( ( ( ( This ( is ( why ( ( many animals ) ( are territorial ) ) ) ) ) ; ) ( they ( defend ( ( their territory ) ( which ( contains ( those resources ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT This)) (VP (VBZ is) (SBAR (WHADVP (WRB why)) (S (NP (JJ many) (NNS animals)) (VP (VBP are) (ADJP (JJ territorial))))))) (: ;) (S (NP (PRP they)) (VP (VBP defend) (NP (NP (PRP$ their) (NN territory)) (SBAR (WHNP (WDT which)) (S (VP (VBZ contains) (NP (DT those) (NNS resources)))))))) (. .)))
This is why many animals are territorial; they defend their territory which contains those resources.
(ROOT (S (NP (NP (NNS Animals)) (SBAR (WHNP (WDT that)) (S (VP (VBP defend) (NP (PRP$ their) (NN area)))))) (VP (VBP are) (ADVP (RB generally)) (VP (VBN known) (PP (IN as) (NP (NN territorial))))) (. .)))