Grammar Test A Answer 11

11. No one has ever questioned why a large fruiting tree or piles of sugarcane trigger a binge of sex among bonobos and why they tend to French kiss when someone throws a cardboard box into their enclosure.


1) This is a compound noun clause. "why a large fruiting tree or piles of sugarcane trigger a binge of sex among bonobos" is tied to "why they tend to French kiss when someone throws a cardboard box into their enclosure." This means that the second signal word ("why") is needed to make the parallel structure complete.