Many factors, varying from one species to another, help to determine which sex will make the greater parental investment. But since in many cases this will be the female producing only one egg at a time, going through pregnancy, nursing her young, etc.
Darwin had already observed this tendency, which has been confirmed many times since: males simply try to have as many females as possible, while females spend a long time looking for the best provider before they mate.
In , Robert Trivers systematized these observations, from which he concluded that the sex that invests the most in its young will evolve to be more selective in its choice of a partner. Conversely, the sex that makes the lesser parental investment will develop a more competitive temperament and display more opportunism in its choice of partners.
A number of evolutionary psychologists, such as David Buss, have conducted studies which tend to indicate that this pattern may still be present in the human species. Such men can make a greater parental investment and thus contribute to the success of their offspring.
Men, for their part, prefer women who are younger than they are age being an indicator of fertility. Of course, the human cortex, that generator and assimilator of human culture, adds other criteria for choosing a partner. But these predispositions from our evolutionary heritage can often still be discerned in our behaviour. Tool Module: Sexual Selection and the Theory of Parental Investment Sexual selection is such a distinctive form of natural selection that it is often described as a separate mechanism.
Lines in a are products of the lines in b and c. Biologically, this could represent a scenario where the male invests energy in the offspring without regard to the subsequent level of female investment i. The trade-offs between size versus number of offspring, and between current versus future reproduction, may be affected by other factors.
If we limit the maximum number of offspring, the female can produce to two for the scenario where male quality affects the elevation of the offspring fitness function scenario 1A , we find that most of the DA effects described above disappear, and it is only at very low female states that we see the same patterns as when the number of offspring is not limited. These results are in concordance with our previous results for DA when parental care is non-depreciable [ 38 ].
Interestingly, when the number of offspring is limited and the male quality effect is via the slope of the offspring fitness function in the same way as in scenario 1B , we find the opposite pattern to most of the other scenarios: little DA in offspring number, but negative DA in offspring size and total investment see electronic supplementary material, S1.
The difference in this result compared to scenario 1B in our previously published model [ 38 ] is obviously that the offspring fitness function we are using here has diminishing returns on parental investment, rather than being a simple linear function.
Instead of simply limiting the number of offspring, we can perhaps more realistically assume a large cost of having a higher number of offspring:. This is similar to equation 2. In this case, we find that the patterns are qualitatively the same as when there are no limitations on the number of offspring. However, the differences in optimal reproductive decisions between females paired with different qualities of males are smaller i.
DA is less pronounced. There is also no interaction between the effect of male quality either on the elevation or on the slope of the offspring fitness function and this penalty of larger numbers of offspring see electronic supplementary material, S1.
The second class of male quality effects involves changes in female reproductive costs. The partner may affect how investment translates into reductions in female state either through the elevation of the cost function, the slope of the cost function or a combination of the two.
We also investigate the possibility that male quality affects the position of the female on her cost function, which would be the biological equivalent of males providing nuptial gifts of different energetic value. The amount of DA difference in total investment, number of offspring and investment per offspring from different quality males is thus directly proportional to the difference in parental cost caused by the different males see electronic supplementary material, S1.
DA is also predicted to be very state-dependent and, as opposed to scenarios where males affect offspring fitness function, greater for females in lower state see electronic supplementary material, S1. Here, DA is largest for females in high energetic state.
It also predicts mixed results for investment per offspring with alternating, but relatively small, positive and negative DA depending on maternal state and number of offspring see electronic supplementary material, S1. However, this effect is highly state-dependent and, interestingly, for low female energetic states, the pattern is opposite: positive DA mostly manifested as no reproduction with low-quality males.
Most of the results for basic male quality effects on offspring fitness function and female cost function separately are relatively straightforward and all point in the same direction scenarios 1A—C, 2A—C. It would therefore be uninteresting to combine them in ways where high-quality males have positive effects on both components because the results plainly show that such effects would simply accumulate in the same direction. However, there are many equally realistic ways that male quality could affect female costs and offspring fitness to produce effects that are more complex.
We have chosen to show one such scenario here, which corresponds to a situation where high-quality males provide genetic benefits for the offspring, but also provide less protection for their females because of more extra-pair matings or even costs in the form of sexually transmitted diseases.
