They don't bite, they don't make a mess on the carpet and they don't cost anything to feed. The appeal of virtual animals is clear.

Both cute and attentive, they treat you as if you are the centre of their world - without any vet's fees.

Continues Below↓

Videogames involving animals are perfect family fun, and come in many forms. Virtual pets have been particularly popular - more than 70 million Tamagotchi toys, the Japanese handheld digital pet, have been sold, while the Nintendogs series, in which the player looks after a virtual puppy on their DS, has been a major seller for Nintendo since its launch in 2005.

Other beastly adventures have involved giving players the chance to run a zoo - the Zoo Tycoon titles - or shape the lives of wild animals, such as the SimAnimals franchise. For the developers, they pose a unique challenge - given that these games live and die on the appeal of their stars, recreating nature can be a tough job.

The new title gives animal fans the chance to play the latest in a litter of new animal-inspired titles. World of Zoo, available on the DS, PC and Wii, lets gamers get closer to the animals and more involved in their lives than is possible in real life. For the game, US developer Blue Fang - the company behind the Zoo Tycoon titles which first went on sale in 2001 - had its work cut out, creating more than 90 species across 11 animal "families", with one of the first tasks being to decide how realistic the creatures would be.

"We really wanted World of Zoo to be a fun, high-paced game," says Steve Gargolinski, who worked on the animals' AI (artificial intelligence). "However, one thing that we learned during our trips to the zoo is that real animals often do a lot of lying around."

To deal with this issue, some leaps of imagination had to be made. "We needed certain animals to do things that they never do in the wild," says Gargolinski. "Giraffes don't roll over - but we thought it would be fun for them to do it. So our animators sat down with the challenge of answering the question, 'If giraffes did roll over, what would it look like?' "

The result, Gargolinski hopes, is believable, if not realistic - something that was the aim of the game.

One of the key figures in attempting this goal is Bruce Blumberg, an expert in animal learning and behaviour.

Not only does Blumberg have a wealth of academic experience - he is former professor at the Massachusetts Institute of Technology (MIT) Media Lab and currently teaches at the Harvard University Extension School - he is also a keen dog owner who competes in shows with his border terrier, Scuppers.

Since animal games are all about keeping the creatures happy, players need to be able to tell how they are feeling - a challenge for Blumberg, who worked on how this could be communicated through the animals' animation.

"One of the first things I did was take videos of me with my dog, doing things like holding up a bone or pretending I was going to throw a ball," says Blumberg. "Then we would analyse it frame-by-frame - it would get people thinking about the conversation that is going on and how we know what the animal is thinking."

In World of Zoo the animals are split into enclosures, so only interact with members of their own animal "family".

The team behind SimAnimals, however, had a different challenge. The series of games - the second of which, SimAnimals: Africa, was released earlier this month - are set in the wild, meaning developer Electronic Arts has had to work out how the animals would react with each other.

By studying animals' natural behaviour - what they eat, whether they hunt or are hunted - its creators came up with what Sam Player, executive producer, calls "a recipe for each animal for how they'll behave". The results, he says, are animals with their own realistic behaviour patterns.

"The game is a living, breathing eco-system," says Player. "If you want to sit back and watch it like it was a terrarium, you could, and the animals would take care of their own needs." Still, like World of Zoo, some changes to reality had to be made. "If we went with super-realism with our animals, we would basically have a game where they sleep, eat and try to kill each other," says Player.

Of course, getting the virtual animals' behaviour spot-on is useless if poor animation reminds the player that they are just a bunch of pixels. Interestingly, Blue Fang did not attempt photo-realistic graphics for World of Zoo, instead using cel-shaded animation, and the resulting cartoonish look, claims Blumberg, helps to involve the player in the animals.

"I think our animals, even though they may be less realistic than in some games, are more believable and have more organic credibility because the movement really works," he says. "We focused on animations that tell a story to the player. We want them to roll over in a way that is communicating whether they are sad or happy."

Of course, basic visuals did not stop the Tamagotchi becoming a craze, but now the virtual pet concept has moved on with EyePet, an innovative game that places the pet in the room with you.

It works by using a webcam to produce a live video feed of you in your living room, on to which the pet is superimposed, and the player can then interact with the critter using hand gestures.

When it came to designing the pet itself, the developers had to create something that would be able to fulfil all the activities they were including in the game.

"We wanted to make our pet believable and something that people would be able to relate to, but our pet also has some special things that it can do," says Russell Harding, the producer of the game.

"Rather than being a mythical creature, we wanted something that was still very animalistic, so that people would know how to respond to it, but can also draw and sing, which is not totally animalistic behaviour."

As a result, they created a pet that is like a cross between a monkey, a puppy and a kitten, although with human facial expressions because, says Harding, they wanted it to be expressive and avoid "text and voice-overs popping up".

All these are features that help create a relationship between the player and the animal, said Gargolinski.

Whether an animated creature in a game can inspire the same depth of feeling as a living, breathing organism is debatable, but they do offer unique experiences that just are not feasible in reality. And of course, a break from those vet's fees.