Editing Concealed ovulation (section)

==Evolutionary hypotheses==

[[evolutionary psychology|Evolutionary psychologists]] have advanced a number of different possible explanations for concealed ovulation.<ref name="Schroder 1993">{{Cite journal | last1 = Schoroder | first1 = I. | title = Concealed ovulation and clandestine copulation: A female contribution to human evolution | doi = 10.1016/0162-3095(93)90026-E | journal = Ethology and Sociobiology | volume = 14 | issue = 6 | pages = 381–389 | year = 1993 }}</ref>  Some posit that the lack of signaling in some species is a trait retained from evolutionary ancestors, not something that existed previously and later disappeared. If signaling is supposed to have existed and was lost, then it could have been merely due to reduced adaptive importance and lessened selection,<ref name="Burt 1992">{{cite journal | author=Burt, Austin | title =  'Concealed ovulation' and sexual signals in primates | journal = Folia Primatologica | volume = 58 | issue = 1 | pages = 1–6  |date=June 1992 | doi = 10.1159/000156600| pmid = 1618432 }}</ref> or due to direct adaptive advantages for the concealment of ovulation. Yet another possibility (regarding humans specifically) is that while highly specific signaling of ovulation is absent, human female anatomy evolved to mimic permanent signaling of fertility.<ref name="Szalay and Costello 1991">{{cite journal |author1=Frederick S. Szalay |author2=Robert K.Costello | title = Evolution of permanent estrus displays in hominids | journal = Journal of Human Evolution | volume = 20 | pages = 439–464   |date=June 1991 | doi = 10.1016/0047-2484(91)90019-R | issue=6}}</ref>

===Paternal investment hypothesis===

The [[paternal investment]] hypothesis is strongly supported by many evolutionary biologists.<ref name="Schroder 1993" /> Several hypotheses regarding human evolution integrate the idea that women increasingly required supplemental paternal investment in their offspring.  The shared reliance on this idea across several hypotheses concerning human evolution increases its significance in terms of this specific phenomenon.

This hypothesis suggests that women concealed ovulation to obtain men's aid in rearing offspring.  Schröder<ref name="Schroder 1993" /> summarizes this hypothesis outlined in Alexander and Noonan's 1979 paper:  if women no longer signaled the time of ovulation, men would be unable to detect the exact period in which they were fecund.   This led to a change in men's mating strategy:  rather than mating with multiple women in the hope that some of them, at least, were fecund during that period, men instead chose to mate with a particular woman repeatedly throughout her menstrual cycle.  A mating would be successful in resulting in conception when it occurred during ovulation, and thus, frequent matings, necessitated by the effects of concealed ovulation, would be most evolutionarily successful. A similar [[Bipedalism#Provisioning model|hypothesis]] was proposed by Lovejoy in 1981 that argued that concealed ovulation, reduced canines and [[bipedalism]] evolved from a reproductive strategy where males provisioned food resources to his paired female and dependent offspring.<ref>{{Cite journal|last=Lovejoy|first=C. Owen|date=1981-01-23|title=The Origin of Man|journal=Science|language=en|volume=211|issue=4480|pages=341–350|doi=10.1126/science.211.4480.341|issn=0036-8075|pmid=17748254|bibcode=1981Sci...211..341L}}</ref><ref>{{Cite journal|last=Lovejoy|first=C. Owen|date=2009-10-02|title=Reexamining Human Origins in Light of Ardipithecus ramidus|journal=Science|language=en|volume=326|issue=5949|pages=74–74e8|doi=10.1126/science.1175834|issn=0036-8075|url=http://doc.rero.ch/record/211449/files/PAL_E4439.pdf|pmid=19810200|bibcode=2009Sci...326...74L|s2cid=42790876}}</ref>

