40:
417:, 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.
388:, 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.
356:: 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.
376:
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
216:
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
149:
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
425:
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.
513:. 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
434:
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 –
497:
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.
283:
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).
1571:
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"
243:
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
145:
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
275:
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?
1846:
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".
978:
820:
Davis DL, Gottlieb MB, Stampnitzky JR (April 1998). "Reduced ratio of male to female births in several industrial countries: a sentinel health indicator?".
1943:
1581:
Doerr ED, Doerr VA (2006). "Comparative demography of treecreepers: evaluating hypotheses for the evolution and maintenance of cooperative breeding".
651:
381:
693:(April 1967). "Extraordinary sex ratios. A sex-ratio theory for sex linkage and inbreeding has new implications in cytogenetics and entomology".
612:
272:
Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
1093:
1009:
927:
565:
339:. A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio.
587:
353:
330:
314:
Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
1152:
Danforth B (1991). "Female
Foraging and Intranest Behavior of a Communal Bee, Perdita portalis (Hymenoptera: Andrenidae)".
298:
Fisher's principle (1930) explains why in most species, the sex ratio is approximately 1:1. His argument was summarised by
1714:
1281:
Pike TW, Petrie M (October 2005). "Maternal body condition and plasma hormones affect offspring sex ration in peafowl".
1741:
Arnold KE, Griffith SC, Goldizen AW (2001). "Sex-biased hatching sequences in the cooperatively breeding noisy miner".
886:
Coney NS, Mackey WC (1998). "The Woman as Final
Arbiter: A Case for the Facultative Character of the Human Sex Ratio".
311:
A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
570:
1669:
Orians GH, Milewski AV (August 2007). "Ecology of
Australia: the effects of nutrient-poor soils and intense fires".
987:
266:
1230:
Ferguson MW, Joanen T (April 1982). "Temperature of egg incubation determines sex in
Alligator mississippiensis".
150:
100 older women; across all ages, the global population is nearly balanced, with 101 males for every 100 females.
414:
323:
The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
259:
115:. However, many species deviate from an even sex ratio, either periodically or permanently. Examples include
667:
247:. He defines five major questions, both for his book and the field in general (slightly abbreviated here):
1978:
629:
602:
146:
919:"Parental Investment and the Control of Sexual Selection: Predicting the Direction of Sexual Competition"
269:, what is the equilibrium allocation of resources to male versus female function in each breeding season?
555:
208:
293:
108:
1712:
Noske RA (1986). "Intersexual niche segregation among three bark-foraging birds of eucalypt forests".
1891:
1487:
1239:
1188:
1118:
918:
757:
702:
502:
478:
1362:
Correns C (1928). "Bestimmung, Vererbung und
Verteilung des Geschlechter bei den hoheren Pflanzen".
1968:
1525:"Persistence of an extreme male-biased adult sex ratio in a natural population of polyandrous bird"
302:(1967) as follows, assuming that parents invest the same whether raising male or female offspring:
112:
67:
39:
395:, maternal body condition can influence the proportion of daughters in the range from 25% to 87%.
1973:
1963:
1915:
1863:
1834:
1818:
1784:
1694:
1651:
1598:
1554:
1316:
Correns C (1922). "Geschlechtsbestimmung und
Zahlenverhaltnis der Geschlechter beim Sauerampfer (
1298:
1263:
1212:
1134:
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726:
529:, adult sex ratios are equally or more male-biased than in those cooperative species, such as
457:
317:
Therefore the genes for male-producing tendencies spread, and male births become more common.
1899:
1882:(January 1973). "Natural selection of parental ability to vary the sex ratio of offspring".
1855:
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1723:
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997:
936:
895:
829:
775:
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673:
597:
592:
550:
320:
As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
744:
Kobayashi K, Hasegawa E, Yamamoto Y, Kawatsu K, Vargo EL, Yoshimura J, et al. (2013).
1772:
1616:
Kokko H, Jennions MD (July 2008). "Parental investment, sexual selection and sex ratios".
1179:
Kiørboe T (May 2006). "Sex, sex-ratios, and the dynamics of pelagic copepod populations".
