( ( ( ( ( Animals ( ( can not ) ( make ( their ( own food ) ) ) ) ) , ) so ) ( we ( call ( them heterotrophs ) ) ) ) . )
(ROOT (S (S (NP (NNS Animals)) (VP (MD can) (RB not) (VP (VB make) (NP (PRP$ their) (JJ own) (NN food))))) (, ,) (IN so) (S (NP (PRP we)) (VP (VBP call) (SBAR (S (NP (PRP them)) (VP (VBZ heterotrophs)))))) (. .)))
Animals cannot make their own food, so we call them heterotrophs.
(ROOT (S (SBAR (IN Because) (S (NP (DT all) (NNS animals)) (VP (VBP require) (NP (NP (DT an) (JJ external) (NN source)) (PP (IN of) (NP (NN food))))))) (, ,) (NP (PRP they)) (VP (VBP are) (VP (VBN called) (S (ADJP (JJ heterotrophic))))) (. .)))