Pack hunter - WikiHQ

thumb|300px|[[Lions working together to take down a large Cape buffalo. ]]

A pack hunter or social predator is a predatory animal which hunts its prey by working together with other members of its species. When hunting cooperation is across two or more species, the broader term cooperative hunting is commonly used.

A well known pack hunter is the gray wolf; humans too can be considered pack hunters. Other pack hunting mammals include chimpanzees, dolphins including orcas, lions, dwarf and banded mongooses, as well as spotted hyenas. Avian social predators include the Harris's hawk, butcherbirds, three of four kookaburra species and many helmetshrikes. Other pack hunters include ants like army ants, the goldsaddle goatfish, and occasionally crocodilians.

Pack hunting is typically associated with cooperative breeding and its concentration in the Afrotropical realm is a reflection of this. Most pack hunters are found in the southern African savannas, with a notable absence in tropical rainforests and with the exception of the wolf and coyote, higher latitudes. or that the environment's inherent unpredictability due to ENSO or IOD events means that in very bad conditions it will not be possible to raise the young necessary to prevent declining populations from adult mortality. It is also argued that Africa's large area of continuous flat and open country, which was even more extensive while rainforest contracted during glacial periods of the Quaternary, may have helped encourage pack hunting to become much more common than on any other continent.

Around 80–95% of carnivores are solitary and hunt alone. Groups that hunt cooperatively, at least some of the time, include mammals such as wolves and wild dogs, lions, spotted hyenas, chimpanzees and humans; archosaurs such as crocodilians as well as large marine fishes such as groupers and moray eels. Some non-avian theropod dinosaurs, such as Albertosaurus sarcophagus, may have displayed pack behaviour.

Evolution of cooperative hunting

thumb|Transient [[orcas]]

Understanding how cooperative hunting could evolve requires considering the circumstances that would make it beneficial.

Evolutionary models

Conventional model: Packer and Ruttan Game Theory

In 1988, the ecologists Craig Packer and Lore Ruttan surveyed documented instances of cooperative hunting to make a game-theoretical model to explain under what circumstances cooperative hunting might evolve. In their model, individuals can engage in one of four hunting strategies: falcons, and fossa due to cooperative hunting. For example, aplomado falcons increase their efficiency of capture when hunting in pairs because the pairs are twice as successful as hunting alone.

Economics of group size

Many populations capable of forming cooperative hunting groups may not necessarily do so if their group is too small or too large for this behavior to be favorable. Group size is an important indicator of specific instances of cooperative hunting, as the prey must be large enough and the hunting group small enough to provide enough food for all individuals. Another important consideration is that when groups grow larger, there is a greater chance that individuals will engage in a cheater strategy. If there is consistently too much cheating in a group, individuals will prefer to hunt alone so they do not have to share their meal with freeloaders.

There may be costs that set an upper limit on group size. The net benefits increase as a function of pack size, as more animals in a group are theoretically able to obtain more meat. However, in African wild dogs, researchers found that the most common group size was not the group size that maximized net benefits. Once the attack begins the wolves have specific roles in the hunt, based on age, gender and social standing, as well as its particular experience and abilities. The youngest wolves frequently do nothing more than observe and learn from the sidelines. Speedy, lightly built females often take on herding roles, darting back and forth in front of prey, causing confusion and preventing escape. Slower but more powerful males are able to take down a large animal more aggressively and quickly.

Aplomado falcons

Aplomado falcons monitor each other's movements during hunts. Males and females always perform the same task in every situation. They begin perched together and the males initiate and give a sharp "chirp" vocalization to signal for the female to follow suit. When chasing birds on the ground, the females follow right behind the birds in the bushes and the males swoop in from overhead to make the kill. Within these pairs, males and females are consistently assigned to a particular role.

Chimpanzees

In Taï, chimpanzee, individuals participate as a driver, blocker, chaser, or ambusher. Drivers follow the prey without trying to catch up with it. Blockers place themselves in a tree to block the progression of the prey. Chasers move quickly after the prey to catch up with it. Finally, ambushers anticipate the escape route of the prey long enough in advance to force it back towards the chasers or down into the lower canopy. Blocking and ambushing are thought to require much more cognitive effort in anticipating the future movements of the prey, and they are thus rewarded with a larger proportion of meat after a successful hunt. These two roles correlate positively with the age of the chimpanzee as the cognitive function necessary to perform these tasks is thought to increase with age. Furthermore, individuals can change roles during the same hunt or maintain their same role during the entire process.

Fossa

thumb|upright|right|The usually solitary fossa sometimes hunts cooperatively.

