Vertebrate reproductive evolution is laid bare by the ever-erect, spring-loaded alligator penis


By Justin Havird, Department of Biological Sciences, Auburn University


“If you think about it in a reductionist manner, well, all penises should be really similar, shouldn’t they?”


That’s according to Professor Brandon Moore, a reproductive biologist at Louisiana Tech University. Although Moore probably spends more time thinking about penises than most of us do, it is clear that only a few simple characteristics are needed for a properly functioning penis. For example, minimum requirements might include a rigid shaft to enter the female reproductive tract and a tube to deliver sperm.


But it turns out there are a menagerie of penis forms out there.


Moore notes that, rather than a relatively uniform population of simple penis forms, “you see the exact opposite. Some even have spines, pieces sticking out, bulbous bits.” As a reproductive biologist, Moore is interested in understanding the complexities of internal fertilization in different evolutionary lineages, which includes penis anatomy.


Moore and colleagues have recently been studying one of the most bizarre penises in the animal kingdom to date – a spring-loaded phallus that is always erect.


Which species’ females are on the receiving end of this anatomical sling-shot? None other than the American alligator. And thanks to a chance meeting between penis specialists – and an unexpected dissection subject – Moore and colleagues have found that the alligator's penis is unlike any previously described penis, a finding with implications for evolutionary theory, conservation, and the mystery of prehistoric phalli. 


Although alligators have been captivating audiences for decades as wrestling opponents, school mascots, and even reality TV stars, one piece of their anatomy has remained a mystery until recently.


Earlier researchers were at a loss when it came to explaining the mechanism of the alligator penis.


“Effectively they said, we have no idea how this will work,” says Diane Kelly, a pioneer in the study of penis mechanics.


Kelly, a reproductive morphologist at the University of Massachusetts, Amherst, met Moore at the 2012 Society for Integrative and Comparative Biology Annual Meeting, at what Kelly now refers to as the “alligator penis mini-symposium”. They quickly decided to combine forces to unravel the mystery of the alligator phallus.  


Kelly proposed a radical hypothesis for how alligator penises might work. During cadaver dissections, she was surprised to find no muscles attached to the penis, unlike the arrangement typical to vertebrates. Instead, a set of muscles formed a sling around the end of the penis, with strong tendons directly attaching to the penis.


Thinking back to the way Achilles tendons store energy in hopping kangaroos, Kelly proposed that when the muscles forming the sling contract, the tendons would elongate, extending the penis outside the cloaca.


Much like a sling-shot, the elongated tendons would store potential energy. When the muscles forming the sling relax, the tendons would immediately shorten, pulling the penis back inside the body. 


Support for Kelly’s hypothesis came a few years later in a remote part of a bayou in Louisiana where Moore and colleagues were performing a routine dissection of an adult male.


An alligator dissection, ventral side up. Photo courtesy of Dr. Brandon Moore.




Moore recalls: “The first time it happened only one or two people saw it, and they were like, ‘What just happened?’”


What had happened would largely confirm Kelly’s hypothesis. A chance probe had hit the nerve responsible for contracting the sling’s muscles, causing the penis to shoot out of the cloaca. The phallus whipped back inside the body cavity just as quickly. 


“It literally was a surprise,” explains Moore. “Then we said, ‘Let’s see if we can do this again.’”


Although it is still unclear why the alligator has evolved this unique sling-shot action, Moore has one idea for why the penis is quickly withdrawn back into the body.


“You don’t want it hanging out in the swamp – so when you’re done with it, pull it back in.” 


A closer look at the microanatomy of the penis revealed that minimizing environmental exposure could be very important. Unlike mammalian phalli, seminal fluid is delivered through an open groove on the bottom of the penis called the sulcus, rather than an internalized tube.


Moore and colleagues also looked at collagen and elastin compositions throughout the penis and found that the tip or glans of the alligator penis had a similar architecture to the mammalian penis. So, although the penis always remains erect, the tip might still change shape and configuration when engorged with blood.


In other words, alligators can get a second erection. A recent study of artificial insemination in Australian saltwater crocodiles confirmed this dual-function hypothesis.   


As Moore describes it, “When the phallus comes out, it opens into this really elaborate structure.”


The alligator penis. Photo courtesy of Dr. Brandon Moore.



By changing shape, the glans might be able to come into more direct contact with the female reproductive tract, increasing the likelihood of fertilization. An elaborate, engorged glans might also provide an increased surface area for antimicrobial secretions. After all, the swamp can be a nasty place for gametes.     



(If the video is not visible, right click/click hold the following link and save video file to your computer: Video, mp4 format)

Rapid eversion and retraction of the alligator penis. Video courtesy of Dr. Brandon Moore.



Alligator penises are more than just a penile novelty. Because alligators are close relatives of birds and dinosaurs, their reproductive biology might reveal something about reproduction in these groups. 


For example, there is a tendency for penis loss in recent bird groups, such as songbirds, whereas the ratites - a more ancient group of large, flightless birds that includes emus, ostriches, and kiwis - have penises that superficially resemble that of the alligator. As Moore points out, “If we can understand how the crocodilian phallus works, then we can move onto the derived phalluses within Aves to understand how function and morphology change over evolutionary time.”


When asked about dinosaur penises, Kelly says, “The theropods might have had something similar or intermediate between the crocodilians and the ratites,”


Although it may be tempting to picture theropods like Tyrannosaurus rex walking around with a gigantic spring-loaded penis, Kelly is quick to point out, “Who knows? It doesn’t fossilize.”


Or, at least, we haven’t found one yet. 


In today’s world, the form and function of alligator penises has a more immediate application: the conservation of critically endangered crocodilians. Although American alligators are no longer endangered, about half of the recognized species of crocodilians in the world are either endangered or threatened. For example, fewer than 1,000 Indian gharials remain in the wild.


Understanding the mechanics of reproduction in alligators could aid in captive breeding programs or artificial insemination efforts for these at-risk species. A working knowledge of penis anatomy is a critical first step in such programs.


For now, Moore’s team is working hard to gain this knowledge. “We’re trying a real holistic investigation of this structure.”

And such an investigation might not only help to reveal the mating secrets of an American celebrity, but also advance conservation efforts in the present, shed light on the phalli of the past, and help explain why some groups eventually lose the thing altogether. 



Moore presented the results of this research at the 2014 annual meeting of the Society for Integrative and Comparative Biology in Austin, Texas.




Justin Havird is nearing completion of his PhD in the Dept. of Biological Sciences at Auburn University under Dr. Scott R. Santos. His research centers on the interaction between organisms and their environments and spans fields such as ecology, physiology, genomics, and evolution. His current work utilizes organisms from the anchialine ecosystem towards these pursuits. Justin plans to study mitochondrial and nuclear genome conflict as a post-doctoral researcher beginning in late 2014.