Sex ratio

Last updated

Map indicating the human sex ratio by country. .mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{} Countries with more males than females. Countries with the same number of males and females (accounting that the ratio has 3 significant figures, i.e., 1.00 males to 1.00 females). Countries with more females than males. No data Sex ratio total population 2020.svg
Map indicating the human sex ratio by country.
  Countries with more males than females.
  Countries with the same number of males and females (accounting that the ratio has 3 significant figures, i.e., 1.00 males to 1.00 females).
  Countries with more females than males.
  No data

A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. [2] [3] However, many species deviate from an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites. [4]

Contents

The human sex ratio 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 [5] and by sex-selective abortion and infanticide. Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well. [6] As of 2024, the global sex ratio at birth is estimated at 107 boys to 100 girls (1,000 boys per 934 girls). [7] By old age, the sex ratio reverses, with 81 older men for every 100 older women; across all ages, the global population is nearly balanced, with 101 males for every 100 females. [7]

Types

In most species, the sex ratio varies according to the age profile of the population. [8]

It is generally divided into four subdivisions:

These definitions can be somewhat subjective since they lack clear boundaries.

Sex ratio theory

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 Eric Charnov. [12] He defines five major questions, both for his book and the field in general (slightly abbreviated here):

  1. For a dioecious species, what is the equilibrium sex ratio maintained by natural selection?
  2. For a sequential hermaphrodite, what is the equilibrium sex order and time of sex change?
  3. For a simultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
  4. Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
  5. 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?

Biological research mostly concerns itself with sex allocation 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).

Fisher's principle

Fisher's principle (1930) [2] explains why in most species, the sex ratio is approximately 1:1. His argument was summarised by W. D. Hamilton (1967) [3] as follows, assuming that parents invest the same whether raising male or female offspring:

  1. Suppose male births are less common than female.
  2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
  3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
  4. Therefore the genes for male-producing tendencies spread, and male births become more common.
  5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
  6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.

In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS). [13] This ratio has been observed in many species, including the bee Macrotera portalis . A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio. [14]

Examples in non-human species

Environmental and individual control

Spending equal amounts of resources to produce offspring of either sex is an evolutionarily stable strategy: 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.

Bacteria of the genus Wolbachia cause skewed sex ratios in some arthropod species as they kill males. Sex-ratio of adult populations of pelagic copepods 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 only one mating, sex ratios are strongly skewed towards females. [15]

Several species of reptiles have temperature-dependent sex determination, where incubation temperature of eggs determines the sex of the individual. In the American alligator, 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. [16]

In birds, mothers can influence the sex of their chicks. In peafowl, maternal body condition can influence the proportion of daughters in the range from 25% to 87%. [17]

Dichogamy (sequential hermaphroditism) is normal in several groups of fish, such as wrasses, parrotfish and clownfish. This can cause a discrepancy in the sex ratios as well. In the bluestreak cleaner wrasse, 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.

Domesticated animals

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.[ citation needed ]

Dioecious plants secondary sex ratio and amount of pollen

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 – Rumex acetosa (Polygonaceae), [18] [19] Melandrium album (Caryophyllaceae), [20] [21] Cannabis sativa [22] and Humulus japonicus (Cannabinaceae). [23]

Polyandrous and cooperatively breeding homeotherms

In charadriiform birds, recent research has shown clearly that polyandry and sex-role reversal (where males care and females compete for mates) as found in phalaropes, jacanas, painted snipe and a few plover species is clearly related to a strongly male-biased adult sex ratio. [24] Those species with male care and polyandry invariably have adult sex ratios with a large surplus of males, [24] which in some cases can reach as high as six males per female. [25]

Male-biased adult sex ratios have also been shown to correlate with cooperative breeding in mammals such as alpine marmots and wild canids. [26] This correlation may also apply to cooperatively breeding birds, [27] though the evidence is less clear. [24] It is known, however, that both male-biased adult sex ratios [28] and cooperative breeding tend to evolve where caring for offspring is extremely difficult due to low secondary productivity, as in Australia [29] and Southern Africa. It is also known that in cooperative breeders where both sexes are philopatric like the varied sittella, [30] adult sex ratios are equally or more male-biased than in those cooperative species, such as fairy-wrens, treecreepers and the noisy miner [31] where females always disperse.

