Do females really select the best penis to mate with? Well, Harvard scientists working with international collaborators have discovered that the penis of a Caribbean lizard may be evolving faster than their other physiological traits.
This is a result of sexual selection that contributes to genetic as well as genital diversity in the species and leads to questions as to why this occurs:
- "Do females, who have the power of mate choice, select the “best” penis for them?"
- Or, is this diversity the result of a sexual feature raised with male lizards looking to reduce female control of reproduction?
Well, lets get into the dirty details. Lizard penises are different from human ones. For one thing, male lizards (and others in the order Squamata) don’t just have one penis. They have a pair of organs called hemipenes.
Don’t get too excited, they usually only use one at a time, and hemipenes means “half penis.”
So anyway, these organs are kept inside the body for day to day activities, and then one becomes externalized by erectile tissue for reproductive purposes.
Let's Take a Closer LookThe scientists who performed the study used comparative methods to ask whether there was more variability in hemipenis shape (length and width), than of non-genital traits like limbs and dewlaps—a flap of skin under a lizard’s throat. This is an important question to answer because increased diversity would imply an increased evolutionary rate for genitalia.
What they found, when they compared measurements is that many very closely related lizards have hemipenes very different from each other!
Scientists were able to calculate the rate of change necessary to produce such different genital features and put this information in the context of a relative lack of diversity in limbs and dewlaps to conclude that the genitalia of Caribbean Anoles lizards evolve approximately 6x faster than other morphological traits.
Genitalia is frequently used to differentiate similar species, but no one has done a quantitative measure of increased evolutionary rate until now.
The speed of evolution is so different that it begs the question, why? Well, the scientists who performed this study assumed that the rapid evolution of male genitalia was a result of co-evolution with the female and proposed two hypotheses:
- Either the evolution occurred through “cryptic female choice” where females drive selection through personal preference.
- Or “sexually antagonistic coevolution” an attempt on the part of male lizards to reduce the female influence on reproduction.