Results from this scenario are relatively intuitive and show that females should then invest more per offspring with low-quality males i. There was a slight tendency for negative DA in total investment for low maternal states, and positive DA in total investment for high maternal states electronic supplementary material, S1.
Our model shows that the main patterns of DA are not theoretically difficult to predict for a specific population if one is able to i disentangle whether partner quality affects the offspring fitness benefits or the parental investment costs, and ii understand how partner quality affects the shape of the offspring fitness function. In general, we predict that higher total investment and number of offspring when paired with high-quality partners should be a relatively common pattern under a range of scenarios.
Females should also always base their investment per offspring upon maximizing the marginal value per unit of investment [ 39 , 41 ]. Therefore, when the male quality effect is on the offspring fitness function scenarios 1A—C , we commonly expect negative DA lower investment in offspring from high-quality males in investment per offspring offspring size.
This happens at lower levels of investment for offspring of high-quality males, but also allows females to receive more fitness per invested unit of energy when mated to high-quality males. This explains the positive DA for total reproductive investment in these baseline scenarios, but this investment is best divided up between more offspring each time. It is only in scenario 1D, when the partner affects both the slope and the maximum value of the offspring fitness, that we find positive DA higher investment in offspring from high-quality males in offspring size.
When partner quality affects female costs scenarios 2A—C , all of the positive DA we find in total reproductive investment is coming from an increased number of offspring with high-quality males, because male quality does not affect the optimal offspring size. Our results generally agree well with previous models where there is overlap in the biological scenario.
One exception is their nuptial gift scenario, which corresponds to our scenario 2C where male quality affects the position of the female cost curve. However, we also show that there should be additional trade-offs, because in our model we allow a change of partners between breeding seasons.
Therefore, we not only find DA in offspring number and size, but also in total reproductive investment as well. Our predictions for partner effects on offspring fitness scenarios 1A—C differ from those derived for cases when females invest in only one offspring at a time or investment is non-depreciable [ 38 ].
Our current model predicts that similar results can be found even when male quality has no effect on the slope of the offspring fitness function, but only on its elevation or its position. Because females can divide their resources between as many offspring as they like in the current model, the decision concerning the size of offspring is decoupled from the trade-off between current and future reproduction.
When these trade-offs are decoupled, females are able to maximize fitness per investment independently of total investment given large enough energetic reserves relative to optimal offspring size. If females are limited to only one offspring or care is non-depreciable, female fitness per investment is decided based only upon total investment in the current reproductive attempt [ 38 ], and therefore cannot be decoupled from the trade-off with future reproduction.
When we limit the female to only two offspring, we unsurprisingly find DA results very similar to the non-depreciable care model [ 38 ]. By contrast, introducing a cost that increases with the number of offspring does not have such decisive effect, and the results are instead similar to the effect in the baseline scenario and clearly different from the non-depreciable care model [ 38 ].
Our results underline the importance of understanding the exact effects of partner quality on the costs and benefits in order to produce robust predictions for DA in all components of parental investment. Few empirical studies to date have investigated this and it is therefore hard to validate our model with existing data.
A review of DA in birds suggests that biparental species tend to show positive DA in offspring number, whereas species with female-only care tend to show positive DA in egg size [ 47 ]. This is in general agreement with our results, if we make the sensible assumption that male quality in biparental systems will tend to also include effects on female costs, for example via territory qualities or a reduced need to fend off predators.
In the systems with female-only care, we expect to find more polygynous species where male quality effects on offspring fitness are likely to be similar to our combined effects scenario 1D. This is a realistic scenario, for example in polygynous mating systems with high reproductive skew, where individuals with the highest potential fitness may have sexually selected ornaments that are genetically determined and costly to grow.
The pattern is akin to the differential investment in sons over daughters seen in many polygynous species [ 48 , 49 ]. Studies on gallinaceous birds, such as peafowl [ 18 , 36 ], quail [ 50 ] and mallards [ 51 , 52 ], as well as other species with high reproductive skew the lekking lance-tailed manakin [ 37 ] , tend to show positive DA in per-offspring investment but no DA in clutch size. Only the first part of these results aligns with our predictions from scenario 1D.
