Mechanisms of Behavior

Table of Contents

Overview: How Behavior Is Produced and Controlled

Behavior arises from the interplay of an animal’s body, nervous system, hormonal system, and environment. In this chapter, the focus is on the mechanisms that make behavior possible: how stimuli are received, processed, and turned into movements; how internal states like hunger or fear change behavior; and how these processes can be studied. The specific roles of “Innate Behavior” and “Learned Behavior” are treated in their own chapters; here we look at the machinery behind both.

Key questions:

How do stimuli from the environment trigger behavior?
How are actions organized into coordinated movement patterns?
Which internal factors (hormones, motivation, biological clocks) shape behavior?
How do simple and complex nervous systems generate behavior?

From Stimulus to Response: Stimulus–Response Chains

Many behaviors can be roughly described as stimulus–response sequences:

Stimulus: A change in the internal or external environment (light, sound, smell, internal nutrient level, etc.).
Reception: Specialized sense organs or receptor cells detect the stimulus.
Processing: Nerve cells and networks evaluate the incoming information.
Decision / Selection: One or more responses are selected from many possibilities.
Execution: Muscle cells, glands, or other effectors carry out the response.

This model is a simplification, but it helps to structure the mechanisms involved.

Receptor Organs and Modalities

Receptors transform specific kinds of energy or substances into nerve signals. Different receptor types respond to different modalities:

Mechanoreceptors: Respond to pressure, vibrations, stretch (e.g., touch receptors, hair cells in ears, stretch receptors in muscles).
Photoreceptors: Respond to light (eyes and simpler light-sensitive spots).
Chemoreceptors: Respond to chemicals (taste and smell, but also internal receptors for CO₂, O₂, glucose).
Thermoreceptors: Respond to temperature changes.
Nociceptors: Respond to potentially damaging stimuli (pain).
Electroreceptors and Magnetoreceptors: In some species, detect electric fields or Earth’s magnetic field.

Each receptor type has:

A threshold: the minimum stimulus intensity needed to trigger a signal.
A specificity: best sensitivity for one kind of energy; other forms are usually less effective.

Key Concepts: Sign Stimuli and Releasing Mechanisms

Not all stimuli are equally important. Many behaviors are triggered by very specific cues:

Sign stimulus (key stimulus): A particular feature of the environment that reliably triggers a behavioral pattern, even if other details vary.

Example: In many fish and bird species, a specific color patch, shape outline, or movement triggers aggressive or courtship behavior.

Innate releasing mechanism (IRM): Hypothetical neural mechanism that recognizes a sign stimulus and “releases” a specific behavioral routine. In modern terms, this corresponds to specialized neural circuits tuned to key features.

Sometimes animals react more strongly to exaggerated or artificial versions of a sign stimulus than to a natural one. These are called supernormal stimuli (e.g., birds preferring to brood on an oversized, more vividly colored “egg” model).

Motor Patterns: How Movements Are Organized

Behavior is rarely just a simple reflex. Even “simple” actions like pecking, grooming, or walking require the precise coordination of many muscles.

Motor Programs and Fixed Patterns

Many behaviors are produced by motor programs: neural activity patterns that:

Define the sequence and timing of muscle contractions.
Can run with little modification once started.
Often continue even if the original stimulus disappears (after a brief starting phase).

Some behaviors show the features of fixed action patterns:

Relatively stereotyped form (similar every time).
Initiated by specific sign stimuli.
Often run to completion once started, even if the stimulus is removed.

However, modern research shows that even “fixed” patterns are frequently modulated by feedback and internal state.

Motor Control Circuits

Movements are controlled on several levels:

Spinal cord / segmental ganglia: Generate many basic reflexes and repetitive movements (walking, swimming, flying patterns in invertebrates and vertebrates).
Central pattern generators (CPGs): Neural networks that can produce rhythmic, coordinated outputs (walking, chewing, breathing, wing beats) without needing detailed commands for every movement step.
Higher centers (brain regions): Start, stop, modulate, or combine these patterns depending on context (e.g., whether to run, walk, or stand still).

