Development of Behavior

Behavior does not appear fully formed in most animals. It develops over time as the body grows, the nervous system matures, and the individual interacts with its environment. This chapter focuses on how behavior emerges and changes during the lifetime of an individual organism (its ontogeny). General goals and methods of behavioral research, as well as basic mechanisms of behavior (innate vs. learned), are treated in other chapters; here we only consider how these mechanisms unfold over developmental time.

What “Development of Behavior” Means

In biology, the development of behavior refers to:

How behavioral patterns arise from the interaction of:

Genetic information
Internal physiological and nervous system maturation
External influences such as environment, learning, and social interactions

How the behavioral repertoire changes from embryo or newborn to adult and, in some cases, into old age.

Three key ideas run through this topic:

Behavior is not simply “inborn” or “learned.”
Almost all behaviors result from a combination of genetic predispositions and experience.
Developmental “sensitive periods.”
At certain times, the nervous system is particularly open to specific environmental influences; experiences in these phases can have long‑lasting or irreversible effects.
Matching behavior to life stage.
Different life stages (larva, juvenile, adult) often require different behaviors; development organizes these changes in a coordinated way.

Genetic Basis and Environmental Shaping

Genetic Predispositions for Behavior

Genes do not encode specific behaviors directly (like “build nest A”), but they:

Specify the structure and chemistry of the nervous system
Influence sensory organs and muscles
Shape hormones and their receptors
Bias attention and motivation toward certain stimuli

Thus, genes create predispositions: an animal may be more likely to orient to particular sounds, build a certain type of nest, or respond in a characteristic way to a predator. Without suitable environmental conditions, however, these predispositions may remain incomplete or never expressed.

Environmental Influences and Experience

Environmental factors that can influence behavioral development include:

Physical environment: light, temperature, space, complexity of surroundings
Social environment: presence of parents, siblings, peers, and other species
Nutritional status: quality and quantity of food
Stressors: predators, crowding, disturbance

These influences can:

Trigger or suppress the expression of genetically based behaviors
Shape the details (fine structure) of behaviors
Alter thresholds at which behaviors are shown (e.g., aggression occurring more or less easily)

Interaction: Nature and Nurture Together

Behavioral development emerges from interaction between genes and environment. Some illustrative patterns:

Canalization:
Some behaviors appear very reliably despite varied environments. They are strongly genetically stabilized (e.g., basic sucking behavior in newborn mammals).
Plasticity:
Other behaviors are highly variable and shaped by experience (e.g., many aspects of song in songbirds, or hunting techniques in some predators).

Different behaviors lie along a continuum between these extremes.

Sensitive and Critical Periods

During development, certain phases are especially important for acquiring or stabilizing specific behaviors.

A critical period is a relatively short, well‑defined time window during which a particular experience must occur for normal development of a behavior. Outside it, the behavior cannot develop properly or at all.
A sensitive period is a somewhat broader window during which experience has especially strong influence, although learning may still be possible later, just less effectively.

Imprinting as a Model Process

Imprinting is a special form of early learning with the following features:

It occurs during a narrow early period (often shortly after birth or hatching).
It requires specific stimuli (e.g., movement and sound of a parent).
It leads to long-lasting, often irreversible changes in behavior.

Two well-known forms:

Filial imprinting:
Young animals (e.g., ducklings, goslings) learn to recognize and follow the parent or parent‑like object encountered in the critical period. This shapes later social behavior and species recognition.
Sexual imprinting:
Later partner preferences may be influenced by what the young animal experienced as “normal” adult individuals of its species (appearance, song, smell etc.).

Imprinting illustrates how innate readiness (the tendency to follow a moving object) is combined with specific early experience (which object is followed) to form a stable behavior.

Other Examples of Sensitive Periods

Bird song development:
Young songbirds often must hear adult song during a specific juvenile period to later produce a normal song. If isolated then, their song remains abnormal even if they hear conspecific song as adults.
Human language acquisition:
Children learn language most easily during early childhood. Severe deprivation of language input in this period leads to long-lasting deficits, even if input improves later.

Maturation and the Role of the Nervous System

Not all behavioral change is due to learning. Part of behavioral development is maturation: the unfolding of genetically programmed changes in the body and nervous system.

Maturation vs. Learning

Maturation includes:

Growth and myelination of nerve fibers
Formation and pruning of synaptic connections
Hormonal changes (e.g., at puberty)
Development of muscles and sensory organs

Learning involves:

Modifying the strength and pattern of neural connections in response to experience
Storing information from interactions with the environment

Many behaviors appear only when both maturation and relevant experience have occurred. For example:

Young animals may practice motor patterns (play-fighting, clumsy flying) that only become effective behaviors once their muscles and nervous system are sufficiently mature.
Sexual and parental behaviors often appear only after hormonal changes associated with sexual maturation.

