Why is the human sex ratio 1:1 at birth?

Having found a dramatic shift in sex ratio at birth in one species of mammal, I have for decades kept an eye on discussions as to whether the human sex ratio at birth differs in some circumstances from 1 male: 1 female. Mammals in theory have two ways of controlling the sex ratio at birth: (i) at conception, or (ii) later by selectively reducing the number of young in utero. We found the latter mechanism at work in the guinea-pig. Since human litter size usually equals the  number of eggs and rarely exceeds 1 and since gestation is relatively long, it always appeared that if there were to be any maternal or paternal genetic control of the sex of the offspring it would have to be at the time of conception, rather than by selective death and reabsorption of embryo or fetus.

In the 1980s I once did the experiment of asking biologists from different disciplines why they thought the human sex ratio is 1:1. The reproductive biologists replied that it was just the result of random segregation of the sex chromosomes. The sex of a human offspring depends on whether it inherits and X or a Y chromosome from its father. Random segregation will, on average, result in a 1:1 ratio. By contrast, the evolutionary biologists said that the sex ratio is explained by Sir Ronald Fisher’s Principle: with the sex of a subject to genetic variation, the the sex ratio will always stabilise at 1:1. I will not repeat the simple explanation that can be found here.

Shifts of the of sex ratio at birth in various animals has been explained in terms of the Trivers-Willard hypothesis: parents which possess a heritable trait that benefits the lifetime reproductive success of one sex will bias the sex ratio towards that sex. There have been various claims in studies of human populations along these lines; for example, male-biased sex ratios in taller, wealthier, high status parents with the offspring likely to be more successful in competition for a mate. However, such claims have been controversial because of the statistical methods used and the results have often not borne out when larger samples were taken from the population.

Both the Fisher Principle and Trivers-Willard rely on there being genetic variation in the sex ratio, in other words that a bias towards one sex or the other is heritable. Thus a key test is to look for heritability in a very large human population. On standard scale of 0 to 1, ‘0’ denotes that a trait is not heritable while ‘1’ all differences in a trait can be explained entirely by genetic variation. Many traits fall somewhere between those two extremes.

But what does determine human sex ratio? It would be predicted if sex ratio is a heritable trait then Fisher’s Principle would apply. By contrast, if it is not heritable then simple Mendelian segregation of the sex chromosomes would suffice as an explanation.

Over the years there have been all sorts of suggestions and claims that the tendency in a family to produce offspring completely or partially biased to one sex is hereditary, and that particular genes could be involved. However, these conclusions have been criticised because the sample sizes were small and the statistical inferences drawn were invalid.

A recent, important paper has tackled the problem by using data from the entire population born in Sweden in and after 1932. That was 3,543,243 individuals and their 4,753,269 children. The results of the analysis were clear. There was no evidence of heritability at all. The calculated heritability was 0. In other words, there was no need to invoke Fisher’s Principle since with no heritability there can be no Fisher.

The authors summed up their results:

In sum, all of our results are consistent with the simple explanation that variation in offspring sex ratio in humans is due to unbiased Mendelian segregation of sex chromosomes during spermatogenesis and unbiased fertilization. The slight excess of male births is likely to be due to a general difference in survival of male and female embryos in the womb, the reasons for which are not yet understood.

Looks like my reproductive biology colleagues were right. Pity I can’t tell most of them; they are long dead.

The question now, of course, is whether the same conclusion, that the human sex ratio at conception is simply the outcome of random segregation of the sex chromosomes, applies to other or to all mammals? And are the statistically-robust demonstrated shifts in sex ratios at birth in some species and in certain environmental conditions all the result of differential loss of embryos and fetuses in utero? I shall permit myself to guess that the answers are ‘Yes’ and ‘Yes’ even though, as with previous human studies, there have been claims that the answer to the first question is ‘No’.

Zietsch BP, Walum H, Lichtenstein P, Verweij KJH, Kuja-Halkola R. 2020 No genetic contribution to variation in human offspring sex ratio: a total population study of 4.7 million births. Proceedings of the Royal Society B 287: 20192849. http://dx.doi.org/10.1098/rspb.2019.2849

Peaker M, Taylor E. 1996. Sex ratio and litter size in the guinea-pig. Journal of Reproduction and Fertility 108, 63-67.

Ludwig Glauert. From Sheffield to the Extinct and Extant Fauna of Western Australia

Ludwig Glauert
(National Library of Australia)

Ludwig Glauert was, I discovered when in The Kimberley of Western Australia, a kingpin of biological science in Western Australia in the early years of the 20th Century. For many years he was at the Western Australian Museum and for many of those years he was the only resident scientist in the state investigating the geology, particularly the palaeontology, and zoology of Western Australia. As well as his investigations of the extinct marsupial megafauna he became the expert on the reptiles of the state.

