Sperm competition is humans:
2 conspicuous features of mammalian sperm
1) high numbers and prevalence of sperm plugs
2) High-proportions of deformed ones
The deformed ones have the 'kamikaze' role. They are not in the best of conditions and so sacrifice their chance at the egg in order to form the framework of sperm plugs. These block the utero-tubal and cervico-uterine junctions so sperm has greater chance of reaching egg.
The volume of the uterus is 1cm3.
Suggestions are that sperm plugs may allow selective passage of sperm, and that ejaculation occurs in spurts so the non-deformed sperm are first and the kamikaze are released later.
Malformed sperm are not adaptive and most malformations occur with inbreeding. And amazingly only 0.001% of sperm are actually capable of fetilisation!! 2000 out of every ejaculate.
It is predicted more sperm should be inseminated if there is a greater risk of double mating. Double mating means when the female is copulating with more than one males. In humans the male sperm lives for approx 5 days in the female tract and those females who mate with two different males within a 5 day period are double-maters.
Strong negative correlation between % times spent together and number of sperm inseminated. More sperm produced if there is greater risk of double-mating. Males didn't seem to realise they were wearing a condom and studies also suggested that males inseminate more sperm in higher weight females.
Advantaged of EPC
EPC is extra-pair-copulations, and for males is obvious- more offspring will be produced!
For females is the advantage just 'sexy sons'? Do females actively promote sperm competition?
Most intra=pair copulations occure towards the end of the menstrual cycle. Most extra-pair copulations occur just before the fertile period (day 12-14). EPC make up 5-7% of total copulations in the fertile period. 25% EPCs unprotected as opposed to 15.5% IPC's unprotected.
This is when the offspring are identified as being biologically fathered by someone other than the man who believes he is the father.
Wedell, Gage and Parker (2002) confirmed that males adjust sperm numbers according to reproductive status of female. There is a risk of competition and females can actually be selective in there sperm uptake! Muscular contractions and ejection (hens do this). Some males who targert EPC's have larger testis.
So overall the sperm competition is dependant upon the female and the female actively promotes this competition!