The invisible half of biodiversity — How male-biased collections skew research and species conservation

We will probably never find out what the females of Schomburgk’s deer (Rucervus schomburgki) looked like. In historical drawings, travel reports, and museum collections, only remains of males are found—with their unique, many-branched antlers. Females likely carried none, and apparently collectors in the 19th century considered them too inconspicuous to preserve. Today, the species is extinct, and the appearance of the hinds remains a mystery forever. Yet especially for extinct animal species, museum collections form their last “habitat”. Without them, not only would the animal itself disappear, but also the knowledge of its existence.

The absence of female specimens even had farther-reaching consequences. In Thailand, the belief persisted for a long time that there were no female Schomburgk’s deer at all. Instead, it was assumed that the males would mate with female Hainan eld’s deer (Panolia siamensis), and the offspring would be assigned—depending on their appearance—either to Schomburgk’s deer or to the eld’s deer. Out of an observational gap, a separate explanation emerged.

Isn’t it the mission of natural history collections to depict the diversity of life in all its facets? Probably yes—but reality looks different. More than 95% of all animal species are invertebrates: insects, spiders, snails, worms, or corals. In museums, however, the groups that matter far less numerically in nature dominate of all things birds and mammals. And even within these already overrepresented animal classes, a sex bias becomes visible: Males are collected far more often than females. The imbalance therefore does not end with the choice of animal groups; it continues within species.

A structural imbalance

The fate of Schomburgk’s deer is not an isolated case. In natural history collections, male animals are still systematically overrepresented today. Especially among birds—but also among mammals—the picture is dominated by males.

A study by Cooper et al. (2019) analyzed more than 2.5 million bird and mammal specimens from five international museums. The result is clear: Since the late 19th century, the imbalance has hardly changed. Among birds, only around 40% of identified specimens are female; among mammals, just under 48%. In absolute numbers, that corresponds to 143,905 more male than female bird specimens and 40,468 more male mammal specimens.

The imbalance is especially pronounced in species with striking males—for example birds-of-paradise, or deer and antelopes whose magnificent feathers, antlers, or horns directed collecting interest toward males.

But the bias shows up not only in individual species, but across entire orders. In nearly all bird orders, males dominate. Only among tinamous was the share of female specimens slightly higher (50.4%). Collections are particularly male-coded among pigeons and doves (36.8% female), hummingbirds and swifts (37.2%), and passerines (38.4%). Especially for hummingbirds, this contrast is striking, because in the wild, more females than males are observed in some populations—yet in collections, males clearly predominate.

Among mammals, the picture is more nuanced. Sloths and anteaters were even female-biased at 71%, and bats were at 52.2%. The latter likely relates to collecting practice: many bat species live in sex-segregated groups outside the mating season—for instance, maternity roosts with almost exclusively females, or male-only roosts. When whole roosts were collected, one sex was inevitably represented disproportionately.

Among even-toed ungulates, by contrast, there is a strong surplus of males (only 39.7% female), even though natural populations usually contain more females. A key reason is the targeted hunting of imposing males. A clear dominance of male specimens is also visible among carnivores.

The question is no longer whether an imbalance exists, but why it has remained in place for generations.

Animal traits and their impact on collecting

Alongside social factors, biological characteristics also influenced which animals were collected. Cooper et al. (2019) show that body size, the degree of sexual dimorphism, and ornamental traits had a measurable effect on the sex ratio in collections.

• Body size:
  In birds, a pronounced size difference between the sexes tended to lead to a slightly higher share of females—possibly because females were easier to catch. In mammals, the effect was the reverse: large, imposing males were hunted intentionally, which reduced the share of females.
• Sexual dimorphism:
  The more males and females differed, the stronger the bias in favor of males—especially among even-toed ungulates and carnivores. Where, by contrast, females are larger or more conspicuous, for example in some birds of prey, they were also more often represented in collections (44.6% instead of an average of about 40%).
• Ornamental traits:
  The more striking the males, the more invisible the females. In birds with particularly colorful plumage—for example passerines or birds-of-paradise—males clearly dominate in collections. Similar patterns hold for mammals: horns, antlers, tusks, or manes made males the preferred hunting trophy, strongly reducing the share of female specimens.

Why do males dominate?