When I first saw the name Glauert I assumed he was a museum-minded German or Swiss national who had made his way to an opening in Australia. I was wrong. Glauert was born in Sheffield. Not only was he born in Sheffield he was educated at the precursors of the University of Sheffield—Firth College and the Technical School. Ludwig Glauert, fellow Sheffield graduates, was one of us.

Ludwig Glauert’s father, Johann Ernst Luis Heinrich, had been born in Germany in 1846. There he married a German-born Englishwoman, Amanda Watkinson, in 1877. They then moved to Sheffield where Ludwig was born on 5 May 1879. In the censuses Ludwig’s father is described as a hardware merchant and exporter of steel cutlery. In 1881 he was the employer of four men and three women. It would seem that Glauert senior, who was known as Louis, was known in the city before 1979 since he was admitted as a Freemason in 1872. He became a British subject in 1893.

The very short biographies of Ludwig state that he was trained as a geologist and spent four years as a demonstrator at Firth College. If I read the history correctly geology was taught as an offshoot of mining, possibly wholly by evening classes. Glauert then worked for his father (in the 1901 census he is listed as a ‘merchant’s clerk’).

Geology at the proto-university of Sheffield was driven by somebody special: Henry Clifton Sorby FRS (1826-1908). Sorby was born in Sheffield, the grandson of a Master Cutler and descendant of the first Master Cutler. He was largely self-taught and was active in research in chemistry, metallurgy and biology as well as in geology. At the age of 31 he was elected to the Royal Society. Sorby never married and lived with his widowed mother. After her death he bought a yacht to spend the summer months on the estuaries of rivers in eastern England, making observations and collecting samples.

Ludwig was not the only scientist in the family. His brother, Hermann Glauert FRS (1892-1934) was a mathematician and head of the aerodynamics department at the Royal Aircraft Establishment, Farnborough. His sister, Elsa, was a noted mathematician at Cambridge; she could sit the examinations but not, of course, being a she, receive a degree. 

Ludwig Glauert was active in the Sheffield Naturalists’ Club, serving on its Council along with academics from the university (Alfred Denny (1860-1947), for example, the first Professor of Biology), amateur naturalists and geologists. The goings-on of the Club were reported in the local press.

Ludwig married Winifred Aimee Beresford in Sheffield in 1907. He then took a job with the geological survey of the Mines Department of Western Australia as palaeontologist based at the museum in Perth. Then, in 1910, he was appointed to the staff of the museum, as the director's scientific assistant. In 1914 he was promoted to Keeper of Geology and Ethnology. 

For six years between 1909 and 1915 he worked in the field on the Pleistocene limestone of the Margaret River Caves, investigating the remains of several species of extinct marsupials and monotremes and of eastern Australian and Tasmanian mammals whose presence in Western Australia had never been suspected.

The story of the Margaret River Caves by Lindsay Hatcher includes the following description of how Glauert became involved as well as of the species and specimens found there:

Reports of the discovery of Mammoth Cave date back to as early as 1895. Mammoth Cave was located by Surveyor Mr Marmaduke Terry in September 1900, and explored by Tim Connelly and Ned Dawson, with Ned being the first to go through the cave and discover the “back door”. Tim conducted unofficial tours through the cave until 1904 when it was officially opened as a tourist cave. He also named the cave “The Dawn of Creation” perhaps due to the expanse of light reflecting off the stream in winter or maybe because of the abundance of fossils found in the cave. 

In 1904 Edgar Robinson; superintendent of the caves and cave guide Tim Connelly were constructing a walkway roughly below the largest solution pipe some 50 metres into the cave (i.e. near the top platform). One of these gentlemen unearthed some rather odd bones. In the same year Connelly notified his good friend Colonel Le Souef of the find. At the time Le Souef had considerable standing within the scientific community as he had been responsible for establishing the Perth Zoological gardens in the 1890’s.

Le Souef in turn notified Mr Bernard Woodward – Director of the W.A. Museum. At this time no one was actively working in Palaeontology and very little work was being done in Archaeology. Bernard Woodward contacted his cousin Mr H.P. Woodward who was working in the Mines Dept., with the fledgling Geological Survey of W.A. It turned out that H.P. Woodward had on staff a young graduate, just out of university and freshly arrived from England, (the Midlands he believed) by the name of Ludwig Glauert. 

Glauert was seconded from the Mines Dept. to the Museum with the brief of Palaeontological Research in the entire South-West. During the years of 1909-1915 two sites in Mammoth Cave; the “Le Souef” and the “Glauert” sites were excavated by the W.A. Museum. Many bones of extinct animals including megafauna bones were found; the fossil material was removed and is now stored in the W.A. Museum. 

