Zynga confirmed the names of four mobile game companies that it has acquired in recent months, according to Reuters.

The company did not  disclose the sums paid for those companies, which make smartphone and tablet games. In an interview with Reuters, Zynga senior vice president David Ko said that it had acquired German company Gamedoctors in December. Gamedoctors, based in Bielefeld near Hanover, created mobile game ZombieSmash.

Zynga also purchased Page44 Studios, which is based in San Francisco, in September. That studio created the “World of Goo” game for Apple’s iOS (iPhone, iPad, and iPod Touch).

Zynga acquired San Francisco-based HipLogic in August. Ko also confirmed Zynga bought New York based Astro Ape Studios in August to develop new titles. (That acquisition was actually previously disclosed, but it is included in Reuters’ list of four new acquisitions). Zynga is dominant on Facebook but is trying to grow its share in mobile, where other players such as Electronic Arts and Gameloft grabbed market share earlier.