Poison frog. Image credit: Rebecca D. Tarvin (CC BY 4.0)
For most animals, the ability to deter predators is vital for survival. Some organisms, such as poison frogs, use bad tasting or toxic chemicals to ward off predators. In the 1990s, scientists discovered that poison frogs acquire their defensive alkaloid chemicals from the mites, ants and other arthropods they eat.
Many poison frog species use bright or contrasting colors to advertise their defenses to predators; this strategy is known as ‘aposematism’. Aposematic frogs have evolved biochemical mechanisms to transport, store and even modify the alkaloid toxins. Although aposematism has evolved independently in three poison frog clades, most of the frogs in this family are dull-colored. These dull-colored frogs are generally assumed to not be able to accumulate alkaloid toxins from their diet. However, very little is known about how animals evolve to be able to use chemicals they eat as toxins to defend themselves.
To learn more about this phenomenon, Tarvin et al. screened different ‘undefended’ frog lineages for alkaloids to determine whether the frogs lacked them, as previously assumed. The researchers used highly sensitive chromatography and mass spectrometry, techniques that can detect and identify specific compounds in chemical mixtures, even at very low concentrations.
The results showed that nearly every ‘undefended’ poison frog had alkaloids, but at substantially lower levels than aposematic species. Tarvin et al. propose that these frogs do not have the transport or storage systems that aposematic frogs employ to use the toxic alkaloids they consume. Rather, the dull-colored frogs accumulate alkaloids passively. This ‘passive accumulation’ appears to be a stepping stone on the path to evolving the ability to accumulate toxins from the diet. Tarvin et al. also found that all of the studied poison frogs ate ants and mites, the main arthropod sources of alkaloids in poison frogs.
The findings of Tarvin et al. suggest that specialized diets are not enough to explain how poison frogs evolved the ability to accumulate toxins. Changes in toxin absorption, distribution, metabolism and excretion are also required for frogs to be able to use alkaloids from their diet as poison. This provides new insights into the evolution of chemical defense in poison frogs and could help researchers to better understand how this type of defense evolved in other animals.
