29:
406:, there is only one male for every group of 6-8 females. If the male fish dies, the strongest female changes its sex to become the male for the group. All of these wrasses are born female, and only become male in this situation. Other species, like clownfish, do this in reverse, where all start out as non-reproductive males, and the largest male becomes a female, with the second-largest male maturing to become reproductive.
377:, for example, females are hatched from eggs incubated between 27.7 to 30 °C (81.9 to 86.0 °F), whereas males are hatched from eggs 32.2 to 33.8 °C (90.0 to 92.8 °F). In this method, however, all eggs in a clutch (20–50) will be of the same sex. In fact, the natural sex ratio of this species is five females to one male.
345:: if the general population deviates from this equilibrium by favoring one sex, one can obtain higher reproductive success with less effort by producing more of the other. For species where the cost of successfully raising one offspring is roughly the same regardless of its sex, this translates to an approximately equal sex ratio.
365:
is usually skewed towards dominance of females. However, there are differences in adult sex ratios between families: in families in which females require multiple matings to keep producing eggs, sex ratios are less biased (close to 1); in families in which females can produce eggs continuously after
205:
males to females in a population, and is therefore derived from a subset of the individuals included when calculating the ASR. Although conceptually distinct, researchers have sometimes equated the ASR with the OSR, particularly in experimental studies of animals where the difference between the two
138:
and infanticide. Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well. As of 2024, the global sex ratio at birth is estimated at 107 boys to 100 girls (1,000 boys per 934 girls). By old age, the sex ratio reverses, with 81 older men for every
414:
Traditionally, farmers have discovered that the most economically efficient community of animals will have a large number of females and a very small number of males. A herd of cows with a few bulls or a flock of hens with one rooster are the most economical sex ratios for domesticated livestock.
502:. This correlation may also apply to cooperatively breeding birds, though the evidence is less clear. It is known, however, that both male-biased adult sex ratios and cooperative breeding tend to evolve where caring for offspring is extremely difficult due to low secondary productivity, as in
423:
It was found that the amount of fertilizing pollen can influence secondary sex ratio in dioecious plants. Increase in pollen amount leads to decrease in number of male plants in the progeny. This relationship was confirmed on four plant species from three families –
486:
species is clearly related to a strongly male-biased adult sex ratio. Those species with male care and polyandry invariably have adult sex ratios with a large surplus of males, which in some cases can reach as high as six males per female.
272:
rather than sex ratio, sex allocation denoting the allocation of energy to either sex. Common research themes are the effects of local mate and resource competition (often abbreviated LMC and LRC, respectively).
1560:
Allainé, Dominique; Brondex, Francine; Graziani, Laurent; Coulon, Jacques and Till-Bottraud, Irène; "Male-biased sex ratio in litters of alpine marmots supports the helper repayment hypothesis"
232:
Sex ratio theory is a field of academic study which seeks to understand the sex ratios observed in nature from an evolutionary perspective. It continues to be heavily influenced by the work of
134:
is of particular interest to anthropologists and demographers. In human societies, sex ratios at birth may be considerably skewed by factors such as the age of mother at birth and by
264:
When does selection favour the ability of an individual to alter its allocation to male versus female function, in response to particular environmental or life history situations?
1835:
Nishimura K, Jahn GC (1996). "Sex allocation of three solitary ectoparasitic wasp species on bean weevil larvae: sex ratio change with host quality and local mate competition".
967:
809:
Davis DL, Gottlieb MB, Stampnitzky JR (April 1998). "Reduced ratio of male to female births in several industrial countries: a sentinel health indicator?".
1932:
1570:
Doerr ED, Doerr VA (2006). "Comparative demography of treecreepers: evaluating hypotheses for the evolution and maintenance of cooperative breeding".
640:
682:(April 1967). "Extraordinary sex ratios. A sex-ratio theory for sex linkage and inbreeding has new implications in cytogenetics and entomology".
