The evolution of concealed ovulation and extended female sexuality may be traced back to early hominids and is thought to have had a role in the evolution of the human species. Concealed ovulation is the adaptation most likely evolved to allow females to control their reproductive cycles while preventing males from monopolizing their reproductive resources.
The absence of observable symptoms or cues of ovulation in female primates, including humans, is called concealed ovulation. In contrast, some species, such as cats and dogs, show apparent ovulation symptoms, such as vaginal enlargement or discharge. The emergence of hidden ovulation is assumed to have influenced the evolution of prolonged female sexuality in monkeys.
Extended female sexuality refers to female primates' capacity to participate in sexual behavior. At the same time, they are not fertile, unlike many other species, where mating is limited to a short window when the female is in oestrus. This prolonged sexuality is assumed to have evolved to boost male investment in kids since males would be more inclined to stick around and give resources if they did not know when the female would become fertile again.
Females who could conceal their ovulation cycles successfully were more likely to live and breed, passing the trait down to their offspring through natural selection. Due to this adaptation, females could be more discriminating about whom they mated with and when boosting their odds of successful mating and offspring. As a result, enlarged female sexuality, or the ability to get sexually aroused and feel sexual pleasure, evolved. This change made females more appealing to potential mates, increasing the likelihood of successful mating. One hypothesis for the evolution of concealed ovulation and extended female sexuality is the "male provisioning" hypothesis. Another hypothesis is the "sperm competition" hypothesis.
The male provisioning theory proposes that the necessity for male parental investment drove the evolution of concealed ovulation and prolonged female sexuality in primates, including humans. According to this theory, female monkeys developed to conceal their fertility to convince males to offer resources and assistance even when infertile. This would boost the chances of the female kids surviving and prospering.
Conversely, males would have less motivation to supply resources during non-fertile times if they could only identify when a female was fertile. This hypothesis suggests that the evolution of concealed ovulation and prolonged female sexuality enabled females to build long-term ties with males, resulting in pair bonding and cooperative parenting, which would have increased offspring survival.
Furthermore, the male provisioning hypothesis proposes that this extended sexuality allowed females to choose mates based on factors other than fertility, such as paternal care, social status, or genetic compatibility, thereby increasing their offspring's survival and reproductive success. Females improve the probability that males will offer resources consistently, even when they are not fertile, by concealing their fertility. In summary, the male provisioning hypothesis proposes that the evolution of concealed ovulation and extended female sexuality was driven by the need for males to provide resources and support even during infertility, resulting in the formation of long-term bonds and cooperative parenting, ultimately improving offspring survival and reproductive success.
In 2009, researchers released a study in the journal "Proceedings of the Royal Society B" that looked at the association between male provisioning and female fertility in a population of wild baboons (Papio cynocephalus) in Kenya. Female baboons were studied for behavioral and physiological data, including indices of male provisioning and female fertility.
The study revealed that male baboons supplied much more food and grooming to females during infertility than during fertility. This shows that male baboons can detect infertility in females and change their provisioning behavior accordingly. The study also found that female baboons who received more food and grooming from males had higher fertility levels. This suggests that male provisioning may enhance female fertility and increase the chances of successful reproduction.
According to the sperm competition hypothesis, the evolution of concealed ovulation and extended female sexuality in primates, including humans, was driven by the need to increase male rivalry for access to a female's eggs. Females developed to have extended sexuality to encourage males to compete for fertilization opportunities, according to this theory.
Furthermore, the sperm competition hypothesis proposes that by mating with numerous males, females can select which sperm to employ for fertilization, increasing the selection of high-quality sperm. This might include sperm from males with robust immune systems or genetic compatibility, boosting the chances of generating healthy babies.
The sperm competition hypothesis also proposes that extended sexuality may minimize infanticide, a behavior in many monkey species in which males kill offspring sired by other men. Females generate ambiguity about the paternity of their young by having several mates, which may inhibit infanticide by males who are confused if the offspring are their own.
The impact of sperm rivalry in the evolution of prolonged female sexuality in two closely related monkeys, the mandrill (Mandrillus sphinx) and the drill (Mandrillus leucophaeus), was explored in one study published in the journal "Animal Behaviour" in 2008. Throughout their reproductive cycles, the researchers collected fecal samples from female mandrills and drills and analyzed the samples' DNA to identify the offspring's paternity.
The study's findings revealed that females in both species mated with many men during a single reproductive cycle, and the paternity of children was frequently unknown. This suggested that in both species, sperm rivalry played a role in the evolution of expanded female sexuality.
The evolution of concealed ovulation and extended female sexuality has been a critical factor in the evolution of the human species. It has enabled females to have more control over their reproductive cycles and be more selective about whom they mate with and when. This has allowed them to increase the chances of successful mating and offspring, increasing the species' overall success.
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