Fossa (Cryptoprocta ferox) are the first documented example of cooperative hunting in solitary species, as fossa are some of the least social carnivores. These spiders do not monopolize certain parts of the web, meaning any individual can use a variety of locations to lie in wait for prey. When prey is captured in the field, it usually requires at least two spiders to be brought back to the nest to be shared among the rest of the colony.

Humans

thumb|Hunting Buffalo by [[Alfred Jacob Miller, c. 1859]]

Research at Lamalera village in Indonesia has shown how cooperative hunting can develop in pre-industrial societies. This village relies on traditional whale hunting techniques for its livelihood.

Interspecific

Groupers (Plectropomus pessuliferus marisrubri) and giant moray eels (Gymnothorax javanicus) provide compelling evidence for interspecific cooperative hunting. Groupers visit moray eels at their resting places and provide visual signals (such as a head shake) to engage morays in the hunt. These associations are non-random and appear to be motivated by the hunger level of the groupers. Groupers were able to capture prey five times more quickly with morays present because the eels could sneak through crevices and corner prey items; additionally, morays that hunted alone were never successful because they did not have a grouper present to lead them to the prey. Thus, the hunting success of groupers and giant moray eels is greater for both species than when hunting alone.

Implications for cognition

Cooperative hunting is sometimes thought to reflect advanced cognitive processes, such as foresight, planning, and theory of mind and involve complex communication between hunters. However, several lines of evidence indicate that many instances of cooperative hunting rely on simple principles and can be observed in species without large brains or advanced cognitive abilities. Nevertheless, cooperative hunting occurs at different levels of complexity, and the most advanced levels may reflect a higher level of cognitive ability. In addition, frequent and successful pack hunting may depend on a higher level of social harmony, complexity, or intelligence, which may facilitate concerted group activities. In general, not much data has been collected on this topic, and new technologies and equipment may allow the collection of enough observations to answer these questions.

Levels of complexity

Many species, including spiders, have been observed to take down prey in groups.

Arguments against cognitive complexity

While it is easy to attribute complex cognitive processes to animals hunting in a coordinated manner, this apparently complex behaviour may be explained in terms of more simple mental operations. For example, the hunting tactics of wolves, which involve fanning out and encircling prey, are argued to have been replicated in a computer simulation where the wolves were programmed with two simple rules: (1) Get within a minimum safe distance from the prey (2) Once this distance is achieved, move away from hunting partners. Ambushing is also argued to have been represented in this simulation by wolves who begin randomly at different locations. This simulation matches the coordination level of complexity, leaving open the question of the cognitive processes necessary for collaboration.

Another argument consists of observations that several animals not usually thought of as cognitively complex have been observed to hunt cooperatively, and in some cases collaboratively. For example, grouper fish have been observed to recruit giant moray eels into collaborative hunts, where their complementary hunting strategies increase the feeding success of both. The gestures performed by the grouper fish fulfill the criteria for a referential gesture, a signalling behaviour previously only observed in humans, great apes, and ravens. In addition, cooperative and collaborative hunting has been observed in other unlikely candidates, such as crocodiles and Cuban boas.

Using AI modeling, a Nagoya University research team led by Kazushi Tsutsui found that cooperative hunting behavior could be reliably predicted using a reinforcement learning stimulation, a type of learning common to animals with lower levels of cognitive complexity. They concluded that "elaborate coordination can be achieved through a relatively simple decision process of mapping between observations and actions via distance-dependent internal representations formed by prior experience."

Arguments for cognitive complexity

In contrast to social carnivores, who normally hunt cooperatively and only occasionally take on complementary roles, chimpanzees in Taï National Park regularly hunt collaboratively. They have been observed to use four different specialized roles, including ambushing, blocking, chasing, and driving. Christophe Boesch has argued that this level of collaborative complexity is indicative of several advanced cognitive processes. For example, chimps must successfully attribute physical abilities to their prey and use this information to predict which tree they may reach and when. In addition, chimps require knowledge of their partner's roles and the information they possess about the prey's location in order to infer the direction of the prey and adjust their behaviour accordingly.

Even more advanced than these predictive abilities may be the ability to participate in collaborative activities with shared goals and intentions, which Michael Tomasello calls shared intentionality. Tomasello claims that this ability involves unique mental representations and a level of cognitive and social complexity only achieved by humans. However, Boesch counters this by pointing out that Taï chimps fulfill all the hallmarks of shared intentionality, bringing into question either the uniqueness or the complexity of this cognitive process.

While the majority of group hunting behaviours do not seem to take much cognitive complexity, it has been observed that species that receive a large portion of their food from cooperative hunts tend to have a complex or harmonious social structure. This includes many of the prototypical social carnivores, such as wolves and wild dogs. Bonobos were likewise found to be better at a cooperative task with a food reward than other, more aggressive chimpanzees.