How animals eat themselves invisible

The animal kingdom is full of incredible examples of camouflage. For instance, many animals optically resemble objects found in their environments such as sticks or leaves, or have colour patterns that permit them to cryptically blend into their surroundings to hide from predators or prey.

Although the study of animal camouflage has a long history in ecology, almost everything we know about it comes from studies of visual mechanisms, probably due to our own reliance on sight. However, unlike us, many animals mainly interact with the world using senses other than vision, such as smell. A dog’s sense of smell, for example, is thousands of times more acute than ours. As predators often use smells to locate prey, even visually camouflaged prey can stick out if they still smell strongly like lunch.

But evolution has a tendency to produce countermeasures; an animal could employ some mechanism to mask or diminish its odour, either by limiting the smell it produces, or altering it to match smells present within its habitat. One way this could happen is via the diet: odour producing chemicals from the food transferred to the animal that causes their smells to match – similar to what happens if you eat a lot of garlic.

This has been shown in some caterpillars that live on the plants they also use for food. Chemicals in the plants, when digested, are transferred to the caterpillars, making them indistinguishable from the plant itself, causing predatory ants to walk right over them, oblivious.

Other than this fascinating example, evidence for similar processes in other animals has remained scarce, meaning it’s unknown how commonplace “chemical camouflage” is.

Underwater disguises

However, in our new study, published by the Royal Society, we have discovered that a small colourful fish, the harlequin filefish, uses a similar mechanism in order to mimic coral and hide from hungry predators. The harlequin or orange-spotted filefish is found on coral reefs throughout the Indian and Pacific oceans, including at Lizard Island in the northern Great Barrier Reef where we conducted this work.

It is very selective about what it eats, choosing to nibble on only certain corals. It also uses these corals as habitat at night, hiding in among the coral branches making it very hard to spot. To determine if these filefish smell like the coral they eat, we recruited coral-dwelling crabs to test if these crabs could tell the difference between the smell of filefish and corals. These tiny crabs live tucked away in among the coral, with each type of coral having its own species of crab.

In the study, the crabs could distinguish between filefish that were fed different corals, preferring those fish who fed on the coral the crabs lived in. In fact, the filefish smelled so strongly of coral that sometimes the crabs were attracted to the fish instead of coral itself.

Smelling like coral is all well and good, but it’s not much use if you still end up being eaten. To test if smelling like coral reduced the ability of predators to detect filefish in the surrounding habitat, we ran a second experiment, this time using large predatory blue-spotted rock cod.

Cod, filefish and corals were put in a tank, with the filefish hidden from the cod. When the filefish diet matched the corals in the tank, the cod stayed tucked away in their cave inside the tank. However, when the filefish diet didn’t match the corals in the tank, the cod were restless, suggesting they smelled food. This implication is that matching the smell of your habitat could be a good way to avoid drawing unwanted attention.

This is the first evidence of an animal other than an insect camouflaging its odour using chemicals derived from its diet. The next step will be to determine exactly how filefish can smell like coral and much work remains to understand this set of biological processes.

We know very little about chemical camouflage in animals, and the study shows how important it is for researchers to examine camouflage from the perspective of the animals involved, rather than our own. The findings also open up the possibility that a wide range of different animals could also be disguising themselves in similar ways – right under our noses.

This article is published in collaboration with The Conversation. Publication does not imply endorsement of views by the World Economic Forum.

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Author: Rohan Brooker is a Postdoctoral Fellow in Biology at Georgia Institute of Technology.

Image: A Medusa Rhizostomeae is seen underwater near the Ukrainian Black Sea port of Odessa, September 1, 2012. REUTERS/Gleb Garanich.