803:
582:
526:
518:
474:
451:
142:
31:
858:
1895:
1491:
1243:
1192:
1122:
761:
706:
1875:
1727:
690:
560:
299:
1754:
1431:
809:. U.S. Department of Health and Human Services, National Center for Health Statistics.
158:
In most species, the sex ratio varies according to the age profile of the population.
1957:
1682:
1629:
1541:
1524:
607:
506:
490:
462:
437:
116:
1919:
1867:
1838:
1698:
1655:
1602:
1302:
1216:
384:, where incubation temperature of eggs determines the sex of the individual. In the
30:"Gender balance" redirects here. For gender balance as a socio-political issue, see
1558:
1267:
1138:
1081:
1067:
913:
481:
and sex-role reversal (where males care and females compete for mates) as found in
442:
244:
1594:
1294:
111:, for evolutionary reasons this is typically about 1:1 in species which reproduce
1001:
714:
1903:
1879:
617:
538:
534:
120:
235:
These definitions can be somewhat subjective since they lack clear boundaries.
203:(ASR), which is defined as the ratio of adult males to females in a population.
1200:
899:
522:
406:
104:
17:
833:
352:
Spending equal amounts of resources to produce offspring of either sex is an
1948:
1944:
CIA listing of sex ratios for individual countries (including age divisions)
1165:
514:
486:
482:
410:
398:
369:
364:
359:
252:
1690:
1647:
1550:
1509:
1208:
1059:
940:
789:
255:
species, what is the equilibrium sex ratio maintained by natural selection?
1911:
1830:
1775:, Banister J (December 1996). "Five decades of missing females in China".
1418:
Riede W (January 1925). "Beiträge zum
Geschlechts-und Anpassungsproblem".
1259:
841:
730:
401:(sequential hermaphroditism) is normal in several groups of fish, such as
373:
1403:
1382:
44:
1859:
1822:
1500:
1475:
1034:"Adult sex ratio variation: implications for breeding system evolution"
770:
745:
722:
530:
510:
402:
392:
333:(ESS). This ratio has been observed in many species, including the bee
135:
1788:
1638:
1050:
1033:
780:
372:
species as they kill males. Sex-ratio of adult populations of pelagic
1251:
1130:
948:
521:. It is also known that in cooperative breeders where both sexes are
494:
1813:
1796:
501:
Male-biased adult sex ratios have also been shown to correlate with
1476:"The evolution of sex roles in birds is related to adult sex ratio"
1109:
Maynard Smith J, Price GR (1973). "The logic of animal conflict".
959:
100:
746:"Sex ratio biases in termites provide evidence for kin selection"
413:. This can cause a discrepancy in the sex ratios as well. In the
377:
only one mating, sex ratios are strongly skewed towards females.
123:
804:"Trend Analysis of the sex Ratio at Birth in the United States"
1445:
131:
127:
66:
number of males and females (accounting that the ratio has 3
391:
In birds, mothers can influence the sex of their chicks. In
119:
species, periodically mating organisms such as aphids, some
1523:
Kosztolányi A, Barta Z, Küpper C, Székely T (August 2011).
996:. Cambridge: Cambridge University Press. pp. 366–382.
262:, what is the equilibrium sex order and time of sex change?
1671:
Biological
Reviews of the Cambridge Philosophical Society
1339:
Rychlewski JE, Zarzycki K (1975). "Sex ratio in seeds of
430:
Dioecious plants secondary sex ratio and amount of pollen
672:. Oxford: Clarendon Press. pp. 141–143 – via
928:
Proceedings of the Royal
Society B: Biological Sciences
867:. The Central Intelligence Agency of the United States
279:
Biological research mostly concerns itself with sex
986:
977:
917:
469:Polyandrous and cooperatively breeding homeotherms
1777:Proceedings of the American Philosophical Society
1032:Székely T, Weissing FJ, Komdeur J (August 2014).
1343:as a result of sparse or abundant pollination".
308:Suppose male births are less common than female.
161:It is generally divided into four subdivisions:
979:"Operational Sex Ratios and Mating Competition"
43:Map indicating the human sex ratio by country.