261:
Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
370:
601:
328:. A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio.
303:
Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
1141:
Danforth B (1991). "Female
Foraging and Intranest Behavior of a Communal Bee, Perdita portalis (Hymenoptera: Andrenidae)".
287:
Fisher's principle (1930) explains why in most species, the sex ratio is approximately 1:1. His argument was summarised by
1270:
Pike TW, Petrie M (October 2005). "Maternal body condition and plasma hormones affect offspring sex ration in peafowl".
1730:
Arnold KE, Griffith SC, Goldizen AW (2001). "Sex-biased hatching sequences in the cooperatively breeding noisy miner".
875:
Coney NS, Mackey WC (1998). "The Woman as Final
Arbiter: A Case for the Facultative Character of the Human Sex Ratio".
300:
A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
1082:
998:
1658:
Orians GH, Milewski AV (August 2007). "Ecology of
Australia: the effects of nutrient-poor soils and intense fires".
976:
916:
1219:
Ferguson MW, Joanen T (April 1982). "Temperature of egg incubation determines sex in
Alligator mississippiensis".
139:
100 older women; across all ages, the global population is nearly balanced, with 101 males for every 100 females.
312:
The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
576:
554:
342:
319:
1703:
104:. However, many species deviate from an even sex ratio, either periodically or permanently. Examples include
656:
236:. He defines five major questions, both for his book and the field in general (slightly abbreviated here):
908:"Parental Investment and the Control of Sexual Selection: Predicting the Direction of Sexual Competition"
559:
258:, what is the equilibrium allocation of resources to male versus female function in each breeding season?
255:
1701:
Noske RA (1986). "Intersexual niche segregation among three bark-foraging birds of eucalypt forests".
907:
403:
1351:
Correns C (1928). "Bestimmung, Vererbung und
Verteilung des Geschlechter bei den hoheren Pflanzen".
1514:"Persistence of an extreme male-biased adult sex ratio in a natural population of polyandrous bird"
291:(1967) as follows, assuming that parents invest the same whether raising male or female offspring:
248:
384:, maternal body condition can influence the proportion of daughters in the range from 25% to 87%.
1305:
Correns C (1922). "Geschlechtsbestimmung und
Zahlenverhaltnis der Geschlechter beim Sauerampfer (
1967:
618:
591:
135:
518:, adult sex ratios are equally or more male-biased than in those cooperative species, such as
544:
306:
Therefore the genes for male-producing tendencies spread, and male births become more common.
282:
197:
97:
1871:(January 1973). "Natural selection of parental ability to vary the sex ratio of offspring".
309:
As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
1880:
1476:
1228:
1177:
1107:
746:
733:
Kobayashi K, Hasegawa E, Yamamoto Y, Kawatsu K, Vargo EL, Yoshimura J, et al. (2013).
691:
491:
467:
1605:
Kokko H, Jennions MD (July 2008). "Parental investment, sexual selection and sex ratios".
1168:
Kiørboe T (May 2006). "Sex, sex-ratios, and the dynamics of pelagic copepod populations".
792:
8:
1957:
847:
101:
56:
1884:
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1111:
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1962:
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1904:
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1807:
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1716:
1683:
1640:
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1287:
1252:
1201:
1123:
1052:
933:
852:
707:
637:
374:
324:
1743:
1420:
798:. U.S. Department of Health and Human Services, National Center for Health Statistics.
147:
In most species, the sex ratio varies according to the age profile of the population.
1896:
1815:
1675:
1671:
1632:
1618:
1535:
1530:
1513:
1494:
1244:
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446:
1908:
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1827:
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1644:
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1291:
1205:
373:, where incubation temperature of eggs determines the sex of the individual. In the
19:"Gender balance" redirects here. For gender balance as a socio-political issue, see
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586:
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470:
and sex-role reversal (where males care and females compete for mates) as found in
1583:
1283:
100:, for evolutionary reasons this is typically about 1:1 in species which reproduce
28:
1761:
990:
703:
571:
515:
507:
463:
440:
131:
20:
1892:
1864:
679:
549:
288:
224:
These definitions can be somewhat subjective since they lack clear boundaries.
