All the others are smooth. But the thumbnails have ridges-about 0.10’ apart-what are the function of these-do they strengthen the nails? 
I’m also interested in seeing the answer - mine have them, too.
I have them too, but on all my fingers. They seem to be related to age…
I have them on all my fingernails too. The nails feel smooth though. Why do you think it’s related to age robcaro?
I’m 19 and have had them though all my teen years.
Because I never had them until I was 70 or so.
Actually I saw my internist for these several years ago and he told me they were because the cuticle was damaged. I tend to pick my cuticles and over time these little horrizontal (to the end of my thumb) ridges formed. He said if I stop the picking I’ll lose the ridges over time. This worked…Well as long as I don’t pick.
I googled “thumb ridges” in Google Images and chose the ones that looked like mine… this also gives the answer you are looking for. Same as my internist.
My sister and I have always had ridges on our thumbnails (not the other nails). We thought it might be a vitamin deficiency, but it never changes no matter what we eat. “Just genetic,” we figure.
Strangely, I have them on my right thumb and both pinkies.
Are iPods allowed on a domestic (US) flight? Not just the new security rules, but in general?
Does your iPod have ridges too? Stop picking at it. 
My thumbnails have horizontal ridges too. And I’m a terrible cuticle-nibbler, so I can see that as being a plausible explanation.
I have vertical ridges on all my fingernails. After googling it, I found out that it’s not a cause for concern and may increase with age. However, horizontal grooves might indicate a problem. Sucks to be you, Antigen and Phlosphr. 
I saw a dermotologist for the same condition, which he called Trachonychia, which is derived from the Greek word for “gee your nails look awful”. According to MD, usually indicative of nothing, but in some cases could be indicative of psoriasis.
This is not medical advice. Talk to your doctor or pharmacist. Use only as directed. Refrigerate after opening. Trained driver on closed course. Apply to underarms only.
I don’t bite my nails or pick at my cuticals but I have these ridges on my right hand thumb.
I asked my doctor a while back what they were and he said “We (I assume the medical profession) have no idea what causes them”
Vertical ridges are normal on any nails.
Horizontal ridges have several causes, from nail biting to diseases as serious as Lupus. Horizontal lines, along with ridges are of signs seen in heavy metal poisonings, like lead and arsenic.
If you’re concerned, show your healthcare provider.
If these are age-related, I should be having them on all fingers & toes by now.