Why did Karana discover had happened to the old bull and to its body?

From the text:

The tide was low when I reached it. Far up on the slope lay the body of the old bull. Gulls had picked his bones clean, but I found what I had come for.
Some of the teeth were as long as my hand and half its width. They were curved at the tops and some were broken, but when I had ground the best of them down with sand, I had for my work four good spear points, broad at the bottom and very sharp at the ends.
I made two more spears from these points and at last was ready to go to the cave of the wild dogs.