So, today temp got up to 43° and sunny. Doggie and I were in the backyard - I was alternately pruning and throwing the "squawky chicken" for Ami. We were supposed to also go for a late afternoon walk but even just chasing after the chicken and just wandering around after me (3/4 of acre fenced yard and he is never more than 40' away from me, go figure) he was panting heavily. He would eat snow but declined water unless it was from my hand. Did not decline pieces of kibble, though   That boy likes his tummy! Yesterday, when we got back from 3.4 mile walk, also panting heavily and visibly tired so I decided to abandon this afternoon's walk and wait until after dark when it will be cooler. Am I being overprotective? I'm thinking that this "heat" is sudden and since he is acclimated to single digit (and below!) temps, that the warmer weather is too much all at once.

Any thoughts?

I dont think Heat would be in issue, considering there is plenty of dogs here living in Warmer states. My pup got pretty lazy when it was 75-85 degrees last year. I would suspect still maybe tired from the previous day hike or walk. How old is your pup?

It is completely possible though, how does the pup act to the temperatures in your house?

Ami is about 19 months old (shelter dog). He is completely outdoor dog - sleeps in unheated outbuilding at night in covered crate with layers of dog bed, blankets and old wool sweaters. Days he is on trolley with dog house and deep bed of straw. So, he really is totally acclimated to single digits. He won't even go in doghouse unless it's below 15° AND windy or in single digits and precipitating. As for tired from yesterday - he normally does 3-3.5 miles early, another 2 - 3 late afternoon/evening. We missed this morning's walk (Mom had bad night, short sleep, went back to bed after taking care of dog - food, pee, poop, long belly scratch etc.) The few times he has been in house he is panting within minutes. Also in car - we can't turn on heat and have to leave window partially open. He has a massively thick undercoat at the moment (when that blows, the earth will change its axis  ). I'm thinking 43° and sunny is too hot (I had to shed multiple layers - worked outside in just sweatshirt and pants. I think I've become husky, too...) I guess I'll see how it goes this evening when I walk him - I can always just turn back if he still seems stressed.