Table of Contents
What Is Innate Behavior?
Innate behavior comprises patterns of action that appear in a species-typical way without prior learning or individual experience. They are:
- Genetically programmed (heritable)
- Largely similar in all normal individuals of a species
- Triggered by specific internal and external stimuli
- Often fully functional at first performance (no “practice” needed)
Innate behaviors form an important contrast and complement to learned behaviors (covered in a separate chapter). In reality, many behaviors have both innate and learned components; here we focus on the innate basis.
Key criteria often used to recognize an innate behavior:
- Stereotypy: The behavior is performed in a relatively fixed, predictable way.
- Species specificity: Same (or very similar) form in all individuals of a species.
- No prior learning required: Appears even in naive animals with no relevant experience.
- Trigger dependence: Appears only in response to certain key stimuli.
- Developmental regularity: Emerges at a characteristic age or stage of development.
Examples include sucking in newborn mammals, web-building in spiders, fixed courtship dances in some birds, and egg-rolling in geese.
Types and Components of Innate Behavior
Although innate behavior can seem very diverse, several common categories and components can be distinguished.
Reflexes
A reflex is a simple, fast, and automatic response to a specific stimulus, mediated by a relatively simple neural circuit (reflex arc).
- Characteristics:
- Involuntary and immediate
- Highly stereotyped (same movement pattern each time)
- Generally resistant to fatigue and variation
- Examples:
- Knee-jerk reflex in humans when the patellar tendon is tapped
- Withdrawal reflex: pulling a hand away from a hot surface
- Pupillary light reflex: pupil constricts in bright light
Reflexes form a basic building block of innate behavior. More complex innate actions can be seen as being built from, or coordinated with, simpler reflexes.
Fixed Action Patterns (FAPs)
A fixed action pattern is a more complex, coordinated movement sequence that, once started, usually runs to completion without being altered by further stimuli.
- Core properties:
- Species-typical, stereotyped sequence (same pattern each time)
- Triggered by a specific stimulus (sign stimulus or releaser)
- Relatively independent of feedback: The pattern often continues even if the stimulus is removed.
- Often goal-directed: Achieves a specific biological function, such as feeding or mating.
- Classic examples:
- Egg-rolling in greylag geese: if an egg is just outside the nest, the goose uses its beak to roll it back with a stereotyped movement. If the egg is removed during the movement, the goose usually continues the rolling behavior to completion as if the egg were still there.
- Courtship displays in many fish and birds: complex sequences of movements (e.g., bowing, spreading feathers) that follow a genetically fixed order.
FAPs form a central concept of innate behavior: they show that animals can have inborn motor programs for complex actions, not just simple reflexes.
Taxis and Kinesis
Innate orientation behaviors help organisms find or avoid regions of the environment. Two important types are taxis and kinesis.
- Taxis: Directed movement toward or away from a stimulus source.
- Positive taxis: Movement toward a stimulus (e.g., positive phototaxis: some insects flying toward light).
- Negative taxis: Movement away from a stimulus (e.g., earthworms avoiding light – negative phototaxis).
- The direction of movement is related to the direction of the stimulus gradient.
- Kinesis: Change in movement activity (speed or turning rate) depending on stimulus intensity, but not direction.
- Orthokinesis: Change in speed when stimulus intensity changes.
- Klinokinesis: Change in turning frequency with stimulus intensity.
- Example: Woodlice move faster and turn more often in dry conditions; in moist areas (their preferred environment) movement slows and turning decreases, so they remain there longer—without actually “heading toward” moisture.
These innate orientation behaviors improve survival (e.g., finding food, escaping predators, maintaining suitable temperature or humidity).
Instinctive Behavior Sequences
Older literature often uses the term instinct for complex forms of innate behavior that fulfill important survival and reproductive functions. In a more precise sense, “instinctive behavior” usually refers to coordinated, multi-step sequences that include several fixed action patterns and often internal motivation states.
- Examples:
- Nest-building behavior in many birds
- Spider web construction
- Complex courtship and mating sequences in insects or fishes
- Hunting sequences in certain predators
Instinctive behaviors may be influenced in details by experience, but the overall structure and main steps are innate.
Stimuli That Trigger Innate Behavior
Innate behaviors do not occur randomly; they are tied to specific releasing conditions. Understanding these conditions is central to understanding when and how innate behaviors appear.
Sign Stimuli (Key Stimuli)
A sign stimulus (or key stimulus) is a specific stimulus pattern that reliably triggers a particular innate behavior.
- Properties:
- Often quite simple in form (e.g., a certain shape, color contrast, sound pattern).
- Usually extracted from a complex environment by the sensory system.
- One and the same stimulus can trigger different behaviors, depending on the internal state of the animal.
- Examples:
- Red spot on the beak of some gull species: pecking by chicks at this red spot triggers parental feeding.
- Specific egg shape and color pattern: triggers egg-rolling behavior in geese.
- Courtship song of male crickets: recognized by females and triggers phonotactic approach (a type of positive phonotaxis).
Supernormal Stimuli
A supernormal stimulus is an exaggerated version of a sign stimulus that can elicit an even stronger innate response than the normal natural stimulus.
- Examples:
- Birds sometimes prefer to incubate a very large, artificially painted egg over their own, normal-sized eggs.
- Chicks peck more often at an artificial beak with an oversized red spot than at the real parental beak.
