Foraging
Encyclopedia
Definitions and significance of foraging behavior
Foraging is the act of searching for and exploiting food resources. It affects an animal's fitnessFitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
because it plays an important role in an animal's ability to survive and reproduce. Foraging theory is a branch of behavioral ecology
Behavioral ecology
Behavioral ecology, or ethoecology, is the study of the ecological and evolutionary basis for animal behavior, and the roles of behavior in enabling an animal to adapt to its environment...
that studies the foraging behavior of animals in response to the environment in which the animal lives.
Behavioral ecologists use economic models to understand foraging; many of these models are a type of optimality model. Thus foraging theory is discussed in terms of optimizing a payoff from a foraging decision. The payoff for many of these models is the amount of energy an animal receives per unit time, more specifically, the highest ratio of energetic gain to cost while foraging. Foraging theory predicts that the decisions that maximize energy per unit time and thus deliver the highest payoff will be selected for and persist. Key words used to describe foraging behavior include; 1) Resources, the elements necessary for survival and reproduction and yet have a limited supply, 2) A predator, any organism that consumes others and 3) Prey, an organism that is eaten in part or whole by another.
Behavioral ecologists first tackled this topic in the 1960’s and 1970’s. Their goal was to quantify and formalize a set of models to test their null hypothesis that animals forage randomly. Important contributions to foraging theory have been made by:
- Eric CharnovEric CharnovEric L. Charnov is an American evolutionary ecologist. He is best known for his work on foraging, especially the marginal value theorem, and life history theory, especially sex allocation and scaling/allometric rules. He is a MacArthur Fellow and a Fellow of the American Academy of Arts and...
, who developed the marginal value theoremMarginal value theoremIn behavioral ecology, the marginal value theorem considers an optimally foraging animal exploiting resources distributed in patches and that must decide when to leave a patch to start searching for a fresh one. The animal is assumed to have evolved to optimize a cost/benefit ratio: searching for...
to predict the behavior of foragers using patches; - Sir John KrebsJohn KrebsJohn Richard Krebs, Baron Krebs FRS is a world leader in zoology and more specifically bird behaviour. He is currently the Principal of Jesus College, Oxford University...
, with work on the optimal diet model in relation to titTitmouseThe tits, chickadees, and titmice constitute Paridae, a large family of small passerine birds which occur in the northern hemisphere and Africa...
s and chickadees; - John Goss-Custard, who first tested the optimal diet model against behavior in the field, using redshankCommon RedshankThe Common Redshank or simply Redshank is an Eurasian wader in the large family Scolopacidae.- Description and systematics :...
, and then proceeded to an extensive study of foraging in the Common Pied Oystercatcher.
Factors influencing Foraging Behavior
Several factors affect an animal’s ability to forage and acquire highly profitable resources. These include learning and genetic composition.Learning
LearningLearning
Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
is defined as an adaptive change or modification of a behavior based on a previous experience. Since an animal’s environment is constantly changing, the ability to adjust foraging behavior is essential for maximization of fitness. Studies in social insects have shown that there is a significant correlation between learning and foraging performance. In nonhuman primates, young individuals learn foraging behavior from their peers and elders by watching other group members forage and by copying their behavior. Observing and learning from other members of the group ensure that the younger members of the group learn what is safe to eat and become proficient foragers.
One measure of learning is ‘Foraging innovation’ which is defined as an animal consuming new food or using a new foraging technique in response to their dynamic living environment. Foraging innovation is considered learning because it involves behavioral plasticity on the animal’s part. The animal recognizes the need to come up with a new foraging strategy and introduce something it has never used before to maximize his or her fitness (survival). The forebrain size has been associated with learning behavior such that animals with larger brain sizes are expected to be better learners. . A higher ability to innovate has been linked to larger forebrain sizes in North American and British Isle birds according to Lefebvre et al. (1997) . In this study, bird orders that contained individuals with larger forebrain sizes displayed a higher amount of foraging innovation. Examples of innovations recorded in birds include following tractors and eating frogs or other insects killed by it and using swaying trees to catch their prey.
Genetics
Types of Foraging
Foraging can be categorized into two main types. The first is solitary foraging, when animals forage by themselves. The second is group foraging. Group foraging includes when animals can be seen foraging together when it is beneficial for them to do so (called an aggregation economy) and when it is detrimental for them to do so (called a dispersion economy).Solitary Foraging
Solitary foraging is when animals find, capture and consume their prey alone. Individuals can manually exploit patches or they can use tools to exploit their prey. Animals may choose to forage on their own when the resources are abundant which can occur when the habitat is rich or when the number of conspecifics foraging are few. In these cases there may be no need for group foraging . In addition, foraging alone can result in less interaction with other foragers which can decrease the amount of competition and dominance interactions an animal deals with. It will also ensure that a solitary forager is less conspicuous to predators . Solitary foraging strategies characterize many of the phocids (the true seals) such as the elephant and harbor seals. An example of an exclusive solitary forager is the South American species of the harvester ant, Pogonomyrmex vermiculatus .Tool use in Solitary foraging
Some examples of tool useTool use by animals
Tools are used by some animals, particularly primates, to perform simple tasks such as the acquisition of food, or grooming. Originally thought to be a skill only possessed by humans, tool use requires some level of intelligence. Primates have been observed exploiting sticks and stones to...
include dolphins using sponges to feed on fish that bury themselves in the sediment, New Caledonian crows that use sticks to get larvae out of trees, and chimpanzees that similarly use sticks to capture and consume termites.
