Parental investment , in evolutionary biology and evolutionary psychology , is any parental expense (e.g. time, energy, resources) that benefits the offspring . [1] [2] Parental investment can be made by both men and women (bi-parental care), women alone (exclusive maternal care) or single men (exclusive paternal care ). Care can be provided at any stage of the offspring’s life, from prenatal (such as egg protection and incubation in birds, and placental nutrition in mammals) to postnatal (such as feeding and protecting offspring).
Parental investment theory, a term coined by Robert Trivers in 1972 , predicts that the sex that invests more in its offspring will be more selective when choosing a mate, and the sex that invests less in the sex until it mates. There will be intra-sexual competition for access. This theory has been influential in explaining sex differences in sexual selection and mate preferences throughout the animal kingdom and in humans .
history
In 1859, Charles Darwin published On the Origin of Species . [3] It introduced the concept of natural selection to the world as well as related theories such as sexual selection . For the first time, evolutionary theory was used to explain why females are “coy” and males are “spirited” and compete with each other for females’ attention. In 1930, Ronald Fisher wrote The Genetic Theory of Natural Selection , [4] in which he introduced the modern concept of parental investment, introduced the sexy son hypothesis , and introduced Fisher’s theory . In 1948,Angus John Bateman published an influential study of fruit flies in which he concluded that because female gametes are more expensive than male gametes, the reproductive success of females was limited by the ability to produce eggs and the reproductive success of males. Access to women was limited. [5] In 1972, Trivers continued this thinking with his proposal for the parental investment theory, which describes how parental investment influences sexual behavior. They concluded that the sex with higher parental investment would be more selective when choosing mates, and the sex with the lower investment would compete intra-sexually for mating opportunities. [2]In 1974, Trivers expanded the parental investment theory to explain parent-child conflict, the conflict between investments that are optimal from a parent versus offspring perspective.
parental care
Ancestral investment theory is a branch of life history theory . Parental investment is first considered by Ronald Fisher in his 1930 book The Genetic Theory of Natural Selection , [7] in which Fisher argued that parental expenditure on both sexes of offspring should be equal. Clutton-Brock expanded the concept of parental investment to include the cost of any other component of parental fitness. [ citation needed ]
Male dunks do not discriminate between their own young and that of other males in polygyny or polygamous systems. They increase their reproductive success through feeding the offspring in relation to their access to the female during the mating period , which is usually a good predictor of paternity . [8] This indiscriminate parental care by males is also seen in redlip blennies .
In some insects, the investment of the male parent is given as a marital gift. For example, ornate insect females receive a spermicide containing nutrients, sperm and protective toxins from the male during copulation. This gift, which can be up to 10% of the male’s body mass, constitutes the total parental investment provided by the male.
In some species, such as humans and many birds, the offspring are altruistic and unable to sustain themselves for extended periods after birth . In these species, males invest more in their offspring than the male parent is precocial , since the species’ reproductive success would otherwise suffer.
The benefits of parental investment to the offspring are large and are associated with effects on condition, growth, survival and ultimately the reproductive success of the offspring. For example, in the cichlid fish Tropheus moorii , a female has too much parental investment in her young because she has mouthbroods young and while mouthbrooding, all the nutrition she takes in goes to feed the young and she is dominant starves himself. In doing so, her babies are bigger, heavier and faster without her. These benefits are very beneficial as it reduces their risk of being eaten by predators and size is usually the determining factor in conflicts over resources. [11]However, such benefits may come at the expense of the parent’s ability to reproduce in the future, for example, through increased risk of injury when protecting the offspring against predators, and increased opportunities for mating during the rearing of the offspring. loss, and increase the time interval until the next breeding.
A special case of parental investment is when the young are in need of nurturing and protection, but the genetic parent does not actually contribute to the effort of raising their offspring. For example, in Bombus terrestris , sometimes sterile female workers will not breed on their own, but will raise their mother’s offspring. This is common in social haploidy caused by haploidy, whereby males are haploid and females are diploid. This ensures that the sisters are more related to each other than their offspring, encouraging them to help their mother raise their children on their own. [12]
Overall, parents are chosen to maximize the difference between benefits and costs, and parental care will develop when benefits outweigh costs.
parental conflict
Breeding is expensive. Individuals are limited in the extent to which they can devote time and resources to producing and raising their children, and such expenditure may even be detrimental to their future condition, survival and further reproductive output. However, such spending is generally beneficial to the offspring, increasing their status, survival and reproductive success. These differences can lead to conflict between the parents. Parents are naturally chosen to maximize the difference between benefits and costs, and parental care occurs when the benefits significantly outweigh the costs.
