For magicians apprentice--Shattered glass on the Otter Enclosure at Bremerhaven Zoo in Germany

Bud, I have no idea how this could have happened. Surely Otter's aren't physically strong enough to do this by hitting it. Otters often play with small rocks as they are swimming on their back's but I don't think they are capable of a large enough rock to do this damage. Maybe something from outside fell or hit the glass. Because Bremerhaven Zoo is located on the mouth of the Weser River with low temperatures in the winter time, possibly it was caused by that. Your guess is as good as mine.