Frogs and their diets: some interesting exceptions

Frogs perform a variety of valuable ecological roles throughout the world. For example, anurans play a crucial role in controlling insect populations in ecosystems throughout the world. While most frog species are generally regarded as insectivores, this is an oversimplification and there exist some fascinating exceptions to this ‘rule’.

While most adult frogs primarily feed on a wide variety of insects, anurans within their larval (or tadpole) phases are often omnivorous and will help themselves to a variety of animal derived foods as well as plant materials. Some tadpoles of a specialized group of dart frogs (Oophaga spp.) participate in oophagy, or egg eating. This interesting feeding strategy requires parental investment from the tadpoles’ mother and involves her laying unfertile eggs as a food source for her tadpoles as they mature. Some other tadpole species participate in more brutal feeding strategies than their omnivorous and herbivorous counterparts. An example of such a species is Spea multiplicata, or the Mexican spadefoot toad. This species exhibits phenotypic plasticity within their larval phase. Some spadefoot tadpoles will trend towards a more typical herbivorous tadpole diet and other tadpoles within the same cohort will develop broader heads, a shorter gut, and a larger, more defined jaw structure. These larger tadpoles mature quickly and their diet is carnivorous. These carnivorous tadpoles will often cannibalize their smaller, herbivorous siblings.  Often, tadpoles’ diets will change dramatically once they metamorphose into adult frogs. Metamorphosis can change an herbivorous tadpole with a long, plant digesting, gut system into the shortened gut of an insectivorous adult frog over the course of a few days.

But anuran diets are not only interesting within their larval stages. For example, poison dart frog species within Central and South America bioaccumulate toxins found within insects they eat throughout their native range. These insects are thought to be slightly toxic themselves from ingesting toxic plant materials. Through each trophic level, these toxins are concentrated within the tissues for the insects and finally within the tissues of the dart frogs. One of these concentrated toxic alkaloids is known as batrachotoxin is found in a few species of beetle, birds, and frogs. This alkaloid is one of the most potently toxic alkaloids known to science and acts as both a neurotoxin and cardiotoxin.

While this process of toxin bioaccumulation is fascinating, my favorite example of interesting anuran diets goes to a strange frog inhabiting the eastern coast of Brazil. Xenohyla truncata is commonly known as Izecksohn’s Brazilian treefrog and is the only known frugivorous frog in the world. These amphibians are endemic to shrubby, dry habitats on coastal Brazil known as restingas. This species is closely associated with various bromeliad species found in this habitat including Neoregelia cruenta and Anthurium harrisii. Researchers found that the seeds ingested by Xenohyla truncata were still viable after traveling through the digestive tract of these frogs and this species could be sustained in human care on a strictly diet of fruits native to the restinga.

These studies indicate that this Brazilian frog species contributes to seed dispersal within its native range. The association of frog to bromeliad is a unique example of mutualism, where the animal benefits from the fruit and the plant benefits by having their seeds transported to new areas of potential growth. Seed dispersal is common within the animal kingdom, yet this is the only known case of amphibian seed dispersal known to science. Most herpetologists would quickly mention tortoises or iguanas when referring to seed dispersers. While, these species are extremely effective at seed dispersal, it is fascinating to find such an unprecedented example of frugivory and seed dispersal within the amphibian world.

Ben M.
Keeper I, Herpetology