link to SALMON home page  Lorenz Hydraulic Model of Motivation
Author Paul Kenyon


We know from everyday experience that the more motivated we are, the more responsive we are to stimuli in our environment. For example, if I am very hungry I am more likely to eat something that I am not particularly fond of, just to satisfy my hunger.

In this simulation you can explore how differences in motivation affect behaviour in the presence of various stimuli.
More specifically, you will see how the courtship behaviour of male fish is a function of:

This VRML simulation on this page is loosely based on a study of courtship in male guppies carried out by Baerends et al. (reference: Behaviour, 8, 249-334, 1955).

The results are used to explain several important theoretical ideas from classical ethology:

Lorenz's Hydraulic Model

Lorenz' model describes the relationship between:

  • behaviour (fixed action patterns)
  • motivation (action specific energy)
  • external stimuli (sign or releasing stimuli)
This animation is based on the type of (static) diagram used in many textbooks to explain Lorenz' model
conventional view of Lorenz' model

Fixed action patterns (FAPs) are relatively stereotyped behaviours (i.e. they seem to run like clockwork) exhibited by all members of a species under appropriate conditions.
FAPs are normally seen when an animal in an appropriate motivational state is exposed to the appropriate external stimulus - this stimulus was called a releaser or sign stimulus by ethologists.
In this model male fish exhibit three different FAPs. This collection of FAPs is called an ethogram.

One of your tasks will be to identify the three FAPs which make up the ethogram of male behaviour in the simulation.

However this task is not straightforward because the actual FAP exhibited by the male depends upon:

In the simulation the colour of the male's body reflects his internal motivation.

Later on you can explore the simulation to determine which male body colour denotes:

In the simulation, the behaviour and size of the female represents her releaser value:

The simulation allows you to determine which female behaviour acts as a :

for male courtship behaviour

Getting started

Install Cortona VRML Client!If  the full size simulation has not loaded, then you need to install the Cortona VRML plugin  into your web browser 

vrml screenshot01

The first thing to do is to distinguish between male and female fish in the simulation. This is relatively easy:
  • There is a school of male fish on the left hand side of the screen.
  • The female fish is not visible until you activate the model by clicking on one of the triangles / cones (green, orange or purple) on model
  • Click any cone
  • Did you see the female appear and disappear? 
  • If not try again, or click another cone. 
Do not worry if you cannot understand the reasons for all the other effects you see. All will be revealed!


The coloured cones (green, orange and purple) represent sign stimuli or releasers. The cones differ in size and colour to reflect their relative strength. In this model there are three releasers of increasing potency:

  1. weak - green
  2. medium - orange
  3. strong - purple

Task #1. When you click on one of these cones several things happen in the model.
Concentrate on these changes:

  • the cone is placed onto a scale pan
  • the behaviour of the female fish depends on which cone you select. 
  • these female behaviours include:
    • swimming forward
    • swivels  her body from side-to-side
    • swivels her body up-and-down

Try to identify the three releasers (female behaviours) and determine their relative strength. 

Exploratory activities

Focus on the behaviour of male fish when they encounter females that differ in attractiveness. 

I think you will find it easier to explore the reactions of the fish by starting with the blue fish and discovering how he reacts to each of the female releaser stimuli. Record your results.

Then observe and record the behaviour of the red and yellow males.
Finally, rank order the motivation of the various males.Remember that the more motivated an animal is, the more likely he is to respond to a relatively weak stimulus.
You are now in a position to appreciate the law of heterogeneous summation.

The law of heterogeneous summation

In this simulation you have seen how two very different (heterogeneous) factors (male internal motivation) and external releasing stimuli add together (summate) to influence behaviour; the behaviour of the male fish is an example of the law of heterogeneous summation in action.
Similar additive effects of stimuli have been observed in field investigations of animal behaviour. One nice example is work by Baerends and Kruijt demonstrating that egg retrieval by herring gulls is a function of both egg colour and pattern; these variables obey the law of heterogeneous summation. (reference: Stimulus selection. In Constraints on Learning: Limitations and Predispositions, eds Hinde & Stevenson-Hinde, 23-50,Academic Press, London,1973).
A web page describing these results is available (see Follow up activities).

Action specific energy

This far we have not focussed on the mechanics of Lorenz's model - it has been noisily flushing in the background. We can now turn our attention to the various components of the 'toilet'.
In Lorenz's model motivation increases with the passage of time. This motivation is specific for one type of behaviour (e.g. either feeding, or fighting or sexual behaviour). This specific source of motivational energy is called action specific energy. It is represented by the accumulation of water in the blue reservoir. The reservoir is filled by a tap. In the model on this page, the tap only turns after the fish have exhibited courtship behaviour.

Points to ponder:

The reason I did not make the tap turn continuously is that doing so would place a burden on your computers ability to display the model - animation eats processing cycles!
However Lorenz postulated that the reservoir was being continuousy filled with water in order to expain a phenomenon which he called vacuum activity.