Neural Mechanisms of Behavior

The nervous system is central for behavior mechanisms. Details of nervous system structure are covered elsewhere; here we focus on how it functions in generating behavior.

Neurons and Synapses as Building Blocks

Neurons communicate via:

Electrical signals along the axon.
Chemical signals at synapses using neurotransmitters.

Features relevant to behavior:

Convergence: Many inputs combine onto one neuron; behavior can therefore integrate multiple stimuli.
Divergence: One neuron influences many downstream cells; a single stimulus can cause widespread changes (e.g., startle response).
Excitation and inhibition: Neurons can either increase or decrease the activity of target cells, allowing flexible selection and suppression of behaviors.

Neural Circuits and Decision-Making

Behavior emerges from activity patterns in neural circuits:

Reflex arcs: Simple circuits with a receptor, one or a few interneurons, and a motor neuron. Important for fast, automatic responses (withdrawal from pain, postural adjustments).
Sensorimotor loops: Feedback from muscles, joints, and skin constantly updates the nervous system, refining ongoing actions.
Action selection networks: Competing neural groups represent different possible behaviors (e.g., flee vs. freeze vs. explore). The currently strongest network suppresses others and determines what the animal does.

Decision-making in animals is thus not usually conscious deliberation, but a competition of neural networks shaped by:

Current sensory input.
Internal state (motivation, hormonal state).
Previous experience (learning).

Motivation and Internal States

External stimuli alone do not determine behavior. Whether and how an animal reacts depends strongly on its motivation and other internal states.

Motivation Systems

Motivation is the internal readiness to perform certain behaviors. Mechanistically, it arises from:

Internal sensors: Receptors measuring, for example, blood sugar, salt concentration, hormone levels, fatigue.
Integrative centers in the brain that compare:

Current internal state with “setpoints” (desired values).
Current state with memory of past outcomes.

Important motivation types:

Hunger and thirst.
Sexual motivation.
Exploration (curiosity).
Social contact or avoidance.
Fear and anxiety.

Often, multiple motivational systems are active at once and compete. The strongest wins and determines the behavior.

Feedback and Exhaustion of Motivation

Motivation is not static; it changes through:

Negative feedback: Performing the behavior reduces the underlying drive.

Example: Eating reduces hunger, which lowers food-seeking behavior.

Positive feedback (in some cases): Performing a behavior initially can increase the tendency to continue (e.g., social interactions that become more attractive as they proceed).
Satiation and fatigue: Repeated performance of the same behavior often leads to decreasing responsiveness to the original stimulus (habituation and motivational decline).

These feedback loops stabilize behavior and prevent endless repetition of unnecessary actions.

Hormonal Influences on Behavior

Hormones are chemical messengers produced by endocrine glands (or equivalent tissues) and transported via body fluids. They act more slowly and diffusely than nerve impulses but can change the overall readiness for certain behaviors.

General Features

Hormonal effects on behavior include:

Increasing or decreasing sensitivities of neurons to stimuli.
Changing the intensity or probability of certain behaviors.
Coordinating long-term changes, such as seasonal behavior patterns.

Examples of hormonal control mechanisms (detailed hormone systems are treated in other chapters):

Sex hormones: Influence courtship, mating, parental care, territorial behavior.
Stress hormones: Prepare the body for fight-or-flight, increasing vigilance and responsiveness to danger cues.
Metabolic hormones: Affect activity level, foraging behavior, and migration readiness.

Hormones and Environmental Cues

Hormone levels often change in response to environmental factors:

Day length and light: Can regulate the onset of breeding seasons, migration, hibernation, or molting via hormonal cascades.
Social environment: Presence or absence of conspecifics (especially dominant individuals) can alter hormone secretion and thereby aggression, submission, or reproductive behavior.

Biological Rhythms and Behavioral Timing

Many behaviors follow rhythms, coordinated by internal clocks.

