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When asked questions of animal behavior such as why animals see, even grade school children can answer that vision helps animals find food and avoid danger. Biologists have three additional explanations: Evolution by natural selection is the cause of sight, the mechanics of the eye are the cause, and even the process of an individual’s development is the cause. Although these answers may be very different, they are consistent with each other. However, biologists have sometimes talked past each other until in the 1960s Nikolaas Tinbergen delineated the four questions—or categories of explanations—of animal behavior, based on Aristotle's four types of causes. This schema constitutes a basic framework of the overlapping behavioral fields of ethology, behavioral ecology, sociobiology, and evolutionary psychology.

Four categories of questions and explanations

The first two categories pertain to the evolution of the species and the second two pertain to the individual.

Evolutionary (ultimate) explanations

1 Function (adaptation)

Darwin’s theory of evolution by natural selection is the only scientific explanation for why an animal’s behavior is usually "well designed" (or seemingly so) for survival and reproduction in its environment.[1] Many examples are well-known. For instance, birds fly south in the winter to find food and warmth, and mammalian mothers nurture their young, thereby having more surviving offspring.

Ultimate function corresponds to Aristotle's formal cause. <3

2 Phylogeny

“Phylogeny” captures all evolutionary explanations other than function/adaptation.[2] There are several reasons why natural selection may fail to achieve optimal design (Mayr 2001:140-143; Buss et al. 1998). One entails random processes such as mutation and environmental events acting on small populations. Another entails the constraints resulting from early evolutionary development. As many characteristics are retained over the course of phylogeny, each organism harbors characteristics of various (phylogenetic) ages. This applies equally to anatomy and behavior. Reconstructing the phylogeny of a species often makes it possible to understand the "uniqueness" of recent characteristics: Earlier phylogenetic stages and (pre-) conditions which persist often also determine the form of more modern characteristics. For instance, the vertebrate eye (including the human eye) has a blind spot, whereas octopus eyes do not. In those two lineages, the eye was originally constructed one way or the other. Once the vertebrate eye was constructed, there were no intermediate forms that were both adaptive and would have enabled it to evolve without a blind spot.

Proximate explanations

3 Causation (proximate mechanisms)

Here are several prominent classes of proximate mechanisms:

  • Brain: Broca’s area, a small section of the human brain, is key to the use of grammar (The Language Instinct).
  • Hormones are chemicals used to communicate among cells of an individual organism. Testosterone, for instance, stimulates aggressive behavior in a number of species.
  • Pheromones are chemicals used to communicate among members of the same species. Some species (e.g., dogs and some moths) use pheromones to attract mates.

Proximate mechanisms correspond to Aristotle's material cause.

In examining living organisms biologists are confronted with diverse levels of complexity (e.g. the chemical level, the physiological level, the psychological level, the social level). Subject of investigation are functional relations between cause and effect within and between the levels. Within the scope of (behavioral) physiology they examine, inter alia, hormonal and neuronal aspects such as the influence of social and ecological conditions on the release of certain transmitters and hormones, and the effects of such releases on behavior. In mammals, stress during birth has a tokolytic (contraction-suppressing) effect. Findings regarding the basic levels are a (proximate) prerequisite for understanding superior levels. However, awareness of the chemical messengers of nerve cells (transmitters) is not enough to understand the superior levels of neuroanatomic circuit diagrams or behavior: "The whole is more than the mere sum of its parts." - All levels must be considered as being equally important (cf. "Laws about the Levels of Complexity" of Nicolai Hartmann; see also Transdisciplinarity).

4 Development (ontogeny)

In the latter half of the twentieth century, social scientists debated whether human behavior was the product of nature (genes) or nurture (environment in the developmental period, including culture). The consensus among biologists now is that behavior is the product of gene-environment interaction, in which the whole can be more than the sum of the parts, that is, the genetic and environmental components. By way of contrast, tallness may simply be the sum of “tall genes” and an environment rich in food.

An example of interaction (as distinct from the sum of the components) involves familiarity from childhood. In a number of species, individuals prefer to associate with familiar individuals but prefer to mate with unfamiliar ones (Alcock 2001:85-89). By inference, genes affecting living together interact with the environment differently from genes affecting mating behavior. A homely example of interaction involves plants: Some plants grow toward the light (phototropism) and some away from gravity (gravitropism). Such species react differently to the same environment because of different genes.

Many forms of developmental learning have a critical period, for instance, for imprinting among geese and language acquisition among humans. In such cases, genes determine the timing of the environmental impact.