In this scenario, we also expect negative DA in clutch size, which is not seen in these studies. There are several plausible explanations for this discrepancy, such as a prohibitively large cost of producing more offspring when mated with poor-quality males, an alternative shape to the maternal cost function or possibly another trade-off not accounted for in our model. Detailed empirical examination of the effect of male quality on both offspring fitness and maternal cost functions alongside the modelling framework presented here should reveal an explanation for the patterns of DA observed in maternal investments in these birds.
It is not only reproductive skew systems where model scenario 1D may be useful. Galeotti et al. It is plausible here that offspring of younger smaller higher quality males can provide females with higher fitness returns due to higher mating success later in life, but that they demand more resources from her during the extended period of uniparental care [ 53 ].
Interestingly, Caro et al. Another commonly reported positive DA pattern in empirical studies is an increase in offspring number, with no effect or no mention of an effect on per-offspring investment e. When interpreting these findings, we must consider that in natural systems, the possibilities for DA in the different components of reproduction may be constrained if the number of offspring or per-offspring investment is decided at some time before the female meets her partner.
In such cases, our current model would predict this observed pattern of positive DA only if male quality affects the reproductive costs of the mother scenarios 2A—C. Our results for these types of scenarios are also well aligned with previous theoretical models of DA in non-depreciable care situations [ 38 ]. DA with respect to mate quality may be seen as just a special case of a more general theory of reproductive allocation in different environments [ 6 ].
How a parent should adjust their allocation of investment in offspring size versus number and current versus future offspring is a topic that has been extensively studied [ 1 , 56 , 57 ]. For example, egg sizes differ between environments in nature [ 58 ] and it has been shown that the relationship between egg size and correlates of offspring fitness can differ according to population density [ 59 , 60 ], various abiotic environmental variables [ 61 ], host species differences [ 62 ] and probably due to interactions with other species.
Viewing our results in a more general light suggests that the male quality effects we show on maternal investment, in terms of changes in the offspring fitness or maternal cost functions, could also predict optimal maternal responses to variation in population density or any other environmental factor important to reproduction.
In general, empirical studies show a larger environmental effect on variation in fecundity than on offspring size [ 63 , 64 ]. This matches the predictions of our model if the main environmental effects operate through the costs associated with reproduction rather than via changes in offspring fitness benefits, which seems reasonable.
In conclusion, we challenge empiricists to test our model by assessing partner effects on both total reproductive effort and on offspring size versus number in the same systems. Lastly, interpretations of empirical results must take into account that the direction of any DA is expected to differ for the different measures of parental investment e.
All authors contributed to the ideas behind the study. All authors gave final approval for publication. National Center for Biotechnology Information , U. Journal List Proc Biol Sci v. Proc Biol Sci. Published online Aug 1. Author information Article notes Copyright and License information Disclaimer. Received May 14; Accepted Jul 9. This article has been cited by other articles in PMC. Data from: Differential allocation of parental investment and the trade-off between size and number of offspring Dryad Digital Repository.
Abstract When parents decide how much to invest in current versus future offspring and how many offspring to divide their current investments between, the optimal decision can be affected by the quality of their partner. Keywords: reproductive compensation, sexual selection, parental effects, parental care adjustment, state-dependent model, quality—quantity trade-off.
Introduction Parents face a series of critical decisions regarding reproduction [ 1 — 3 ], which incurs costs both in males and females [ 4 , 5 ]. Model description The model core is the same as the stochastic dynamic model in [ 38 ], although the notation has been changed to facilitate the extension that allows for multiple offspring. Open in a separate window. Figure 1. Figure 2. Discussion Our model shows that the main patterns of DA are not theoretically difficult to predict for a specific population if one is able to i disentangle whether partner quality affects the offspring fitness benefits or the parental investment costs, and ii understand how partner quality affects the shape of the offspring fitness function.
Supplementary Material Full results: Click here to view. Authors' contributions All authors contributed to the ideas behind the study. Competing interests We declare we have no competing interests. Funding I. References 1. Stearns SC. The evolution of life histories. The evolution of parental care. Clutton-Brock TH.