Glauert first completed the “Le Souef” dig – at the base of the old solution pipe. It was from this site that a Giant Echidna (Zaglossus hacketti), a Short-faced Kangaroo (Simosthenurus occidentalis) and a Wombat (Vombatus hacketti) were found. The almost complete wombat skeleton was found in the solution pipe, suggesting it perished in the original pipe which now lies on top of the rockpile. Glauert then moved to the north wall to what is known as the “Glauert” dig.

The material in which the bones were embedded comprised two groups; the lower series consisted of reddish coarse sand containing fragments of wood and gastropod shells in addition to the bones, with occasional bands of black loamy soil of 25mm thickness. Layers of stalagmite (flowstone) often enclosed the bones, wood fragments and bearing casts of eucalyptus leaves were not uncommon. One of these layers was completely covering the sediments, thus protecting the animal remains. The upper layer was a sandy bed which was yellowish in colour; the bones it contained were much fresher in appearance compared with the lower sediments. 

Glauert believed that the bone bearing deposit was a remnant of a mass of bone breccia which at one time partly filled the large chamber. This remnant was protected by a coating of flowstone for many years until the protection was undermined by the stream flowing through the cave and much of the material with its priceless store of animal remains was washed away and lost to science. 

Excavations produced a sizeable fossil collection, some 10,000 specimens; his total excavation amounted to 30 cubic metres of soil. Unfortunately the stratigraphical relationship was poorly documented, probably due to inadequate resources and time constraints, making any assessment of relative ages of the material extremely hard. 

The assemblage contains 34 vertebrate species, most of which are small and typical of the south-west today. Several types of large extinct animals are represented i.e. Megafauna. These include the Giant Echidna (Zaglossus hacketti), Wombat (Vombatus hacketti), Wallaby (Wallabia kitcheneri), the giant extinct diprotodontid (Zygomaturus trilobus), extinct browsing kangaroos (Simosthenurus occidentalis and Simosthenurus brownei) and the Marsupial Lion (Thylacoleo carnifex). Other groups of animals represented are those which still occur in Eastern Australia or Tasmania: the Koala (Phascolarctos cinereus), Tasmanian Devil (Sarcophilus harrisii) and Tasmanian Tiger (Thylacinus cynocephalus). The south-west corner seems to have changed very little even though the giant marsupial fauna has disappeared. 

Ludwig Glauert in the Western Australian Museum with remains of fossil marsupials from Mammoth Cave,
Margaret River
Photograph by EL Mitchell first published in the Western Mail 19 June 1914
National Library of Australia

News of his work at Margaret River reached Sheffield. The Sheffield Daily Telegraph of Friday 8 April 1910 reported the findings under the headline, ‘Proves Useful in Australia, which reminded readers: ‘The matter is of interest in this district, apart from its scientific value, on account of the excellent work done in connection with it by Mr. Ludwig Glauert who will be remembered as a Sheffield student and as a demonstrator in geology at the University of Sheffield’.

Glauert’s interests extended far beyond paleontology. He studied aboriginal culture, publishing newspaper articles under the pen-name, ‘Jay Penne’ and studied the extant fauna of Western Australia as well as the extinct.

In October 1917 Glauert enlisted in the Australian Imperial Force for service in the latter stages of the First World War as a field company engineer in the 51st Infantry Battalion. The reinforcements for this Western Australian battalion sailed for Europe on the troopship HMAT Aeneas.  51st Battalion was part of the counter-attack at Villers Bretonneux on 24/25 April 1918 during which action the Battalion lost 389 men in two days.

After the war ended he lectured to servicemen and studied Australian material in the Natural History Museum in London.  On his return to Perth and the museum in 1920, he became keeper of the biological collections. In 1927 he was appointed Curator and then, in 1954, Director.

In his spare time he acquired an arts degree from the University of Western Australia in 1928. He collected and he wrote extensively on the fauna. Building up the museum and its collections was impeded in the inter-war years by lack of money. For many years Glauert was the only scientist; he had one technician and a taxidermist. He used his private income to buy books for the museum.

Glauert’s enthusiasm encouraged interest in the general public and especially the young. He made the museum a meeting place for the professional and amateur societies and clubs with members interested in some or all aspects of natural history—mirroring in many cases the organisations that had existed in Sheffield at the turn of the century. His articles, books, lectures and broadcasts evoked interest which, in turn, brought in specimens, often caught and presented to its owner by that scourge of Australian wildlife, the domestic cat.

He continued to work on reptiles and scorpions after his retirement.

Ludwig Glauert died on 1 February 1963, aged 83.