A serious case of...
Why does Duolingo gives us "Trotz DER Elefanten sind sie gekommen" instead of Trotz DEN Elefanten sind sie gekommen"?
I second what christian said. In this case, the dative sounds really awkward to me (although I live in the southern part of Germany where these dative constructions usually are used more frequently than in the rest of Germany). @margamanterola: You're right that sometimes also the dative is used with 'trotz' but that's more in constructions like 'trotz heftigem Regen' or 'trotz alledem'. If the object is used with an article it's virtually always better to stick to the genitive. See here for more information: http://www.duden.de/rechtschreibung/trotz