Re: Drive by science
There are quite a lot of animal species in which the male's penis is shaped in such a way as to make it an effective tool for removing any sperm that might already be present in the female's reproductive tract. There are species in which males have a penis that can literally scoop a competitor's sperm out of the female's reproductive tract.
In the case of humans, it seems that the enlarged glans with its flared corona at the base helps to displace any sperm that might be present on the forward stroke, and then pull it out on the back stroke. Some studies have claimed that when couples have sex after a period of separation (when the woman may have had the opportunity to mate with other men while she and her partner were separated), the man is more likely to thrust deeply. Some have speculated that this may be an unconscious behavioral mechanism to help him more effectively remove any competitor's sperm that might be present.
Of course, in some species, males go to greater lengths to try to ensure that a female's eggs are not fertilized by a competitor's sperm. For instance, in some arthropod species, males secrete a glue-like substance after ejaculation, effectively gluing the female's vagina shut and thus preventing her from mating with any other males. In other arthropod species, the male's penis actually breaks off in the female's vagina after he inseminates her, plugging the vagina and ensuring that she will be unable to mate with any other males.
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