Why do we Care About Frog Sex?

Museums provide us with snapshots of biological and ecological information reaching far into the past. These collections, stored worldwide, contain many kinds of flora and fauna that can be used in research. When carrying out large-scale studies it is important to know what it is you are working with and, equally importantly, where it is from. Research must consider different variables depending on what is being studied, such as sex, age, or location. Because of this, it is important to have as much information about your information as possible.

But why do we care about frog sex? In a recent paper published in the Biological Journal of the Linnean Society, Wainwright et al. at the Natural History Museum, London, investigated the problem of sex bias in museum collections. Their focus was on herpetological samples (amphibians and reptiles) and their metadata (data about the specimens themselves), ultimately aiming to understand whether it is representative of natural wild populations. But how do you achieve this? As mentioned, it is important to have as much detail about your information as possible, answering questions such as, how old was the specimen when caught? Where was it caught? What time of year was it? How big is it? Here, Wainwright et al. were particularly interested in the male to female ratio of specimens.

For some animals it is easy to tell the difference between females and males, and therefore possible to account for sex in research. For example, brightly coloured male birds of paradise are easily distinguishable from their duller female counterparts: an example of sexual dimorphism. Amphibians and reptiles, on the other hand, can be comparatively very difficult to sex—at least not without dissection.

In their study, Wainwright et al. carried out a large-scale data analysis of herpetological specimen data, covering almost 6,000,000 specimens from 341 museums worldwide, taken from the Global Biodiversity Information Facility database (GBIF). The goal? To discover the proportion of female and male animals on record, using standardised names and specimens spanning from 1800–2021.

It is important to acknowledge that biological sex exists on a spectrum. Some reptiles and amphibians display characteristics of multiple sexes, and some individuals have the capacity to change from one to another. However, for simplicity, the researchers adopted a binary system. Most historical collections were labelled in this way, and that makes up a large proportion of the specimens recorded by GBIF.

Outside the categories of male and female, Wainwright et al. included a third category for specimens: no recorded sex. This turned out to be important.

Wainwright et al. were careful with which species they included. Some species had so few specimens that the sex data skewed completely in favour of one sex or the other. This can occur as a result of how the specimens were collected; some reptiles and amphibians develop their sex based on the temperature at which the eggs were maintained, known as Temperature Dependent Sex Determination (TSD)—a phenomenon most famously seen in sea turtles. As such, if the entirety of a species’ recorded specimens were collected in one small area during a short period of time, it is possible to only collect one sex. Additionally, some species are parthenogenetic (where an embryo can develop without fertilisation), meaning there are only females. Cases such as these were removed from analysis.

To avoid including specimens with unrepresentative proportions, Wainwright et al. discarded any species that contained fewer than 10 recorded specimens. The cut- off point would have been higher, but the median number of sexed species was as low as eight, so this would be limiting their sample size significantly.

The analysis showed a larger proportion of herpetological specimens were male: 53% of reptiles and 61% of amphibians. Wainwright et al. propose several reasons that these data could be skewed and weigh up how likely they are.

Many species of amphibians and reptiles exhibit sexual dimorphism, though not quite like birds of paradise. For example, females are often larger than males. However, Wainwright et al. investigated the metadata of species that exhibited strong sexual size dimorphism (SSD) and found that this did not have a significant impact on the sex ratios in museum specimens. Proportions vary slightly based on the year of collection, and rather than evening out over time, the proportion of female specimens actually decreases.

Yet one thing did appear to influence sex bias: song. The males of many frog species produce a mating call, or sing territorially, making them easier to locate and capture. While it is true that females also vocalise in many species, male mating calls far exceed the limit of female sounds. It is therefore not difficult to imagine collectors more easily locating males when in the field.

Some species did have higher proportions of female specimens, such as sea turtles, because collection and observation methods of these species tend to focus on nesting and egg laying sites, typically on land. Even so, the overall skew was towards male specimens in both amphibians and reptiles.

With all these factors accounted for, it is important to discuss the most significant discovery—93.9% of reptiles and 97% of amphibians had no associated sex data at all. Just 5% of GBIF’s herpetological museum specimens had accompanying sex metadata. This is an important finding, as the inability to control for sex in herpetological datasets could be having wide reaching and confounding effects on research that relies on specimens from GBIF and similar databases worldwide.

The paper does note that it is worth remembering GBIF does not necessarily represent the actual metadata relating to each individual specimen in their respective museums, only what has been uploaded to their online database. It is possible that the information is held somewhere, but this is unhelpful when carrying out research on the limited data available without visiting every museum to find out what, if any, additional information is held onsite.

Fixing this disparity, and more vitally this lack of available information, would be a monumental undertaking. Wainwright et al. have hope that with better funding to both museums and field researchers, we can recover our frog sex deficit (FSD) and learn more about the diverse species around us via those we have collected and preserved.