Birds aren’t the only animals that can make a top-grossing mobile game.

Pocket Gems, one of the handful of mobile game developers that has recently taken funding from a top-tier venture capital firm, just passed 28 million downloads with its casual, free-to-play title Tap Zoo leading the way. While the company has five apps out, Tap Zoo is one that’s been a fixture at the top of the charts for virtually all of its seven-month existence.

The game, which lets players manage a virtual zoo by buying and breeding animals, has been in the Top 10 grossing games for 27 out of the last 31 weeks. It’s the #1 highest-grossing app today in the U.S. (The company isn’t disclosing other metrics like MAU or DAU.)

“We like to think about games that are going to have a universal appeal,” said chief executive Daniel Terry. “They have to be easily accessible and attract a huge audience with a lot of diversity.”

There are a couple factors that have led to Tap Zoo’s success. First, it’s a free game that monetizes with in-app purchases of virtual currencies, meaning the game’s downloads are probably at least ten times what they would be if Tap Zoo were a paid app. The game doesn’t use in-your-face pressure to get its players to buy currency with invasive interstitials. But there are a number of animals and items that a player simply cannot get without having “Stars,” a virtual currency in the game that they can only get through in-app purchases or downloads of other apps. Pocket Gems constantly refreshes the content with new animals. Plus there are other unique features like cross-breeding, which leads to entirely made-up animals like ligers and more.

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