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.
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