Written by Roger Downie
Froglife and University of Glasgow
Back in July 2020, Isabel Byrne, Chris Pollock and I wrote a Croaking Science article that described the Centrolenidae, a neotropical family of 158 species known as glass frogs because of their transparency. In that article, we focused on reproduction in glass frogs. Here I discuss new work on their transparency and its contribution to their survival.
The importance to animals of body surface colours and patterns has long been studied. A key work is Hugh Cott’s book Adaptive Coloration in Animals (1940). Broadly, animals have evolved two different strategies: a) aposematic coloration, where they possess some protection, such as toxicity, and advertise this by being conspicuously coloured and patterned: in amphibians, poison-arrow frogs are an obvious example; b) cryptic coloration, where colours and patterns in some way blend with the background, making the animal difficult to detect: such methods of camouflage may match the background, or disrupt edges, or mimic background features. Cott undertook his doctoral research while lecturing at the University of Glasgow and was supervised by John Graham Kerr, the regius professor of Zoology who had developed an interest in the military uses of camouflage during World War 1. Cott’s thesis (1938) was titled The problem of adaptive coloration with special relevance to the Anura, and he moved to the University of Cambridge soon after completing it. Both he and Kerr were frustrated by the military authorities being unwilling to accept scientific advice on camouflage during both World Wars.
Transparency as a means of camouflage is fairly common in aquatic habitats, but is rare on land. For vertebrates, the red coloration of the haemoglobin in blood is a particular problem. In water, ice fish and eel larvae avoid this problem by having no red blood cells. In glass frogs, new research by Taboada et al. (2022) demonstrates a different strategy: when at rest and requiring concealment from predators, they hide their red blood cells.
Glass frogs have highly transparent muscles and ventral skin, and the major internal organs: heart, liver and digestive organs, lie within mirrored sacs containing reflective guanine crystals which reflect the incident light. However, when frogs are active, their blood vessels are easily visible because of the circulating red blood cells. Taboada and colleagues compared active, awake glass frogs with individuals asleep and resting on leaves. They found that asleep frogs transmitted 34-61% more light than active individuals, making them more transparent, and that red blood cells were essentially absent from most of the circulatory system when the frogs were asleep. To find where the red cells were, they had to use the technique of Photo-acoustic microscopy, which can penetrate into solid tissue and visualise its contents. They found that the red cells were located in extensible sinuses in the liver, increasing liver volume by 40%. The transition from asleep to active in terms of red cell distribution took about 60 minutes. Measurements of the oxygen content of the blood showed that this decreased by 31% during sleep, suggesting a possible negative aspect of the red cell hiding process. The research team carried out comparative measurements on three other kinds of arboreal frog: none of them showed the transparency found in glass frogs, or the concealment of red blood cells when at rest. The authors speculate that the mechanism underlying red cell hiding in glass frogs may exploit a widespread ability in frogs, where other species have been shown to regulate respiration by temporary storage of red cells in the liver.
In another report, Barnett et al. (2020) question the use of the word ‘transparency’ as applied to glass frogs, since their dorsal green pigmentation renders them ‘translucent’, rather than transparent. Barnett and colleagues showed that translucent model frogs offered protection against avian predators and that translucency made detection of individuals slower.
This work is a reminder of the many ways in which the physiology of amphibians is unusual, such as the ability of some species to survive freezing, helping them to survive in a changing world.
Thanks to Malcolm Kennedy for drawing my attention to the Taboada et al. paper.
Barnett, J.B. et al. (2020). Imperfect transparency and camouflage in glass frogs. PNAS 117, 12885-12890.
Byrne, I., Pollock, C. and Downie, J.R. (2020). See-through frogs are worth a look. Froglife eNewsletter Croaking Science July 2020.
Taboada, C. et al. (2022). Glass frogs conceal blood in their liver to maintain transparency. Science 378, 1315-1320.