The overrepresentation of males is no accident. It is the result of historical hunting practices, aesthetic preferences, and biological differences—shaped above all by the colonial collecting practices of the 18th and 19th centuries. Expeditions searched for “spectacular” specimens for European museums—animals that appeared large, conspicuous, and impressive.

Colonial big-game hunting was strongly trophy-oriented. Animals with mighty antlers, imposing horns, manes, or particularly colorful plumage were collected. In many species, such traits are especially pronounced in males. They were considered representative of a species, while females were mentioned, documented, or preserved less often.

Even in species where both sexes carry horns, this pattern is visible. From the Kouprey (Bos sauveli)—an extremely rare or possibly extinct wild cattle species of Southeast Asia—most preserved skulls come from male animals, even though females also had horns. What mattered was not only the presence of horns, but their size, shape, and trophy value.

Similar structures can be found in African antelopes, Asian wild cattle, or deer: collections reflect the preferences of colonial big-game hunting and continue to shape our image of these species today.

Biological factors also play a role. In many species, males are more conspicuous, more territorial, and more mobile during the mating season. Courtship behavior, territory defense, and song increase their visibility—and thus the likelihood of being collected. Females, by contrast, often remain hidden during breeding and rearing of young. Collections therefore capture not only species diversity but also visibility.

Practical aspects mattered as well. Females are often more plainly colored or harder to distinguish from juveniles. In older collections, sex was also not always documented carefully. And in some regions, it was considered inappropriate to shoot females with young—which further increased the surplus of male specimens.

Natural history collections are not a neutral reflection of biological reality. They are a mirror of historical selection decisions—and for a long time, what was collected was above all what stood out. What stood out was usually the males. This imprint persists to this day and influences research, species conservation, and our understanding of species.

The male “standard” and the “other”

A historical example illustrates how deeply the male “standard” was embedded in natural history. When the ornithologist Alfred Newton received a female specimen of the Rodrigues parakeet (Psittacula exsul) in 1871, he described it as a new species. Although he could have published an illustration of the female, he wrote: “As it is unfortunately a female, I refrain from doing so.” He wanted to wait for a male specimen. Here, it is not only a specimen that is being judged, but a sex.

Because waiting took too long, Newton finally had an illustration made in 1875—based on the female. Shortly afterward, a male specimen was sent to him, one of the last of its species. Today, both a male and a female have been preserved.

Statistically, this hierarchy also shows up: Female type specimens are represented far less often. In most cases, the male was considered the reference form, while the female was treated as a deviation or an add-on. Newton’s wording makes this attitude explicit.

This male-coded perspective goes beyond collecting practice. Jack Ashby from the University Museum of Zoology in Cambridge points out that only about 8% of bird species named after people are named after women. The Rodrigues parakeet also carries, alongside its geographic name, the designation “Newton’s parakeet”—named after Alfred Newton. Even Rodrigues Island was named after a man.

This hierarchy also continued in the history of science itself. That women have always been significantly involved in natural history but rarely became visible is shown by the example of Elizabeth Gould. She is best known as the wife of the famous naturalist John Gould, but not for her more than 600 scientific illustrations that appeared in her husband’s publications. Her own contribution remained in the background for a long time.

Why are female birds less colorful than males?

When people think of birds-of-paradise, they picture shimmering colors and spectacular ornamental feathers. But this splendor is shown almost exclusively by males. What the females look like is known by only a few, because they are often much more plainly colored—and for that very reason far less present in illustrations, collections, and popular portrayals.

Of numerous bird-of-paradise forms that were first described as distinct species and later recognized as hybrids, often only a single type specimen exists—almost always a magnificently colored male. Examples include Bensbach’s, Blood’s, or Elliot’s bird-of-paradise. The females of these forms are usually entirely absent from the record.

The pronounced difference between the sexes is not a coincidence, but the result of sexual selection and differing ecological demands.

In many bird species, females invest more in breeding and rearing young. They spend longer at the nest and therefore benefit from inconspicuous camouflage coloration, which reduces the risk of detection by predators. Males, by contrast, often compete for access to females. Bright colors, courtship behavior, and song increase their chances in competition—even if they increase the risk of being detected.

The apparent “plainness” of females is therefore not a deficit form, but a distinct, evolutionarily stable strategy. Ironically, this adaptive inconspicuousness favored their underrepresentation in collections. What was a survival advantage in evolution led to systematic invisibility in collecting practice.