Vacuum activity

Lorenz coined the term vacuum activity to describe behaviour which apparently occurs in the absence of any external stimulus. In the hydraulic model, action specific energy can accumulate to such a high level that the pressure of water in the reservoir is capable of pushing open the restraining valve. This causes water to flow into the trough, and out through holes in the floor that represent fixed action patterns. You can see this effect for yourself by clicking on the reservoir (the blue cube in the model). The cones representing sign stimuli disappear to emphasise that what happens next does not depend on their presence. Then the blue fish swim forward across the screen representing vacuum behaviour.

The innate releasing mechanism

When you activate the model:

Lorenz introduced the term innate releasing mechanism (IRM) to describe a central ( located somewhere in the brain) mechanism that handled the link between external stimulus, internal motivation and behavioural output. The scale pan, pulley, trough and outflow pipes in the model correspond to the IRM.

If you look closely you can see that the smallest releaser causes water to flow from one pipe whereas all three pipes are filled when the strongest sign stimulus is activated. This is actually a limitation of the way in which I have constructed the model. As it stands the toilet part of the simulation behaves in a way that corresponds to one colour of fish.

Point to ponder: Can you identify which coloured fish most closely resembles the actions of the hydraulic model?

Behavioural quiescence

An important feature of the model is that after the animal has engaged in a particular behaviour (FAP) there is a period of time when they less likely to respond even if the same stimulus is presented again - behavioural quiescence.
This occurs because the reservoir has been drained of action specific energy. In the simulation this is represented by having the cones (representing sign stimuli) disappear as the reservoir is being refilled with water.
In fact the simulation simplifies the story somewhat. For example, if the fish has just reponded to a weak sign stimulus and only a little water has flowed into the trough, then it would respond if it was presented with another stimulus.

Point to ponder: Can you predict what simuli would be responded to, and what behaviour the various fish would display?

Limitations of Lorenz's Hydraulic Model

Models and theortetical systems are useful because they organise research findings and suggest further experiments. Models often help reveal the principles underlying the organisation of behaviour. But they do not necessarily tell us much about the actual brain mechanisms involved.
Clearly the brain does not operate like a toilet - except in some special circumstances! One of the most powerful criticisms of Lorenz's model is that it does not include any mechanism for learning. There is no way that the consequences of the animal's behaviour can feedback into the system to influence subsequent behaviour.
Lorenz saw no need for it to include feedback. After all he was interested in FAPs - behaviours which by definition do not change over an animal's lifetime. We now know that FAPs do not always spring fully-formed into the animal's behaviour. FAPs are subject to modification during development.
But this is not a criticism of Lorenz, rather it is an illustration of how Lorenz's ideas led to greater understanding. You have got to start somewhere, and the journey will be a lot easier if the first steps are taken by someone of Lorenz's stature.

Lorenz was not blind to the crude nature of his theoretical model.

In 1950 he wrote:
"This contraption is, of course, still a very crude simplification of the real processes it is symbolizing, but experience has taught us that even the crudest simplisms often prove a valuable stimulus to investigation."

Nevertheless the model has now fallen out of favour because it proved impossible to locate structures in the brain where action specific energy accumulated within a reservoir or processes that functioned like the postulated IRM. In other

So what .....

Students often ask "So what has this got to do with human behaviour?"
One answer is that Lorenz's model of motivation has been used as an explanation of human aggression. You will recall from from an earlier section that:

"In Lorenz's model motivation increases with the passage of time. This motivation is specific for one type of behaviour (e.g. either feeding, or fighting or sexual behaviour). This specific source of motivational energy is called action specific energy. It is represented by the accumulation of water in the blue reservoir. "

According to this view the only way to reduce human aggression is to engage in aggressive behaviour. Furthermore there will come a time - according to the model - when aggressive behaviour will be exhibited in the absence of any provoking stimuli, so called vacuum activity.

You might wonder if this view was seriously held. Yes it was. In 1963 Lorenz published a book "On Aggression". It was republished in 1996 by Rouledge with a special introduction by Eric Salzen which is well worth reading. Salzen describes how the controversy raised by Lorenz's ideas culminated in 1986 when UNESCO and the American Psychological Association published a statement which was intended to:

".. dispel the widespread belief that human beings are inevitably disposed to war as a result of innate, biologically determined aggressive traits"

Follow up activity

Draw a graph of the of the behaviour of the male fish in the simulation as a function of:

Compare the simulation results with those found by Baerends et al (1955) which are reproduced in many animal behaviour textbooks e.g.

These books give excellent accounts of classic ethological concepts and discoveries. Alcock provides an excellect insight into the research and thinking of contemporary behavioural biologists in his book Animal Behavior: An Evolutionary Approach (6th Ed. Sinauer, 1998.)

A note to instructors

You may wonder why I have based this teaching resource on such a heavily criticised set of concepts and theoretical model. The answer is that - from a teaching point of view - they provide fertile ground. This resource was designed for use with first year psychology undergraduates as part of module taught by a small team of lecturers who present views on a single topic from different perspectives (biological, behaviourist, cognitive etc.)

My aim is to gently expose students to:

At first I expect students to be confused by what is going on. That is an intentional part of my strategy. I want students to begin to appreciate behavioural complexity and the problems of observation.
Dr Paul KenyonYou may be interested in visiting the SALMON Bookshop which is a participant in the Amazon Associates Program and offers books that I have carefully selected to complement the material covered on this website.
I hope you find this a useful SALMON resource.

Paul Kenyon