On very hot days it's not unusual to chance upon flattened squirrels on sidewalks or streets. The 1st time I saw a squirrel like that it was so flat that I thought it had been run over by a car. But when the squirrel noticed me it got up and ran to the nearest tree.
After the squirrel in the picture above ran away, I walked over to the shady spot where it had been lying and flattened my palm on the asphalt. It felt cool. Apparently asphalt and concrete conduct heat well. So by flattening its belly against such a surface, a squirrel loses body heat and lowers its body temperature. That's a good thing to achieve on a hot day.
Initially I thought the belly-cooling behavior had probably originated among the city-dwelling squirrels within the last 200 years or so. But when I searched the internet for more information, I saw pictures of squirrels lying on soil or wood chips. The behavior may in fact be an ancient one.