No idea there. I also wondered about the whole belly heat deal too as most of these snake are crepuscular and I wouldn't imagine there are many places to find belly heat in a termite mound not to mention, BPs aren't known to sun themselves or sit out in the open on a warm rock.

Maybe the gradient is from termite mounds to abandoned rodent dens? I mean a termite mound would nice and toasty warm while a rodent burrow underground would be very cool.