See also

Humans:

Institutions:

Related Research Articles

<span class="mw-page-title-main">Sexual selection</span> Mode of natural selection involving the choosing of and competition for mates

Sexual selection is a mechanism of evolution in which members of one biological sex choose mates of the other sex to mate with, and compete with members of the same sex for access to members of the opposite sex. These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.

<span class="mw-page-title-main">Sexual dimorphism</span> Evolved difference in sex-specific characteristics

Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consist of most animals and some plants. Differences may include secondary sex characteristics, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection. These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.

<span class="mw-page-title-main">Behavioral ecology</span> Study of the evolutionary basis for animal behavior due to ecological pressures

Behavioral ecology, also spelled behavioural ecology, is the study of the evolutionary basis for animal behavior due to ecological pressures. Behavioral ecology emerged from ethology after Niko Tinbergen outlined four questions to address when studying animal behaviors: What are the proximate causes, ontogeny, survival value, and phylogeny of a behavior?

<span class="mw-page-title-main">Reproductive success</span> Passing of genes on to the next generation in a way that they too can pass on those genes

Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves.

<span class="mw-page-title-main">Parental investment</span> Parental expenditure (e.g. time, energy, resources) that benefits offspring

Parental investment, in evolutionary biology and evolutionary psychology, is any parental expenditure that benefits offspring. Parental investment may be performed by both males and females, females alone or males alone. Care can be provided at any stage of the offspring's life, from pre-natal to post-natal.

Philopatry is the tendency of an organism to stay in or habitually return to a particular area. The causes of philopatry are numerous, but natal philopatry, where animals return to their birthplace to breed, may be the most common. The term derives from the Greek roots philo, "liking, loving" and patra, "fatherland", although in recent years the term has been applied to more than just the animal's birthplace. Recent usage refers to animals returning to the same area to breed despite not being born there, and migratory species that demonstrate site fidelity: reusing stopovers, staging points, and wintering grounds.

In evolutionary biology and evolutionary psychology, the Trivers–Willard hypothesis, formally proposed by Robert Trivers and Dan Willard in 1973, suggests that female mammals adjust the sex ratio of offspring in response to maternal condition, so as to maximize their reproductive success (fitness). For example, it may predict greater parental investment in males by parents in "good conditions" and greater investment in females by parents in "poor conditions". The reasoning for this prediction is as follows: Assume that parents have information on the sex of their offspring and can influence their survival differentially. While selection pressures exist to maintain a 1:1 sex ratio, evolution will favor local deviations from this if one sex has a likely greater reproductive payoff than is usual.

Monogamous pairing in animals refers to the natural history of mating systems in which species pair bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.

Sex allocation is the allocation of resources to male versus female reproduction in sexual species. Sex allocation theory tries to explain why many species produce equal number of males and females.

Cooperative breeding is a social system characterized by alloparental care: offspring receive care not only from their parents, but also from additional group members, often called helpers. Cooperative breeding encompasses a wide variety of group structures, from a breeding pair with helpers that are offspring from a previous season, to groups with multiple breeding males and females (polygynandry) and helpers that are the adult offspring of some but not all of the breeders in the group, to groups in which helpers sometimes achieve co-breeding status by producing their own offspring as part of the group's brood. Cooperative breeding occurs across taxonomic groups including birds, mammals, fish, and insects.

<span class="mw-page-title-main">Sexual conflict</span> Term in evolutionary biology

Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between males and females. In one example, males may benefit from multiple matings, while multiple matings may harm or endanger females due to the anatomical differences of that species. Sexual conflict underlies the evolutionary distinction between male and female.

<span class="mw-page-title-main">Mate choice</span> Mechanism for evolution

Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior. In other words, before an animal engages with a potential mate, they first evaluate various aspects of that mate which are indicative of quality—such as the resources or phenotypes they have—and evaluate whether or not those particular trait(s) are somehow beneficial to them. The evaluation will then incur a response of some sort.

