I this quote from Drs. Kelly and Orbach in this Discover Magazine article really sums harbor porpoises up well:
“An erect harbor porpoise penis goes up to its chin — or, where its chin would be, if it were not a marine mammal,” Kelly said. And their vaginas are the most complex of the four, with “a whole bunch of spiraling folds,” Orbach explained, “which just keep twisting around … a little bit like a corkscrew.”
But these journal articles are also really interesting reads (and they’re both open access, I believe).
Orbach, D.N., Kelly, D.A., Solano, M. and Brennan, P.L., 2017. Genital interactions during simulated copulation among marine mammals. Proceedings of the Royal Society B: Biological Sciences, 284(1864), p.20171265.
This is a series of little illustrated factoids about animal reproduction, posted daily. I’d love to hear your suggestions/ feedback. I also post these on my insta.