Pronounced sexual dimorphism—that is, clear differences between males and females—is especially common in birds. Natural history collectors therefore preferentially took the conspicuous males, which were seen as representative. Thus biological visibility amplified an already existing aesthetic and cultural bias.

Irreversible gaps: sex bias in extinct species

Sex bias becomes especially clear when it comes to extinct species. Because here, gaps can no longer be closed. What was never collected or documented is irretrievably lost.

The analysis by Cooper et al. (2019) shows that even among type specimens—the official reference specimens used to define a species—females are strongly underrepresented: only 27% of bird and 39% of mammal types are female. Females are also less often represented among paratypes, the supplementary comparison specimens. Sex-specific differences were therefore often not systematically integrated into the original description. Especially in extinct species, this distortion remains permanent.

Of Schomburgk’s deer, we know hundreds of antlers from male animals—but not a single confirmed female specimen. What the hinds looked like will likely remain unknown. The same applies to various bird-of-paradise forms that were later recognized as hybrids or whose populations disappeared: usually only male type specimens exist. The females are completely missing.

But gaps do not always run only in one direction. Of the Mauritius blue pigeon (Alectroenas nitidissima), which went extinct in the 1830s, only three skins have been preserved. They lack the bright red facial skin shown in historical illustrations. Paleontologist Julian P. Hume suggested in 2011 that the preserved specimens are likely females. Contemporary reports describe the males as “infinitely more beautiful”, but if Hume is right, not a single specimen of the male appearance remains.

In other cases, what is missing is not the sex itself, but the information about it. Of the Liverpool pigeon (Caloenas maculata), only a single specimen exists in the World Museum in Liverpool; its sex is unknown. Similar is the case for the Dieffenbach rail (Hypotaenidia dieffenbachii), which is also known from only one preserved specimen, without documented sex. Rails usually do not show pronounced sexual dimorphism, but even subtle differences—for example in size or proportions—can no longer be reconstructed without comparison.

Sometimes the female is missing, sometimes the male, sometimes the decisive information. In every case, a lasting gap in knowledge remains. For extinct species, no new fieldwork can close these gaps anymore—the museum collection is the final authority of knowledge here.

Invisible in the museum? Sex bias in exhibitions

Natural history museums and research collections were built up over centuries. For a long time, collectors, hunters, and curators decided which animals would be preserved and displayed. The rule often was: the more striking, the more representative. Accordingly, in many exhibitions, the colorful or imposing males dominate.

Female animals are missing not only in collections, but also often remain in the background in staging. When they are shown, they often appear smaller, more passive, or as an addition to the male. Such portrayals shape an image of nature that is not only biologically incomplete, but also conveys cultural role models.

In Animal Kingdom: A Natural History in 100 Objects, Jack Ashby describes this gender bias in museums on two levels: female specimens are either missing entirely or placed less prominently—for example lower on a shelf or in a more restrained posture. Ashby suspects that taxidermists unconsciously reproduced social gender stereotypes.

This imbalance was empirically documented by museum curator Rebecca Machin in an analysis of the Manchester Museum (2005/2006). At the level of exhibited individuals, in the mammal gallery 71% of the specimens were male and only 29% female. For birds, the ratio was 66 to 34%.

The distortion is even clearer at the level of species representation: 61% of mammal species were represented exclusively by males, 11% exclusively by females, and only 14% by both sexes. Among bird species, almost half were represented exclusively by males, while only 3% were shown only by females.

Staging also followed a pattern: when both sexes were presented together, in around three quarters of cases the male was positioned higher; in 82% of cases it took a more upright, more dominant posture. Accompanying texts reinforced this asymmetry: males were described as hunters, fighters, or protectors, females almost always reduced to the role of mother. Notably, the word “mother” appeared regularly in the exhibition texts, while “father” did not appear at all.

To make this sex bias visible, in 2006, as part of International Women’s Week, Machin temporarily covered numerous male specimens. Suddenly, the few female animals took center stage—and the imbalance became immediately obvious.

Museums are not only scientific archives, but also places of education. Distorted portrayals can therefore cement ideas of “male dominance in nature” in the long term. Natural history museums thus also bear a cultural responsibility: they should critically reflect on their modes of presentation.

There are, however, exceptions. In the extinct New Zealand Huia, the female was morphologically more conspicuous than the male: its long, curved beak differed clearly from the male’s shorter one. In some exhibitions, this peculiarity is taken up and the female is staged more prominently. Such cases, however, remain rare.

