The hunt for an effective, reversible, and socially acceptable male birth control continues. The newest target: The smooth muscle that makes up the tubes connecting the testes to the urethra. This needs to contract in order for sperm to reach their final destination. Now, scientists have shown that you can make mice sterile by eliminating their ability to contract that muscle. The result: A mouse with a dry ejaculation but which is still "pelvis thrusting with appropriate vigor and frequency".
This is a long way from becoming reversible treatment for human gentlemen, though. Right now, probably the most promising male birth control is RISUG, in which a clear polymer gel is injected into the vas deferens. The gel doesn't block the tube up completely, but it does seem to prevent sperm from successfully reaching the urethra and being capable of fertilizing an egg. RISUG is in Phase III clinical trials in India, but, even then, there are still safety questions about it and, so far, it's only been proven to be reversible in tests on non-human primates.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.