Neural and Hormonal Milestones

Key developmental changes that affect behavior include:

Early synapse overproduction and pruning:
Initially, more neural connections are formed than will remain. Repeatedly used connections are stabilized; others are eliminated. Experience influences which connections are kept.
Critical synaptic stabilization phases:
Some connections stabilize only if specific stimuli are encountered in the right time window (e.g., certain visual patterns for normal vision).
Hormonal transitions:

Thyroid hormones, steroid hormones, and others shape brain development.
Puberty‑related hormones trigger courtship, territoriality, and other adult behaviors.

Thus, the timing of behavior development is closely linked to internal developmental schedules.

Early Experience and Long-Term Effects

Experiences in early life often have disproportionate and lasting influence on later behavior.

Early Social Experience

Social interactions with parents and siblings can:

Provide models for species‑typical behaviors (e.g., grooming, play, feeding)
Establish basic social competencies (e.g., recognizing conspecifics, reading signals)
Calibrate aggression levels and social tolerance

Insufficient or abnormal early social experience may lead to:

Deficits in social communication
Overly timid or overly aggressive behavior
Abnormal sexual or parental behaviors

Handling, Stress, and Coping Styles

Gentle handling or moderate, controllable challenges in early life can:

Improve later stress tolerance
Promote exploratory and flexible behavior

Conversely, extreme or chronic early stress can:

Heighten fear and anxiety
Bias behavior toward avoidance or aggression
Alter physiological stress systems in lasting ways

These long-term influences illustrate that behavioral development shapes not only specific actions but also general styles of reacting (often called coping styles or behavioral syndromes).

Play and Practice

Play behavior is especially common in juveniles of many vertebrates (mammals, some birds). From a developmental perspective, play:

Provides practice for later behaviors (e.g., hunting, fighting, courtship)
Enhances motor skills, coordination, and flexibility
Offers safe experimentation with social roles (e.g., switching between chasing and being chased)
Helps calibrate limits and rules (biting inhibition, conflict resolution)

Although play may look purposeless in the moment, it is a crucial part of normal behavioral development. Animals deprived of opportunities to play often show:

Clumsy or inappropriate social behavior
Poor conflict management
Reduced flexibility in novel situations

Behavioral Transitions Between Life Stages

Different life stages often require fundamentally different behaviors. Behavioral development organizes these transitions.

Metamorphosis and Stage‑Specific Behavior

Species with metamorphosis (e.g., many insects, amphibians) show:

Larval behaviors: feeding, avoiding predators, often different habitat use
Adult behaviors: dispersal, reproduction, courtship, territoriality

Neuromuscular systems and hormonal states are reorganized between stages, enabling new behaviors while others disappear or are modified. For example:

Tadpoles (larval frogs) are largely aquatic, herbivorous, and show schooling.
Adult frogs become more solitary, air-breathing, and insect‑eating, with new motor patterns for jumping and mating calls.

Age-Specific Roles in Social Animals

In some social insects and vertebrates, individuals of different ages perform distinct tasks:

Young individuals may stay in the nest/colony (care for brood, clean).
Older individuals forage, defend, or reproduce.

These age‑polyethisms (age‑related task patterns) arise from:

Internal changes (e.g., hormone levels, brain state)
Social regulation (e.g., cues from other colony members or brood)

Behavioral development thus helps integrate individuals into the social structure of the group.

Canalization and Individual Differences

Even under similar genetic and environmental conditions, individuals differ in their behaviors. Developmental processes contribute to both:

Stability within individuals
Variation between individuals

Canalization: Producing Reliable Outcomes

Some developmental pathways are canalized:

They tend to produce the same behavioral outcome even when conditions vary moderately.
This helps ensure that essential behaviors (e.g., basic locomotion, care for offspring) are reliably developed.

Mechanisms behind canalization include:

Redundant neural circuits
Self‑correcting feedback loops between behavior and environment
Robust genetic and hormonal programs

Emergence of Individuality

At the same time, small differences in:

Early experiences
Social position
Random variation in development (so‑called developmental noise)

can lead to individual behavioral profiles (e.g., bolder vs. shyer, more explorative vs. more cautious). These profiles may remain stable over long periods and can influence survival and reproductive success.

Behavioral development, therefore, does not aim at producing identical copies but rather a range of suitable strategies within species‑specific boundaries.

Summary: Key Features of Behavioral Development

Behavior develops over time through interaction of genes, maturation, and experience.
Sensitive and critical periods mark times when particular experiences shape behavior with especially strong and often lasting effects.
Imprinting serves as a central example of early, durable learning that depends on a narrow time window.
Maturation of the nervous and hormonal systems sets the timetable for many behaviors, while learning refines and adapts them to current environments.
Early experiences, including social interactions and exposure to stress, can have large, long-lasting impacts on behavioral tendencies.
Play is a key developmental tool for practicing and shaping complex motor and social behaviors.
Behavioral development coordinates stage-specific behaviors, aligning them with ecological and social demands of each life stage.
Despite canalization, developmental processes generate individual differences, contributing to behavioral diversity within a species.

Subsequent chapters will examine in more detail how specific juvenile behaviors and mechanisms of learning contribute to this developmental process.