# Fixed Action Patterns

Psychology I · Ethology · https://tryals.app/learn/psychology-i/fixed-action-patterns

## Wired-In Behavior

A **Fixed Action Pattern (FAP)** is an innate, stereotyped sequence of behavior that runs to completion once triggered, regardless of environmental changes. Think of it as ballistic, like a fired projectile.

FAPs are triggered by a **sign stimulus** (or **releaser**), a specific environmental feature the animal is tuned to detect. Konrad Lorenz and Niko Tinbergen proposed the **Innate Releasing Mechanism (IRM)** as the neural filter that unleashes the matching FAP.

| Concept | Definition & Classic Example |
| --- | --- |
| **Sign Stimulus** | A specific trigger, such as the red underside of a rival male stickleback. |
| **Supernormal Stimulus** | An exaggerated trigger that yields a stronger response than reality. |

**Common pitfall:** Assuming a FAP can be stopped partway. Once the sign stimulus triggers it, it cannot be aborted, allowing supernormal stimuli to completely hijack the mechanism.

## Mechanics & Supernormal Cues

Tinbergen discovered that herring gull chicks peck harder at a stick with three red stripes than at a realistic gull head. Exaggerated cues simply out-compete reality for the releasing mechanism's attention.

$$\text{Response strength} \propto \text{sign stimulus intensity}$$

This proportionality means that as the cue is exaggerated into a **supernormal stimulus**, the innate response grows stronger.

| Model Type | Stickleback Response |
| --- | --- |
| Neutral grey model (no red) | No lunge or attack |
| Model with red belly patch | Lunge to attack |
| Exaggerated bright-red model | Aggressive attack response |

**Distinction:** A standard sign stimulus elicits normal species-typical behavior, whereas a supernormal stimulus exploits the IRM to produce a response that exceeds natural biological limits.

## Practice questions

6 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Sort each behavior by whether it is a Fixed Action Pattern or flexible, learned behavior.

**Answer:**

- Fixed Action Pattern: A goose keeps rolling toward the nest even after the egg is taken away, A male stickleback attacks any red-bellied model on sight, A gull chick pecks a red spot to beg, in a stereotyped sequence
- Flexible / learned behavior: A dog learns to sit only after being rewarded many times

**Why:** FAPs are innate, stereotyped, and released by a sign stimulus (egg-rolling, red-belly attack, red-spot pecking); behavior that requires reinforcement or flexibly adapts to circumstances (learning to sit, updating a route) is not a FAP. The discriminator is innate-and-ballistic versus learned-and-flexible.

Page: https://tryals.app/practice/psychology-i/fixed-action-patterns/sort-each-behavior-by-whether-it-is-a-fixed-action-pattern-or

### 2. A gull chick pecks harder at a **striped stick** than at a realistic model of its parent's head. Tinbergen called this a supernormal stimulus. What modern phenomenon has the same structure?

A. Optical illusions, which fool the visual system in the same way
B. Learned preferences, which override instinct once experience accumulates
C. Nothing modern, supernormal stimuli occur only in laboratory conditions
D. Engineered food, concentrated beyond anything ancestral

**Answer:** D. Engineered food, concentrated beyond anything ancestral

**Why:** The releasing mechanism was never tracking parents, it was tracking a cue that reliably meant parent. Exceed the cue and the response exceeds too. A crisp is that stick: it wins on the appetite's own criteria, which is why willpower is the wrong frame for it.

Page: https://tryals.app/practice/psychology-i/fixed-action-patterns/a-gull-chick-pecks-harder-at-a-striped-stick-than-at-a-realistic

### 3. Select every statement that is TRUE about sign stimuli and supernormal stimuli.

A. Birds will sometimes prefer incubating an oversized fake egg over their own.
B. A sign stimulus can be a single feature (like the color red), not the whole realistic object.
C. A supernormal stimulus is simply a life-sized, realistic model of the real object.
D. An exaggerated cue can out-compete the real object for the releasing mechanism's response.
E. A fixed action pattern requires reinforcement to become established.

**Answer:** A. Birds will sometimes prefer incubating an oversized fake egg over their own.; B. A sign stimulus can be a single feature (like the color red), not the whole realistic object.; D. An exaggerated cue can out-compete the real object for the releasing mechanism's response.

**Why:** Exaggerated cues out-competing reality, a single feature serving as the trigger, and preferring an oversized egg are all true. A supernormal stimulus is exaggerated (not merely realistic), and a FAP is innate rather than reinforced - so those two are false.

Page: https://tryals.app/practice/psychology-i/fixed-action-patterns/select-every-statement-that-is-true-about-sign-stimuli-and

### 4. A goose that has begun the egg-rolling fixed action pattern will stop mid-movement if the egg is taken away.

**Answer:** False

**Why:** False. The pattern runs to completion regardless, the goose keeps tucking an egg that is no longer there. That is the defining property: a releasing stimulus starts it, and it then executes ballistically without feedback, which is what distinguishes it from goal-corrected behaviour.

Page: https://tryals.app/practice/psychology-i/fixed-action-patterns/a-goose-that-has-begun-the-egg-rolling-fixed-action-pattern-will-stop

### 5. A greylag goose whose egg is taken away mid-retrieval finishes the rolling sweep with nothing under her bill. Why would selection favour a behaviour that carries on after it has plainly become pointless?

A. Because completing the sequence is how she learns to perform it accurately on future occasions.
B. Because a fixed action pattern is a reflex, and no reflex can be interrupted by anything once begun.
C. Because the sequence is released rather than steered.
D. Because the goose cannot perceive the egg's absence, her visual system being suppressed for the duration of the movement.

**Answer:** C. Because the sequence is released rather than steered.

**Why:** It looks like a bug and it is a reasonable trade: checking the target continuously needs sensing, comparison and the ability to abort, and eggs very rarely vanish mid-roll. The empty sweep is the price of a ballistic design, and the moment a behaviour runs into pointlessness it reveals it was never being guided by the goal.

Page: https://tryals.app/practice/psychology-i/fixed-action-patterns/a-greylag-goose-whose-egg-is-taken-away-mid-retrieval-finishes-the

### 6. Order the causal chain of a Fixed Action Pattern, from trigger to completion.

**Answer:**

1. A sign stimulus (releaser) appears in the environment.
2. The Innate Releasing Mechanism detects that specific feature.
3. The IRM unleashes the matching fixed action pattern.
4. The behavioral sequence runs to completion, ballistically.
5. It finishes even if the triggering stimulus has since disappeared.

**Why:** A sign stimulus is detected by the IRM, which releases the FAP, and the sequence then runs to completion regardless of later change. The ballistic, cannot-be-stopped ending is exactly what defines a FAP.

Page: https://tryals.app/practice/psychology-i/fixed-action-patterns/order-the-causal-chain-of-a-fixed-action-pattern-from-trigger-to