“You are what you eat” has become a cliché spouted by parents and diet gurus alike. Of course, while most of us might feel a bit sick after eating all the leftover Halloween candy, we’re not going to turn the yellow of Lemon Heads or start oozing chocolate out of our skin glands. For some members the animal world, however, diet is important for more than nutrients and energy. In fact, many species depend on their food to give them the colors, poison, and unearthly glow for which we know them best. Here are four animals for which “you are what you eat” is a fact of life:
1. The golden poison dart frog goes by the scientific name P. terriblis with good reason. It may be the most poisonous animal in the world – terrible indeed for would-be predators. Despite their diminutive size (up to 55 millimeters long, about two US quarters placed side by side), golden poison dart frogs pack quite a punch. One frog carries enough poison to kill ten to twenty humans. However, unlike many other poisonous animals, golden poison dart frogs outsource their poison production. The frogs isolate and concentrate alkaloid batrachotoxins from the bugs that they eat, a process called sequestration. During digestion, batrachotoxin is shuttled from the frogs’ gut to special skin glands, where it becomes a key ingredient in the deadly slime that coats their skin. Concentrating the poisonous chemicals in their food is a common thread with all poison dart frogs, but P. terriblis’s use of batrachotoxin in particular is what makes it especially deadly. Frogs kept in captivity will lose their toxicity, since their new diet doesn’t usually include the alkaloid-rich bugs crucial to toxin production.
Frogs aren’t the only animal that uses the capture-and-concentrate tactic for making poisons. For example, the monarch caterpillar is also poisonous because of an alkaloid-rich diet (in this case, the leaves of the milkweed plant). Some of this toxin carries over to the adult butterfly, making it an unpleasant meal for predators. There are even several species of birds in Papua New Guinea that use the same strategy, appropriating insect alkaloid toxin and storing it in their skin and feathers.
2. Sometimes an animal’s signature color comes from what it eats – flamingos owe their lovely pink-red hue to a class of pigments called carotenoids. Carotenoids are first produced by plants (beta-carotene is what gives carrots their orange color). Flamingos get their carotenoids from the plankton, algae, and brine shrimp they eat. In fact, a lot of the color variation across flamingo species comes from their food source. Flamingos that feed on carotenoid-producing algae directly are a darker hue than those that get their carotenoids through a second-hand source (like eating shrimp that eat algae). Without this dietary infusion of color, flamingos would be a dull gray or white. Flamingo coloring is a big concern in zoos and aquaria, where the birds don’t get all the carotenoids they would in the wild. To keep their animals more naturally colored (and attractive to visitors), zookeepers may feed them extra prawns or sprinkle their food with a concentrated carotenoid additive. Carotenoid-derived coloring is not uncommon, especially among birds. The red plumage of scarlet ibises and cardinals and the pink bellies of salmon all owe their distinctive color to a diet high in the pigment. Scarlet tanager males are turned (you guessed it) scarlet by a high-carotenoid diet, while the females are an olive-green from the interaction of carotene and another pigment, melanin.
3. Members of the nudibranch sea slug family scavenge both color and toxins from their prey. Among the family are some of the most brightly colored denizens of the ocean – and that’s saying a lot for since nudibranchs make their home in the kaleidoscopic world of coral reefs. These slugs dine on other invertebrates like baby jellyfish, coral polyps, anenomes, and even other sea slugs. Nudibranchs can sequester pigments from their prey and then display them on their own skin, creating a colorful pattern well matched to their environment. Among the brilliant background of a coral reef, the nudibranchs’ rainbow hues are the perfect camouflage.
Considering the fact that many nudibranchs can also recycle more active defense mechanisms, their bright skin is also warning to predators. Nudibranchs that feed on jellyfish can steal their prey’s special stinging cells (nematocysts). The nematocysts travel harmlessly through the slug’s digestive tract and are transported to their back, where the nematocysts begin their new life as a slug defense mechanism. And like poisonous dart frogs, certain nudibranchs concentrate toxins from the sponges they eat to make their own poisonous brew.
4. Aequorea victoria jellyfish, like many other bioluminescent animals, glow a soft fluorescent blue and green thanks to a chemical cascade taking place inside their clear flesh. This species of jellyfish holds a special place in the hearts of many biologists, who use the jelly’s green fluorescent protein (GFP) in everything from imaging to genetic engineering. In fact, the discovery and development of GFP as a tool won several scientists a Nobel Prize in 2008.
For all the glory given to GFP, it’s not actually the piece of the cascade that supplies the jellyfish their glow. GFP simply takes the light from another molecule, coelenterazine, and shifts it from blue to green. Coelenterazine is a type of luciferin, a light-emitting molecule. During the light-producing chemical cascade, coelenterazine is energized to an excited state by the enzyme luciferase. Coelenterazine then returns to an unexcited state by releasing energy in the form of light.
Scientists in the lab have to externally supply coelenterazine to their specimens for them to glow; as it turns out, the A. victoria jellies have to import coelenterazine as well. A. victoria at the Monterey Bay aquarium slowly lost their ability to glow, but when the jellies were fed a diet of bioluminescent plankton, they lit right back up. The luciferase enzyme part of the bioluminescence cascade is coded into the jellyfishes’ DNA. However, they need to scavenge the actual light source from already glowing prey – like having to steal a bulb from a neighbor every time you wanted to turn on your lamp.