Supernormal stimuli demonstrate that innate releasing mechanisms respond to certain simple features, not to “realistic” natural objects as such.
Innate Releasing Mechanisms
The concept of an innate releasing mechanism (IRM) refers to the internal neural processing system that recognizes a sign stimulus and activates the corresponding innate behavior.
- It filters sensory input for key features.
- It connects recognition of these features to the appropriate motor pattern.
- It operates largely without learning, based on genetic programming.
Although the detailed neural basis differs among species and is not always known, the concept helps to explain how specific stimuli select specific responses out of many potential behaviors.
Internal Factors in Innate Behavior
Innate behavior is not triggered by external stimuli alone. Internal conditions strongly modulate whether a given behavior pattern will actually be expressed.
Action-Specific Energy and Motivation
Ethologists proposed that innate behaviors have an internal drive or action-specific energy that builds up over time and is discharged when the corresponding action is performed.
- Basic idea:
- For each behavior (e.g., mating, aggression, feeding), energy accumulates internally.
- A suitable external sign stimulus opens the “valve,” releasing the stored action-specific energy as visible behavior.
- After the behavior, the energy level decreases, and the tendency to perform that behavior drops until energy builds up again.
Although “action-specific energy” is a simplified metaphor, the underlying idea of internal motivation states remains important: an animal may ignore a sign stimulus when not motivated (e.g., satiated predator ignoring prey).
Endogenous Rhythms and Hormonal States
Several internal biological processes influence the readiness for innate behaviors:
- Circadian and seasonal rhythms: Many innate behaviors appear only at certain times of day or year (e.g., mating calls in breeding season, migratory restlessness in birds).
- Hormonal influences: Hormone levels (e.g., sex hormones, stress hormones) modulate sensitivity to stimuli and likelihood of particular innate responses.
- Example: Courtship behaviors in many vertebrates depend on sex hormones reaching certain levels.
- Physiological needs: Hunger, thirst, or reproductive state change which stimuli act as effective releasers.
Thus, innate behavior is best seen as a result of interaction between external stimuli and internal state.
Development of Innate Behavior
Even though innate behaviors are genetically programmed, their expression is linked to development.
Maturation
Many innate behaviors require a certain degree of physical and neural maturation before they can be executed.
- Examples:
- Flying in young birds is based on innate motor programs but depends on wing and muscle development.
- Sexual behaviors typically appear only after sexual maturity, when body and hormone systems are fully developed.
Developmental timing of innate behaviors is itself under genetic control.
Experience-Independent vs. Experience-Modified Innate Behaviors
Innate behaviors differ in how much they can be influenced by experience:
- Purely experience-independent behaviors:
- Appear in full form even in completely naive animals.
- Example: First web built by a young spider, or first time a newly hatched insect performs species-typical movements.
- Experience-modified innate behaviors:
- Have a strong innate basis but can be refined by practice or feedback.
- Example: Hunting techniques in many predators have an innate framework, but efficiency improves with experience.
This makes clear that “innate” does not necessarily mean “unchangeable” or “rigid” over an individual’s life.
Innate Behavior and Survival Value
Innate behaviors exist because they confer evolutionary advantages: they increase the chances that individuals survive and reproduce in their typical environment.
Adaptive Functions
Examples of adaptive roles of innate behavior include:
- Self-preservation:
- Startle and escape responses.
- Avoidance of harmful stimuli (e.g., bitter taste often signals toxins).
- Food acquisition:
- Innate hunting or foraging patterns.
- Innate preferences for certain habitats or microenvironments where food is abundant.
- Reproduction:
- Courtship displays and mating rituals.
- Parental care behaviors such as nest-building, brooding, and feeding offspring.
- Orientation and migration:
- Innate migratory directions in many bird species.
- Innate preferences for certain environmental cues (e.g., sun position, Earth’s magnetic field).
Natural selection shapes which innate behaviors persist: actions that systematically lead to higher reproductive success are more likely to be preserved in the gene pool.
Limits and Costs of Innate Behavior
Innate behaviors are not always perfectly adapted to new circumstances:
- They are optimized for ancestral environments; in changed or artificial contexts, they can become maladaptive.
- Example: Many animals are attracted by artificial lights (a maladaptive form of phototaxis), leading to exhaustion or predation.
- Supernormal stimuli can “hijack” innate preferences (e.g., preference for artificial oversized eggs).
- They are often less flexible than learned behaviors.
- Once a fixed action pattern is triggered, it might not easily adapt to sudden changes in context.
Because of these limitations, many species combine innate patterns with learning processes to achieve a balance between reliability and flexibility.
Interplay Between Innate and Learned Behavior
Although treated separately in this course, innate and learned behaviors are usually intertwined in real animals.
- Innate predispositions for learning:
- Animals are innately biased to learn certain things more easily than others (e.g., taste aversion to harmful foods, imprinting on parents or conspecifics).
- Innate “frameworks” with learned fine-tuning:
- Courtship songs in some birds: an innate template guides the general structure of the song, but individuals refine it through listening and practice.
- Species-specific learning windows:
- Critical or sensitive periods in which experience permanently shapes behavior on an innate basis (e.g., imprinting in some birds).
Understanding innate behavior thus provides a foundation for explaining how learning can operate efficiently and within species-typical constraints.