Sperm competition
[2] Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition.
The elephant seal falls into this category since he can participate in bloody violent matches in order to protect his community and defend his title as the alpha male.
Males may even have to travel long distances during the breeding season in order to find a female, which significantly drains their energy supply.
The studies concluded that fish that were residential had fuller stomachs containing higher quality of prey compared to their migrant counterparts.
[25] This can last up to several hours allowing him to ward off any rival males giving his sperm a high chance to fertilize that female's egg.
Elephant seals are known to engage in bloody battles in order to retain their title as dominant male so that they are able to mate with all the females in their community.
[2] Bumblebee mating plugs, in addition to providing a physical barrier to further copulations, contain linoleic acid, which reduces re-mating tendencies of females.
[31] It has been found that some male mollies (Poecilia) have developed deceptive social cues to combat sperm competition.
The mere display of an armament can suffice to drive away the competition without engaging in a fight, hence saving energy.
[2] In Drosophila, males release seminal fluids that contain additional toxins like pheromones and modified enzymes that are secreted by their accessory glands intended to destroy the sperm that have already made their way into the female's reproductive tract from a recent copulation.
Amounts of protein and carbohydrate intake were tested for its effects on sperm production and quality in adult fruit flies (Diptera: Tephritidae).
Studies showed these flies need to constantly ingest carbohydrates and water to survive, but protein is also required to attain sexual maturity.
[41] These all require a good diet with nutrients for proper gamete production as well as energy for activities, which includes participation in leks.
In addition, protein and carbohydrate amounts were shown to have an effect on sperm production and fertility in the speckled cockroach.
[45] The human penis may have been selected to displace seminal fluids implanted in the female reproductive tract by a rival male.
[45] Specifically, the shape of the coronal ridge may promote displacement of seminal fluid from a previous mating[46] via thrusting action during sexual intercourse.
[45] A 2003 study by Gordon G. Gallup and colleagues concluded that one evolutionary purpose of the thrusting motion characteristic of intense intercourse is for the penis to “upsuck” another man's semen before depositing its own.
[47] Evolution to increase ejaculate volume in the presence of sperm competition has a consequence on testis size.
[52] Proteins present in the female reproductive tract or on the surface of the ovum may influence which sperm succeeds in fertilizing the egg.
Inbreeding depression is considered to be due largely to the expression of homozygous deleterious recessive mutations.
In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between sperm of rival males for achieving fertilization.
[55] In vitro fertilization experiments in the mouse, provided evidence of sperm selection at the gametic level.
Female fruit flies (Drosophila melanogaster) were mated with males of four different degrees of genetic relatedness in competition experiments.
[62] Females of many species of arthropod, mollusk and other phyla have a specialized sperm-storage organ called the spermatheca in which the sperm of different males sometimes compete for increased reproductive success.
Cooperation confers a competitive advantage by several means, some of these include incapacitation of other competing sperm and aggregation of genetically similar spermatozoa into structures that promote effective navigation of the female reproductive tract and hence improve fertilization ability.
Relationships across species between the frequency of multiple mating by females and male testis size are well documented across many groups of animals.
[65] Male chimpanzees that live in a structured multi-male, multi-female community, have large testicles to produce more sperm, therefore giving them better odds to fertilize the female.
Regarding sexual dimorphism among primates, humans fall into an intermediate group with moderate sex differences in body size but relatively large testes.
Female black-winged damselflies are known to mate with several males over the span of only a few hours and therefore possess a receptacle known as a spermatheca which stores the sperm.
[70] This is because there seems to be a cumulative percentage increase in fertilization for the final male, such that the eggs laid in the last oviposition bout are the most successful.
