(Sexual Swimmers)

3 The Swimmer World

Swimmers is an extension of some previous projects by the author, involving stick figures whose motions are controlled by a set of parameters which evolve with the help of a GA. This project takes a step down in dimension (from 3 to 2), but includes a more automated means of evolving locomotion, by imbedding the figures in a dynamic environment, where the fitness landscape is constantly changing, as in Yaeger's Polyworld, and where fitness equals reproduction. These swimmers are situated autonomous agents, who mate autonomously.

This sets them apart from the organisms recently developed by Sims, which - though they are much more complex - are evolved using objective functions determined by design. This model, then, is more open-ended in terms of modeling reproduction. It also ties together the emergence of individual morphology and locomotion with population dynamics. Individual behaviors emerge which have meaning in the context of the dynamic ecosystems within which they evolve. For instance, particular kinds of swimming styles (i.e., aggressive and energy-wasteful, vs. slow and energy-efficient) can affect, and be affected, by environmental conditions (such as food supply - due to the rate at which swimmers must eat to replenish their energy).

In this world there are only swimmers, food bits, and the boundaries of the pond. A goal in this project has been to make the rules of the game as few and as straightforward as possible - motivated by the desire to minimize complexity in Design so as to focus on Emergence. Despite the model's brevity, the individual swimming techniques and overall group dynamics that emerge are sufficiently realistic, and a continual source of novelty. I define "swimming" in this paper to include any activity that a swimmer does which enables it to move through the pond and to orient itself towards a goal which is itself moving. The swimmers have rudimentary perception and reactivity - which is hard-wired. They are not much on brains, but they have sufficiently versatile motor control systems and morphological schemes, for potentially many locomotion styles. The swimmers were designed with the ability to locate and choose food bits and mates that are within their view. They spend most of their lives "pursuing" these goals.

Key parameters were specified to supply an environment from which optimized behavior can emerge. These include:

maximum life span of swimmers
initial population size and area
how far swimmers can see (view radius)
water viscosity
mass of swimmers
rate at which food multiplies
amount of food at the start of the simulation
various levels of food energy exchange and loss
various constraints in swimmer embryology

These settings are all explained in the following sections.

3.1 The Virtual Pond

The world in which the swimmers live is a two-dimensional virtual pond of water. Fluid flow is not modeled within the pond. Instead, interaction with water is reflected in the dynamics of a swimmer, as body parts stroke through the water and affect the swimmer's translational and angular velocities. Effects such as turbulence, wakes, vortices, etc., are not a part of this model. The pond is defined by a square boundary, which the swimmers cannot go beyond.

3.2 The Energy Cycle

During the running of the simulation, the distribution and dissipation of energy is computed throughout the pond. Energy originates from the food bits scattered around the pond. Food multiplies autonomously, at a slow rate - new bits appear at random positions in the pond at regular intervals. When a food bit is eaten by a swimmer, it is stored in the swimmer and increases its energy level by a predetermined amount. A swimmer uses up its stored energy in three ways:

1) Existing. A very small amount of energy is expended as a base-level metabolism.

2) Moving limbs. The more the swimmer exerts force against water, the more energy is used up (swimmers who exert more force against water become hungry more frequently than others).

3) Mating. The act of mating requires that a certain amount of stored energy be lost and transferred to offspring.

When a swimmer's energy level drops below a designated threshold, it assumes the hungry state, and quits whatever else it happens to be doing and begins to search for a nearby food bit. If it finds one, it will pursue this food bit. If it doesn't find a food bit, or cannot swim to a chosen food bit before its energy level reaches zero, it dies of hunger. If a swimmer is pursuing a food bit which another swimmer then eats, it looks for another food bit to eat.

4 Swimmers

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