Parents are equally related to all offspring, and so to optimize their fitness and their chances of reproducing their genes, they must distribute their investments equally between current and future offspring. However, any single offspring are more related to themselves (they have 100% of their DNA in common) than their siblings (siblings typically share 50% of their DNA), this is dependent on the fitness of the offspring. It is best if the parent(s) invest more in them. To optimize fitness, parents will want to invest equally in each offspring, but each offspring will want a greater share of the parent’s investment. Parents are chosen to invest in the offspring as long as the investment in the present offspring is not costlier than investing in the future offspring.
In iteroparous species, where individuals may go through multiple reproductive bouts during their lifetime, an adjustment may exist between current offspring and investment in future reproduction. Parents need to balance the demands of their offspring against their own upkeep. This potentially negative effect of parental care was explicitly formalized in 1972 by Trivers, who originally defined the term paternal investment to mean an individual’s offspring that increases the chances of surviving (and therefore the offspring of) the mother. -Investment by the father (at the cost of reproductive success) the ability of the parent to invest in other offspring .
Penguins are a prime example of a species that makes great sacrifices for its health and well-being in exchange for the survival of its offspring. This behavior, which does not necessarily benefit the individual, but the genetic code from which the individual is derived, can be observed in king penguins. Although some animals display altruistic behaviors toward individuals who are not directly related, many of these behaviors are mostly seen in parent-offspring relationships. When breeding, males remain at the breeding site for five weeks in a fasting-period, waiting for the female to return for her incubation shift. However, during this time period, if there is a delay in returning the female to the breeding ground, the males may decide to release their eggs. [14]
This shows that these penguins initially show a trade-off of their own health, in the hopes of increasing the survival of their eggs. But there comes a time where the cost of a male penguin outweighs the benefit of a successful breeding season. Olof Olsson examined the relationship between how many experiences an individual has in breeding and how long an individual will wait until they have released their eggs. He proposed that the more experienced the individual, the better able he was to replenish his exhausted body’s reserves, allowing him to stay on the egg longer. [14]
The sacrifice of body weight and potential survival of males to increase the chances of survival of their offspring is a trade-off between current reproductive success and the future survival of the parents. [14] This trade-off makes sense with other examples of kin-based altruism and is a clear example of the use of altruism in an attempt to increase the overall fitness of an individual’s genetic material at the expense of the individual’s future survival. .
Mother-offspring conflict in investing
Mother-offspring conflict has also been studied in animal species and in humans. One such case is documented by ethologist Wolf Schiffenhovel in the mid-1970s. Ipo women of West New Guinea engage in a cultural practice in which they give birth outside the village. After the birth of her child, each woman weighed whether she should keep the child or leave the child in a nearby brush, inevitably ending in the death of the child. [15]Survival chances and availability of resources within the village were factors in this decision of whether or not to keep the child. During a pictorial birth, the mother realized that the child was very ill and would not survive, so she wrapped the baby, preparing to release the baby in the brush; However, seeing the child move, the mother untied the child and brought it to the village, demonstrating the change of life and death. [15] This conflict between mother and child resulted in alienation behavior in Brazil, which was observed in the work of Schaper-Hughes, “Many alto children not only remain without a name, but also as long as they cannot walk or talk.” don’t start, until their name is not”, [16]or if a medical crisis has arisen and the infant requires emergency baptism. This conflict between existence, both emotional and physical, prompted a change in cultural practices, resulting in new forms of mother-to-child investment.
alloparental care
Alloparental care, also known as ‘allomothering’, is when a member of a community, in addition to the biological parents of the infant, participates in the provision of care for the offspring. [17] A range of behaviors fall under the term alloparental care, some of which are: carrying, feeding, watching, protecting, and grooming. The stress of alloparental care on the parents, especially the mother, can be reduced, so the negative effects of parental conflict on the mother can be reduced. [18]While the apparent altruistic nature of the behavior may seem to contradict Darwin’s theory of natural selection, because caring for offspring that are not their own would not increase their direct fitness, while raising time, energy and resources would be taken away. Will go The behavior of its offspring can be explained in terms of progressively increasing indirect fitness, since the offspring are likely to be non-offspring kin, hence carrying some of the genetics of the alloparent. [17]
progeny and position direction
Parental investment behavior increases the offspring’s chances of survival, and does not require an underlying mechanism that is compatible with empathy applied to adults, or in situations involving unrelated offspring, and does not require the offspring to be exposed to any By no means altruistic behavior is required. [19] [20] Individuals invested by parents are not more vulnerable to exploitation by other adults.
