The Mystery of Marriage (1932) Online

This educational film (by the director Mary Field, who produced other nature documentaries in a similar wry style) sets out to impart information on the reproductive habits of various creatures to the young mind, thanks to some painstaking time-lapse photography and some close-up film of various animals -- although the more exotic species appear to have been filmed in the Zoo! So far as this goes it is reasonably informative and interesting, and I encountered a few facts about plant strategies for seed dissemination that I hadn't known before. (I did also spot a couple of errors of fact: so far as I'm aware, frogs are not reptiles and alligators are actually very attentive mothers...)

However, the novel and memorable twist here is to reverse the usual sex-education trope in which "the birds and the bees" stand proxy for human activity; in this film, animal behaviour is illustrated by using human examples. The result is often very funny, as in the cuts between praying-mantis females and the icy glances of rival Society ladies, or between the male preening himself and the young man stroking his moustache.