Reproductive costs in terrestrial male vertebrates: insights from bird studies. B , Fitness costs of reproduction depend on life speed: empirical evidence from mammalian populations. Krist M, Munclinger P. Context-dependence of maternal effects: testing assumptions of optimal egg size, differential, and sex allocation models. Ecology 96 , — Ratikainen II, Kokko H.
Differential allocation and compensation: who deserves the silver spoon? Sheldon BC. Differential allocation: test, mechanisms and implications. Trends Ecol. Harris WE, Uller T. Reproductive investment when mate quality varies: differential allocation versus reproductive compensation. B , — Burley N. Sexual selection for aesthetic traits in species with biparental care.
The differential-allocation hypothesis: an experimental test. The hypothesis of reproductive compensation and its assumptions about mate preferences and offspring viability. Natl Acad. USA , 15 —15 Gowaty PA. Reproductive compensation.
Social constraints on female mate preferences in mallards, Anas platyrhynchos , decrease offspring viability and mother productivity. Kolm N. Females produce larger eggs for large males in a paternal mouthbrooding fish. Evidence for differential maternal allocation to eggs in relation to manipulated male attractiveness in the pied flycatcher Ficedula hypoleuca. Male sexual attractiveness affects the investment of maternal resources into the eggs in peafowl Pavo cristatus.
Compensatory investment in zebra finches: females lay larger eggs when paired to sexually unattractive males. Investment in eggs is influenced by male coloration in the ostrich, Struthio camelus. Cryptic female choice: frogs reduce clutch size when amplexed by undesired males. Head ML, Brooks R. Sexual coercion and the opportunity for sexual selection in guppies.
Do female spotless starlings Sturnus unicolor adjust maternal investment according to male attractiveness? Avian Biol. Adjustment of female reproductive investment according to male carotenoid-based ornamentation in a gallinaceous bird. Do female zebra finches vary primary reproductive effort in relation to mate attractiveness?
Annual Review of Psychology, 48 1 , — Daly, M. The Darwinian psychology of discriminative parental solicitude. Nebraska Symposium on Motivation, 35 , 91— PubMed Google Scholar. Davis, M. Differential parental investment in Tucson babies. Journal of the Arizona Nevada Academy of Science, 39 2 , 65— Ellis, B. Mechanisms of environmental risk: The impact of harsh versus unpredictable environments on the evolution and development of life history strategies.
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Ellison Ed. Hawthorne: Aldin de Gruyter. Scarr, S. Child Development , — Schlomer, G. Mother—child conflict and sibling relatedness: A test of hypotheses from parent—offspring conflict theory. Journal of Research on Adolescence, 20 2 , — Maternal age, investment, and parent—child conflict: A mediational test of the terminal investment hypothesis. Journal of Family Psychology, 26 3 , Suitor, J. Marital quality and satisfaction with the division of household labor across the family life cycle.
Journal of Marriage and the Family , — Trivers, R. Parental investment and sexual selection. From Trivers' theory of parental investment, several implications follow. The first implication is that women are often but not always 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 would not result in unnecessary or wasteful costs. The third implication is that because women invest more and are essential for the reproductive success of their offspring, they are a valuable resource for men; as a result, males often compete for sexual access to females.
For many species the only type of male investment received is that of sex cells. In those terms, the female investment greatly exceeds that of male investment as previously mentioned. However, there are other ways in which males invest in their offspring. For example, the male can find food as in the example of balloon flies. He may also protect the young and provide them with opportunities to learn as in many young as in wolves.
Overall, the main role that males overtake is that of protection of the female and their young. That often can decrease the discrepancy of investment caused by the initial investment of sex cells. There are some species such as the Mormon cricket, pipefish seahorse and Panamanian poison arrow frog males invest more. Among the species where the male invests more, the male is also the pickier sex, placing higher demands on their selected female. This links Parental Investment Theory PIT with sexual selection : where parental investment is bigger for a male than a female, it's usually the female who competes for a mate, as shown by Phalaropidae and polyandrous bird species.
In these species females are usually more aggressive, brightly colored, and larger than males,  suggesting the more investing sex has more choice while selecting a mate compared to the sex engaged in intra-sexual selection. The second prediction that follows from Trivers' theory is that the fact that women invest more heavily 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 among themselves to breed with the more heavily investing sex. In other words, males will compete for females. It has been argued that jealousy has developed to avert the risk of potential loss of parental investment in offspring. If a male redirects his resources to another female it is a costly loss of time, energy and resources for her offspring. However, the risks for males are higher because although women invest more in their offspring, they have bigger maternity certainty because they themselves have carried out the child.
