The Hydraulic Model of Instinctive Behaviour

Lorenz's Theory of Aggression

Lorenz stated aggression was a necessary, adaptive response in animals. By displaying aggressive behaviours, the animal in question has a higher chance of securing the means for its survival.

• These include food, territory, and the right to mate.

Lorenz believed aggression occurred due to a build-up of pressure released by a sign stimulus, resulting in specific behaviours common across the species (FAPs triggered by IRMs, as discussed above). Aggression is a means to secure survival, so it usually does not result in animals fighting to the death, interestingly enough.

Lorenz famously stated, in how his models related to human behaviour:

Innate Releasing Mechanisms in Ethology

Innate releasing mechanisms (IRMs) in ethology are hardwired neural networks in the brain that recognise specific stimuli ( sign stimuli or releases ) to trigger fixed action patterns (FAPs), i.e., a sequence of actions encoded in response to these specific stimuli.

The hydraulic model of instinctual behaviour brings these ideas together to illustrate how this is implemented in an animal, considering the animal’s motivation and aggressive displays.

The Hydraulic Model in Psychology

A hydraulic model in psychology is a physiological or psychological model that proposes that when pressure builds in a system (i.e. a human), the built-up energy will need to be released.

Hydraulic Model of Aggression Definition

The hydraulic model of aggression visualizes Lorenz's idea on how aggressive behaviours occur in animals, specifically in that aggression builds up in a reservoir over time, and a sign-stimulus or releasor unleashes the built-up aggression, resulting in aggressive behaviours. It involves a reservoir (drive), a release mechanism, a stimulus which "unplugs" the reservoir, and resulting behaviours upon release.

Interestingly, Lorenz’s hydraulic model of instinctual behaviour goes back to the work of Freud. He believed that aggression was an inevitable outcome because, in his view, animals, especially males, are biologically programmed to fight for what they deem necessary for their survival (as we discussed above). Take a look at the diagram below:

Fig. 2 - Konrad Lorenz’s psychohydraulic model of motivation shows how aggression and motivation builds in animals (1950)¹.

Lorenz believed that aggression increases over time as the urge to be aggressive builds up in animals. Once the internal pressure to be aggressive builds up enough, a sign stimulus releases it, triggering aggression.

The reservoir is emptied in this case and is more sensitive the longer the time that has passed since the last release. Let's explore how the hydraulic model of instinctive behaviour links to motivation in animal behaviour, specifically, how fluid in the model builds up and drives the mechanism.

Motivation in Animal Behaviour

Motivation is the fluid that accumulates in the reservoir and becomes the drive to act in this mechanism.

1. Action-specific energy or pent-up aggression accumulates in the reservoir, and the sign stimulus serves as the trigger. In this case, the stimulus is the weight that clogs the reservoir.

2. A sign stimulus will ‘release’ the pent-up reservoir, resulting in FAPs, and FAPs can vary depending on how much the reservoir releases. The spouts indicate different behaviours, and different levels of release result in different behaviours.

Once this occurs, the animal’s aggression level drops (known as behavioural quiescence) until the pent-up aggression builds back up, and the process repeats.

Motivation increases over time, so the reservoir builds up over time. Motivation in animal behaviour is specific to the behaviours it triggers (e.g., when a species needs to mate or secure food).

Rhoad and Kalat (1975) observed the aggressive behaviour of male Siamese fighting fish (Betta splendens) in response to other male fish (recognised by their bright colours), a mirror image of the fish, a moving model, and a stationary model. They exposed the fish to each stimulus for one hour per day.

Rhoad and Kalat (1975) found that the fish would puff up in response to any stimulus, exhibiting similar behaviours. The mirror image elicited a response most effectively, followed by the moving and stationary models. However, another fish is still more effective than the mirror image, and its effectiveness does not depend on the stimuli’s order.

• After repeated exposure, the aggressive behaviours decreased rapidly.
• However, the fish were still alert to the stimuli, but they actively avoided them instead of being aggressive (habituation), which could signify how the ‘reservoir’ needs to replenish itself.

Vacuum Activity and Spontaneous FAPs

In Lorenz’s model, either the stimulus causes the release of pent-up aggression, leading to FAPs, or the pressure from the reservoir spontaneously discharges (usually due to a lack of sign stimulus' or releasors).

Known as vacuum activities, the pent-up aggression has built up to a point where it must be released and does so in the absence of external stimuli. It makes its way out and leads to a FAP.

Fig. 3 - Aggression may occur spontaneously when not released, and internal pressures build up, according to Lorenz.

Evaluation of Lorenz’s Hydraulic Model (1950)

Konrad Lorenz’s theories are essential for multiple reasons, namely through their contributions to understanding behaviour in animals, specifically aggression, and acting as a model for comparative aggression in humans.

However, the model has a few problems, which we will address here:

Arms et al. (1979) measured spectators watching aggressive sports on scales of hostility. Male and female participants were exposed to stylised aggression (professional wrestling), realistic aggression (ice hockey), and competitive non-aggressive events (i.e., swimming).

They found aggression and hostility increased rather than decreased after viewing the events. However, this was not the case for the non-competitive events.