Surging Testosterone Levels Discovered in Woolly Mammoth Tusk
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A collaborative team of researchers from the University of Michigan (U-M) has analyzed traces of sex hormones in extinct woolly mammoths’ tusks. The study, published in Nature, is the first to provide molecular evidence that male mammoths experienced musth.
Aggression, restlessness and unpredictability – it must be musth
Healthy male elephants experience a natural phenomenon known as musth, where they demonstrate aggressive and unpredictable behaviors for varying periods of time in adulthood. Musth – which can last for two to three months – is characterized by a surge in reproductive hormones such as testosterone. Male elephants in musth present with a gloopy secretion of temporin – a hormone-rich substance – leaking from the temporal gland on either side of their head.
What is testosterone?
Testosterone is a steroid hormone that is key for the development of male vertebrates’ reproductive tissues.
Damaged skeletal remains from extinct relatives of the modern elephant have suggested that these mighty creatures also engaged in aggressive “musth battles”. Dr. Michael Cherney, research affiliate at the U-M Museum of Paleontology and a research fellow at the U-M Medical School, is the lead researcher on a new project that provides molecular evidence for this phenomenon. The research team show that testosterone levels can be detected in the growth layers of male mammoth and elephant tusks, providing an opportunity to analyze surges in these hormones over time.
“Because musth is associated with markedly increased testosterone in modern elephant serum (10–20 times higher than observed between musth episodes), it provides a starting point for assessing the feasibility of using hormones preserved in tusk growth records to investigate temporal changes in endocrine physiology,” the researchers write.
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Applying steroid mass spectrometry in paleoendocrinology
Cherney and colleagues analyzed tusk samples from an adult African bull elephant that is believed to have been approximately 30–40 years old when it was killed in Botswana in the 1960s. Tusks from two adult woolly mammoths (one male and one female) were also obtained for analysis. The male woolly mammoth is estimated to have lived to be 55 years old, and radiocarbon dating suggests it roamed Siberia approximately 33,291–38,866 years ago. The female woolly mammoth tusk specimen, discovered in Wrangel Island, is estimated to be aged between 5,597–5,885 years old.
CT scans conducted across various laboratories at the U-M School of Dentistry were used to determine the annual growth increments in the tusks. The researchers hypothesized that chemical compounds – such as hormones – could be “stored” in dentin, a mineralized tissue found in teeth. As tusks are essentially front teeth that continue to grow out of the mouth, dentin can be recovered from the interior of the tusk and, using a tiny drill bit, Cherney and team were able to grind out half-millimeter–wide samples of dentin from different monthly intervals of the tusks’ growth.
"Tusks hold particular promise for reconstructing aspects of mammoth life history because they preserve a record of growth in layers of dentin that form throughout an individual's life," says study co-author Professor Daniel Fisher, a curator at the U-M Museum of Paleontology and professor in the Department of Earth and Environmental Sciences.
The lab of endocrinologist Professor Richard Auchus at the U-M Medical School was tasked with determining how to extract molecules from within the tusk dentin for analysis. They adopted mass spectrometry, a technique that measures the mass-to-charge ratio (m/z) of atoms and/or molecules in a sample, which can be used for molecule quantification and identification when coupled with other analytical methods, such as liquid chromatography.
"We had developed steroid mass spectrometry methods for human blood and saliva samples, and we have used them extensively for clinical research studies. But never in a million years did I imagine that we would be using these techniques to explore 'paleoendocrinology’," says Auchus, who is the study’s co-author. "We did have to modify the method some, because those tusk powders were the dirtiest samples we ever analyzed.”
Annual increases in testosterone found in woolly mammoth specimen
The chemical analysis of the woolly mammoth dentin revealed annual increases in testosterone levels that sometimes rose to 10 times above baseline levels. These peaks, says Cherney, are consistent with the peaks that were observed from the African bull elephant’s tusk: "Temporal patterns of testosterone preserved in fossil tusks show that, like modern elephants, mature bull mammoths experienced musth," he notes. As anticipated, the testosterone levels detected in the female woolly mammoth tusk did not show much variation and was typically below the lowest levels recorded from the male mammoth’s tusk.
“When Mike (Cherney) showed me the data from the elephant tusks, I was flabbergasted. Then we saw the same patterns in the mammoth – wow!" exclaims Auchus.
It's important to note that only one male woolly mammoth tusk was obtained for analysis in this study. To this end, Cherney says that the researchers hope to conduct experiments on other tusks using the same methods, and this data will help to characterize how mammoth musth compares to musth in living organisms. "We are also interested in seeing how it varied across time and space. These data will be useful for testing hypotheses related to population dynamics and the impacts of various environmental factors during the ice age, both when mammoths were thriving and when their populations were faltering prior to extinction," Cherney explains.
Setting the stage
The U-M team believe their results “set the stage” for forthcoming studies that can use steroid hormones preserved in dentine to study the biology of modern and extinct mammals. Technology Networks asks Cherney whether there are any limitations to this work that the group wish to overcome going forward: "Testosterone is clearly preserved in these mammoth tusks, but although they are very old, these permafrost remains are also about as well-preserved as fossils come. We don't yet know how well testosterone and other steroids preserve in other environments, so that will be the focus of future studies," he says. As these are organic compounds, they are susceptible to degradation, but teeth are a stable mineralized tissue that preserves well in the fossil record. "We think it could also contain steroids in other preservational contexts," Cherney adds.
As for the research team's next steps, Cherney says that they plan to analyze the tusks of females and juveniles to see what other aspects of life-history they can see using steroids preserved in tusks. "Things like calving records, ages of sexual maturity could really enhance our understanding of the paleobiology of these animals and of the factors that led to their extinction," he notes. "We are also looking at other species to (among other things) test the limits of testosterone preservation in fossils."
Dr. Michael Cherney was speaking to Molly Campbell, Senior Science Writer for Technology Networks.
Reference: Cherney M, Fisher D, Auchus R, et al. Testosterone histories from tusks reveal woolly mammoth musth episodes. Nature. 2023. doi: 10.1038/s41586-023-06020-9