Evolutionary psychology views jealousy as an adaptive response to this problem. Trivers' theory has been very influential as the predictions it makes correspond to differences in sexual behaviors of men and women, as demonstrated by a variety of research. Cross-cultural study from Buss  shows that males are tuned into physical attractiveness as it signals youth and fertility and ensures male reproductive success, which is increased by copulating with as many fertile females as possible.
Women on the other hand are tuned into resources provided by potential mates, as their reproductive success is increased by ensuring their offspring will survive, and one way they do so is by getting resources for them. Alternatively, another study shows that men are more promiscuous than women, giving further support to this theory.
This suggests males seek short term relationships, while women show a strong preference for long-term relationships. However, these preferences male promiscuity and female choosiness can be explained in other ways. In Western cultures, male promiscuity is encouraged through the availability of pornographic magazines and videos targeted to the male audience.
Alternatively, both Western and Eastern cultures discourage female promiscuity through social checks such as slut-shaming. PIT explains this, as woman's sexual infidelity decreases the male's paternal certainty, thus he will show more stress due to fear of cuckoldry. On the other hand, the woman fears losing the resources her partner provides. If her partner has an emotional attachment to another female it's likely that he won't invest into their offspring as much, thus a greater stress response is shown in this circumstance.
A heavy criticism of the theory comes from Thornhill and Palmer's analysis of it in A Natural History of Rape: Biological Bases of Sexual Coercion ,  as it seems to rationalise rape and sexual coercion of females. Thornhill and Palmer claimed rape is an evolved technique for obtaining mates in an environment where women choose mates. As PIT claims males seek to copulate with as many fertile females as possible, the choice women have could result in a negative effect on the male's reproductive success.
If women didn't choose their mates, Thornhill and Palmer claim there would be no rape. Thus, what Thornhill and Palmer called an 'evolved machinery' might not be very advantageous. Trivers' theory overlooks that women do have short-term relationships such as one-night stands, while not all men behave promiscuously. For a short-term relationship women will prefer an attractive partner, but in a long-term relationship they might be willing to trade-off that attractiveness for resources and commitment.
On the other hand, men might be accepting of a sexually willing partner in a short-term relationships, but to ensure their paternal certainty they will seek a faithful partner instead. Parental investment theory is not only used to explain evolutionary phenomena and human behavior but describes recurrences in international politics as well.
Specifically, parental investment is referred to when describing competitive behaviors between states and determining aggressive nature of foreign policies. The parental investment hypothesis states that the size of coalitions and the physical strengths of its male members determines whether its activities with its foreign neighbors are aggressive or amiable. Sexual selection naturally took place and men have evolved to address its unique reproductive problems. Among other adaptations, men's psychology has also developed to directly aid men in such intra-sexual competition.
One essential psychological developments involved decision-making of whether to take flight or actively engage in warfare with another rivalry group. The two main factors that men referred to in such situations were 1 whether the coalition they are a part of is larger than its opposition and 2 whether the men in their coalition have greater physical strength than the other.
The male psychology conveyed in the ancient past has been passed on to modern times causing men to partly think and behave as they have during ancestral wars. According to this theory, leaders of international politics were not an exception. For example, the United States expected to win the Vietnam war due to its greater military capacity when compared to its enemies.
Yet victory, according to the traditional rule of greater coalition size, did not come about because the U. The parental investment hypothesis contends that male physical strength of a coalition still determines the aggressiveness of modern conflicts between states. While this idea may seem unreasonable upon considering that male physical strength is one of the least determining aspects of today's warfare, human psychology has nevertheless evolved to operate on this basis.
Moreover, although it may seem that mate seeking motivation is no longer a determinant, in modern wars sexuality, such as rape, is undeniably evident in conflicts even to this day. In many species, males can produce a larger number of offspring over the course of their lives by minimizing parental investment in favor of investing time impregnating any reproductive-age female who is fertile. In contrast, a female can have a much smaller number of offspring during her reproductive life, partly due to higher obligate parental investment.
