Nature is both endlessly inventive and endlessly bone-headed. You want proof? The male flour beetle can, by accident, impregnate his mate with his rival’s sperm.
It’s not unusual for females of a species to mate with several males. Getting to such females before anyone else does isn’t always an option, so mating success (in terms of number of offspring produced) goes to males that have adaptations which remove or neutralize their rivals’ sperm. Animals will often have either penises or mating techniques that flush or scrub out the sperm of other males.
The flour beetle is a great example of this. In this species, the second male to mate with a female generally fertilizes two-thirds of her eggs. Researchers believe that this is because the male’s genitalia removes the sperm of the last male to mate with the female.
But there is a wrinkle. The second male flour beetle may be able to clean out the female, but he has no way to clean the first male’s sperm off his own genitalia. He’s no more monogamous than the female, so after mating successfully he goes to find another mate. If he does mate, with someone else’s sperm on his genitals, it turns out exactly as you would expect. Although the male does fertilize most of the new female’s eggs with his own sperm, he fertilizes some of the eggs with his rival’s sperm as well. According to a paper on the subject of flour beetles, Fertilization by Proxy, this is not a rare phenomenon.
“Non-self sperm may be translocated back into the reproductive tracts of new, previously unmated females, where the translocated sperm go on to gain significant fertilization success. We found that, in 45 out of 204 crosses, sperm translocation occurred and in these 45 crosses over half of the offspring were sired by spermatozoa which had been translocated between females on the male genitalia.”
So the female is knocked up by a male she’s never met, and the male has done a lot of work to father someone else’s children.
[Source: Fertilization by Proxy]