Topic: Defence in frogs: toxins and camouflage
The many striking examples of convergence most famously include the case of mimicry, but the question of defence also extends to the use of toxins (and venoms), such as alkaloids, where we also find molecular convergence.
“Eat or be eaten” is one of the unavoidables, and unsurprisingly nearly all organisms go to considerable lengths to avoid ending up as dinner. Of key importance is either obvious warning (such as the aposematic colours of a wasp or the rattle of a rattle-snake) or camouflage. Not only are there many striking examples of convergence, most famously in the case of mimicry, but the question of defence also extends to the use of toxins (and venoms). Frogs show particularly interesting convergences in terms of warning colouration and/or toxins.
The poisonous frogs of Madagascar (mantellids) are strikingly convergent on the famous Neotropical poison frogs (dendrobatids) and there are further convergences within each family. Whilst the dendrobatids are evidently monophyletic, there is an interesting correlation between degree of toxicity and colouration that has evolved independently several times. From an evolutionary perspective this observation has several important consequences. First, it can be shown that this correlation has emerged on different time scales, some are relatively recent but at least one is much more ancient. Evidently this represents the repeated shift to adopting a specialized diet from which the toxic chemicals are derived (notably from ants, termites and mites). In addition, the correlation between toxicity and colouration may reflect a physiological pre-requisite linked to high levels of metabolic activity (high aerobic/low anaerobic). Incidentally, sequestration of chemicals for defensive purposes is by no means restricted to the dendrobatid frogs, but in the amphibians alone has evolved independently at least three other times (apart from the mantellids also in the Australian myobatrachid Pseudophryne and apparently a South American bufonid). These examples of convergences in toxicity are also important in the case of evidence for molecular convergence, especially as the chemicals employed are typically alkaloids. Not only have these been recruited independently by many groups of frogs (typically, as noted, by dietary sequestration), but also in groups as diverse as birds, insects, tunicates, and plants. The evolutionary association between chemical defense and dietary specialization is, however, apparently unique to the dendrobatids.
So one way of engaging in defence is to stand out conspicuously and/or employ toxic chemicals to advertise your danger and/or distastefulness. As noted this has evolved many times. But there are other more subtle ways. Some frogs are leaf-sitters, slow-moving and difficult to disturb; they have very ingeniously found a way to become similar to the Invisible Man. Moreover, frogs have evolved this trick independently several times. By matching the reflectance of their skin to that of the leaf on which they happen to be sitting, both of which are in the near infrared, so the frog becomes much more difficult to spot if the eyes of the predator, such as a snake, are tuned to this wavelength. It is also possible that the frog becomes thermally cryptic, potentially vital because of the heat sensory capacity of snakes, which is of course convergent with some beetles.
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Map of Life - "Defence in frogs: toxins and camouflage"
January 23, 2018