A related concept is labeled “biased learning” (Alcock 2001:101-103) and “prepared learning” (Wilson, 1998:86-87). For instance, after eating food that subsequently made them sick, rats are predisposed to associate that food with smell, not sound (Alcock 2001:101-103). Many primate species learn to fear snakes with little experience (Wilson, 1998:86-87).[3]

See developmental biology and developmental psychology.

File:4 behavior questions.png

Explanations of Animal Behavior: Causal Relationships; Adopted from Tinbergen (1963)

Causal relationships

The figure shows the causal relationships among the categories of explanations. The left-hand side represents the evolutionary explanations at the species level; the right-hand side represents the proximate explanations at the individual level. In the middle are those processes’ end products—genes (i.e., genome) and behavior, both of which can be analyzed at both levels.

Evolution, which is determined by both function and phylogeny, results in the genes of a population. The genes of an individual interact with its developmental environment, resulting in mechanisms, such as a nervous system. A mechanism (which is also an end-product in its own right) interacts with the individual’s immediate environment, resulting in its behavior. Here we return to the population level. Over many generations, the success of the species’ behavior in its ancestral environment (or more technically, the environment of evolutionary adaptedness) may result in evolution as measured by a change in its genes.

In sum, there are two processes—one at the population level and one at the individual level—which are influenced by environments in three time periods.


Returning to the initial issue of why we see, here are explanations by the four categories:

  • Ultimate (functional): To find food and avoid danger.
  • Phylogeny: The vertebrate eye initially developed with a blind spot, but the lack of adaptive intermediate forms precluded the loss of the blind spot.
  • Development: Neurons need the stimulation of light to wire the eye to the brain (Moore, 2001:98-99).
  • Proximate (mechanistic): The lens of the eye focuses light on the retina visual system.

The Westermarck effect is the lack of sexual interest in one’s siblings (Wilson, 1998:189-196):

  • Ultimate (functional): To discourage inbreeding, which decreases the number of viable offspring.
  • Phylogeny: Found in a number of mammalian species, suggesting initial evolution tens of millions of years ago.
  • Development: Results from familiarity with another individual early in life, especially in the first 30 months for humans. The effect is manifested in nonrelatives raised together, for instance, in the Israeli kibbutz system..
  • Proximate (mechanistic): Little is known about the neuromechanism.

Use of the four-question schema

The four-question schema is used as the central organizing device in some texts but not others. For instance, it is used in one of the most widely used animal behavior texts (Alcock, 2001) but not in one of the most widely used evolutionary psychology texts (Buss, 2004:12). An advantage of the schema is that it highlights gaps in knowledge, analogous to the role played by the periodic table in the early years of chemistry.

For nonbehavioral aspects of biology (e.g., anatomy), three of the four questions are applicable. Only the proximate mechanism is not relevant, as it cannot be an explanation for itself.

See also



  1. The literature conceptualizes the relationship between function and evolution in two ways. On the one hand, function and evolution are often presented as separate and distinct explanations of behavior (Nikolaas Tinbergen, ethology, Cartwright 2000:10; Buss 2004:12). On the other hand, the definition of adaptation, a central concept in evolution, is a trait that is functional to the reproductive success of the organism and that is the result of natural selection; that is, function and evolution are inseparable. Given this, it is best to conceptualizes function as an evolutionary explanation. The term “function” is preferable to “adaptation”, because it is understandable to students prior to an explanation of evolution.
  2. ”Phylogeny” often emphasizes the evolutionary genealogical relationships among species (Alcock 2001:492; Mayr, 2001:289) as distinct from the categories of explanations. Although the categories are more relevant in a conceptual discussion, the traditional term is retained here.
  3. ”Biased learning” is not necessarily limited to the developmental period.


  • Alcock, John (2001) Animal Behavior: An Evolutionary Approach, Sinauer, 7th edition. ISBN 0-87893-011-6.
  • Buss, David M. (2004) Evolutionary Psychology: The New Science of the Mind, Pearson Education, 2nd edition. ISBN 0-205-37071-3.
  • Krebs, J.R., Davies N.B. (1993) An Introduction to Behavioural Ecology, Blackwell Publishing, ISBN 0-632-03546-5.
  • Moore, David S. (2001) The Dependent Gene: The Fallacy of ‘Nature vs. Nurture’, Henry Holt. ISBN 0-8050-7280-2.
  • Pinker, Steven (1994) The Language Instinct: How the Mind Creates Language, Harper Perennial. ISBN 0-06-097651-9.
  • Tinbergen, Niko (1963) "On Aims and Methods in Ethology," Zeitschrift für Tierpsychologie, 20: 410-433.
  • Wilson, Edward O. (1998) Consilience: The Unity of Knowledge, Vintage Books. ISBN 0-679-76867-x.

External links

Diagrams on Tinbergen's four questions