Jack rabbits probably avoid overheating by staying in the shade through the hottest part of the day - if possible in a shallow depression (to avoid the heat of the ground) and in a breeze to encourage heat loss from those ears.
But its cooler in the shade, so body heat will be lost from those ears anyway, whether there's a breeze or not. In late afternoon and early evening, when its cooler outside, the jack rabbit can go looking for food without risk of overheating.
