How to allow visitors to see forum topics but not answers unless they register and login.

I have a WP MS (3.5.1) and BP (1.7) network with a main site for WooCommerce, one child site for BP functionality and currently one child site for blogging.

I would like to configure my forums so that a visitor to the site can read the blog posts and see the topics in the forums but can't see the replies unless they are registered and logged in. I'm pretty sure this can be done but I don't know how to do it.

Any help would be greatly appreciated!