THERE'S a reason cats aren't called man's best friend - and it's all in their genes, according to a new study.
Cats are simply not as domesticated as dogs despite sharing households with humans for at least 9,000 years, researchers at the Washington University School of Medicine have found.
In fact, the main reason they stick around at all is because they like getting rewards.
Scientists working on the cat genome sequencing project compared the genomes of domestic cats and wild cats to analyse how they differed.
And although they discovered that in certain areas there were significant variations, they concluded that felines are only "semi-domesticated".
Senior author Wes Warren, an associate professor of genetics at The Genome Institute at Washington University, said: "Cats, unlike dogs, are really only semi-domesticated. They only recently split off from wild cats, and some even still breed with their wild relatives.
He added that the researchers were "surprised" to find any DNA evidence of cats' domestication at all.
The scientists found changes in the domestic cats' genes that other studies have shown are involved in behaviours such as memory, fear and reward-seeking.
These types of behaviours, particularly reward-seeking, are generally thought to be important in the domestication process.
"Humans most likely welcomed cats because they controlled rodents that consumed their grain harvests," said Warren.
"We hypothesised that humans would offer cats food as a reward to stick around."
This mean that cats that would have preferred to lead a solitary life had an extra incentive to stick around and, over time, humans would have selected the most docile creatures to keep as pets.
The cat genome sequencing project originally began in 2007 for the purposes of studying hereditary diseases in domestic cats.
Cat or dog? What is the better pet?
This poll ended on 21 November 2014.
Dog. Man's best friend for a good reason
Cat. They are far more cuddly
Dog. Cats kill too much wildlife
Don't care. Just keep you cat inside!
This is not a scientific poll. The results reflect only the opinions of those who chose to participate.
The researchers sequenced a domestic female Abyssinian cat named Cinnamon that had degenerative eye disorder and whose lineage could be traced back several generations.
But in order to learn more about the domestication process, researchers also sequenced the genomes of purebred domestic cats.
They found that the main difference between these cats and their wild counterparts lay in features such as specific hair colours and fur patterns, as well as facial structures and docility.
For example, the Birman breed has characteristic white paws which could be traced to two small changes in a gene associated with hair colour.
However, when it comes to key characteristics such as eating a carnivorous diet and having an excellent sense of smell, the researchers found that there was no difference in the genome.
"Our results suggest that selection for docility, as a result of becoming accustomed to humans for food rewards, was most likely the major force that altered the first domesticated cat genomes."
The research appears in the Proceedings of the National Academy of Sciences: Early Edition.
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