The push against single-use plastic has some emotive mascots, including the iconic turtle with a straw in its nose. The working hypothesis about why turtles are so attracted to plastic is that plastic drifting in the ocean can look a lot like jellyfish. But how does that explain turtles caught up in, and eating, other kinds of plastic, like that straw?

A paper published in Current Biology this week has an alternative explanation: it’s the smell of plastic, not the look, that attracts turtles. Rather, it's the smell of organisms that latch on to the plastic. The authors found that turtles were attracted to the smell of plastic coated in goopy ocean organisms just as much as they were attracted to the smell of food. That’s useful information for starting to figure out how to mitigate the effects of ocean plastic on predators like turtles.

Delicious stinky plastic

Plastic floating around in the ocean pretty quickly gets gooped up by all kinds of organisms that start to grow on them. This “biofouled” plastic has been found to emit dimethyl sulfide, an organic compound that plankton releases in large quantities. Animals use this smell as a signal that food may be present, and seabird species that rely on this as a food cue have been shown to ingest more plastic than species that use the smell cue less.

Still, that’s just one piece of evidence suggesting that smell is an important explanation of ocean plastic consumption. Turtle biologist Joseph Pfaller and his team wanted to look for a second piece of evidence by testing experimentally whether turtles responded to plastic in the same way they did to food.

So they took a bunch of plastic water bottles from Walmart and left them floating in the ocean for five weeks to get properly biofouled. At the same time, they collected 15 loggerhead turtle hatchlings to keep in tanks for a few months. When the turtles were a few months old, they used their feeding times to expose them to a range of different smells to see how they responded.

Sniffer turtles

Turtles use a range of airborne cues—including dimethyl sulfide—to alert them to the presence of food. When they get a whiff of something that smells like the prospect of lunch, they keep their “nares” (turtle nostrils) out of the water for longer to help them home in on the source of the smell.

Using a piping system that delivered different smells into each turtle’s tank area, Pfaller and his colleagues exposed each turtle to four different smell stimuli: deionized water, clean plastic, biofouled plastic, and turtle food. Each turtle was exposed to one smell per day, with the smells presented in random order.

To measure how the turtles responded, the researchers looked at two different measures: the amount of time each turtle spent with its nares out of the water, and how many breaths they took. They found that there was no difference in response between the clean plastic and deionized water, and no difference between biofouled plastic and food. The turtles also spent significantly more time with their nares out of the water when they smelt both the food and the biofouled plastic, compared to when they were given the aroma of clean plastic and water. This suggests that biofouled plastic smelled like food to the turtles.

The results for the number of breaths wasn’t so clear-cut—the turtles breathed more frequently for food and biofouled plastic than for clean plastic, but they also had a high number of breaths for the deionized water control. That could cast doubt on the researchers’ conclusions, but it could also just mean that turtles don’t change how much they breathe in response to food smells—or it could even be a result of the noisy data that comes with small samples.

Not just jellyfish

Taken along with the evidence on bird plastic consumption, this result offers a second piece of evidence that smell plays an important role in animals’ attraction to plastic. Because the researchers only used a small number of turtles, so their results will need confirmation through more studies in the future to be sure that the data wasn't too noisy to give a reliable result.

Still, the results have the advantage of explaining something that otherwise makes no sense: why turtles and other sea creatures are chowing down on such a huge variety of plastic, not just the kinds that might look like prey. More work exploring this explanation could help to build a better understanding of how animals are interacting with plastic—and how to stop them from being killed by it.

Current Biology, 2020. DOI: 10.1016/j.cub.2020.01.071 (About DOIs).