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Physiological Evidence of Evolution of Short-Term Mating
Mate selection is a complex process influenced by biology, culture, and environment. Individuals seek long-term or short-term partners, depending on hormones, life stage, and self-perception. Women generally prefer long-term relationships but are always involved in short-term ones. Although most men end up in long-term relationships, most men prefer short-term mating.
A strategy becomes more attractive depending on the number of available partners responding to it and the lack of many domestic competitors using it. Recent speed dating studies show fewer differences between men and women than previous studies using other methods (e.g., self-report). However, whether this finding can be replicated in populations with more diverse levels of attractiveness and SES remains to be seen.
Studies suggest that some people lean more towards short-term relationships (e.g., short-term flirting, one-night stands), others aspire to long-term relationships (e.g., marriage, cohabitation), and some more significant are still available for relationships of both types.
Although both sexes are engaged in non-binding sex, men are more likely to desire short-term relationships than women, possibly because of historical asymmetries in the costs and benefits of short-term mating. Since women bear most of the burden during pregnancy, childbirth, and postpartum, short-term sexual intercourse has resulted in higher costs (low parental investment, infectious diseases, violence, etc.) against ancestral women than men.
Biological Evidence of Short-Term Mating
Men and women are more similar in what they want in a short-term partner but differ in what they want in a long-term partner. Asexual animals pass all of their chromosomes, and therefore, all copies of every gene, to their offspring. In contrast, by meiosis, sexually reproducing diploid animals have two copies of each chromosome but pass on only one copy of each chromosome to eggs or sperm.
This means that a sexually reproducing diploid animal only passes on half of its total genes to its offspring. Despite the cost of losing half of the ability to pass on genes to the next generation, sexual reproduction is much more common than asexual reproduction in animals because it offers several evolutionary advantages.
The main advantage of sexual reproduction comes from genetic recombination. Genetic recombination allows the offspring of an organism to be genetically diverse. Sexual reproduction increases the chances of beneficial mutations and the inability to propagate harmful mutations. Genetic diversity within a group of progeny is favorable when the local environment changes. This idea becomes clear when we consider organisms that can reproduce sexually and asexually.
For example, aphids will promote asexual reproduction when their environment is stable. When the environment gets cold, most aphids reproduce sexually because sexual reproduction produces eggs resistant to freezing and can be halted during the winter. Genetic diversity may also lead to evolutionary defenses against parasites and disease. The mud snail, Potamopyrgus antipodarum hosts several fluke parasites.
Sexual individuals of this species are more common in areas with a high risk of tapeworm infection. In areas of low risk of infection, asexual individuals have replaced sexual individuals. This suggests that the genetic diversity gained from sexual reproduction is necessary for this species to resist parasites, as clonal individuals may not readily survive in areas where parasites are abundant.
Sexual reproduction usually involves the evolutionary distinction of males and females. Females typically produce far fewer gametes (eggs) than males and invest more in each. Males, on the other hand, produce many gametes (sperm) and invest little in each. Substantial differences in gamete investment between the sexes lead to, in some cases, opposing reproductive strategies between the sexes. Females may be more interested in mate selection than males due to their high gametophyte cost.
Physiological Evidence for Short-Term Mating
Several studies have discussed the evolutionary advantage of these attractive physical cues. For example, the degree of fat storage (as reflected by BMI) and its distribution pattern (reflected by WHR) can signal a selected mate's health status and fertility. Body fat storage is an indicator of fertility, pregnancy, and lactation. Furthermore, the distribution of body fat is related to the sex hormone status of individuals.
Recent research has shown that individual differences in psychological traits can also lead to differences in mate preferences. In line with this research, only facial stimuli have been used as a proxy for attractiveness. Few studies have examined the association between psychological traits and attractiveness indices—other physical indicators such as WHR and BMI. In addition to the impact of gender-typical psychographic characteristics on the attractiveness assessment, partner preferences in short- and long-term relationships largely depend on gender.
Historically, evolutionary psychologists were among the first to consider short-term mating of offspring as an adaptive reproductive strategy (most other psychologists have considered itResearch has suggested that females pursuing short-term mating prioritize physical attractiveness to facilitate the mating acquisition of good genes. Although it is known that physical characteristics are important in mate selection, the study examining the importance of physical characteristics in assessing male fitness has yet to be easily discovered. Strategic pluralism suggests that women engage in short-term sexual relationships when the benefits of doing so outweigh the costs.
Testicle Size as an Evidence
A multitude of physiological indicators points to the history of many matings. The size of men's testicles provides one hint. Big testes are often formed as a result of solid sperm competition, which occurs when sperm from two or more men fill the reproductive tract of one female at the same time due to copulation with two or more males. Sperm rivalry puts men under pressure to generate big ejaculates containing much sperm. The bigger, sperm-laden ejaculate has an advantage in displacing the ejaculate of other males inside the woman's reproductive system in the race to the prized egg.
Males have far larger testes compared to their body weight than gorillas and orangutans. Male testes account for 0.018 percent of gorilla body weight and 0.048 percent of orangutan body weight. In comparison, human male testes account for 0.079 percent of men's body weight, which is 60% higher than orangutans and more than four times that of gorillas when body size is considered. Men's huge testes give evidence that women in human evolutionary history occasionally had sex with more than one guy within a few days. Without sperm competition, this size of testes would not have evolved.
Moreover, it implies that both sexes pursued short-term mating at times. However, humans do not have the enormous testes of any primate. The volume of human testes is far less than that of the very promiscuous chimp, whose testes account for—more than 269 percent of its body weight. The volume of human testes is far less than that of the very promiscuous chimp, whose testes account for—more than 269 percent of its body weight.
Variations in Sperm Insemination
Variations in sperm production and insemination further indicate the evolutionary occurrence of casual mating. Thirty-five couples agreed to provide ejaculates resulting from sexual intercourse, from either condoms or flow back, the gelatinous mass of seminal fluid spontaneously discharged by a woman at various points after intercourse, in a study to determine the effect on sperm production of separating mates from each other.
Each couple's partners had been separated for varied amounts of time. Men's sperm counts increased considerably as the couple's time apart increased since their previous sexual contact. The longer men spent apart, the more sperm they inseminated in their spouses when they eventually had intercourse.
Men inseminated 389 million sperm each ejaculate on average when the couples spent 100% of their time together. However, when couples spent just 5% of their time together, males inseminated 712 million sperm each ejaculate, nearly doubling the number. The amount of sperm inseminated rises when other men's sperm is present simultaneously in the wife's reproductive tract due to the chance for extramarital sex afforded by the couple's separation and the period since the man's previous ejaculation did not affect the rise of sperm insemination.
Even if the male masturbated to climax while apart from his wife, he still inseminated more sperm when reunited if he had been separated from her for an extended period. The husband's increased sperm insemination after lengthy separation assures that his sperm has a better shot in the race to the egg by crowding out or displacing a potential interloper's sperm. The fact that males have a physiological mechanism that increases sperm count when their wives may have had opportunities to be unfaithful suggests that humans had extramarital affairs at some point in their evolutionary past.
Mate selection can be measured empirically, but the mechanisms underlying mate selection are not always well understood. Advances in molecular genetic technology have allowed mate selection research to focus on behavioral observations to understand mating preferences at the molecular level. Mate selection based on genetic factors may evolve to avoid inbreeding and maximize the offspring's genetic quality.
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