Never mind Bitcoin mania taking the doyen of virtual currencies into the stratosphere. The real online craze to get with right now is to breed virtual cats.

Net-based cousins of the Tamagotchi digital pet launched in Japan 20 years ago, "CryptoKitties" are wowing owners of Ethereum, one of Bitcoin's main rivals.

With some 50,000 sold for more than $6.6 million (5.6 million euros) in total, according to the creators' website, the critters are steadily collaring the blockchain, the digital virtual ledger on which so-called crypto currencies run.

Ethereum owners use their virtual cash to buy the creatures, then to bring them up and feed them while taking care generally to look after their well-being.