How do the curators know that these are humans in masks and not animals?
Visually it is possible to infer that these are masked human figures, because of their body. You can see that the upright position of the figures suggest that these are humans in disguise. If you look closely, you can see human ears beneath the masks, and on the back of the sculpture you can see that the figures have human feet and backsides. The back of the slab is decorated with eighteen crouching jaguars. The jaguar may have been a totem animal or clan symbol of the deceased. You can compare the rendering of the jaguar bodies to those of the human figures in masks.