<span class="mw-page-title-main">Parental care</span> Behavior in animals of taking care of offspring

Parental care is a behavioural and evolutionary strategy adopted by some animals, involving a parental investment being made to the evolutionary fitness of offspring. Patterns of parental care are widespread and highly diverse across the animal kingdom. There is great variation in different animal groups in terms of how parents care for offspring, and the amount of resources invested by parents. For example, there may be considerable variation in the amount of care invested by each sex, where females may invest more in some species, males invest more in others, or investment may be shared equally. Numerous hypotheses have been proposed to describe this variation and patterns in parental care that exist between the sexes, as well as among species.

Fisher's principle is an evolutionary model that explains why the sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females. A. W. F. Edwards has remarked that it is "probably the most celebrated argument in evolutionary biology".

In the evolutionary biology of sexual reproduction, operational sex ratio (OSR) is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate, or alternatively the local ratio of fertilizable females to sexually active males at any given time. This differs from physical sex ratio which simply includes all individuals, including those that are sexually inactive or do not compete for mates.

Bateman's principle, in evolutionary biology, is that in most species, variability in reproductive success is greater in males than in females. It was first proposed by Angus John Bateman (1919–1996), an English geneticist. Bateman suggested that, since males are capable of producing millions of sperm cells with little effort, while females invest much higher levels of energy in order to nurture a relatively small number of eggs, the female plays a significantly larger role in their offspring's reproductive success. Bateman's paradigm thus views females as the limiting factor of parental investment, over which males will compete in order to copulate successfully.

<span class="mw-page-title-main">Evolution of eusociality</span> Origins of cooperative brood care

Eusociality evolved repeatedly in different orders of animals, notably termites and the Hymenoptera. This 'true sociality' in animals, in which sterile individuals work to further the reproductive success of others, is found in termites, ambrosia beetles, gall-dwelling aphids, thrips, marine sponge-dwelling shrimp, naked mole-rats, and many genera in the insect order Hymenoptera. The fact that eusociality has evolved so often in the Hymenoptera, but remains rare throughout the rest of the animal kingdom, has made its evolution a topic of debate among evolutionary biologists. Eusocial organisms at first appear to behave in stark contrast with simple interpretations of Darwinian evolution: passing on one's genes to the next generation, or fitness, is a central idea in evolutionary biology.

A biological ornament is a characteristic of an animal that appears to serve a decorative function rather than a utilitarian function. Many are secondary sexual characteristics, and others appear on young birds during the period when they are dependent on being fed by their parents. Ornaments are used in displays to attract mates, which may lead to the evolutionary process known as sexual selection. An animal may shake, lengthen, or spread out its ornament in order to get the attention of the opposite sex, which will in turn choose the most attractive one with which to mate. Ornaments are most often observed in males, and choosing an extravagantly ornamented male benefits females as the genes that produce the ornament will be passed on to her offspring, increasing their own reproductive fitness. As Ronald Fisher noted, the male offspring will inherit the ornament while the female offspring will inherit the preference for said ornament, which can lead to a positive feedback loop known as a Fisherian runaway. These structures serve as cues to animal sexual behaviour, that is, they are sensory signals that affect mating responses. Therefore, ornamental traits are often selected by mate choice.

Inbreeding avoidance, or the inbreeding avoidance hypothesis, is a concept in evolutionary biology that refers to the prevention of the deleterious effects of inbreeding. Animals only rarely exhibit inbreeding avoidance. The inbreeding avoidance hypothesis posits that certain mechanisms develop within a species, or within a given population of a species, as a result of assortative mating and natural and sexual selection, in order to prevent breeding among related individuals. Although inbreeding may impose certain evolutionary costs, inbreeding avoidance, which limits the number of potential mates for a given individual, can inflict opportunity costs. Therefore, a balance exists between inbreeding and inbreeding avoidance. This balance determines whether inbreeding mechanisms develop and the specific nature of such mechanisms.

