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Crafty cats coax their owners into giving them what they want by using a special purr that humans just can't ignore, says new research from Sussex published in Current Biology journal on 15 July.

The team of Sussex psychologists discovered that cat owners find this "solicitation" purr irresistible because a high-frequency element embedded within it, similar to a cry or meow, subtly triggers a sense of urgency. By employing such an embedded "cry", cats appear to be exploiting innate tendencies that humans have for nurturing offspring. However, in this case the felines subtly bury their "feed me" messages in an otherwise pleasant purr.

Lead author Dr Karen McComb was inspired to initiate the study because her own cat, Pepo, had the knack of consistently waking her up in the mornings with insistent purring.

She says: "I wondered why this purring sounded so annoying and was so difficult to ignore. Talking with other cat owners, I found that some of them - including co-author Anna Taylor - also had cats who showed similar behaviour."

Karen and her team set up an experiment that tested human responses to the different purring types. She says: "When humans were played purrs recorded while cats were actively seeking food at equal volume to purrs recorded in non-solicitation contexts, even those with no experience of cats judged the 'solicitation' purrs to be more urgent and less pleasant."

The crucial factor in determining whether a purr was rated as urgent or pleasant was an unusual high-frequency element - reminiscent of a cry or meow - embedded within the naturally low-pitched purr. When the team re-synthesised purrs to remove the embedded cry (and left other characteristics unchanged), the urgency ratings for these purrs decreased significantly.

"The embedding of a cry within a call that we normally associate with contentment is quite a subtle means of eliciting a response - and solicitation purring is probably more acceptable to humans than overt meowing."

Not all cats, however, use this solicitation purring: "It seems to most often develop in cats that have a one-on-one with their owners rather than in large households where there is a lot going on and such purring might get overlooked. Meowing seems to be more common in these situations."

Meanwhile, those that did use solicitation purring and were recruited to help with the research were not always cooperative. Karen says: "Cats exhibit this behaviour in private with their owners, typically at anti-social times, such as first thing in the morning. They also tend to clam up or leave when strangers turn up. We had to train owners to use the equipment to record both the solicitation and non-solicitation purrs that we needed in their own homes.

"I've worked on communication and cognitive abilities in a wide range of mammals, including elephants and lions, but domestic cats were one of the most challenging subjects to date."

Cats, it seems, will always be cats!

Listen to examples of the cat purrs