Prehistoric Caribbean peoples used pigments made partly of bird or bat guano to create extensive cave art.
Jago Cooper of the British Museum in London and his colleagues found thousands of artworks in dozens of caves on Mona Island, near Puerto Rico. Scanning electron microscopy, spectroscopy and other analyses show that artists crafted some images by scooping out the soft deposits on the cave walls. Other motifs were painted using pigments composed of both guano from the cave floor and plant-based binders that the artists brought to the caves.
The images — depicting faces, animals and geometric designs — were made as early as the eleventh century AD, according to radiocarbon dating and other methods. Many artworks are found far from cave entrances, meaning ancient people ventured deep into dark zones to work.
The authors say that their analytical techniques can be applied to artworks at a wide variety of archaeological sites.