Deep in the forests of South and Central America you may struggle to spot the tiny glass frog. Discovered in the 1920s, glass frogs (family Centrolenidae) have the fascinating ability to become almost entirely transparent when they sleep during the day.
Scientists were captivated by their discovery and began to theorise on how this came about. They hypothesised that the frog was pushing the red blood cells out of its blood vessels, but where were they going? Since red blood cells absorb light, the scientists used spectroscopy to discover that while the frog slept, its red blood cell concentration was lower. The concentration increased after exercise.
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