Companies in ailing sectors like energy and retail are adopting poison pills as their shares tank. We tracked down 41 recent moves — and the activist investors they're fighting off.

· Over the past two months, at least 41 companies adopted poison pill contracts or filed to put them to a shareholder vote. 
· Poison pills limit the stakes activist investors can take by giving the company the ability to sell new stock to existing shareholders at a discount if a trigger is hit.
· The measure was rarely...