<span class="mw-page-title-main">Polyandry in animals</span> Class of mating system in non-human species

In behavioral ecology, polyandry is a class of mating system where one female mates with several males in a breeding season. Polyandry is often compared to the polygyny system based on the cost and benefits incurred by members of each sex. Polygyny is where one male mates with several females in a breeding season . A common example of polyandrous mating can be found in the field cricket of the invertebrate order Orthoptera. Polyandrous behavior is also prominent in many other insect species, including the red flour beetle, the adzuki bean weevil, and the species of spider Stegodyphus lineatus. Polyandry also occurs in some primates such as marmosets, mammal groups, the marsupial genus' Antechinus and bandicoots, around 1% of all bird species, such as jacanas and dunnocks, insects such as honeybees, and fish such as pipefish.

References

  1. ↑ Data from the CIA World Factbook . Map compiled in 2021, data from 2020.
  2. 1 2 Fisher RA (1930). The Genetical Theory of Natural Selection. Oxford: Clarendon Press. pp. 141–143 – via Internet Archive.
  3. 1 2 Hamilton WD (April 1967). "Extraordinary sex ratios. A sex-ratio theory for sex linkage and inbreeding has new implications in cytogenetics and entomology". Science. 156 (3774): 477–488. Bibcode:1967Sci...156..477H. doi:10.1126/science.156.3774.477. JSTOR   1721222. PMID   6021675.
  4. ↑ Kobayashi K, Hasegawa E, Yamamoto Y, Kawatsu K, Vargo EL, Yoshimura J, et al. (2013). "Sex ratio biases in termites provide evidence for kin selection". Nature Communications. 4: 2048. Bibcode:2013NatCo...4.2048K. doi: 10.1038/ncomms3048 . hdl: 2123/11211 . PMID   23807025.
  5. ↑ "Trend Analysis of the sex Ratio at Birth in the United States" (PDF). U.S. Department of Health and Human Services, National Center for Health Statistics.
  6. ↑ Davis DL, Gottlieb MB, Stampnitzky JR (April 1998). "Reduced ratio of male to female births in several industrial countries: a sentinel health indicator?". JAMA. 279 (13): 1018–1023. doi:10.1001/jama.279.13.1018. PMID   9533502.
  7. 1 2 "Field Listing—Sex ratio". CIA Factbook . The Central Intelligence Agency of the United States. Retrieved 2024-04-18. (2023 estimates)
  8. ↑ Coney NS, Mackey WC (1998). "The Woman as Final Arbiter: A Case for the Facultative Character of the Human Sex Ratio". Journal of Sex Research. 35 (2): 169–175. doi:10.1080/00224499809551930.
  9. ↑ Parker GA, Simmons LW (1996). "Parental Investment and the Control of Sexual Selection: Predicting the Direction of Sexual Competition" (PDF). Proceedings of the Royal Society B: Biological Sciences . 263 (1368): 315–321. doi:10.1098/rspb.1996.0048. JSTOR   50614 . Retrieved 24 December 2022.
  10. 1 2 Kvarnemo C, Ahnesjö I (2002). "Operational Sex Ratios and Mating Competition" (PDF). In Hardy IC (ed.). Sex Ratios: Concepts and Research Methods (PDF). Cambridge: Cambridge University Press. pp. 366–382. doi:10.1017/CBO9780511542053.019. ISBN   9780521818964 . Retrieved 24 December 2022.
  11. ↑ Székely T, Weissing FJ, Komdeur J (August 2014). "Adult sex ratio variation: implications for breeding system evolution". Journal of Evolutionary Biology. 27 (8): 1500–1512. doi: 10.1111/jeb.12415 . PMID   24848871. S2CID   8350737.
  12. ↑ Charnov EL (1982). Sex Allocation. Princeton: Princeton University Press. ISBN   9780691083124.
  13. ↑ Maynard Smith J, Price GR (1973). "The logic of animal conflict". Nature. 246 (5427): 15–8. Bibcode:1973Natur.246...15S. doi:10.1038/246015a0. S2CID   4224989.