[[extended female sexuality|Continuous female sexual receptivity]] suggests human sexuality is not solely defined by reproduction; a large part of it revolves around conjugal love and communication between partners.  Copulations between partners while the woman is pregnant or in the infertile period of her menstrual cycle do not achieve conception, but do strengthen the bond between these partners.  Therefore, the increased frequency of copulations due to concealed ovulation are thought to have played a role in fostering [[pair bond]]s in humans.<ref name="Benagiano and Mori 2009">{{Cite journal
 | last1 = Benagiano | first1 = G.
 | last2 = Mori | first2 = M.
 | title = The origins of human sexuality: Procreation or recreation?
 | journal = Reproductive Biomedicine Online
 | volume = 18
 | pages = 50–59
 | year = 2009
 | issue = Suppl 1
 | pmid = 19281665
 | doi=10.1016/s1472-6483(10)60116-2
}}</ref>

The pair bond would be very advantageous to the [[reproductive fitness]] of both partners throughout the period of pregnancy, lactation, and rearing of offspring.  Pregnancy, lactation and caring for post-lactation offspring require vast amounts of energy and time on the part of the woman.  She must at first consume more food, then provide food to her offspring, while her ability to forage is reduced throughout.  Supplemental male investment in the mother and her offspring is advantageous to all parties.  While the man supplements the woman's limited gathered food, the woman is enabled to devote the necessary time and energy to the care of their offspring.  The offspring benefits from the supplemental investment, in the form of food and defense from the father, and receives the full attention and resources of the mother.  Through this shared parental investment, both man and woman would increase their offspring's chances for survival, thereby increasing their reproductive fitness.   In this way, natural selection would favor the establishment of pair bonds in humans.  To the extent that concealed ovulation strengthened pair bonding, selective pressure would favor concealed ovulation as well.

Another, more recent, hypothesis is that concealed ovulation is an adaptation in response to a promiscuous mating system, similar to that of our closest evolutionary relatives, [[bonobos]] and [[Common chimpanzee|chimpanzees]]. The theory is that concealed ovulation evolved in women to lessen paternity certainty, which would both lessen the chances of infanticide (as a father is less likely to kill offspring that might be his), and potentially increase the number of men motivated to assist her in caring for her offspring ([[partible paternity]]). This is supported by the fact that all other mammals with concealed ovulation, such as dolphins and [[gray langur]]s, are promiscuous, and that the only other ape species that have multi-male communities, as humans do, are promiscuous. It is argued that evidence such as the [[Coolidge effect]], showing that a man does not seem to be naturally geared towards sexual mate-guarding behavior (that is, preventing other males from having access to his sexual partner), supports the conclusion that sexual monogamy (though perhaps not [[Monogamy#Varieties in biology|social monogamy]] and/or pair bonding) was rare in early modern humans.<ref name="Ryan">{{cite book|author=Christopher Ryan Ph.D|author2=Cacilda Jethá Ph.D|name-list-style=amp|title=Sex at Dawn|isbn =978-0-06-220794-4 |publisher=HarperCollins|year=2012}}</ref>

===Reduced infanticide hypothesis===
This hypothesis suggests the adaptive advantage for women who had hidden estrus would be a reduction in the possibility of [[infanticide]] by men, as they would be unable to reliably identify, and kill, their rivals' offspring.<ref name="Schroder 1993" /> This hypothesis is supported by recent studies of wild [[Gray langur|hanuman langurs]], documenting concealed ovulation, and frequent matings with males outside their fertile ovulatory period.<ref name="Heisterman et al 2001">{{Cite journal | last1 = Hestermann | first1 = M. | last2 = Ziegler | first2 = T. | last3 = Van Schaik | first3 = C. P. | last4 = Launhardt | first4 = K. | last5 = Winkler | first5 = P. | last6 = Hodges | first6 = J. K. | doi = 10.1098/rspb.2001.1833 | title = Loss of oestrus, concealed ovulation and paternity confusion in free-ranging Hanuman langurs | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 268 | issue = 1484 | pages = 2445–2451 | year = 2001 | pmid =  11747562| pmc = 1088898}}</ref> Heistermann et al. hypothesize that concealed ovulation is used by women to confuse paternity and thus reduce [[infanticide in primates]].  He explains that as ovulation is always concealed in women, men can only determine paternity (and thus decide on whether to kill the woman's child) probabilistically, based on his previous mating frequency with her, and so he would be unable to escape the possibility that the child might be his own, even if he were aware of promiscuous matings on the woman's part.