Trivers’ parental investment theory
Parental investing as defined by Robert Trivers in 1972 [21]There is an investment by the parent in the offspring that increases the offspring’s chances of survival and therefore reproductive success at the expense of the parent’s ability to invest in other offspring. A large parental investment substantially reduces the chances of the parent investing in the other offspring. Parenting investments can be divided into two main categories: mating investments and parenting investments. Mating input involves the sexual act and invested sex cells. Parenting investment is the time and energy spent raising offspring after conception. Females’ parental investment in both mating and parenting efforts is much greater than that of males. In terms of sex cells (eggs and sperm cells), the female’s investment is huge, while males produce thousands of sperm cells which are supplied at the rate of twelve million per hour. [22]
Females have a fixed supply of about 400 ova. In addition, fertilization and conception occur in females, an investment that exceeds the male’s investment of just one sperm cell. Furthermore, for females, an act of mating can result in a nine-month commitment such as a human pregnancy and subsequent commitments related to parenting such as breastfeeding. From Trivers’ theory of parental investment, several implications emerge. The first implication is that women are often not the more investing sex. The fact that they are often the more investing sex leads to the second implication that evolution favors females who are more selective of their mates to ensure that intercourse will not result in unnecessary or wasteful costs. . The third implication is that because females invest more and are essential to the reproductive success of their offspring, They are a valuable resource for men; As a result, males often compete for the sexual access of females.
Men as the more investing gender
For many species only one type of male input is obtained which is the sex cells. In those words, female investment is much higher than male investment as mentioned earlier. However, there are other ways in which males invest in their offspring. For example, male balloon flies can find food as in the example. [23] He can find a safe environment for the female to feed or lay eggs, for example in many birds. [24] [25]
He can also protect the young and provide them with opportunities to learn like wolves. Overall, the main role of men is to protect women and their children. This can often reduce the investment discrepancy caused by the initial investment of sex cells. There are some species such as Mormon cricket, pipefish seahorse and Panamanian poison arrow frog males invest more. In species where the male is more invested, the male is also the selector sex, placing high demands on his chosen female. For example, the female they choose most often has 60% more eggs than the rejected females. [26]
This combines parental investment theory (PIT) with sexual selection: where parental investment is greater for a male than for a female, it is usually the female that competes for a mate. is, as shown by Phalaropidae and polygynous bird species. Females in these species are generally more aggressive, brightly colored, and larger than males, [27] suggesting that the more invested sex has more choice when selecting a mate, which is inter-individual. The sex involved in sexual selection is compared.
Women as a valuable resource for men
The second prediction from Trivers’ theory is that the fact that females invest more in offspring makes them a valuable resource for males as it ensures the survival of their offspring which is the driving force of natural selection. Therefore, the sex that invests less in offspring will compete with each other for reproduction with the sex with more investment. In other words, men will compete for women. It has been argued that jealousy evolved to avert the risk of potential harm to the parent’s investment in the offspring. [22]
If a male redirects his resources to another female it is a costly loss of time, energy and resources for his offspring. However, the risk is higher for men because although women invest more in their offspring, they have greater maternal certainty because they have given birth to the child themselves. However, males can never have 100% paternal certainty and therefore risk investing resources and time in genetically unrelated offspring. Evolutionary psychology views jealousy as a favorable response to this problem.
Application of Trivers principle in real life
Trivers’ theory has been very influential because it predicts that it corresponds to differences in the sexual behavior of men and women, as demonstrated by various research. Buss (1989) [28]Cross-cultural studies from the U.S. show that males are tied into physical attractiveness because it signals youthfulness and fertility and ensures male reproductive success, which is increased by copulating with more fertile females. . Females, on the other hand, are included in the resources provided by potential mates, as ensuring the survival of their offspring increases their reproductive success, and one way of doing this is by obtaining resources for them. Alternatively, another study suggests that men are sexier than women, lending further support to this theory. Clark and Hatfield [29]found that 75% of men were willing to have sex with a stranger, compared to 0% of women. On the other hand, 50% of women agreed to go on a date with a stranger. This suggests that men seek short-term relationships, while women show a stronger preference for long-term relationships.
