical morphological characteristics of other ungulates, but recent discoveries indicate that they were also descended from early artiodactyls . Ungulates are typically herbivorous and many employ specialized gut bacteria to enable them to digest cellulose, though some members may deviate from this: several species of pigs and the extinct entelodonts are omnivorous, while cetaceans and
5625-487: Was initiated from within an individual's domain of danger, he would be attacked and likely killed. The risk of predation to each individual was, therefore, correlated to the size of his domain of danger. Frog jumping in response to the water snake was an attempt to lower the domain of danger. Hamilton also went on to model predation in two-dimensions, using a lion as an example. Movements that Hamilton proposed would lower an individual's domain of danger were largely based on
#676323