David Attenborough's Natural Curiosities

Sir David Attenborough examines the mysteries behind some of nature's most fascinating creatures and plants. Animals are usually either male or female and behave in a way that is characteristic of their gender. But in nature, there are curious exceptions. Female hyenas look and behave like males and male seahorses actually get pregnant and give birth. Why have these two animals swapped their sexual identities? Spiders spin intricate webs using silk, an incredibly strong material, and some birds can weave complicated nests from strips of leaves. What are the processes behind the building of these complex structures, and could we adapt spider silk to benefit us?