In evolutionary biology, we learnt about forces of natural selection and sexual selection. Menno Schilthuizen book “Nature’s Nether Regions: What the sex lives of bugs, birds and beasts tell us about evolution, biodiversity and ourselves” deals with a specific aspect of sexual selection: genitalia evolution!
How to tell beetle and spiders apart?
If you study spiders, beetles and other “bugs”, you know it’s really difficult to tell species apart. They all look the same. This might not be a problem that you face all the time, but it certainly is a problem that taxonomists working in natural history museum needs to solve.
Before the advent of barcoding technology, morphology is the only reliable tool these taxonomists had. Very quickly, they learnt that if you dissect the genitalia of the individuals that look alike, they turned out to be very different. Hence, genitalia is the best way to tell species apart!
Why so many different shapes and function?
Reproduction is often viewed (by us) as a cooperative activity: male and female ‘working’ together to produce the best babies. BUT from evolutionary biologist’s perspective, reproduction is a “never-ending evolutionary tango, a dance marathon that conserves elements of both battle and ballet.” – wrote Menno in the book.
Because of this constant action-reaction, weird (again in our mind’s eyes) adaptation like mating plugs, poisonous semen, penis spines and many others examples are rampant in the natural world.
Of course, females are not the passive receiver of all these male adaptations, females (a little bit more hidden and difficult to study compared to males) have evolved countermeasures like sperm dumping, sperm removal and complicated coiling of their vaginas to get an upper hand of ‘choosing’ the best sperms for her offspring.
What about Hermaphrodites?
Flatworms and snails are hermaphrodites, meaning they have both penis and vagina. Surely there wouldn’t be any incentive to invent all these weird penises and vaginas to combat each other. Turned out that I was wrong.
The reproductive organs in hermaphrodites are as weird (and more violent) than the ones that were already covered. In the marine flatworms, they have a pair of dagger-like penises. When they encounter another flatworm, both of them wants to inseminate the other, so they “engage in a duel of striking and parrying”, until one of them stab the other with the love dart that transfer sperms.
Because they are still a limited number of eggs (even in hermaphrodites), most of the hermaphrodites prefer to be male so that they can inseminate as many other hermaphrodites as possible, hence the evolution of dart-like penises that sometimes killed its partner.
Where do we stand?
Most of the examples of genitalia evolution covered in the book are weird to human. But Menno ends the book by reminding us that our genital function and shapes are also the product of evolution. We (and the rest of bugs, birds, beasts) formed a continuum in the natural world.