Circadian and Other Rhythms

Key rhythm types:

Circadian rhythms: ≈24-hour cycles (sleep–wake, activity patterns, feeding times, hormone levels).
Ultradian rhythms: Shorter than 24 hours (feeding bouts, certain hormonal pulses).
Infradian rhythms: Longer than 24 hours (menstrual cycles, seasonal migrations, reproduction).

These rhythms are generated by internal biological clocks (clock genes and specialized brain areas). They are:

Endogenous: They persist even in constant conditions (e.g., constant darkness).
Entrained by environmental cues (zeitgebers), especially light, temperature, and social signals.

Function for Behavior

Biological rhythms:

Optimize behavior to predictable environmental cycles (day/night, seasons).
Coordinate different physiological processes with each other.
Contribute to phenomena like jet lag and seasonal changes in behavior when zeitgebers change.

Sensory Filtering and Selective Attention

Animals are bombarded with sensory information, but they only react to a small fraction. Mechanisms that limit and prioritize information include:

Sensory filtering at receptors: Many receptors are adapted to specific ranges and changes (e.g., responding mainly to moving rather than stationary stimuli).
Preprocessing in early neural centers: Already in early neural stages, unimportant signals can be weakened or blocked.
Selective attention: Higher brain areas can enhance or suppress processing of certain stimuli, depending on motivation and context.

Example: A hungry animal pays more “attention” to food-related cues than to irrelevant background noises.

This selective processing prevents overload and helps focus on biologically relevant information.

Conflict, Displacement, and Redirection Behaviors

When multiple motivations and stimulus–response tendencies collide, characteristic behaviors can appear that give insights into underlying mechanisms.

Conflict Behavior

If an animal is simultaneously motivated for incompatible actions (e.g., attack vs. flee), one may observe:

Ambivalent behavior: Alternation or mixture of elements from both behavior types (e.g., an animal approaches and withdraws repeatedly).
Displacement activities: Apparently irrelevant behaviors appear, often self-directed (e.g., grooming, pecking at the ground) during conflict or frustration. Mechanistically, when strong but blocked motivational systems cannot express their “proper” behavior, other easily triggered motor programs are released instead.

Redirection

Sometimes a behavior intended for one target is redirected to another, safer or more accessible target:

Example: An animal threatened by a dominant conspecific may direct aggression not at the dominant but at a subordinate or an object.

Such patterns illustrate how competing motivational and motor systems can be reorganized in real time.

Mechanisms in Simple vs. Complex Nervous Systems

Basic principles of behavioral control appear in both simple and complex animals, but their implementation differs.

Invertebrate Models

Many classic insights into behavior mechanisms come from animals with relatively simple nervous systems:

Individual identified neurons can sometimes be linked to specific behaviors.
Central pattern generators and motor programs have been mapped in detail in some insect and crustacean species.
Simple reflex chains can produce surprisingly flexible and adaptive behavior when combined and modulated.

Vertebrates and Higher Brain Centers

In vertebrates (including humans), behavior mechanisms involve:

More layered and specialized brain regions (e.g., structures involved in emotion, memory, and planning).
Greater capacity for learned modifications of innate programs.
More complex action selection processes, though still built on the same basic principles: sensory input, internal state integration, neural competition, hormonal modulation, and motor execution.

Integrating Mechanisms: From Components to Whole Behavior

Any real-world behavior typically combines:

Sensory input (stimuli, sign stimuli, supernormal stimuli).
Central processing (neural circuits, motivation systems, biological clocks, learning).
Hormonal state (long-term modulation, seasonal and developmental changes).
Motor execution (motor programs, central pattern generators, feedback control).

Studying mechanisms of behavior thus requires:

Careful observation and experimental manipulation of stimuli.
Recording of neural and hormonal activity when possible.
Analysis of how changes in internal state alter the same external behavior.

These mechanisms form the basis for the more specific topics treated in the chapters on coordination of movements, innate behavior, and learned behavior.