192:(ASR), which is defined as the ratio of adult males to females in a population.
1189:
888:
1946:
822:
596:
495:
479:
451:
426:
341:
Spending equal amounts of resources to produce offspring of either sex is an
105:
1937:
1933:
CIA listing of sex ratios for individual countries (including age divisions)
1154:
418:
1679:
1636:
1539:
1498:
1197:
1070:
1048:
929:
902:
778:
431:
244:
species, what is the equilibrium sex ratio maintained by natural selection?
233:
1900:
1819:
1764:, Banister J (December 1996). "Five decades of missing females in China".
1407:
Riede W (January 1925). "Beiträge zum
Geschlechts-und Anpassungsproblem".
1248:
830:
719:
390:(sequential hermaphroditism) is normal in several groups of fish, such as
1868:
606:
527:
523:
109:
1392:
1371:
33:
1848:
1811:
1489:
1464:
1023:"Adult sex ratio variation: implications for breeding system evolution"
759:
734:
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511:
395:
322:(ESS). This ratio has been observed in many species, including the bee
93:
1777:
1627:
1039:
1022:
769:
361:
species as they kill males. Sex-ratio of adult populations of pelagic
1240:
1119:
937:
510:. It is also known that in cooperative breeders where both sexes are
503:
475:
471:
457:
399:
387:
358:
353:
348:
241:
1802:
1785:
490:
Male-biased adult sex ratios have also been shown to correlate with
1465:"The evolution of sex roles in birds is related to adult sex ratio"
362:
1098:
Maynard Smith J, Price GR (1973). "The logic of animal conflict".
735:"Sex ratio biases in termites provide evidence for kin selection"
519:
499:
402:. This can cause a discrepancy in the sex ratios as well. In the
391:
381:
366:
only one mating, sex ratios are strongly skewed towards females.
124:
732:
793:"Trend Analysis of the sex Ratio at Birth in the United States"
483:
1434:
1511:
948:
89:
55:
number of males and females (accounting that the ratio has 3
380:
In birds, mothers can influence the sex of their chicks. In
108:
species, periodically mating organisms such as aphids, some
1512:
Kosztolányi A, Barta Z, Küpper C, Székely T (August 2011).
985:. Cambridge: Cambridge University Press. pp. 366–382.
251:, what is the equilibrium sex order and time of sex change?
112:
1660:
Biological
Reviews of the Cambridge Philosophical Society
1328:
Rychlewski JE, Zarzycki K (1975). "Sex ratio in seeds of
419:
Dioecious plants secondary sex ratio and amount of pollen
120:
116:
808:
661:. Oxford: Clarendon Press. pp. 141–143 – via
917:
Proceedings of the Royal
Society B: Biological Sciences
336:
1729:
1462:
1020:
856:. The Central Intelligence Agency of the United States
268:
Biological research mostly concerns itself with sex
1091:
1327:
1097:
975:
966:
906:
458:Polyandrous and cooperatively breeding homeotherms
1766:Proceedings of the American Philosophical Society
1021:Székely T, Weissing FJ, Komdeur J (August 2014).
1944:
1332:as a result of sparse or abundant pollination".
331:
297:Suppose male births are less common than female.
150:It is generally divided into four subdivisions:
1433:
968:"Operational Sex Ratios and Mating Competition"
964:
32:Map indicating the human sex ratio by country.
1863:
1657:
1218:
1143:Annals of the Entomological Society of America
466:birds, recent research has shown clearly that
1834:
1783:
1760:
1604:
901:
1458:
1456:
1454:
184:The tertiary sex ratio is equivalent to the
960:
958:
874:
1920:. Association of Professional Geographers.