However, these preferences (male engagement and female selection) can be explained in other ways. In Western cultures, male intercourse is encouraged through the availability of pornographic magazines and videos targeted to male audiences. Alternatively, both Western and Eastern cultures discourage female involvement through social checks such as slut-shaming. [30]
PIT (Paternal Investing Theory) also explains the pattern of sexual jealousy. [22] When men visualize their partner as being sexually infidel (having sex with someone else), they were more likely to show a stress response, and women perceived their partner as emotionally unfaithful (having sex with another woman). showed more stress when imagining being in love with). PIT explains this, because the female’s sexual infidelity lowers the male’s paternal certainty, thus showing more stress than fear of adultery. On the other hand, the woman has a fear of losing the resources provided by her partner. If his partner has an emotional attachment to another female, chances are he will not invest as much in their offspring, thus showing a greater stress response to the situation.
A heavy critique of the theory is Thornhill and Palmer’s A Natural History of Rape: Biological Bases of Sexual Correspondence , [31]in its analysis as it seems to rationalize the rape and sexual coercion of women. Thornhill and Palmer claimed that rape is an evolved technique of obtaining mates in an environment where women choose mates. As PIT asserts that males seek to copulate with as many fertile females as possible, the choice of females can result in a negative impact on the male’s reproductive success. If the women did not choose their mates, Thornhill and Palmer claim that there would be no rape. This ignores various socio-cultural factors, such as the fact that not only infertile women are raped – 34% of minor rape victims are under the age of 12, [32]Which means they are not of fertile age, thus there is no evolutionary advantage in rape. them. 14% of rapes in England occur on men, [33] which cannot increase a man’s reproductive success because no conception will occur. Thus, what Thornhill and Palmer called ‘evolved machinery’ may not be very beneficial.
Versus Sexual Strategies
Trivers’ theory ignores that women tend to have short-term relationships such as one-night stands, while not all men behave inappropriately. An alternative explanation for PIT (Paternal Investment Theory) and mate preferences would be Busch and Schmidt’s sexual strategy theory. [34] SST argues that both sexes pursue short-term and long-term relationships, but look for different qualities in their short-term and long-term partners. Women will prefer an attractive partner for a short-term relationship, but in a long-term relationship they may be willing to trade that attraction for resources and commitment. Men, on the other hand, may accept a sexually inclined mate in a short-term relationship, but to ensure their paternal certainty they will instead seek a loyal mate.
international politics
Parental investment theory is used not only to explain evolutionary phenomena and human behavior, but also to describe recidivism in international politics. In particular, parent investment is referred to when describing competitive behavior between states and when determining the aggressive nature of foreign policies. The paternal investment hypothesis states that the size of the alliance and the physical strength of its male members determine whether its activities with its foreign neighbors are aggressive or sociable. [35]According to Trivers, the investment of the parents of males has been relatively low, and therefore forced them into more competitive conditions on limited reproductive resources. Sexual selection occurred naturally and males have evolved to overcome their own unique reproductive problems. Among other adaptations, the psychology of males has also evolved to directly assist males in such inter-sexual competition. [35]
An essential psychological development involved deciding whether to fly or to actively engage in combat with another rival group. The two main factors that men noted in such situations were (1) whether the alliance they were part of was greater than its opposition and (2) whether the men in their alliance had more physical strength than the other. Is. The male psychology expressed in antiquity has been passed on to modern times, whereby men think and behave partly as they did during ancestral wars. According to this theory, the leaders of international politics were no exception. For example, the United States was expected to win the Vietnam War because of its greater military capability than its enemies. However, according to the traditional rule of more aligned size,[35]
The paternal investment hypothesis argues that the male physical strength of alliances still determines the aggression of modern conflicts between states. While the idea that male physical strength is one of the least determining aspects of today’s warfare, human psychology has evolved to operate on this basis. Furthermore, although it may seem that the mate seeking motivation is no longer a determinant, sexuality in modern wars, such as rape, is undeniably evident in conflicts today.
