Question about plural accusative

[nwln2]

Can someone tell me why in this sentence 'den' instead of 'die' is used?

Das Essen ist den Elefanten willkommen.
In accusative case, use "den" for masculine nouns like "Elefanten"

[kragi]

"Elefanten" is dative, not accusative. Question: "Wem ist das Essen willkommen?". Answer: "den Elefanten". "Willkommen" goes with dative.

[kmj2l]

Doing a search on "Elefanten" brings up a number of previous discussions on this topic: This one in particular is informative: http://duolingo.com/#/comment/135 It seems that "Elefant" takes on "-en" even when singular, in the accusative, which was news to me. I do wish they would be a little more explicit about these strange grammatical particulars.

Edit: After some searching, here is a page with further useful information, including how to identify such nouns: http://www.vistawide.com/german/grammar/german_nouns03.htm