1915:
1786:"Five decades of missing females in China"
1569:
1463:Liker A, Freckleton RP, Székely T (2013).
1269:
674:
672:
206:values may not always be readily apparent.
1801:
1626:
1529:
1488:
1451:
1391:
1134:
1077:. Princeton: Princeton University Press.
1038:
978:Sex Ratios: Concepts and Research Methods
768:
758:
658:The Genetical Theory of Natural Selection
318:In modern language, the 1:1 ratio is the
16:Ratio of males to females in a population
1598:
1409:Flora oder Allgemeine Botanische Zeitung
1140:
955:
678:
650:
648:
27:
1369:
1350:
1304:
1167:
1069:
1063:
1014:
669:
642:. Map compiled in 2021, data from 2020.
409:
371:temperature-dependent sex determination
1945:
654:
602:Sex-selective abortion and infanticide
220:— ratio in post-reproductive organisms
1700:
1406:
645:
276:
181:— ratio in sexually mature organisms
1784:Coale AJ, Banister J (August 1994).
1444:(cit.: Plant Breeding Abstr., 1934,
842:
840:
337:Environmental and individual control
59:, i.e., 1.00 males to 1.00 females).
227:
13:
1753:
1717:10.1111/j.1442-9993.1986.tb01396.x
785:
14:
1979:
1926:
1918:Techniques for Sex Ratio Analysis
1744:10.1111/j.0908-8857.2001.320303.x
837:
369:Several species of reptiles have
1672:10.1111/j.1469-185x.2007.00017.x
1619:10.1111/j.1420-9101.2008.01540.x
1531:10.1111/j.1420-9101.2011.02305.x
357:cause skewed sex ratios in some
1723:
1694:
1651:
1607:Journal of Evolutionary Biology
1563:
1554:
1518:Journal of Evolutionary Biology
1505:
1427:
1400:
1363:
1344:
1321:
1298:
1263:
1212:
1161:
1027:Journal of Evolutionary Biology
530:where females always disperse.
1440:8th Congr. Jap. Ass. Adv. Sci.
965:Kvarnemo C, Ahnesjö I (2002).
895:
868:
802:
726:
630:
577:List of countries by sex ratio
343:evolutionarily stable strategy
320:evolutionarily stable strategy
1:
1704:Australian Journal of Ecology
1584:10.1016/j.anbehav.2005.10.017
1421:10.1016/S0367-1615(17)32904-X
1284:10.1016/j.anbehav.2004.12.020
624:
332:Examples in non-human species
1938:A review of sex ratio theory
991:10.1017/CBO9780511542053.019
704:10.1126/science.156.3774.477
7:
1916:Rath RM, Mishra AK (2005).
1893:10.1126/science.179.4068.90
1448:, â„– 3, p. 248, ref. â„– 768).
560:XY sex-determination system
533:
10:
1986:
555:Trivers–Willard hypothesis
280:
256:simultaneous hermaphrodite
65: Countries with more
41: Countries with more
36:
18:
1311:Biologisches Zentralblatt
1190:10.1007/s00442-005-0346-3
889:10.1080/00224499809551930
848:"Field Listing—Sex ratio"
404:bluestreak cleaner wrasse
92:of males to females in a
51: Countries with the
1732:Journal of Avian Biology
823:10.1001/jama.279.13.1018
249:sequential hermaphrodite
161:— ratio at fertilization
142:
877:Journal of Sex Research
1372:"Optimal sex ratio in
1334:Acta Biol Crac Ser Bot
930:10.1098/rspb.1996.0048
619:Gender Balance Council
592:Demographic transition
201:(OSR) is the ratio of
136:sex-selective abortion
81:
1469:Nature Communications
1353:Handb. Vererbungswiss
1155:10.1093/aesa/84.5.537
974:. In Hardy IC (ed.).
905:, Simmons LW (1996).
