A chemically-camouflaged frog

By Matthew Cobb

Social insect colonies rely heavily on chemical signalling to identify members of the colony, and conversely to detect intruders. These communication systems are generally very effective, but as that great scientist Professor Ian Malcolm put it, ‘Life will find a way’. If there’s a locked system, somewhere a pesky but perspicacious parasite will find a way to crack it.

Caterpillars of the Maculinea genus – also known as Alcon butterflies – hatch on the ground near Myrmica ant nests, and are picked up by the workers. The ants take home what tastes/smells like one of their babies, except this is a carnivorous cuckoo that will munch its way through their larvae…

An unusual example of such chemical camouflage was discovered in 2013 by a group of German and Swiss researchers, led by Mark-Oliver Rödel of the Museum für Naturkunde Berlin. It’s unusual because, as the title of this post indicates, it involves a frog.

The West African Rubber Frog (Phrynomantis microps) is found throughout west Africa, and can often be found living in the underground nests of the ponerine ant Paltothyreus tarsatus (aka the African stink ant).

These are pretty aggressive ants about 2 cm long, which pack a nasty bite and an even more powerful sting. They can be found pretty much throughout sub-Saharan Africa, where they play a very important ecological role. Most ponerine ants have quite small nests of a few hundred individuals, but in a paper published in 2013 my pal Christian Peeters found that some P. tarsatus nests can be as large as 5,000 individuals.

Here’s a rather small picture of Christian with a box of these alarmingly large ants:

Although the ants are notoriously aggressive, they don’t seem to bother about the Rubber Frog, as shown by this photo:

You can see how the ants seem rather bemused by the frogs in this video stitched together from the Rödel paper by a YouTube user – the first part shows ants with an adult frog, the final section with a froglet:

Other frogs, and other arthropods, are immediately attacked by the ants when they encounter them. However, when dead mealworms or live termites were covered in extracts from the frog’s skin, they were generally ignored by the ants, or at least it took much longer for the first bite to be administered:

When Rödel’s group examined the chemical composition of the frog’s skin, they found it contained two novel peptides – short proteins, each 9 or 11 amino acids long – with a proline-phenylalanine pair at the end. When termites were covered with either or both of these peptides, the ants took significantly longer to attack them, suggesting these are indeed the active ingredients on the frog’s skin:

This finding is doubly surprising – most instances of chemical camouflage involve cuticular hydrocarbons, which many arthropods use for communicating (for example, these are involved in the case of the Alcon Blue caterpillars described above). In the case of Phrynomantis microps, not only were novel peptides involved, no hydrocarbons could be detected on the frog’s skin, even though the animals were living in a hydrocarbon-rich environment in the ants’ nest.

What’s in it for the frog? Protection from predators (you’d have to be very foolhardy to take on the ants) and possibly protection from dessication during the dry season. They may also eat some of the ant larvae, although that is speculation on my part.

What’s in it for the ants? Probably nothing. If the frogs found a way to hack their chemical communication system, but at low or zero cost to the ants, then it won’t matter. If there’s a substantial cost to the ants, then you would expect a chemical arms race to begin – any ant nest that used a slightly different system of communication would not sustain the cost of the frog in the room.

The final point about this rather neat piece of biology, which flowed from a field observation, is that it’s opened up a new area of study in chemical communication in ants, and potentially a way of placating aggressive insects.

You see, Professor Malcolm was right:

 

Rödel M-O, Brede C, Hirschfeld M, Schmitt T, Favreau P, Stöcklin R, et al. (2013) Chemical Camouflage– A Frog’s Strategy to Co-Exist with Aggressive Ants. PLoS ONE 8(12): e81950

Peeters C, U. Braun U & Hölldobler B (2013) Large Colonies and Striking Sexual investment in the African Stink Ant, Paltothyreus tarsatus (Subfamily Ponerinae) African Entomology, 21(1):9-14. (Abstract)