Artificial Insemination – How’s a Girl to Choose?
Upon first consideration it might seem somewhat un-natural for a human female to attend a sperm bank for the purposes of propagating her genetic lineage. Yes, the natural sex part is removed from the equation; however, when it comes to the selection of a donor she can be choosy with respect to several physical and behavioral characteristics like race, physical health, and even the IQ of the male with the winning seed. In a perfect world we would all define the most important characteristics for our mates, find mates with said characteristics and procreate in order to obtain offspring with said characteristics. However, reality in the natural world is harsh, whether you are human or otherwise, and sometimes things just don’t work out optimally. In organisms where multiple males compete and copulate with a single female (polyandrous sexual system), females are often coerced into sexual activity with males that they wouldn’t otherwise choose (see ‘Not tonight honey, I have a headache’). What’s a female to do if some un-desirable sperm happens to find its way into her reproductive tract?
Cryptic female choice (CFC) refers to the power of the female to bias sperm use towards that of preferred males, despite the availability of sperm from other (sub-optimal) males. Females in several species have evolved ways to allow for the sperm of certain males to be the successful fertilizer of the precious eggs, not entirely unlike selecting such seed from a catalogue in a fertility clinic. For example, female freshwater guppies (Poecilia reticulata) overwhelmingly prefer to mate with males that have bright body coloration, specifically with large orange spots1. Do they posess the ability to swing the insemination odds in the favor of a good looking suitor? It appears that they do. In laboratory experiments, female guppies were given a choice to mate with an intermediately colored male in two situations: 1) when he was the more attractive candidate (i.e. when he was paired with a dull colored indivudial), and 2) when he was the less attractive candidate (i.e. when he was paired with a very brightly colored individual). In both cases the only male that had access to the female was the intermediately colored one, the comparative indivudials were visible by the female but not accessible. The results were clear: the intermediately colored males inseminated 68% more sperm into females when they were perceived as the more attractive candidate1. The mechanism by which this happens is as yet unclear, but there is no question that females exercised some control over the number of sperm that were successfully transferred to her reproductive tract subsequent to a copulation event. If she mated with an attractive male she kept more of his sperm, simple as that.
Another example of females manipulating the insemination success of various types of sperm comes from the feral fowl Gallus gallus domesticus (aka wild chickens). These organisms have a complex social system, with males being in an intricate hierarchy of social dominance. Females prefer to copulate with dominant males (not with subordinate ones); however, the underdogs still undertake copulations, often violently coercing the female in order to do so. Fortunately, the females have been found to get the last laugh: analysis of the fertilization success of dominant vs subordinate males showed that females eject the ejaculates of the latter subsequent to copulation2. So although the subordinate males utilize their strength to force copulations upon unwilling females, their chances at paternity are limited by the fact that she can subsequently discard his donation in favor of one that she actively seeks out.
In the natural world there is an abundance of examples of females biasing paternity in favor of specific male phenotypes or social ranks, kind of like a human female in a sperm bank selecting the seed of a successful entrepreneur over an unemployed couch surfer. However, the major difference lies in the fact that in the natural world females are capable of undertaking such selection without the intervention of human-invented fertility procedures. Even the lowly female chicken (who has proven to be more than just the ‘dumb’ animal we eat for dinner) displays a level of sophistication that seems unattainable for the Homo sapien. In species where coersion is commonplace (and I would argue that our species is no exception), it is extremely advantageous for females to employ mechanisms to avoid having offspring that are fathered by undesirable sperm. If that means making a well-informed decision after perusing a brochure from a sperm bank over a hasty choice after a few drinks at a night club, I’ll vote for the former.
1Pilastro, A., Simonato, M., Bisazza, A. and Evans, J.P. 2004. Cryptic female preference for colorful males in guppies. Evolution 58: 665-669.
2Pizzari, T. and Birkhead, T.R. 2000. Female feral fowl eject sperm of subdominant males. Nature 405: 787-789.