Female bottlenose dolphins have evolved a defence against unwanted pregnancy - complicated vaginas.
When male bottlenose dolphins are looking to breed, they form packs of two to four dolphins.
They then surround a female dolphin and take turns with her. The female has little choice over who mates with her.
But researchers have found the females have evolved a way of choosing who fathers their dolphin babies.
They made moulds of the dolphins' vaginas, which showed "unusual vaginal folds, spirals, and recesses".
"There's this unparalleled level of vaginal diversity that we had no idea existed before," researcher Dara Orbach told New Scientist magazine.
By inflating dolphin penises and inserting them into the moulds, they could scan how they fit together.
Their study concluded the females can move during mating to send an unwanted male's penis the wrong way and prevent a successful fertilisation.
"She may not choose who she mates with, but might be able to choose which male or, more precisely, which sperm, fertilises her egg," Janet Mann, of Georgetown University, told New Scientist.