When it comes to the penis bone, mammals around the world have a varying degree of stoutness, from the blue whale whose bone measures an intimidating 2.5 meters (8 feet) to the shrew who scrambles in at just 6 millimeters (0.25 inches). Somewhere along the way, though, humans lost their penis bone, or baculum, and when it comes to reproduction, well, size and shape really might matter.

At least, those are the findings of a new study published in Proceedings of the Royal Society B that set out to find why some animals still have bones in their boners.

First of all, we should inform you that the baculum is one of the most morphologically diverse bones in the world – and not just in size.

“The reason natural historians are so fascinated by bacula is because they have lots of unusual features: odd ridges and grooves, strange curvature and bizarre-shaped tips,” said Manchester Metropolitan University researcher and lead author Charlotte Brassey, PhD, in a statement.

“However, all of the previous attempts to study the baculum have simplified the bone into very basic measurements of length and width, and have ignored all this important shape information.”

This mineralized bone is located in the glans of some mammals' (including primates) penises and can come in all shapes. For example, the ground squirrel’s mushroom tip resembles the innards of a venus fly trap, while the elephant’s anterior trunk resembles, well, its other trunk. (If you’re really curious, there’s an entire Reddit list about the amazing corkscrew penis bones found in birds).

So, why then did humans get stiffed in the game of penis bone evolution? To better understand the purpose of these bones, researchers first needed to study them in animals that still have them today. To do so, they generated 3D models of penis bones belonging to more than 70 species and ran simulations normally used in structural engineering to replicate the kinds of “loads” sustained by various penises during sex in order to “test the mechanical performance of the penis bone.”

`Animals with a baculum have them because when they throw down, they throw down hard. Previous research suggests the privy bone is useful in “vaginal friction”, a hypothesis that claims the penis bone became more robust to help get a rather large penis into a smaller female. However, the researchers suggest that the bone serves as an evolutionary tactic to help the penis stay put in a safe way and to withstand a potentially long mating sesh (sorry, human guys).

“Some carnivorous mammals such as the fossa, a cat-like animal that is most famous as the villains in the animated movie Madagascar, can mate for up to three hours continuously and our research suggests that the penis bone is relatively stronger in such species that mate for very long durations,” said Brassey.

Furthermore, it appears the baculum also protects the urethra when sperm is delivered for fertilization.

The researchers say their findings could have implications for animal conservation programs, especially those with captive animals that struggle to reproduce in zoos and other facilities, as well as animal husbandry and welfare. Understanding how the bone works could also help explain why some species (read: us) are so successful without it.