739:Nature Communications
545:Operational sex ratio
198:operational sex ratio
31:
492:cooperative breeding
410:Domesticated animals
216:quaternary sex ratio
1885:1973Sci...179...90T
1837:Journal of Ethology
1481:2013NatCo...4.1587L
1438:. Sieb. et. Zuce".
1393:10.1038/hdy.1967.50
1370:Mulcahy DL (1967).
1233:1982Natur.296..850F
1182:2006Oecol.148...40K
1112:1973Natur.246...15S
751:2013NatCo...4.2048K
696:1967Sci...156..477H
494:in mammals such as
438:(Caryophyllaceae),
167:secondary sex ratio
57:significant figures
1849:10.1007/BF02350089
1490:10.1038/ncomms2600
760:10.1038/ncomms3048
655:Fisher RA (1930).
638:CIA World Factbook
375:American alligator
325:Macrotera portalis
283:Fisher's principle
277:Fisher's principle
177:tertiary sex ratio
98:Fisher's principle
96:. As explained by
82:
1436:Humulus japonicus
1227:(5860): 850–853.
1040:10.1111/jeb.12415
924:(1368): 315–321.
817:(13): 1018–1023.
690:(3774): 477–488.
447:Humulus japonicus
157:primary sex ratio
80:
79:
1975:
1921:
1912:
1860:
1831:
1805:
1782:Also printed as
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1747:
1727:
1721:
1720:
1698:
1692:
1691:
1655:
1649:
1648:
1630:
1602:
1596:
1595:
1572:Animal Behaviour
1567:
1561:
1558:
1552:
1551:
1533:
1524:(8): 1842–1846.
1509:
1503:
1502:
1492:
1460:
1449:
1443:
1431:
1425:
1424:
1404:
1398:
1397:
1395:
1367:
1361:
1360:
1348:
1342:
1341:
1330:Rumex acetosa L.
1325:
1319:
1318:
1302:
1296:
1295:
1272:Animal Behaviour
1267:
1261:
1260:
1241:10.1038/296850a0
1216:
1210:
1209:
1165:
1159:
1158:
1138:
1132:
1131:
1120:10.1038/246015a0
1095:
1089:
1088:
1067:
1061:
1060:
1042:
1033:(8): 1500–1512.
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1012:
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1007:
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981:
973:
970:
962:
953:
952:
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944:
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865:(2023 estimates)
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861:
844:
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806:
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797:
789:
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782:
772:
762:
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723:
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663:Internet Archive
652:
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587:Groom kidnapping
582:Bride kidnapping
540:Evolution of sex
436:Melandrium album
228:Sex ratio theory
218:
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171:— ratio at birth
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1943:
1942:
1929:
1924:
1879:(4068): 90–92.
1803:10.2307/2061752
1756:
1754:Further reading
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572:Human sex ratio
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516:varied sittella
508:Southern Africa
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441:Cannabis sativa
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203:sexually active
188:adult sex ratio
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132:human sex ratio
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21:Gender equality
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1307:Rumex acetosa
1301:
1293:
1289:
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1281:
1278:(4): 745–51.
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1000:9780521818964
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480:painted snipe
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464:charadriiform
455:
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452:Cannabinaceae
449:
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427:Rumex acetosa
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351:of the genus
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106:parthenogenic
103:
99:
95:
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87:
75: No data
68:
58:
54:
45:than females.
44:
35:
30:
26:
22:
1917:
1876:
1872:
1843:(1): 27–34.
1840:
1836:
1793:
1789:
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1731:
1725:
1708:
1702:
1696:
1663:
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1323:
1314:
1310:
1306:
1300:
1275:
1271:
1265:
1224:
1220:
1214:
1176:(1): 40–50.
