They have the support of large investors. They conduct joint developments with the Eth. Soon strong growth is expected 
Is there any evidence that They conduct joint developments with the Eth?
There is ongoing collaboration between Zcash and Ethereum. Here's just one example:
https://blog.ethereum.org/2017/01/19/update-integrating-zcash-ethereum/
There's a lot of information on the web about possible integrations and collaborations, it's definitely something that's going on.