sexual selection
In many species, males can produce large numbers of offspring during their lives, reducing parental investment in favor of investing the time to impregnate any reproductive-age female who is fertile. In contrast, a female may produce a small number of offspring during her reproductive life, partly due to the investment of a more constrained parent. Females will be more selective (“selective”) than males, selecting males with good fitness (eg, genes, high status, resources, etc.), in order to overcome any lack of direct parental investment from the male. be able to help. , and therefore increase reproductive success. Robert Trivers’ theory of parental investment predicts that the sex with the greatest investment in breastfeeding, parenting, and protecting the offspring will be the more discriminatory in intercourse; and that the sex that invests less in offspring,[36] )।
In species where both sexes are highly invested in parental care, mutual selection is expected to arise. An example of this is seen in crested oculettes, where the parents share equal responsibility in incubating their single egg and raising the chick. In Crested Occlets, both sexes are ornate. [37]
Parental investment in humans
Humans have evolved biologically and behaviorally increasing levels of parental investment. The fetus requires a high investment from the mother, and the altruistic newborn requires a high investment from a community. Species whose newborns are unable to walk on their own and require parental care have a high degree of altruism. Human babies are born unable to take care of themselves and require additional parental investment after birth to survive. [38]
maternal investment
Trivers (1972) [2]hypothesized that more biologically bound investment would predict more voluntary investment. Mothers invest an impressive amount of money in their children even before they are born. The time and nutrients needed for a fetus to develop, and the risks associated with delivering these nutrients and childbirth, is a huge investment. To ensure that this investment is not wasted, mothers are likely to invest after the birth of their babies, to ensure that they survive and be successful. Compared to most other species, human mothers give more resources to their offspring even before childbirth, at high risk to their own health. It is associated with the development of a slower life history, with fewer, larger offspring being born after a longer interval, which requires increased parental investment.
The developing human fetus—and the brain in particular—needs nutrients to grow. In the later weeks of gestation, as brain growth progresses, the fetus receives the nutrients it needs. [41]The most aggressive placental phenotype in rodents and primates is hemocorial placenta, in which the chorion destroys the uterine epithelium and has direct contact with maternal blood. Other placental phenotypes are separated from the maternal bloodstream by at least one layer of tissue. The more aggressive placenta allows for more efficient transfer of nutrients between mother and fetus, but it also comes with risks. The fetus is able to release hormones directly into the mother’s bloodstream to “demand” the increased resources. This can lead to health problems like pre-eclampsia for the mother. During childbirth, detachment of the placental chorion can cause excessive bleeding. [42]
The obstetric dilemma also makes birth more difficult and results in increased maternal investment. Humans have evolved both bipedalism and large brain size. The development of bipedalism changed the shape of the pelvis, and narrowed the birth canal, at the same time the brain was developing to become larger. The decreased size of the birth canal means babies are born earlier in development, when their brains are smaller. Humans give birth to children with 25% developed brains, while other primates give birth to babies with 45-50% developed brains. [43] A second possible explanation for early birth in humans is the energy required to develop and maintain a large brain. Supporting a gestationally large brain requires energy that the mother may be unable to invest. [44]
Obstetric dilemmas make birth challenging, and a distinctive feature of humans is the need for assistance during childbirth. The altered shape of the bipedal pelvis requires that the babies leave the birth canal away from the mother, unlike all other primate species. This makes it more difficult for the mother to clear the baby’s breathing passages, ensure that the umbilical cord doesn’t wrap around the neck, and freely pull the baby without tilting the baby’s body the wrong way. [45]
Human beings also need sociality for the need of birth attendant. To guarantee the presence of a birth attendant, humans must congregate in groups. It has been controversially claimed that humans are social, [46] like ants and bees, characterized by relatively high parental investment, cooperative care of young, and division of labor. It is not clear which evolved first; Sociability, bipedalism, or birth appearance. Bonobos, our closest living relatives with chimpanzees, have high female sociability and births among bonobos are also social events. [47] [48] Sociability may have been a prerequisite for birth attendance, and bipedalism and birth attendance may have evolved as far back as five million years ago.
