How well do you know your animal kingdom anatomy? Order these animals by their penis size — longest first!
Oh and just in case you were wondering...here's a Sperm Whale's todger.
A Barnacle releasing his beast.
And of course a duck, horrifying us all with its corkscrew cock.
Please forgive me for what your eyes can now, never unsee.
Nature's Nether Regions: What the Sex Lives of Bugs, Birds, and Beasts Tell Us About Evolution, Biodiversity, and Ourselves (Penguin Group, 2014) By Menno Schilthuizen
Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15 521 men (BJU International, 2004) By David Veale, Sarah Miles, Sally Bramley, Gordon Muir and John Hodsoll
Equine Reproductive Physiology, Breeding and Stud Management, (Cambridge University Press) By Mina C. G Davies Morel
Male Reproductive Dysfunction (Jaypee Brothers Medical Publishers, 2011) By SC Basu
Pheromones and Animal Behavior: Chemical Signals and Signatures (Cambridge University Press, 2014) By Tristram D. Wyatt
Sperm Competition in Humans: Classic and Contemporary Readings (Springer Science + Business Media, Inc, 2006) By Todd K. Shackelford, Nicholas Pound
Dr Alexander Arkhipkin tells the BBC
Arctic Animals and Their Adaptations to Life on the Edge (Tapir Academic Press, 2005) By Arnoldus Schytte Blix
Wild Mammals of North America: Biology, Management, and Conservation (The John Hopkins University Press, 2003) By George A. Feldhamer, Bruce C. Thompson, Joseph A. Chapman
Mammal Anatomy: An Illustrated Guide (Marshall Cavendish Corporation, 2011) By Marshall Cavendish Corporation