Field Notes
Lizard Pecker Pics v. Human Willy Pics
Why we can identify different species by looking at their penes?
Penises. Penes. Genitalia of the male persuasion. While these structures are all too familiar to half the human population, within the entire animal kingdom there exists a wide diversity of male-centric copulatory organs. In fact, this diversity of penile protrusions is so broad, that it can actually help us identify species.
Let’s take a step back.
What is a penis?
All jokes aside, a penis is the sexual organ for biological males of a species. While penises may serve dual function in some species (e.g., urination as well as intercourse) their primary purpose is to pass on genetic material in the form of sperm.
However, throughout the animal kingdom, the diversity of penes is absolutely incredible. Feline fornicators possess penile spikes to “rake” the vaginal walls and trigger ovulation. Dolphin dongs are prehensile, allowing them to grab and maneuver their particularly dextrous phallus. Bed bug boners are saber like and have been known to literally stab females to death with their armored member.
Reptilian Pricks
But within the complex world of penises, pickles, and peckers, snakes and lizards (we’ll refer to them as squamates here) offer some of the most extreme penis development. This complexity is so pronounced in fact, species that appear completely identical are separated into two once you dig into their phallic propagators.
But what’s so special about squamate schlongs? First, let’s do squamate penis biology 101.
Squamate Schlong 101
These organisms have 2 penises called hemipenes (see pic). These hemipenes are kept inverted within the tail of the organism until being everted outwards. Squamates may evert their hemipenes for sexual reproduction however, some species may evert their hemipenes as a defense mechanism against particularly homophobic predators. (Writer’s note: This is a joke. The only animals with demonstrated homophobia are human beings).
Each hemipene possesses an outer groove called the sulcus spermaticus. This groove transports sperm along the outside of the hemipene (as opposed to inside). Generally, these traits hold true for all snakes and lizards.
Decorative Shafts
However across some 11,000 species of squamates (source: Reptile database), the morphology of hemipenes varies wildly. Many species possess ornamentation such as hooks, spines, dimples, frills, and grooves (see figure). Further, the basic morphology of hemipenes can vary wildly between even closely related species. In 2012, a species of Anolis lizard was discovered to actually be 2 species based partly on hemipene morphology. The species, A. tropidogastor exhibits large, bulbous, and bilobed hemipenes while A. gagei possesses small, thin, and unilobed hemipenes (see Kohler et. al. 2012 figure 1). As a note, it is not about the size of the hemipene, it’s about how it’s used to delimit species. In fact, deciding species through hemipene morphology is exceedingly common in the world of squamate systematics.
But Why is This?
Well part of that equation may be how quickly hemipenes diversify. In fact, Anolis lizards undergo genital evolution 6 times faster than nongenital evolution. In essence, their penes diversify, divide, and delineate several times faster than the rest of their body. This finding implies that hemipenes are under some form of evolutionary pressure to diversify at an accelerated rate. While the exact process that causes this rapid evolution has yet to be discovered, two hypotheses are posited.
Just Tell Me What You Want!
One hypothesis is described simply as cryptic female choice. Here, females are selecting for males that possess some desirable trait. The trait the male hemipene possess is not known by the lizard. This could be anything from size, shape, or presence /absence of certain ornamentation. However, what is important to know is that this choice isn’t advertised. In essence, the choice is a “hidden” one and males have no way of knowing if their pene is the “correct” pene.
For example, say that male hemipene A is large and bulbous while hemipene B is small and narrow. Female genitalia may be structured in such a way that only Hemipene A would allow for successful fertilization. As the structure of the female genitalia is presumably unknown before copulation, the choice is cryptic.
Morphology is one avenue for cryptic female choice, but this can also include selection for certain sperm, keeping or removing “plugs” that prevent another male from mating with the female, or really any number of traits (see Eberhard 2010 for more examples).
Constant Phallic Fluxx
The other hypothesis is that of sexually antagonistic coevolution. In this hypothesis, males and females are at an evolutionary arms race, er, genitalia race. In this scenario both sexes are adapting to traits that the other is evolving. Females evolve some trait that the males must overcome. Once overcome, the females then evolve some trait to prevent or hinder the males from mating. And the cycle continues on and on, with both males and females evolving traits in response to one another. Essentially, this evolution of genitalia and sexual systems is stuck on a neverending treadmill of evolution. However, the exact mechanisms under which this hypothesis would act in lizards has yet to be studied.
Regardless of which hypothesis is acting, we know that squamate hemipenes evolve rapidly. This rapid evolution causes substantial differences between different groups of lizards and snakes. As these differences accumulate and the different groups diverge from each other further and further the two groups may ultimately become separate species.
And we can tell they’re separate species by looking at their penes.