What are the consequences when males are overrepresented in collections?

An imbalance in natural history collections does not remain without consequences. If above all males were collected and documented, this still distorts research, species conservation strategies, and our understanding of species today.

A striking example is the golden-winged warbler (Vermivora chrysoptera). In 2019, ornithologist Ruth Bennett discovered that males and females use different habitats in winter: male birds overwinter in moist montane forests, females in lower regions that are heavily affected by deforestation. Between 2000 and 2016, females therefore lost twice as much habitat as males. For a long time, this difference went unaddressed because conservation programs did not include sex-specific habitat use.

Bennett and her team also found that up to two thirds of threatened North American migratory birds use different habitats or elevations depending on sex—but this is considered in fewer than 10% of conservation programs. Anyone who looks only at the “average species” overlooks key ecological differences.

Behavioral research, too, was long male-coded. For decades, the assumption prevailed that in birds only males sing. Riebel et al. (2019) showed that in large sound archives only 0.01 to 0.03% of recordings are explicitly labeled as female song. Today, we know that in around two thirds of songbird species, females also sing. If you analyze only male voices, you get an incomplete picture of communication, territorial behavior, and mate choice. Since 2017, the project Female Bird Song has been documenting songs of female birds specifically to close this research gap.

Biases also affect long-term morphological and genetic studies. Historical specimens are often used to analyze changes in body size or pollutant loads over decades. But because males and females differ in size, metabolism, and diet, environmental toxins are taken up, stored, or released through reproductive processes differently depending on sex. If sex is not systematically considered in such analyses, apparent trends can emerge—for example a supposed “shrinking” of a species in the course of climate change, or shifts in pollutant load. Such effects can be artifacts of an unbalanced collection stock.

The bias doesn’t end in the museum display case

The historical collection bias continues in the digital age. On platforms such as eBird, Xeno-Canto, the Macaulay Library, GBIF, or image databases, conspicuous males are likewise often overrepresented. They are photographed more often, their songs recorded more frequently, and their traits documented in greater detail. Females more often remain unidentified or unrecorded.

This distortion directly affects modern research. AI-assisted identification systems learn from available training data. If these mostly show males or contain their songs, recognition rates for females drop. Population models and habitat projections also rely on such datasets. If female records are systematically missing, a distorted picture of distribution, population size, and ecological requirements of a species emerges. The historical bias is thus unintentionally perpetuated.

Even modern genetic sexing methods reach limits. A study by Hall et al. (2025) showed that in all examined populations there were sex-reversed individuals—birds whose genetic sex did not match their anatomical traits. Such biological peculiarities do not explain the historical surplus of males in museums. They do, however, underscore how important careful, methodologically robust sex determination is in research and species conservation.

Biased research: what needs to change

Museums are considered archives of biodiversity. But they are not neutral repositories—they are historical products of their time. That males are overrepresented in many natural history collections is well documented. Researchers at the Natural History Museum and other institutions now point out themselves that this imbalance can influence scientific analyses. The debate about bias in collections is thus part of a broader discussion about data quality and scientific responsibility.

The key question is: Is this imbalance actively corrected today? According to Cooper and her team, so far only inadequately.

An unbalanced sex ratio directly affects research results—from morphological comparisons and ecological models to conservation strategies. If sex is not systematically considered, an image of the “average species” emerges that in fact depicts only one half.

First steps toward correction are emerging. These include the targeted addition of female individuals to collections, consistent documentation of sex—if necessary retroactively via DNA analysis—as well as new standards for type specimens, in which alongside a male holotype a female paratype should also be considered.

Exhibitions must change as well. Female animals are often missing there or reduced to their role as mother. As Rebecca Machin has shown, staging and accompanying texts often convey stereotypical attributions. Museums are not only scientific archives, but public places of education. They should make existing distortions transparent, revise forms of presentation, and make missing females visible instead of glossing over them.

The necessary change also affects research practice. Female bird song must be documented as systematically as male song. Capture methods should not automatically favor territorial males. And in ecological or genetic studies, sex should not be treated as a marginal variable, but as a fundamental category.

Only when both sexes are considered equally does a more complete picture of biodiversity emerge—and thus a robust basis for effective species conservation. Anyone who looks exclusively at conspicuous males is not describing the species, but only a slice of its biological reality.

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