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There is a sex bias in amphibian and reptile natural history collections
There are more male specimens of amphibians and reptiles in museum collections than females.
Whilst this in itself can have implications for collections, what is perhaps more surprising is that the vast majority of herpetology records on global databases are unsexed, severely limiting the types of studies that can be done on them.
Natural history museums around the world contain a huge wealth of information about the life with which we share this planet.
This information is vital to our understanding of the impacts of the biodiversity crisis and how to prevent it. But in recent years there has been a growing recognition that these collections are not as objective as has often been assumed.
A new study looking at the amphibians and reptiles held in hundreds of natural history collections around the world has found that there is an underlying sex bias, with the collections generally containing more male specimens than females. With sex influencing many aspects of how a species lives and what they look like, any skew towards one sex or the other could potentially alter any studies conducted on them.
Dr Natalie Cooper is a Principal Researcher at the Natural History Museum who led this latest study looking into these underlying sex biases in natural history collections.
‘We were interested in understanding whether you see the sex biases in amphibians and reptiles in the same way we showed previously you see a male bias in birds and mammals,’ explains Natalie. ‘We found that there is a bias, and we do see slightly more males than females in the collections, even in groups where you might expect there to be more females.’
The study has been published in the scientific journal Biological Journal of the Linnean Society.
The team looked at the data for over 6,000,000 specimens held in the global database of natural history specimens. ©The Trustees of the Natural History Museum, London
Sex bias in natural history collections
This is not the first time that an underlying sex bias has been found in natural history collections. Previous studies have found that there is a skew towards males in mammal and bird collections, as collectors frequently favoured individual animals with the brightest feathers and largest horns.
But reptiles and amphibians offer an interesting comparison. Many species within these groups do vastly different things with their sex. For example, many species can reproduce via parthenogenesis, meaning that females can produce offspring on their own without the input of any males, while the sex of many other species is often determined by the temperature at which the eggs are incubated.
‘Then there’s also reverse sexual size dimorphism,’ explains Natalie. ‘We’re quite used to big males and small females because we’re often thinking about it in a mammalian context, but in frogs for example, the females are often bigger than the males.’
Yet despite these wide variations in ecologies and behaviours, the analysis still found that there were, on average, more male amphibians and reptiles in natural history collections, even in groups in which it might be expected that there would be more females.
There are more male frog specimens than females in the collections, likely due to collectors homing in calling frogs to catch them ©Shutterstock/EcoPrint
The reasons for this are seemingly fairly straight forward. For example, when it comes to lizards, it is frequently the males that will stand out more when displaying to females making them more of a target for collectors, while for frogs it is the males that do most of the calling when trying to attract mates.
Dr Jeff Streicher is the Senior Curator in Charge of reptiles and amphibians at the Natural History Museum and is also an author of the study.
‘I thought there would be a big bias towards male frogs,’ explains Jeff. ‘This is because when people are collecting frogs in the field, they will find most species when the frogs vocalise. They’ll only hear the male frogs calling and so female specimens are typically much rarer in collections.’
In some groups, however, there were interesting reversals. Within the sea turtles there is actually a sex bias towards more female specimens in the collections, which makes sense when each year it is the females that return to beaches to lay their eggs, and so making them an obvious target for collectors.
There is an interesting reverse in the sex bias for sea turtles, as nesting females were more likely to be caught by collectors ©The Trustees of the Natural History Museum, London
But the fact that the collections have an overall slight male-bias is not the most surprising discovery.
‘For me the biggest result from the study is that there is just very little information at all about the sex of amphibian and reptile specimens,’ says Jeff. ‘The majority of specimen records that are on these big global databases lack sex information, and that really limits their utility.’
Limiting research
Out of the six million reptile and amphibian specimens registered on the Global Biodiversity Information Facility (GBIF) database which contains the records from hundreds of museums around the world, the researchers were shocked to find that an astonishing 95% had no sex data associated with them at all. There are some good reasons for this, mainly in that it can be exceedingly difficult to sex many species of reptiles and amphibians, even in fairly common and well-studied animals.
‘The bearded dragon is generally considered to be highly sexually dimorphic at least partly because the males can get very big,’ explains Dr Marc Jones, the Curator of fossil reptiles and amphibians at the Natural History Museum who was also involved with the research. ‘However, telling the difference between small males and big females can be really hard.’
‘In the field we generally estimate the sex of bearded dragons according to anatomical traits such as body size, beard, and the head width, but when we’ve then compared those estimates to genetic results we don’t have a 100% success rate.’
‘So if you are not used to looking at these animals, then it is not easy to sex them.’
The future of the tuatara is threatened by climate change, but a lack of sex data for historical specimens makes it impossible to study recent changes to their sex ratio ©Shutterstock/Nelida Zubia
In addition to this, Marc also suggests that in some cases researchers may have simply thought that the sex of the specimen in front of them was so obvious there was no need to officially note it down. Regardless of its cause, the fact that most specimens remain unsexed means that there are some questions that are now impossible to ask.
For example, Natalie and Marc were interested in looking at how the climate crisis might impact the survival of the lizard-like tuatara. The endangered reptiles are confined to a handful of islands off the coast of New Zealand, and because their sex is determined by the temperature at which the eggs are incubated, there are concerns that raising temperatures might cause a shift in the reptile’s sex ratio. But as Natalie explains, ‘we just can’t do those investigations, unfortunately, without any sex data.’
Yet even for those specimens which are sexed, the underlying male sex bias can also cause issues. This is because males and females have different genetics, morphologies and behaviours, which in turn influences a range of aspects of their lives. This means it is hard to generalise aspects of a species’ life history if there is a sex skew in either direction.
In the future, the most straightforward solution would be to train more people in taxonomy to create more experts who can effectively and accurately sex museum species and then upload the records onto the global databases. But this will take time and money.
In the meantime, researchers simply need to be aware of this issue and try and adjust any future research to account for it.
