How can we use foxes and rabbits help transmit secret messages? No animal cruelty is necessary - a new technique encodes messages using a predator-prey model normally used to predict the changes in animal populations. This advance in steganography could revolutionize the art of secrecy.

Cryptography is the art of disguising messages so they're incomprehensible to anyone but the recipient, while steganography is the art of disguising messages' very existence. One famous example of steganographic communication comes from Greek historian Herodotus, who told the story of a slave whose head was shaved bald and then tattooed with a message. When the slave's hair grew back, nobody suspected the tattoo's existence, allowing him to travel freely to the recipient, shave his head, and reveal the message.

In our information age, steganographers more often conceal information within a digital file. The file may look like an innocent photo, except that the ones and zeroes in the code of certain pixels have been modified. One technique involves taking a digital image and imperceptibly brightening certain pixels - perhaps every tenth, or hundredth, or ninety-third - to hide another image. Similarly, you could have a text message where letters correspond to the amount of the color shift. Although this technique effectively hides secrets from the naked eye, computer programs can pick out the message from the mess by detecting the changed pixel distribution.

But now there's a new steganographic method that can fool computers as well as people. It's described in Physical Review E, and uses predator-prey models to further disguise a hidden message.

In general, a predator-prey model uses mathematical equations to predict how a population of, say, rabbits and foxes will wax and wane over time. We would expect that as the rabbit population increases, they create more prey for the foxes, whose population will increase in response. But as more and more foxes eat the rabbits, the rabbit population declines, decreasing the foxes' food supply and forcing their population to decline as well. Based on common-sense rules like these, predator-prey models use different equations to predict the exact details of this cycle, taking into account the environment, the availability of resources, and the initial populations of each species.