A study of nasty and nice lab rats has scientists on the verge of knowing the genes that separate wild animals like lions and wolves from their tame cousins, cats and dogs.

Unlike their wild ancestors, house pets and other domesticated animals share the trait of tameness, meaning they tolerate or even seek out human presence. New research, which is published in the June issue of the journal Genetics and involved the interbreeding of friendly and aggressive rats, reveals gene regions that influence the opposing behaviors.

"I hope our study will ultimately lead to a detailed understanding of the genetics and biology of tameness," said researcher Frank Albert of the Max Planck Institute for Evolutionary Anthropology in Germany. "Maybe we'll then be able to domesticate a few of those species where humans have historically not been successful like the wild African Buffalo."

And we can possibly understand more about the furry creatures in our homes.

"If you think about dogs, they are such amazing animals. When you compare a dog with a wild wolf, a wolf has no interest in communicating [with] or tolerating humans," Albert told LiveScience. "If you're lucky a wolf in the wild wouldn't care about you. But a dog does care and they even seek human presence."

He added, "Dogs were all wolves at some point. How did they become these animals that need humans to exist?"

Breeding rats

The roots of this study date back to 1972 when researchers in Novosibirsk, in what is now Russia, caught a large group of wild rats around the city. Back at the lab, the researchers arbitrarily separated the rats into two groups. In one group, called the tame rats, the scientists then mated the friendliest rats, those that tolerated humans, with one another, and in the other group they mated the most aggressive rats with each other.

Demeanor in rats is tested with the glove test, in which a human hand protected by a metal glove approaches a caged rat. The tame rats tolerate the hand and even sometimes toddle across it. Aggressive rats try to escape, scream, attack and bite the person's hand. The rats even perform boxing moves, standing on their hind legs while sort of punching the human hand away.

The experiment is going on to this day, with two generations bred each year, resulting in a team of extremely tame rats and a team of very aggressive ones.

Nice genes

To figure out the genes behind the rat behaviors, Albert and his colleagues interbred a few of the tamest rats with a few of the aggressive rats and then interbred the resulting pups. That way, the rats would have a mix of genes from both types of parents.

So if two rats had matching genes in one region of their genomes but differing tameness behaviors, the researchers could rule out this genetic region as responsible for the behaviors. The inverse is also true.

First, behavior tests teased out which rats were naughty and which were nice. Then, the researchers ran genetic tests. While the results don't reveal specific tameness genes, the researchers have pinpointed sets of genes responsible for tameness.

Further breeding and testing will hopefully uncover the exact genes linked with certain rat behaviors.