Not a fan of lemmings, since everything we know about them in popular culture is incorrect - they don't commit mass suicide - and is suicide really the image we want for ourselves? @Lemminary has the right idea.
It's a myth popularized by Walt Disney in a documentary where they made the claim and dumped a bunch of lemmings off a cliff into the sea to prove they did just that.
If it makes you feel better, lemmings don't actually commit suicide like you're imagining and Disney made it all up. They basically threw lemmings off a cliff to get the footage they wanted.