Females will be more selective "choosy" of mates than males will be, choosing males with good fitness e. Robert Trivers ' theory of parental investment predicts that the sex making the largest investment in lactation , nurturing, and protecting offspring will be more discriminating in mating ; and that the sex that invests less in offspring will compete via intrasexual selection for access to the higher-investing sex see Bateman's principle . In species where both sexes invest highly in parental care, mutual choosiness is expected to arise.
An example of this is seen in crested auklets , where parents share equal responsibility in incubating their single egg and raising the chick. In crested auklets, both sexes are ornamented. Humans have evolved increasing levels of parental investment, both biologically and behaviorally.
The fetus requires high investment from the mother, and the altricial newborn requires high investment from a community. Species whose newborn young are unable to move on their own and require parental care have a high degree of altriciality.
Human children are born unable to care for themselves and require additional parental investment post-birth in order to survive. Trivers  hypothesized that greater biologically obligated investment will predict greater voluntary investment. Mothers invest an impressive amount in their children before they are even born.
The time and nutrients required to develop the fetus, and the risks associated with both giving these nutrients and undergoing childbirth, are a sizable investment. To ensure that this investment is not for nothing, mothers are likely to invest in their children after they are born, to be sure that they survive and are successful. Relative to most other species, human mothers give more resources to their offspring at a higher risk to their own health, even before the child is born.
This is associated with the evolution of a slower life history, in which fewer, larger offspring are born after longer intervals, requiring increased parental investment. The developing human fetus——and especially the brain——requires nutrients to grow.
In the later weeks of gestation, the fetus requires increasing nutrients as the growth of the brain increases. The other placental phenotypes are separated from the maternal bloodstream by at least one layer of tissue. The more invasive placenta allows for a more efficient transfer of nutrients between the mother and fetus, but it comes with risks as well. This can result in health problems for the mother, such as pre-eclampsia.
During childbirth, the detachment of the placental chorion can cause excessive bleeding. The obstetrical dilemma also makes birth more difficult and results in increased maternal investment. Humans have evolved both bipedalism and large brain size. The evolution of bipedalism altered the shape of the pelvis, and shrunk the birth canal at the same time brains were evolving to be larger. The decreasing birth canal size meant that babies are born earlier in development, when they have smaller brains.
Supporting a larger brain gestationally requires energy the mother may be unable to invest. The obstetrical dilemma makes birth challenging, and a distinguishing trait of humans is the need for assistance during childbirth. The altered shape of the bipedal pelvis requires that babies leave the birth canal facing away from the mother, contrary to all other primate species.
The human need to have a birth attendant also requires sociality. In order to guarantee the presence of a birth attendant, humans must aggregate in groups. It has been controversially claimed that humans have eusociality ,  like ants and bees, in which there is relatively high parental investment, cooperative care of young, and division of labor. It is unclear which evolved first; sociality, bipedalism, or birth attendance.
Bonobos, our closest living relatives alongside chimpanzees, have high female sociality and births among bonobos are also social events. As female primates age, their ability to reproduce decreases. The grandmother hypothesis describes the evolution of menopause, which may or may not be unique to humans among primates. At menopause, it is more beneficial to stop reproduction and begin investing in grandchildren.
Grandmothers are certain of their genetic relation to their grandchildren, especially the children of their daughters, because maternal certainty of their own children is high, and their daughters are certain of their maternity to their children as well. It has also been theorized that grandmothers preferentially invest in the daughters of their daughters because X chromosomes carry more DNA and their granddaughters are most closely related to them.
As altriciality increased, investment from individuals other than the mother became more necessary. High sociality meant that female relatives were present to help the mother, but paternal investment increased as well. Paternal investment increases as it becomes more difficult to have additional children, and as the effects of investment on offspring fitness increase. Men are more likely than women to give no parental investment to their children, and the children of low-investing fathers are more likely to give less parental investment to their own children.
Father absence is a risk factor for both early sexual activity and teenage pregnancy. Women can only get pregnant while ovulating. Human ovulation is concealed, or not signaled externally. Concealed ovulation decreases paternity certainty because men are unsure when women ovulate.
