What are researchers to do when some 4,000 bathing and defecating hippos in Kenya’s Mara River are destroying the water quality and undermining an important water study?

Call in the calvary? Better yet, a fleet of autonomous airboats disguised to look like crocodiles developed and operated by a CMU spinout, Platypus.

That’s exactly what happened this spring when Yale University forestry researchers, who have been studying the water quality on the Mara River since 2008, realized that they needed to take drastic measures to get to the bottom of the river and study the situation.

The problem, says Paul Scerri, associate research professor in CMU’s Robotics Institute and president of Platypus, is that the hippos spend long days wallowing and defecating in the river. Each hippo produces about 22 pounds of wet dung each day, which settles to the bottom.

When the river is at high tide, oxygen levels downriver crash and massive fish kills can occur, generally because hippo feces are being flushed from the hippo pools, the researchers say.

By dressing up the airboats to look like crocodiles—a suggestion made by a Maasai guide assisting the researchers—they fooled most of the hippos into staying away. This allowed the airboats to safely skim the surface and take measurements of the river bottom where the hippo dung had settled.

“Hippos are very territorial and aggressive and have been known to attack boats,” explains Yale researcher Amanda Subalusky. “We have to work with armed rangers when we get in the river anywhere, but we would never be able to get into a hippo pool.”

Of course, there was a close call when one suspicious hippo gave chase briefly.

“Those were 30 seconds that none of us will forget,” said Scerri.

The data is still being analyzed, but researchers believe the data collected will be meaningful. The project was sponsored, in part, by Project Olympus and supported by the National Geographic Society, Switzer Fellowship, World Wildlife Fund, National Science Foundation and Yale.