I was wondering if anyone was going to mention that.
There are two primary factors that create, for you and me, the idea that artificial light may have been used. (It wasn't.) The first factor is that one step in my post-processing was to brighten the heads to immediately lead the viewer's eye to that part of the photo. The other factor is that I changed the color balance of the elephants. For whatever reason that I don't understand, it resulted in the heads changing more than the torsos even though I selected the entire body of both elephants.
On a related issue, my friend who has been photographing elephants for years told me to expect the color balance to be thrown off when photographing elephants. I didn't understand what he could be referring to when he first mentioned it, so we discussed it in great detail. As I began photographing elephants, I realized that the Nikon sensor really does have a problem with displaying a consistent color balance of an elephant's skin. (My friend and his wife use Nikon equipment, as do I.) I was shooting at 3 frames per second and was really surprised to see the wide range of different tonalities within the set of photos. So, I simply resorted to what I like doing best regardless of the situation: making the photo what I want it to be and ignoring how accurate or inaccurate reproduction of the colors is compared to how I or anyone remembers them in the scene.