New tusk-analysis techniques reveal increased testosterone in male woolly mammoths

Testosterone levels are found in mammoth and elephant tusks.

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The presence of hormones in biological media reveals endocrine activity on various timelines related to growth, reproduction, illness, and stress. While different tissues store steroid hormones accumulated over time, the serum provides immediate circulating concentrations. In current and historical contexts, the presence of hormones in keratin, bones, and teeth has been examined; however, the biological significance of these findings is up for debate.

A new study by the University of Michigan shows the first direct evidence that adult males experienced musth, a testosterone-driven episode of heightened hostility towards competing males. 

Previous blood and urine investigations on male elephants revealed increased testosterone levels during musth. From bone damage, fractured tusk tips, and other oblique lines of evidence, it has been assumed that musth battles occurred in extinct relatives of modern elephants.

However, the new study is the first to demonstrate that testosterone levels are recorded in the growth layers of mammoth and elephant tusks. 

In a Siberian woolly mammoth tusk preserved by permafrost, researchers from the University of Michigan and their international colleagues report yearly recurring testosterone surges that can be up to 10 times higher than baseline levels. Over 33,000 years ago, an adult male mammoth lived on the Earth.

According to the researchers, the mammoth tusk’s testosterone peaks are compatible with musth-related testosterone peaks identified in the tusk of an African bull elephant. The word “musth” is derived from the words for drunk in Hindi and Urdu.

Study lead author Michael Cherney, a research affiliate at the U-M Museum of Paleontology and a research fellow at the U-M Medical School, said, “Temporal patterns of testosterone preserved in fossil tusks show that, like modern elephants, mature bull mammoths experienced musth.” 

The new study shows that testosterone and other steroid hormones, integrated into dentin, the hard tissue that constitutes the roots of all teeth, are present in current and ancient tusks.

This discovery established dentin as a valuable hormone storage site and sets the way for future developments in paleoendocrinology. 

The researcher said, “In addition to broad applications in zoology and paleontology, tooth-hormone records could support medical, forensic, and archaeological studies.”

Hormones are signaling molecules that aid in controlling behavior and physiology. The main sex hormone in male vertebrates, testosterone, is part of the steroid hormone family. It moves through the bloodstream and gathers in different tissues.

Scientists have previously examined steroid hormones found in the hair, nails, bones, and teeth of humans and animals. However, the significance and worth of such hormone records have been under constant review and discussion.

According to the researcher of a recent Nature paper, their discoveries should help change this since they show that steroid records in teeth can sometimes preserve useful biological information for thousands of years. 

Study co-author Daniel Fisher, a curator at the U-M Museum of Paleontology and professor in the Department of Earth and Environmental Sciences, said, “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.” 

Fisher, who is also a professor at the U-M Department of Ecology and Evolutionary Biology, said, “Because musth is associated with dramatically elevated testosterone in modern elephants, 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 investigation used two adult woolly mammoths, a male and a female from Siberia, and one adult African bull elephant’s tusk. The samples were obtained legally and with the necessary authorizations.

The researchers used The tusks’ annual growth increments were located using CT scans, and contiguous half-millimeter-wide samples of dentin growth that roughly corresponded to monthly intervals were ground using a very small drill bit. This milling operation created powder, which was gathered and chemically examined. 

This milling operation created powder, which was gathered and chemically examined. To extract hormones from tusk dentin for mass spectrometric measurement, the study required brand-new procedures that Rich Auchus, an endocrinologist at the University of Michigan, developed in his lab. 

Auchus, professor of internal medicine and pharmacology at the U-M Medical School, said, “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 we would use these techniques to explore paleoendocrinology. We had to modify the method because those tusk powders were the dirtiest samples we had ever analyzed. I was flabbergasted when Mike (Cherney) showed me the data from the elephant tusks. Then we saw the same patterns in the mammoth—wow!”

When a hunter killed the African bull elephant in 1963, it was thought to be between 30 and 40 years old. The male woolly mammoth lived to be roughly 55 years old, according to estimations. A diamond-mining business in Siberia found its right tusk in 2007.

According to radiocarbon dating, the animal was alive between 33,291 and 38,866 years ago. 

The University of Michigan-led testosterone study discovered that the average testosterone level was lower than the lowest values recorded in the male mammoth’s tusks and that the testosterone levels from the female woolly mammoth’s tusk exhibited minimal fluctuation over time. 

The tusk was found on Wrangel Island, which is presently isolated from northeast Siberia by a sea that was once connected to it during glacial periods of lower water levels.

The average testosterone level in female woolly mammoth tusks was lower than the lowest values recorded for male mammoth tusks, in contrast to the male tusks, which showed no fluctuation over time.

Co-author Selcer said, “This is one of the reasons we come to work every morning at the University of Michigan: to make discoveries that empower us to see the world in new ways,” “The project shows you the importance of both collaboration across schools—thanks to Mcubed 3.0—and of the university’s instrumentation infrastructure.”

The results of this study could be utilized to look into fossil evidence for smaller-toothed animals, such as humans and other hominids. 

The researcher wrote, “With reliable results for some steroids from samples as small as 5 mg of dentin, these methods could be used to investigate records of organisms with smaller teeth, including humans and other hominids. Endocrine records in modern and ancient dentin provide a new approach to investigating reproductive ecology, life history, population dynamics, disease, and behavior in modern and prehistoric contexts.”

Journal Reference:

  1. Cherney, M. D., Fisher, D. C., et al. Testosterone histories from tusks reveal woolly mammoth musth episodes. Nature. DOI: 10.1038/s41586-023-06020-9
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