Solitary Foraging and Optimal Foraging Theory
The theory scientists use to understand solitary foraging is called optimal foraging theoryOptimal foraging theory
Optimal foraging theory is an idea in ecology based on the study of foraging behaviour and states that organisms forage in such a way as to maximize their net energy intake per unit time. In other words, they behave in such a way as to find, capture and consume food containing the most calories...
. Optimal foraging theory (OFT) was first proposed in 1966, in two papers published independently, by Robert MacArthur
Robert MacArthur
Robert Helmer MacArthur was an American ecologist who made a major impact on many areas of community and population ecology....
and Eric Pianka
Eric Pianka
Eric Rodger Pianka is an American biologist, whose work includes herpetology and evolutionary ecology. His textbook, Evolutionary Ecology is considered a classic, and his writings for the general public and television appearances have made him an influential figure.-Youth:Pianka was born in...
, and by J. Merritt Emlen. This theory argues that because of the key importance of successful foraging to an individual's survival, it should be possible to predict foraging behavior by using decision theory
Decision theory
Decision theory in economics, psychology, philosophy, mathematics, and statistics is concerned with identifying the values, uncertainties and other issues relevant in a given decision, its rationality, and the resulting optimal decision...
to determine the behavior that an “optimal forager” would exhibit. Such a forager has perfect knowledge of what to do to maximize usable food intake. While the behavior of real animals inevitably departs from that of the optimal forager, optimal foraging theory has proved very useful in developing hypotheses for describing real foraging behavior. Departures from optimality often help to identify constraints either in the animal's behavioral or cognitive repertoire, or in the environment, that had not previously been suspected. With those constraints identified, foraging behavior often does approach the optimal pattern even if it is not identical to it. In other words, we know from Optimal Foraging Theory that animals are not foraging randomly even if their behavior doesn’t perfectly match what is predicted by OFT.
Versions of OFT
There are many versions of optimal foraging theory that are relevant to different foraging situations. These models generally possess the following components according to Stephens et al. 2007;
- currency: an objective function, what we want to maximize , in this case energy over time as a currency of fitness
- decision: set of choices under the organism’s control , or the decisions that the organism exhibits
- constraints: “an organism’s choices are constrained by genetics, physiology neurology, morphology and the laws of chemistry of physics”
Some of these versions include:
1. The optimal diet model, which analyzes the behavior of a forager that encounters different types of prey and must choose which to attack. This model is also known as the prey model or the attack model. In this model the predator encounters different prey items and decides whether to spend time handling or eating the prey. It predicts that foragers should ignore low profitability prey items when more profitable items are present and abundant . The objective of this model is to identify the choice that will maximize fitness. How profitable a prey item is depends on ecological variables such as the time required to find, capture, and consume the prey in addition to the energy it provides. It is likely that an individual will settle for a trade off between maximizing the intake rate while eating and the search interval between prey .
2. Patch selection theory, which describes the behavior of a forager whose prey is concentrated in small areas known as patches with a significant travel time between them. The model seeks to find out how much time an individual will spend on one patch before deciding to move to the next patch. To understand whether an animal should stay at a patch or move to a new one, think of a bear in a patch of berry bushes. The longer a bear stays at the patch of berry bushes the less berries there are for that bear to eat. The bear must decide how long to stay and thus when to leave that patch and move to a new patch. The movement will depend on the travel time between patches and the energy being gained from one patch versus that which could be gained if the individual moved to another patch . This is based on the marginal value theorem
Marginal value theorem
In behavioral ecology, the marginal value theorem considers an optimally foraging animal exploiting resources distributed in patches and that must decide when to leave a patch to start searching for a fresh one. The animal is assumed to have evolved to optimize a cost/benefit ratio: searching for...
.
2.1. Central place foraging theory, is a version of the patch model. This model describes the behavior of a forager that must return to a particular place in order to consume its food, or perhaps to hoard it or feed it to a mate
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...
or offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
. Chipmunks are a good example for this model. As travel time between the patch and their hiding place increased, the chipmunks stayed longer at the patch.
