If you have been down to Ituri Forest at the San Dieog Zoo recently, you’ve probably seen our resident hippos playing with a large ball at the bottom of their pool. The keepers are almost as happy to see the hippos interacting with this ball as the hippos seem to be splashing about with it. A few months ago, these large, floating toys were removed from the hippo exhibit pool because they had been damaging the glass from the inside. Since this was one of their favorite toys to play with, a number of employees were trying to come up with an alternative toy strong enough to give the hippos, yet safe enough for the exhibit glass.

The first step was a brand-new, and expensive, buoy. It was soft enough on the glass but apparently not thick enough for the hippos. Jabba, our 24-year-old male, popped the buoy the first morning it was presented to him!

Two team members put their heads together: John Michel, a senior keeper, and Rich Molina, a construction and maintenance worker. The plan was to somehow sink the ball the hippos previously had, thereby keeping it away from the glass. Easier said than done!

The first idea was to fill the ball with gravel. The only problem was that the hippos punctured a hole into the 1/4 inch-thick plastic in short order and the small bits of gravel spilled out into the pool. Since we vacuum the pool every morning, this spilled gravel would eventually be sucked into and ruin the pump used to clean the pool, so the ball was hauled out of the pool and back onto the drawing board.

The second idea, and eventual solution, was to coat the inside of the ball with form oil and then pour 80 pounds (36 kilograms) of concrete into the ball to set. The oil would form a layer inside the ball and the concrete would harden without bonding to the inside of the ball. The ball was then thoroughly rinsed, rolled out to the exhibit, and filled with water. The idea was for the concrete to move independently inside the ball and always rest on the bottom so the ball wobbles and jerks when the hippos push it around.

It DID take a few days for the hippos to go near this new, sinking ball when it was first presented, but once they did, they spent hours pushing it around. They even managed to push it OUT of the pool. No easy task, since the original ball weighed 65 pounds (29 kilograms), then add the 80 pounds of concrete and fill the rest of this 31-inch (79-centimeter) ball up with water. It took a lot of muscle to get a ball weighing several hundred pounds (and wobbling about like a weeble) back into the pool!

So, next time you’re at the Zoo, check out the hippos’ new (and improved) wobbling ball, hopefully located at the bottom of the hippo pool!

Nerissa Foland is a senior keeper at the San Diego Zoo.

Read a profile of our hippos, Jabba and Funani.

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