Good question! Trotz (in spite of/despite) is one of those words that demands a case change. Trotz goes with the genetive case. So the definite articles in genetive are Masculine = des, Feminine =der, Neuter = des, Plural = der. So, since Elefant is masculine, the only option is plural. Here's a link to other genetive demanding words: http://german.about.com/library/blcase_gen2.htm