No, makes sod all difference IME. Same to sweating.

you are fitter hence sweat less?

I find that the fitter I am, the more I sweat... Not really noticed re drinking. Perhaps they just kept wanting to stop due to lack of fitness and used stopping to have a drink as an excuse?

I am (slightly?) fitter than the rider concerned, so am I wrong to conclude that being fitter reduces one’s thirst?

No.

Though being fitter will mean your system is not stressed as much as your riding partner, therefore you will sweat less to cool your body. Also, you could well have been more hydrated before the ride; plus a thousand other variables.

Breathing mouth open vs nose closed

Even at my fittest I sweat quite a lot 😳

I dont think there is a correlation (limited sample of course) between fitness and sweating. I have trained with many (runners) people who can run till exhaustion whilst I am relatively comfortable and I often sweat much more than them.

Thanks for tall the replies, something to think about, but I think CaptJon has it with

Breathing mouth open vs nose closed

Although I often still breath through my mouth, I'm rarely if ever gasping, so it's maybe just a dry mouth/throat psychosomatic thing?