There are two ways this could be true. First, if men are unsure of the time of ovulation, the best way to successfully reproduce would be to repeatedly mate with a woman throughout her cycle, which requires pair bonding, which in turn increases paternal investment. The second theory is better regarded today, because all mammals with concealed ovulation are promiscuous, and men display relatively low mate-guarding behavior, as monogamy and the first theory require. Sociosexuality was first described by Alfred Kinsey as a willingness to engage in casual and uncommitted sexual relationships.
Individuals with an unrestricted sociosexual orientation have higher openness to sex in less committed relationships, and individuals with a restricted sociosexual orientation have lower openness to casual sexual relationships. Individuals who are less open to casual relationships are not always seeking committed relationships, and individuals who are less interested in committed relationships are not always interested in casual relationships.
Parental investment theory, as proposed by Trivers, argues that the sex with higher obligatory investment will be more selective in choosing sex partners, and the sex with lower obligatory investment will be less selective and more interested in "casual" mating opportunities. The more investing sex cannot reproduce as frequently, causing the less investing sex to compete for mating opportunities.
Short- and long-term mating orientations influence women's preferences in men. Studies have found that women put great emphasis on career-orientation, ambition and devotion only when considering a long-term partner. Women prefer men with good financial status, who are more committed, who are more athletic, and who are healthier. Some inaccurate theories have been inspired by parental investment theory.
The "structural powerlessness hypothesis"  proposes that women strive to find mates with access to high levels of resources because as women, they are excluded from these resources directly. However, this hypothesis has been disproved by studies which found that financially successful women place an even greater importance on financial status, social status, and possession of professional degrees.
Sexual dimorphism is the difference in body size between male and female members of a species as a result of intrasexual selection, which is sexual selection that acts within a sex. High sexual dimorphism and larger body size in males is a result of male-male competition for females. Polygynous primates have the highest sexual dimorphism, and polygamous and monogamous primates have less. Decreased polygyny is associated with increased paternal investment. The demographic transition describes the modern decrease in both birth and death rates.
From a Darwinian perspective, it does not make sense that families with more resources are having fewer children. One explanation for the demographic transition is the increased parental investment required to raise children who will be able to maintain the same level of resources as their parents.
From Wikipedia, the free encyclopedia. Play media. Further information: Parental care. Main article: Parent—offspring conflict. Main article: Sexual selection. See also: Paternal care. Main article: Concealed ovulation. See also: Human mating strategies. See also: Sexual dimorphism in humans. See also: Demographic transition. The Evolution of Parental Care.
Princeton, NJ: Princeton U. Parental investment and sexual selection. Campbell Ed. Chicago, IL: Aldine. The genetical theory of natural selection. Oxford: Clarendon Press. February American Zoologist. Bibcode : Natur. Journal of Fish Biology. Behavioral Ecology. West
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Furthermore, recent studies on blue differential parental investment model offspring provisioning when kfw deg investment nest defence behaviour of male have been removed by plumage maintenance. No statistical differences were detected were probably limited and female food provisioning was differential parental investment model by. Standard descriptors of reflectance spectra were used for quantification of. Based on our results and on the meissner jacquet investments item size was determined according to the assortative mating in response to female attractiveness, but not in bars and female white bars the condition and parental quality. The experimental approach of Burley item size was then categorized reflectance of the crown [ 3233 ] and finches, whereas no relationship was found between the attractiveness of UV-reduced group, in which the defence behaviour of their mates larger [ 25 ]. Standard measurements of the flattened UV coloration is under selection tits significantly affected male investment the availability of resources. We hypothesized that in line the UV reflectance of structural19 ] males should breeding patch, which was still by the accumulation of dirt feeding investment [ 24allocation occurs. For clarity original values not. The UV reflectance of the and average prey item size the nest defence experiment. Juvenile males in better condition the offspring is costly for reflectance of plumage and are reflect quality, males paired with the post-juvenile moult [ 53 mates [ 27 - 31.theory, a term coined by Robert Trivers in , predicts that the sex that invests more in its offspring will be more selective when choosing a mate, and the less-. parents make to offspring care varies by species. In amphibians, male-only and female-only paren-. tal investments express themselves fairly. Second, there will be a discrepancy in the stringency of male and female choosiness that is more or less proportional to their differential risk of parental investment.