Sperm Competition

A forewarning: This 1st post will be a work in progress. I'll post a few iterations before having it completed... As usual, please feel free to make suggestions/call me out.

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EDIT: Here is a wild story from a few years ago which shows that males produce better sperm after watching pornography that involves other men. Apparently the mere image of another man in-copula is enough to register high on the sperm competition-scale.

OK, so what it the heck is sperm competition????

Quite literally, sperm competition is the competition between the sperm of 2 (or more) males within the reproductive tract of the female (post-copulatory sperm competition). Especially in animals like humans where only a single ovum is up for grabs, the outcome of their competition is very important. Males with more competitive strategies pass on their genes, while the losers do not. This is by definition, is differential reproductive success. These ideas and a few others were presented initially in Geoff Parker's 1970 paper in The Biological Reviews.

Aside from looking at sperm competition at the level of the individual sperm, people recognized the importance of probability (mostly precopulatory mechanisms). Basically,

1. inseminate more sperm per ejaculate and have a greater chance of being the Lucky guy...
2. Inseminate some moderate number of sperm, but remove the sperm from the last guy.. This mechanism has let to some pretty elaborate "sperm scoopers" in insects, and maybe in humans *see here*. Common morphology includes spines and shovel-like things.
   
3. A differential investment in ejaculation based on the probability of encountering rival male sperm. Testes size is ofter used as a proxy here. Large testes means capable of rapid sperm production. Invest more when the probability is high as a result of mating system or female behavior. A couple of general ideas here.
• Go for the gusto and inseminate large numbers of sperm when your species is characterized by polygyny or promiscuity.
• If there's an indication of infidelity- inseminate an many and as soon as possible.
  

Human sperm competition?

Most definitely...

1. There is ample opportunity for rival males sperm to interact, given the high rates of infidelity characterizing most human societies.
   
2. Humans have relatively large testes, as compared to monogamous species. This is good indirect evidence for sperm competition, assuming that sperm production increases with testes size.
   
3. Even after controlling for frequency of ejaculation, males who have been separated from their partners (and thus enhancing the probability of cuckoldry) inseminate more sperm than those males that have been separated.

Females can manipulate the outcome of sperm competition,

Function of Female Orgasm: Excerpt from Marlene Zuk, full text here

Robin Baker and Mark Bellis, based on sperm competition theory, predict that orgasm functions to manipulate conception by increasing the suction of semen into the reproductive tract. This idea is based on a more general Upsuck hypothesis, that the uterine contractions of orgasm facilitate the movement of semen, but Baker and Bellis draw an explicit analogy to sperm manipulation in other species, particularly insects, in which females may selectively retain sperm from different mates. They predicted that sperm retention would depend on whether or not a woman had an orgasm and when the orgasm occurred (before, during, or after intercourse).

More soon...

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