Researchers studied the shape of tongues and listened to how subjects pronounced words
The distinctive Scottish rolling "R" is being lost as younger Scots no longer pronounce the letter as forcefully, language experts say.
This has led to the softening of words such as "car", "bar" and "fur".
Older Scots also roll the letter in "girl" and "farm", but younger speakers now appear to miss this R out.
Some have blamed softer accents in the media, but a Glasgow University and Queen Margaret University in Edinburgh study suggests a more natural change.
Researchers investigated the latest changes by using ultrasound on a group of young Scots to see how the tongue moves inside the mouth and shapes for words.
Delayed sound
Prof Jim Scobbie, from QMU, led the research with Dr Eleanor Lawson and Prof Jane Stewart-Smith.
Sociolinguist Dr Lawson said: "What we found is that some Scottish speakers are delaying the 'R' gesture, so it's happening in silence afterwards.
"They're not losing it completely - they're still producing it. You just can't hear it the same.
"We found that one group of Scottish speakers are doing one thing with their tongue, and another group are doing something completely different.
"Speakers with a more vernacular Scottish accent seem to delay their 'R' gesture making it hard to hear and others with a less vernacular accent don't but also use a completely different tongue shape, one that we usually associate with American English."
People outside of Scotland may struggle to understand Scots speaking with the softer "R", but the researchers said people within the country who speak in the different styles can still understand each other.
Dr Lawson added: "It's not media or TV, it's more boring than that - it's just a natural sound change. It seems to happen in languages all over the world over time, particularly at the end of words."
