Reproduction Model

created with NetLogo


This model is a representation of how populations change over time and are effected by different changes in their environment. The turtles can reproduce if they meet a turtle of the opposite sex and die when they grow old. If a turtle is below a certain age or above a certain age, however, it will be unable to reproduce even if it meets another turtle. In general, the population will rise exponentially.


In the model there are two different types of turtles, male and female. Each turtle takes a turn following a set of instructions that tells it how to behave.

First, the turtle will move in a random direction. Once it moves, it will then age, and if its age is above it's lifespan, it will die. It then checks to see if there are any other turtles on the spot where it has landed. If there are, the turtle checks to see if the turtles it is touching are of the opposite sex, and if it is at an age where it can mate. If the answer is yes, it will check the birthrate variable to find out what the probability is that it will give birth, and has a chance of giving birth. This cycle repeats for each turtle, and then plots the populations of the turtles on a graph.


In this model, variables that can be changed by the user are to the left of the display and variables being monitored are on the right side.

The SETUP button will set up the model. It will reset the monitors and clear the screen, and then will create a number of turtles equal to the INITIALPOPULATION slider value. It will change each turtle into a male (blue) or female (pink) using the probability chosen in the PERCENTCHANCEGIRL slider.

The GO button will start the simulation and begin the lifecycles of the turtles (for more information, see the HOW TO USE IT SECTION).

The INITIALPOPULATION slider bar will set the number of turtles created the beginning of the simulation. The more, the higher the initial population and the more likely a turtle will find a mate.

The LIFESPAN slider bar will set the number of turns a turtle will live before dying. The higher this, the longer the population of turtles persists.

The BIRTHRATE slider bar will set the probability that a turtle will give birth when it meets a turtle of the opposite sex. The lower, the less likely there will be more turtles.

The PERCENTCHANCEGIRL slider bar sets the probability that any turtles hatched will be a girl. The higher this is, the more likely your turtle will be a girl. More turtles will hatch if there are an equal number of male and female turtles. If the number is uneven, the turtles will die out faster.

The MATURITYAGE slider bar controls how old a turtle has to be in order for it to reproduce. If the turtle is younger than this, it can't reproduce.

The OLDAGE slider bar controls how young a turtle has to be in order for it to reproduce. If the turtle is old than this, it can't reproduce.

The NUMBER-OF-OFFSPRING slider bar controls how many children the turtles will have when they mate.

The POPULATION-CAP slider bar controls the maximum number of turtles that can be in the model.

The TURTLE POPULATIONS graph shows the total number of turtles (green), the number of females (pink) and the number of males (blue).

The monitor POPULATION counts the total number of turtles.

The monitor FEMALES counts the number of females.

The monitor MALES counts the number of males.


Try changing any of the variables outlined above! Look under HOW TO USE IT for information on the effects of changing each variable.

One interesting thing to try changing is the ratio of males to females. You may notice that when there is an even number of males and females, the population grows, but when there are more female or male turtles, the population often dies out.


To the basic format for a population model I added the slider for percent chance female, and I also added the minimum and maximum age for reproduction sliders. To easily see the data collected, I made graphs and monitors for the total population and the populations to females and males.

Another addition I would like to make would be to make a maximum population of turtles, to better simulate what conditions would be like in real life. I would also like to add a food source that the turtles must compete for.


Created by Ada Taylor on April 1, 2008 for the Apprenticeship Program at Shodor Education Foundation.

This page was automatically generated by NetLogo 4.0. Questions, problems? Contact

The applet requires Java 1.4.1 or higher. It will not run on Windows 95 or Mac OS 8 or 9. Mac users must have OS X 10.2.6 or higher and use a browser that supports Java 1.4. (Safari works, IE does not. Mac OS X comes with Safari. Open Safari and set it as your default web browser under Safari/Preferences/General.) On other operating systems, you may obtain the latest Java plugin from Sun's Java site.