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The second instalment in our double bill on the science of sex, answering this question from Robert Turner, a Curio from Leeds: "Why do we only have two sexes?"

Drs Rutherford and Fry look for anomalies in the animal kingdom that go beyond the traditional mechanics of human reproduction. Biologist and author Carin Bondar describes some of the wild and somewhat disturbing ways other animals like to do it.

Take the hermaphrodite sea slug who races to stab its penis into its partner's brain during sex, or the female redback spider who loves to indulge in a spot of post-coital cannibalism.

But the greatest number of different sexes is found in the world of fungi. Some species can have hundreds of distinct mating types. Fungal ecologist Lynne Boddy explains how mushrooms have sex and why on earth they need so many polygamous partners.

Presenters: Hannah Fry, Adam Rutherford
Producer: Michelle Martin.