1154:Annals of the Entomological Society of America
477:birds, recent research has shown clearly that
8:
1469:
1467:
1465:
195:The tertiary sex ratio is equivalent to the
971:
969:
1931:. Association of Professional Geographers.
1797:"Five decades of missing females in China"
1474:Liker A, Freckleton RP, Székely T (2013).
685:
683:
217:values may not always be readily apparent.
1812:
1637:
1540:
1499:
1402:
1088:. Princeton: Princeton University Press.
1049:
989:Sex Ratios: Concepts and Research Methods
779:
769:
669:The Genetical Theory of Natural Selection
329:In modern language, the 1:1 ratio is the
27:Ratio of males to females in a population
1420:Flora oder Allgemeine Botanische Zeitung
661:
659:
38:
653:. Map compiled in 2021, data from 2020.
640:
382:temperature-dependent sex determination
613:Sex-selective abortion and infanticide
231:— ratio in post-reproductive organisms
192:— ratio in sexually mature organisms
7:
1795:Coale AJ, Banister J (August 1994).
1455:(cit.: Plant Breeding Abstr., 1934,
853:
851:
348:Environmental and individual control
70:, i.e., 1.00 males to 1.00 females).
1728:10.1111/j.1442-9993.1986.tb01396.x
25:
1929:Techniques for Sex Ratio Analysis
1755:10.1111/j.0908-8857.2001.320303.x
380:Several species of reptiles have
1683:10.1111/j.1469-185x.2007.00017.x
1630:10.1111/j.1420-9101.2008.01540.x
1542:10.1111/j.1420-9101.2011.02305.x
368:cause skewed sex ratios in some
1618:Journal of Evolutionary Biology
1529:Journal of Evolutionary Biology
1038:Journal of Evolutionary Biology
541:where females always disperse.
1451:8th Congr. Jap. Ass. Adv. Sci.
976:Kvarnemo C, Ahnesjö I (2002).
588:List of countries by sex ratio
354:evolutionarily stable strategy
331:evolutionarily stable strategy
1:
1715:Australian Journal of Ecology
1595:10.1016/j.anbehav.2005.10.017
1432:10.1016/S0367-1615(17)32904-X
1295:10.1016/j.anbehav.2004.12.020
343:Examples in non-human species
1949:A review of sex ratio theory
1002:10.1017/CBO9780511542053.019
715:10.1126/science.156.3774.477
1927:Rath RM, Mishra AK (2005).
1904:10.1126/science.179.4068.90
1459:, № 3, p. 248, ref. № 768).
571:XY sex-determination system
1995:
566:Trivers–Willard hypothesis
291:
267:simultaneous hermaphrodite
76: Countries with more
52: Countries with more
47:
29:
1322:Biologisches Zentralblatt
1201:10.1007/s00442-005-0346-3
900:10.1080/00224499809551930
859:"Field Listing—Sex ratio"
415:bluestreak cleaner wrasse
287:
103:of males to females in a
62: Countries with the
1743:Journal of Avian Biology
834:10.1001/jama.279.13.1018
260:sequential hermaphrodite
172:— ratio at fertilization
888:Journal of Sex Research
1383:"Optimal sex ratio in
1345:Acta Biol Crac Ser Bot
941:10.1098/rspb.1996.0048
630:Gender Balance Council
603:Demographic transition
212:(OSR) is the ratio of
147:sex-selective abortion
92:
1480:Nature Communications
1364:Handb. Vererbungswiss
1166:10.1093/aesa/84.5.537
985:. In Hardy IC (ed.).
916:, Simmons LW (1996).
750:Nature Communications
556:Operational sex ratio
209:operational sex ratio
42:
503:cooperative breeding
421:Domesticated animals
227:quaternary sex ratio
1896:1973Sci...179...90T
1848:Journal of Ethology
1492:2013NatCo...4.1587L
1449:. Sieb. et. Zuce".
1404:10.1038/hdy.1967.50
1381:Mulcahy DL (1967).
