Last night, Stephen Colbert showed up on the Late Show with some grey whiskers.

It wasn't the Dr. Andrew Weil look favoured by his predecessor, David Letterman.

Rather, Colbert's beard was cropped short.

"My wife does not like this beard," Colbert conceded.

So why would Colbert do something like this even if his wife dislikes it?

Ratings, of course.

In the August 20 to 24 period, NBC's Jimmy Fallon's show scored a rare victory, according to the tvbythenumbers website.

It attracted 1.91 million viewers, on average, compared to 1.86 million for Colbert on CBS and 1.69 million for ABC's Jimmy Kimmel.

More crucially, Colbert was trailing both of them in the much desired 18 to 49 demographic.

It's easy to dismiss these numbers because all three shows were in reruns, but that's still enough to alarm network executives.

So suddenly, Colbert grows a beard, alienates his wife, but manages to become a topic of conversation on the first show after Labour Day.

He's a pro, which is why he's lasted so long in such a brutally competitive business.

But Colbert's still got to be careful about Kimmel. That's because late night's second Jimmy is treading on Colbert's traditional turf by now focusing more attention in his monologues on Donald Trump.

Plus, Kimmel has a beard.