Heteropaternal Superfecundation

IVF conceived fraternal twin half-brothers
 born to Dutch couple in 1993

I know your first question is: WTH? Did she make those words up?  

No. Basically the term "heteropaternal superfecundation" is scientific language for the fathering of fraternal twins by two separate sources after multiple eggs have been produced during a woman's ovulation.

I'm a nerd of the highest order. I love biology, genetics, and the like. Today, I listened to the radio and the topic of discussion was "Is monogamy natural".  The co-host of the morning show said she read this book called "Moral Animal", which shows that monogamy is not natural for men, but it is for women. Why? Because men can get multiple women pregnant.

"Biologically speaking only, men are designed to impregnate many women! Survival of the fittest. Look it up."- Yasmin to me on Twitter

My response was that women could indeed get pregnant by multiple men, delivering multiples by different fathers.  So if it is not natural for men, it is neither natural for women. 

A gentleman asked me how it is even possible.  It's simple. If people are not monogamous, and freely sharing themselves with anyone, it happens.  The body can release multiple eggs, not as many as sperm produced by men, but there is also a scientific reason for that. Men release multiple sperm because not every sperm will hit the target. Also, not every sperm that hits the target will bring forth a baby.  So there must be multiple sperm to do the job.

Anyway, I'm going off on a tangent.  How rare is it that multiple fathered offspring come from one birth? In the animal kingdom, it is not very rare at all according to some reports.  Breeders of cats and dogs do it all the time. One litter can have multiple sires. 

What does that have to do with monogamy? Nothing in my opinion, but I didn't make the argument. They say this is a rare occurrence.  But how rare is it really? Or is it just rarely that it gets reported? How many people actually get paternity tests on twins?