In recent decades, optimal foraging theory has often been applied to the foraging behavior of human hunter-gatherer
Hunter-gatherer
A hunter-gatherer or forage society is one in which most or all food is obtained from wild plants and animals, in contrast to agricultural societies which rely mainly on domesticated species. Hunting and gathering was the ancestral subsistence mode of Homo, and all modern humans were...
s. Although this is controversial, coming under some of the same kinds of attack as the application of socio biological
Sociobiology
Sociobiology is a field of scientific study which is based on the assumption that social behavior has resulted from evolution and attempts to explain and examine social behavior within that context. Often considered a branch of biology and sociology, it also draws from ethology, anthropology,...
theory to human behavior, it does represent a convergence of ideas from human ecology
Human ecology
Human ecology is the subdiscipline of ecology that focuses on humans. More broadly, it is an interdisciplinary and transdisciplinary study of the relationship between humans and their natural, social, and built environments. The term 'human ecology' first appeared in a sociological study in 1921...
and economic anthropology
Economic anthropology
Economic anthropology is a scholarly field that attempts to explain human economic behavior using the tools of both economics and anthropology. It is practiced by anthropologists and has a complex relationship with economics...
that has proved fruitful and interesting.
Group Foraging
Group foraging is when animals find, capture and consume prey in the presence of other individuals. In other words it is foraging when success depends not only on your own foraging behaviors but the behaviors of others as well . An important note here is that group foraging can emerge in two types of situations. The first situation is frequently thought of and occurs when foraging in a group is beneficial and brings greater rewards known as an aggregation economy. The second situation occurs when a group of animals forage together but it may not be in an animal’s best interest to do so known as a dispersion economy. Think of a cardinal at a bird feeder for the dispersion economy. We might see a group of birds foraging at that bird feeder but it is not in the best interest of the cardinal for any of the other birds to be there too. The amount of food the cardinal can get from that bird feeder depends on how much it can take from the bird feeder but also depends on how much the other birds take as well.Cost and benefits of group foraging
As already mentioned, group foraging brings both costs and benefits to the members of that group. Some of the benefits of group foraging include being able to capture larger prey , being able to create aggregations of prey, being able to capture prey that are difficult or dangerous and most importantly reduction of predation threat . With regard to costs, however, group foraging results in competition for available resources by other group members. Competition for resources can be characterized by either scramble competition whereby each individual strives to get a portion of the shared resource, or by interference competition whereby the presence of competitors prevents a forager’s accessibility to resources . Group foraging can thus reduce an animal’s foraging payoff .
Group foraging may be influenced by the size of a group. In some species like lions and wild dogs
African Wild Dog
Lycaon pictus is a large canid found only in Africa, especially in savannas and lightly wooded areas. It is variously called the African wild dog, African hunting dog, Cape hunting dog, painted dog, painted wolf, painted hunting dog, spotted dog, or ornate wolf...
, foraging success increases with an increase in group size then declines once the optimal size is exceeded. A myriad number of factors affect the group sizes in different species. For example lionesses (female lions) do not make decisions about foraging in a vacuum. They make decisions that reflect a balance between obtaining food, defending their territory and protecting their young. In fact, we see that lion foraging behavior does not maximize their energy gain. They are not behaving optimally with respect to foraging because they have to defend their territory and protect young so they hunt in small groups to reduce the risk of being caught alone. Another factor that may influence group size is the cost of hunting. To understand the behavior of wild dogs and the average group size we must incorporate the distance the dogs run
Group foraging and the Ideal Free Distribution
The theory scientists use to understand group foraging is called the Ideal free distributionIdeal free distribution
In ecology, an ideal free distribution is a way in which animals distribute themselves among several patches of resources. The theory states that the number of individual animals that will aggregate in various patches is proportional to the amount of resources available in each...
. This is the null model for thinking about what would draw animals into groups to forage and how they would behave in the process. This model predicts that animals will make an instantaneous decision about where to forage based on the quality (prey availability) of the patches available at that time and will choose the most profitable patch, the one that maximizes their energy intake. This quality depends on the starting quality of the patch and the number of predators already there consuming the prey.
Summary
Successful foraging is essential for the survival and reproduction of an organism. Foraging behavior is affected by so many factors and this likely differs across species. Foraging behavior is a phenotype thus it is determined by the genotype of the individual and its environment (availability of resources and the presence of predators). It is important to understand how foraging behavior fits in the context of an organism’s life history and how this in turn affects the foraging decisions organism makes. In times of crisis such as depletion of resources, animals will gain from having foraging innovation abilities to survive. Since there is such a clear link between foraging behavior and fitness it is easy to understand how those behaviors that benefit the organism and help them survive and reproduce will be selected for and passed on. For some organisms this might be the ability to use tools. Without tools the individual might not be able to find the most profitable prey (New Caledonian crows). For others it might be the size of the pollen load an individual collects (Honeybees). For others it might be creating a way to cooperatively hunt schools of fish in the dark ocean (Spinner Dolphins). Every species and every individual is different but the main aim is to find a way to balance maximizing food intake with other aspects of life.
External links
- Institute for the Study of Edible Wild Plants and Other Foragables
- The Big Green Idea Wild Foraging Factsheet
- Caress, Badiday. (2000), The emergence and stability of cooperative fishing on Ifaluk Atoll, for Human Behavior and Adaptation: an Anthropological Perspective, edited by L. Cronk, N. Chagnon, and B. Iro ns, pp. 437–472.