As female primates age, their fertility declines. The grandmother hypothesis describes the evolution of menopause, which may or may not be unique to humans among primates. [49] As women age, the cost of investing in additional reproduction increases and the benefits diminish. At menopause, it is more beneficial to stop breeding and start investing in grandchildren. Grandmothers are certain of their genetic relationship with their grandchildren, especially their daughters’ children, because there is greater maternal certainty of their own children, and their daughters are sure of their motherhood to their children as well. It has also been theorized that grandmothers invest preferentially in their daughters because the X chromosomes contain more DNA and their grandchildren are most closely related to them.[50]
parental investment
As philanthropy grew, investment from individuals other than the mother became more necessary. Higher sociability meant that female relatives were present to help the mother, but paternal investment also increased. Paternal investment increases as it becomes more difficult to produce additional children, and as such the effect of investment on offspring fitness increases. [51]
Men are more likely than women to give their children no parental investment, and children of fathers with less investment are more likely to give their children less parental investment. The absence of a father is a risk factor for both early sexual activity and teen pregnancy. [52] [53] [54] [55] father’s absence increases children’s stress levels, which are associated with earlier onset of sexual activity and short-term mating orientation. [56] [57] [58] [59] [ 60]Daughters of absent fathers are more likely to seek short-term partners, and one theory explains this as a preference for external (non-partner) social support because of the perceived need for committed partners in an uncertain future and high-stress environments. Uncertain availability. [61]
Investment as a predictor of mating strategies
hidden ovulation
Women can get pregnant only during ovulation. Human ovulation is hidden, or not externally indicated. Concealed ovulation reduces paternity certainty because men are uncertain when women ovulate. [62] The development of latent ovulation has been thought to result from an increased need for altruism and parental investment. This can be true in two ways. First, if males are unsure of the timing of ovulation, the best way to successfully reproduce would be to have repeated intercourse with a female throughout her cycle, which requires pair bonding, which in turn Increases parental investment. [63]The second theory states that low paternity certainty will increase paternal investment in polygamous groups, as more males can invest in offspring. The second theory is considered better today, as all mammals with hidden ovulation are the majority, and males exhibit relatively little mate-guarding behavior, as required by monogamy and the first theory.
sexual orientation
Sociability was first described by Alfred Kinsey as a willingness to engage in casual and unrestricted sex. [65] Social sexual orientation describes sociality on a scale from unrestricted to restricted. Individuals with unrestricted social sexual orientation have greater openness to sex in less committed relationships, and individuals with restricted social sexual orientation have less openness to casual sex. [66] [67] However, it is accepted today that sociality does not actually exist on a one-dimensional scale. People who are less open to casual relationships are not always looking for committed relationships, and people who are less interested in committed relationships are not always interested in casual relationships. [68]Short- and long-term mating orientations are modern descriptors of openness to uncommitted and committed relationships, respectively. [69]
Parental investment theory, as proposed by Trivers, argues that sex with high compulsive investment will be more selective in choosing sex partners, and sex with low compulsive investment will be less selective and in “casual” mating opportunities. more interested. The more invested sex may not reproduce as often, causing the less investing sex to compete for opportunities for mating. [21] [70] In humans, females have a higher mandatory input (pregnancy and childbirth) than males (sperm production). [34] Females are more likely to have long-term mating orientations, and males are more likely to have short-term mating orientations. [68]
Short- and long-term mating orientations influence female preferences in males. Studies have found that women place more emphasis on career-orientation, ambition and devotion only when they consider a long-term partner. [71] When marriage is not involved, women place greater emphasis on physical attractiveness. [72] In general, women prefer men whose parents are likely to have a high investment and who have good genes. Women prefer men with good financial standing, who are more committed, who are more athletic, and who are healthier. [73]
Some misconceptions are driven by parental investment theory. The “structural powerlessness hypothesis” [74] proposes that women attempt to find mates with access to high levels of resources because, as women, they are directly excluded from these resources. However, this hypothesis has been disproved by studies that have found that financially successful women attach even greater importance to financial status, social status, and possession of a professional degree.
sexual dimorphism
Sexual dimorphism is the difference in body size between male and female members of a species that results from interracial selection, which is sexual selection that acts within one sex. Higher sexual dimorphism and larger body size in males are the result of male–male competition for females. [76] Primate species in which multiple females and one male form groups have greater sexual dimorphism than species that have multiple females and males, or both one female and one male. Most sexual dimorphism occurs in polygamous primates, and less in polygamous and monogamous primates. [77] [78] Humans have the lowest levels of sexual dimorphism of any primate species, indicating that we have developed a decreasing level of polygamy. Decreased polygamy is associated with increased parental investment.
demographic transition
Demographic change describes modern reductions in both birth and death rates. From Darwin’s point of view, this does not mean that families with more resources have fewer children. One explanation for the demographic transition is the increase in parental investment needed to raise children who would be able to maintain the same level of resources as their parents.
