 Sattler assists the park 's game warden , Robert Muldoon , in a search for survivors , but they only find an injured Malcolm before the Tyrannosaurus returns . Grant , Tim , and Lex spend the night in a tree , and befriend a Brachiosaurus . Later , they discover the broken shells of dinosaur eggs . Grant concludes that the dinosaurs have been breeding , which occurred because of their frog DNA — West African bullfrogs can change their sex in a single @-@ sex environment , allowing the dinosaurs to do so as well , proving Malcolm right . 
