Can (every) guitarist(s) play the bass guitar?

I played upright bass (fretless folk ,jazz and rock), folk guitar (finger picking), rhythm (including electric 12 string) and electric bass (rock and R&B). Currently play uke in a couple of jam groups, just for fun.
YMMV but it can be done.

Are/were you proficient in all?

Based on my brother’s experience, yes. Being good at it, really good, is another story.

High school, playing upright, rated by several music teachers at our school and other schools as best “jazz walking-bass player”. Took two semesters of university music school, jazz improvisation, while in high school.
Play solos on uke at jams and occasional public performances.

Bravo, @Tropical_Willie!
Thank you.

I play guitar but not the bass. I know the general layout of the bass guitar, but those strings are so fat, there really is a separate technique to playing. I do think it would be a reasonably easy transition with some training and practice though.

I started playing guitar when I was 11. I played for 2 years, but then I lost interest. I resumed playing 7 years later, when I was 20.

About 2 years after that I tried playing bass. Plucking the strings and fingering the frets wasn’t that difficult, but it wasn’t as easy as playing the 6-string. I tried to play it like a 6-string lead guitarist, so my playing didn’t have the drive that a good bassist does.

Most guitars have 6 strings. Some have 12, but the difference between playing a 6 and a 12 is less than the difference between playing a 6 and a bass. That’s because the 12 strings are mounted on the same neck as a 6-string, and the extra 6 strings are strung right next to the primary 6, so they don’t require a wider neck.

The bass guitar usually only has 4 strings. Those strings are thicker, and therefore require greater finger strength for plucking and for applying sufficient pressure on the frets. Also, I think a bass guitar neck is thinner, so there’s probably some adjustment to be made there too.

I think a bass guitarist could eventually play a 6-string, but the transition might be slightly more difficult. That’s because from what I’ve seen, most bass guitar work doesn’t require the ability to play chords, whereas that’s an essential skill for most 6-string guitar work.

In summation, it would be a lot easier for either guitarist to play the other guitar type than it would be for someone who’s never played, but a successful transition does require some work.

I play rhythm guitar but not bass.

If you can play guitar you can play bass or at least pass for a bass player. It’s harder to go the other way. I play both but I am more proficient with guitar. I have an 8 string guitar with a .80 for the low #F and a .64 for the low B and I incorporate bass techniques with it at times.

Talent helps in all aspects, on any instrument. When I said “really good,” meant to have talent, to be able to improvise on the fly.

Really interesting information you’ve given me, guys, thanks!