===Sex and reward hypothesis===

Schröder<ref name="Schroder 1993" /> reviews a hypothesis by Symons and Hill, that after hunting, men exchanged meat for sex with women.   Women who continuously mimicked estrus may have benefited from more meat than those that did not.  If this occurred with enough frequency, then a definite period of estrus would have been lost, and with it sexual signaling specific to ovulation would have disappeared.

===Social-bonding hypothesis===

Schröder<ref name="Schroder 1993" /> presents the idea of a "gradual diminution of mid-cycle estrus and concomitant continuous sexual receptivity in human women" because it facilitated orderly social relationships throughout the menstrual cycle by eliminating the periodic intensification of male–male aggressiveness in competition for mates.<ref name="Schroder 1993" /> The extended estrous period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle) has been said to have a similar effect to the lack of a "heat" in women.  While concealed human ovulation may have evolved in this fashion, extending estrus until it was no longer a distinct period, as paralleled in the bonobo, this theory of why concealed ovulation evolved has frequently been rejected. Schröder outlines the two objections to this hypothesis: (1) natural selection would need to work at a level above the individual, which is difficult to prove; and (2) selection, because it acts on the individuals with the most reproductive success, would thus favor greater reproductive success over social integration at the expense of reproductive success.

However, since 1993 when that was written, group selection models have seen a resurgence.<ref name="Koeslag, 1997">{{cite journal | last1 = Koeslag | first1 = J. H. | year = 1997 | title = Sex, the prisoner's dilemma game, and the evolutionary inevitability of cooperation | journal = J. Theor. Biol. | volume = 189 | issue = 1| pages = 53–61 | doi=10.1006/jtbi.1997.0496 | pmid=9398503| bibcode = 1997JThBi.189...53K }}</ref><ref name= "Koeslag, 2003">{{cite journal | last1 = Koeslag | first1 = J. H. | year = 2003 | title = Evolution of cooperation: cooperation defeats defection in the cornfield model | journal = J. Theor. Biol. | volume = 224 | issue = 3 | pages = 399–410 | doi = 10.1016/s0022-5193(03)00188-7 | pmid = 12941597 | bibcode = 2003JThBi.224..399K }}</ref><ref name="Wilson, D. S. 2008">{{cite journal | last1 = Wilson | first1 = D. S. | last2 = Wilson | first2 = E. O. | year = 2008 | title = Evolution 'for the good of the group'. [Article] | journal = American Scientist | volume = 96 | issue = 5| pages = 380–389 | doi=10.1511/2008.74.1}}</ref> (See [[group selection]], [[reciprocal altruism]], and [[kin selection]].)

===Cuckoldry hypothesis===

Schröder in his review writes that Benshoof and Thornhill hypothesized that estrus became hidden after monogamous relationships became the norm in ''[[Homo erectus]]''.<ref name="Schroder 1993" />  Concealed ovulation allowed the woman to mate secretly at times with an alternative man, and thus gain the benefit of his genes for her offspring, while still retaining the benefits of the pair bond with her usual sexual partner.  Her usual sexual partner would have little reason to doubt her fidelity, because of the concealed ovulation, and would have high, albeit unfounded, paternity confidence in her offspring.  His confidence would encourage him to invest his time and energy in assisting her to care for the child, even though it was not his own.  Again, the idea of a man's investment being vital to the child's survival is a central fixture of a hypothesis regarding concealed ovulation, even as the evolutionary benefits accrue to the child, the woman, and her clandestine partner, and not to her regular sexual partner.