1244:1982Natur.296..850F
1193:2006Oecol.148...40K
1123:1973Natur.246...15S
762:2013NatCo...4.2048K
707:1967Sci...156..477H
505:in mammals such as
449:(Caryophyllaceae),
178:secondary sex ratio
68:significant figures
1860:10.1007/BF02350089
1501:10.1038/ncomms2600
771:10.1038/ncomms3048
666:Fisher RA (1930).
649:CIA World Factbook
386:American alligator
336:Macrotera portalis
294:Fisher's principle
288:Fisher's principle
188:tertiary sex ratio
109:Fisher's principle
107:. As explained by
93:
1447:Humulus japonicus
1238:(5860): 850–853.
1051:10.1111/jeb.12415
935:(1368): 315–321.
828:(13): 1018–1023.
701:(3774): 477–488.
458:Humulus japonicus
168:primary sex ratio
91:
90:
16:(Redirected from
1986:
1932:
1923:
1871:
1842:
1816:
1793:Also printed as
1792:
1759:
1758:
1738:
1732:
1731:
1709:
1703:
1702:
1666:
1660:
1659:
1641:
1613:
1607:
1606:
1583:Animal Behaviour
1578:
1572:
1569:
1563:
1562:
1544:
1535:(8): 1842–1846.
1520:
1514:
1513:
1503:
1471:
1460:
1454:
1442:
1436:
1435:
1415:
1409:
1408:
1406:
1378:
1372:
1371:
1359:
1353:
1352:
1341:Rumex acetosa L.
1336:
1330:
1329:
1313:
1307:
1306:
1283:Animal Behaviour
1278:
1272:
1271:
1252:10.1038/296850a0
1227:
1221:
1220:
1176:
1170:
1169:
1149:
1143:
1142:
1131:10.1038/246015a0
1106:
1100:
1099:
1078:
1072:
1071:
1053:
1044:(8): 1500–1512.
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1018:
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992:
984:
981:
973:
964:
963:
957:
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924:
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904:
903:
883:
877:
876:(2023 estimates)
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846:
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674:Internet Archive
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598:Groom kidnapping
593:Bride kidnapping
551:Evolution of sex
447:Melandrium album
239:Sex ratio theory
229:
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182:— ratio at birth
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85:
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51:
45:
21:
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1983:
1954:
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1940:
1935:
1926:
1890:(4068): 90–92.
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1845:
1814:10.2307/2061752
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1765:Further reading
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583:Human sex ratio
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527:varied sittella
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452:Cannabis sativa
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1327:
1323:
1319:
1318:Rumex acetosa
1312:
1309:
1304:
1300:
1296:
1292:
1289:(4): 745–51.
1288:
1284:
1277:
1274:
1269:
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1261:
1257:
1253:
1249:
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1132:
1128:
1124:
1120:
1116:
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1105:
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1097:
1095:9780691083124
1091:
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1069:
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1013:
1011:9780521818964
1007:
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972:
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624:Institutions:
619:
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491:painted snipe
488:
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475:charadriiform
468:
466:
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463:Cannabinaceae
460:
459:
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448:
444:
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438:Rumex acetosa
429:
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362:of the genus
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139:
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129:
125:
122:
118:
117:parthenogenic
114:
110:
106:
102:
98:
86: No data
79:
69:
65:
56:than females.
55:
46:
41:
37:
33:
19:
1928:
1887:
1883:
1854:(1): 27–34.
1851:
1847:
1804:
1800:
1780:
1776:
1746:
1742:
1736:
1719:
1713:
1707:
1674:
1670:
1664:
1621:
1617:
1611:
1586:
1582:
1576:
1567:
1532:
1528:
1518:
1483:
1479:
1456:
1450:
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1413:
1394:
1390:
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1367:
1363:
1357:
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1340:
1334:
1325:
1321:
1317:
1311:
1286:
1282:
1276:
1235:
1231:
1225:
1187:(1): 40–50.
