Scientists love the data they get by attaching electronic tags to animals, but these devices can be a literal drag. For animals that fly or swim, tags can mess up their mechanics and force them to spend more energy. That's what scientists expected to see when they studied dolphins with data loggers suction-cupped to their backs. To their surprise, they found instead that these dolphins refused to work any harder at all. When scientists attach a data-logging tag to an animal, they try to disturb the animal as little as possible. Tags on birds, for example, are limited to a certain percentage of the bird's mass. But there are fewer guidelines for animals that aren't birds, writes Julie van der Hoop, a graduate student in the MIT–Woods Hole Oceanographic Institute joint program. And much of the research on tag drag in marine mammals has been done with computer modeling, not actual animals. Van der Hoop and her coauthors wanted to see how tags affect real, swimming dolphins. Working with Dolphin Quest Oahu, the researchers engineered a setup that would let them measure how tags changed dolphins' energy use. Dolphins were trained to swim laps around an oval-shaped course. In between sets of laps, they came up for breath only in a specific place: underneath a floating dome, which measured how much oxygen the animals were using and how much carbon dioxide they were giving off.
The researchers used computational fluid dynamics to simulate how their suction-cup tags would increase the drag on swimming dolphins. Then they tested four trained dolphins, repeatedly sending them through the course both with and without tags stuck to their backs. When the dolphins surfaced under the dome, the data from their respiration told scientists how fast the dolphins' metabolisms were working.
Because their models showed that tags increased drag, "we’d expected to see an increase in energy use when animals were tagged," van der Hoop says. "We didn’t consider the option that animals would slow down." But that's just what happened. Dolphins with tags swam 11 percent slower than they did without a tag. Metabolic measurements showed no difference in how hard they were working. Faced with the extra drag from a tag, dolphins simply changed their pace so they were exerting the same amount of effort as before.
It's possible that a study with more animals would reveal an overall effect of tags on dolphins' metabolisms, the authors note. But so far, the results show that individuals can adjust their behavior to keep their energy use the same.
What happens if dolphins don't have the option of swimming more slowly? In the wild, an individual might race to pursue prey or keep up with the pod. Van der Hoop says she's now testing what happens when dolphins have to maintain a certain swimming speed. "I like to use the analogy of a backpack," she says. "I hike more slowly with more gear on my back...But if I have to hike quickly—to make it to camp before dark—carrying the heavy pack, I’d burn a lot more energy."
The tags in this study are made for short-term use. If they do make life harder for animals in the wild, the effect is likely to be brief. Besides the welfare of marine animals, though, the study also raises concerns about the integrity of the data that tags are gathering. Scientists tag animals because they want to record their natural behavior. If animals change their behavior in response, it's a problem.
"I think the community has been aware of the limitations of tag data, and important welfare concerns," van der Hoop says. These concerns had come mostly from guesswork, but studies like hers are giving scientists the information they need to build less disruptive tags.
Improving tag design can "help us create better devices, do better science, and better understand the data we collect," van der Hoop says. Scientists will need to work harder to ensure animals are safe and their data is sound—unlike the dolphins, which as we've seen don't like to do any extra work.
Images: top WHOI; bottom van der Hoop et al.
van der Hoop JM, Fahlman A, Hurst T, Rocho-Levine J, Shorter KA, Petrov V, & Moore MJ (2014). Bottlenose dolphins modify behavior to reduce metabolic effect of tag attachment. The Journal of experimental biology PMID: 25324344