Red bumps when hiking for long periods in hot weather

It's probably a heat rash. They are common in hot humid places like here in Japan, especially when you exert yourself. I think they often happen when your body is not acclimated to the heat and the perspiration happens faster than your pores can deal with. I've found that if I continue doing for more than a week whatever exercise I am doing they go away.

I agree with Miguel.
When was young on my first summer cycle holiday without my parents. I got red spots under my socks and shorts. I got an appointment to a local Scottish doc twice but they were in the evening, the spots had vanished each time.

Yeah, if you hadn't have mentioned them not itching I would have immediately suspected chiggers. As miguel said though, it's probably some form of heat rash.

Thanks for your help guys!

The red bumps on your ankles sound to me like one of two things. Either 1) they are from chiggers, microscopic little arthropods (I believe it's actually a mite) that burrow into your skin and cause inflammation in the form of little red bumps; or 2) irritating oils from a plant. Poison ivy is not the only plant that causes dermatitis. Depending on the region you're hiking (the Southwestern United States has a lot), there is a wide variety of other plants that can irritate the skin.

If it were heat rash (or miliaria), you'd probably get the red bumps in a more widespread distribution around your body and not just your lower legs. Heat rash is also supposed to cause itching.

My hunch is that it's chiggers.

Living where I am we have learned about chiggers. If anything they itch. Boy, do they itch! Maybe a heat rash. Maybe time to ask the dermatologist.

"If anything they itch. Boy, do they itch!"

And that's a gross understatement.

"If it were heat rash (or miliaria), you'd probably get the red bumps in a more widespread distribution around your body and not just your lower legs."

Not necessarily, he's getting it in the area that works the most and is most likely covered by socks. AKA the part of his body where the skin is most likely to be at an elevated temperature as compared to the rest of his skin.