1173:
1169:
1163:
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1103:
1099:
1093:
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1065:
1030:
1026:
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1004:. Retrieved
977:
947:– via
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921:
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858:. Retrieved
853:CIA Factbook
851:
814:
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787:
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687:
683:
657:
632:
612:
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565:
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524:treecreepers
489:
461:
445:
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432:Polygonaceae
425:
422:
413:
386:
379:
368:
352:
347:
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323:
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286:
269:
267:
234:Eric Charnov
231:
223:
213:
202:
196:
185:
174:
164:
154:
149:
146:
129:
85:
83:
66:
52:
42:
25:
1415:: 421–452.
1374:Silene alba
1006:24 December
943:24 December
680:Hamilton WD
607:Youth bulge
528:noisy miner
520:fairy-wrens
512:philopatric
69:than males.
1958:Population
1947:Categories
1869:Willard DE
1865:Trivers RL
1790:Demography
1628:1885/54578
1442:: 363–367.
1071:Charnov EL
860:2024-04-18
770:2123/11211
625:References
482:and a few
472:phalaropes
396:parrotfish
270:allocation
94:population
1963:Selection
1953:Sex ratio
1340:: 101–14.
1317:: 465–80.
1170:Oecologia
903:Parker GA
514:like the
504:Australia
498:and wild
468:polyandry
400:clownfish
388:Dichogamy
359:arthropod
354:Wolbachia
242:dioecious
86:sex ratio
1909:29326420
1857:10590797
1828:24724998
1762:Coale AJ
1688:39566226
1680:17624961
1645:14624385
1637:18462318
1592:53165151
1540:21749544
1499:23481395
1475:: 1587.
1380:Heredity
1359:: 1–138.
1292:53185717
1206:13412222
1198:16425044
1073:(1982).
1049:24848871
779:23807025
745:: 2048.
534:See also
526:and the
363:copepods
349:Bacteria
125:termites
110:eusocial
102:sexually
1901:4682135
1881:Bibcode
1873:Science
1820:7828766
1812:2061752
1548:6954828
1477:Bibcode
1257:4307265
1249:7070524
1229:Bibcode
1178:Bibcode
1128:4224989
1108:Bibcode
1057:8350737
831:9533502
747:Bibcode
720:6021675
712:1721222
692:Bibcode
684:Science
566:Humans:
476:jacanas
392:wrasses
382:peafowl
88:is the
67:females
1907:
1899:
1855:
1826:
1818:
1810:
1778:987286
1776:
1686:
1678:
1643:
1635:
1590:
1546:
1538:
1497:
1290:
1255:
1247:
1221:Nature
1204:
1196:
1126:
1100:Nature
1081:
1055:
1047:
997:
936:
829:
777:
718:
710:
500:canids
484:plover
254:For a
247:For a
240:For a
123:, and
73:
63:
49:
39:
1905:S2CID
1853:S2CID
1824:S2CID
1808:JSTOR
1774:JSTOR
1684:S2CID
1641:S2CID
1588:S2CID
1544:S2CID
1288:S2CID
1253:S2CID
1202:S2CID
1124:S2CID
1053:S2CID
983:(PDF)
972:(PDF)
949:JSTOR
938:50614
934:JSTOR
912:(PDF)
796:(PDF)
708:JSTOR
143:Types
113:wasps
90:ratio
43:males
1897:PMID
1816:PMID
1676:PMID
1633:PMID
1536:PMID
1495:PMID
1309:)".
1245:PMID
1194:PMID
1079:ISBN
1045:PMID
1008:2022
995:ISBN
945:2022
827:PMID
811:JAMA
775:PMID
716:PMID
506:and
444:and
398:and
195:The
130:The
121:ants
117:bees
53:same
1889:doi
1877:179
1845:doi
1798:doi
1770:140
1740:doi
1713:doi
1668:doi
1623:hdl
1615:doi
1580:doi
1526:doi
1485:doi
1417:doi
1413:118
1388:doi
1280:doi
1237:doi
1225:296
1186:doi
1174:148
1151:doi
1116:doi
1104:246
1035:doi
987:doi
926:doi
922:263
885:doi
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765:hdl
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700:doi
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462:In
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434:),
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