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 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 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 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 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, 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 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. Individuals take turn in keeping guard, while all others participate in other activities.
Hunting together enables group-living predators, such as wolves and wild dogs, to catch large prey, what 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.
A group of Spinner dolphins is for instances able to force fish into a small confined space, which makes capturing prey particularly easy, as there is no way for the latter to flee. Furthermore, large groups are able to monopolize resources and defend them against solitary animals or smaller groups of the same or different species.
It was shown that larger groups of lions tend to be more successful in protecting prey from hyenas than small ones. In terms of Optimal foraging theory, animals always try to maximize their net energy gain when feeding, because this 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. Since foraging may be energetically costly searching, hunting, handling, etc.
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 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 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 is still unclear. Staying together in groups often brings energetic advantages. Birds flying together in a flock use aerodynamic effects to reduce energetic costs, e.
Another benefit of group living occurs when climate 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 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 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 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.
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