  14. ↑ Danforth B (1991). "Female Foraging and Intranest Behavior of a Communal Bee, Perdita portalis (Hymenoptera: Andrenidae)". Annals of the Entomological Society of America. 84 (5): 537–548. doi:10.1093/aesa/84.5.537.
  15. ↑ Kiørboe T (May 2006). "Sex, sex-ratios, and the dynamics of pelagic copepod populations". Oecologia. 148 (1): 40–50. Bibcode:2006Oecol.148...40K. doi:10.1007/s00442-005-0346-3. PMID   16425044. S2CID   13412222.
  16. ↑ Ferguson MW, Joanen T (April 1982). "Temperature of egg incubation determines sex in Alligator mississippiensis". Nature. 296 (5860): 850–853. Bibcode:1982Natur.296..850F. doi:10.1038/296850a0. PMID   7070524. S2CID   4307265.
  17. ↑ Pike TW, Petrie M (October 2005). "Maternal body condition and plasma hormones affect offspring sex ration in peafowl". Animal Behaviour. 70 (4): 745–51. doi:10.1016/j.anbehav.2004.12.020. S2CID   53185717.
  18. ↑ Correns C (1922). "Geschlechtsbestimmung und Zahlenverhaltnis der Geschlechter beim Sauerampfer (Rumex acetosa)". Biologisches Zentralblatt. 42: 465–80.
  19. ↑ Rychlewski JE, Zarzycki K (1975). "Sex ratio in seeds of Rumex acetosa L. as a result of sparse or abundant pollination". Acta Biol Crac Ser Bot. 18: 101–14.
  20. ↑ Correns C (1928). "Bestimmung, Vererbung und Verteilung des Geschlechter bei den hoheren Pflanzen". Handb. Vererbungswiss. 2: 1–138.
  21. ↑ Mulcahy DL (1967). "Optimal sex ratio in Silene alba". Heredity. 22 (3): 411–423. doi: 10.1038/hdy.1967.50 .
  22. ↑ Riede W (January 1925). "Beiträge zum Geschlechts-und Anpassungsproblem". Flora oder Allgemeine Botanische Zeitung. 118: 421–452. doi:10.1016/S0367-1615(17)32904-X.
  23. ↑ Kihara H, Hirayoshi J (1932). "Die Geschlechtschromosomen von Humulus japonicus. Sieb. et. Zuce". 8th Congr. Jap. Ass. Adv. Sci.: 363–367. (cit.: Plant Breeding Abstr., 1934, 5, № 3, p. 248, ref. № 768).
  24. 1 2 3 Liker A, Freckleton RP, Székely T (2013). "The evolution of sex roles in birds is related to adult sex ratio". Nature Communications. 4: 1587. Bibcode:2013NatCo...4.1587L. doi: 10.1038/ncomms2600 . PMID   23481395.
  25. ↑ Kosztolányi A, Barta Z, Küpper C, Székely T (August 2011). "Persistence of an extreme male-biased adult sex ratio in a natural population of polyandrous bird". Journal of Evolutionary Biology. 24 (8): 1842–1846. doi: 10.1111/j.1420-9101.2011.02305.x . PMID   21749544. S2CID   6954828.
  26. ↑ 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"
  27. ↑ Doerr ED, Doerr VA (2006). "Comparative demography of treecreepers: evaluating hypotheses for the evolution and maintenance of cooperative breeding". Animal Behaviour. 72 (1): 147–159. doi:10.1016/j.anbehav.2005.10.017. S2CID   53165151.
  28. ↑ Kokko H, Jennions MD (July 2008). "Parental investment, sexual selection and sex ratios". Journal of Evolutionary Biology. 21 (4): 919–948. doi:10.1111/j.1420-9101.2008.01540.x. hdl: 1885/54578 . PMID   18462318. S2CID   14624385.
  29. ↑ Orians GH, Milewski AV (August 2007). "Ecology of Australia: the effects of nutrient-poor soils and intense fires". Biological Reviews of the Cambridge Philosophical Society. 82 (3): 393–423. doi:10.1111/j.1469-185x.2007.00017.x. PMID   17624961. S2CID   39566226.
  30. ↑ Noske RA (1986). "Intersexual niche segregation among three bark-foraging birds of eucalypt forests". Australian Journal of Ecology . 11 (3): 255–267. doi:10.1111/j.1442-9993.1986.tb01396.x.
  31. ↑ Arnold KE, Griffith SC, Goldizen AW (2001). "Sex-biased hatching sequences in the cooperatively breeding noisy miner". Journal of Avian Biology. 32 (3): 219–223. doi:10.1111/j.0908-8857.2001.320303.x.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.