1184:
1180:
1174:
1157:
1153:
1147:
1114:
1110:
1104:
1085:
1076:
1041:
1037:
1027:
1015:. Retrieved
988:
958:– via
952:. Retrieved
932:
926:
908:
891:
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881:
869:. Retrieved
864:CIA Factbook
862:
825:
821:
815:
798:
753:
749:
739:
698:
694:
668:
643:
623:
622:
576:
575:
535:treecreepers
500:
472:
456:
450:
446:
443:Polygonaceae
436:
433:
424:
397:
390:
379:
363:
358:
351:
334:
328:
297:
280:
278:
245:Eric Charnov
242:
234:
224:
213:
207:
196:
185:
175:
165:
160:
157:
140:
96:
94:
77:
63:
53:
36:
1426:: 421–452.
1385:Silene alba
1017:24 December
954:24 December
691:Hamilton WD
618:Youth bulge
539:noisy miner
531:fairy-wrens
523:philopatric
80:than males.
1969:Population
1958:Categories
1880:Willard DE
1876:Trivers RL
1801:Demography
1639:1885/54578
1453:: 363–367.
1082:Charnov EL
871:2024-04-18
781:2123/11211
636:References
493:and a few
483:phalaropes
407:parrotfish
281:allocation
105:population
1974:Selection
1964:Sex ratio
1351:: 101–14.
1328:: 465–80.
1181:Oecologia
914:Parker GA
525:like the
515:Australia
509:and wild
479:polyandry
411:clownfish
399:Dichogamy
370:arthropod
365:Wolbachia
253:dioecious
97:sex ratio
1920:29326420
1868:10590797
1839:24724998
1773:Coale AJ
1699:39566226
1691:17624961
1656:14624385
1648:18462318
1603:53165151
1551:21749544
1510:23481395
1486:: 1587.
1391:Heredity
1370:: 1–138.
1303:53185717
1217:13412222
1209:16425044
1084:(1982).
1060:24848871
790:23807025
756:: 2048.
545:See also
537:and the
374:copepods
360:Bacteria
136:termites
121:eusocial
113:sexually
1912:4682135
1892:Bibcode
1884:Science
1831:7828766
1823:2061752
1559:6954828
1488:Bibcode
1268:4307265
1260:7070524
1240:Bibcode
1189:Bibcode
1139:4224989
1119:Bibcode
1068:8350737
842:9533502
758:Bibcode
731:6021675
723:1721222
703:Bibcode
695:Science
577:Humans:
487:jacanas
403:wrasses
393:peafowl
99:is the
78:females
1918:
1910:
1866:
1837:
1829:
1821:
1789:987286
1787:
1697:
1689:
1654:
1646:
1601:
1557:
1549:
1508:
1301:
1266:
1258:
1232:Nature
1215:
1207:
1137:
1111:Nature
1092:
1066:
1058:
1008:
947:
840:
788:
729:
721:
511:canids
495:plover
265:For a
258:For a
251:For a
134:, and
84:
74:
60:
50:
1916:S2CID
1864:S2CID
1835:S2CID
1819:JSTOR
1785:JSTOR
1695:S2CID
1652:S2CID
1599:S2CID
1555:S2CID
1299:S2CID
1264:S2CID
1213:S2CID
1135:S2CID
1064:S2CID
994:(PDF)
983:(PDF)
960:JSTOR
949:50614
945:JSTOR
923:(PDF)
807:(PDF)
719:JSTOR
154:Types
124:wasps
101:ratio
54:males
1908:PMID
1827:PMID
1687:PMID
1644:PMID
1547:PMID
1506:PMID
1320:)".
1256:PMID
1205:PMID
1090:ISBN
1056:PMID
1019:2022
1006:ISBN
956:2022
838:PMID
822:JAMA
786:PMID
727:PMID
517:and
455:and
409:and
206:The
141:The
132:ants
128:bees
64:same
1900:doi
1888:179
1856:doi
1809:doi
1781:140
1751:doi
1724:doi
1679:doi
1634:hdl
1626:doi
1591:doi
1537:doi
1496:doi
1428:doi
1424:118
1399:doi
1291:doi
1248:doi
1236:296
1197:doi
1185:148
1162:doi
1127:doi
1115:246
1046:doi
998:doi
937:doi
933:263
896:doi
830:doi
826:279
776:hdl
766:doi
711:doi
699:156
473:In
465:).
445:),
1960::
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