A Mammoth’s Journey: New isotopic science

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In 2017, John McKay elucidated the history of modern science through the lens of the mammoth, or really, the mammoth hunters, in his book* Discovering the Mammoth: A Tale of Giants, Unicorns, Ivory, and the Birth of a New Science. The difference between what (mainly) European thinkers thought about the meaning of mammoth and other megafauna bones in the early days of discovery and what we knew a decade ago is not merely reflective of the accretion of knowledge and understanding of an observed science. It is much more dramatic than that. For example, a theory thought viable in the 189th century (IIRC, it has been a little while since I read McKay’s book) is that mammoths were still extant, and lived underground as fossorial animals, and could not survive contact with open air. Frozen mammoth carcasses would then represent mammoths that got too close to the surface, accidentally breathed, died, and were frozen in place, partly sticking out. Other early thinking on mammoth and other megafauna remains invoked unicorns and other mythical creatures. We have come a long way.

From what we know about elephants in general (including mammoths), individual mammoths also come a long way from birth to death. I hesitate to use the highly misunderstood word nomadic, which for racist reasons tends to invoke randomness and the strange (and non often existent) practice of never returning to the same place more than once other than by accident. But elephants are potentially nomadic, in the sense that they their daily activity range on average is a tiny fraction of their yearly home range in size, or that every now and then they may move a great distance, depending on resources and other factors. In fact, many modern elephant herds are far from nomadic, either because of the availability of resources or because they are trapped in parks. The point is, understanding the way of the elephant, sensu lato, necessitates understanding their patterns of movement across the landscape, and how they access resources on that landscape, and the relationship between where they move or stay and their social behavior, which in turn links to the animal’s development from birth to old age and ultimate entry, for some, into the fossil record.

And that is informed by a study just out, which takes this journey of understanding a good distance from mid 20th century views of mammal ecology, using techniques initially applied in the 1980s and that have come to full maturity: stable isotopes.

There is a handful of elements that get used a lot in biology, and that occur in nature as a number of different stable isotopes. Atoms make up everything, and exist as distinct immutable elements (unstable elements can change, of course, not the topic of conversation here). But many elements have very slightly different versions, which remain stable, and for most purposes, substitute for each other in chemical reactions But not quite. As these items are moved around in biological systems, passed from one molecular surface to another, or hooked up to each other or other elements to make various molecules, or separated from other atoms, during the process of life itself at the finest level, whether or not a given atom is used or ignored is very slightly biased by which version of the element it is. The difference is so small and insignificant that you can create a biological system in which virtually all of the atoms of a given element are of one stable isotope only, and the system works just fine. But if you provide a biological system with two different isotopes of that atom, let it run for a while, you might find that the tiny molecular machines that make up the works of a cell have been biased for or against one or another element, so the biological mass left at the end — some tissue such as bone or muscle — is made out of a biased subset of those isotopes.

Hand an isotope expert a sample of carbon-containing material and they can tell you if it comes from a sea creature or a land creature based on the isotopes. Hand some stuff from sea mud to an isotope expert and they can tell you if the Oxygen in the skeletal remains of the included dead organisms were formed during a glacial period vs. an inter-glacial period. The utter coolness of isotopic research is unsurpassed. Isotopic research is to animal ecology what genetics is to many other aspects of biology.

So, a large number of researches led by (wait for it…) Matthew Wooller, have applied isotopic analysis to the tusk of an individual woolly mammoth (Mammuthus primigenius) to reconstruct where it went from the time it was born to the time it died, and it lived close to 30 years, in Alaska.

There are two parts to this study. One is to look at the values of the isotopes over time to see how the different life stages went, and the second is to use a fancy simulation to estimate where the animal actually went in Alaska. In my view, the first part is a very important and likely stable result that, when added to other similar results from other individuals, will eventually contribute in a major and long lasting way to an understanding of mammoth life history. The second part is very cool but is necessarily more speculative. For example, it requires faith in a particular reconstruction of ecology of the region at the time, which itself is necessarily speculative.

The following graphic shows the isotopic history of this mammoth.

High vs low values for each element reflect the bias between one or another isotope (not the amount of that element in the sample). The meaning of a shift in isotope can be very controversial or more straightforward. The overall squiggliness of the lines reflects a combination of strong seasonal variation and stochastic or multi-year variation in various aspects of ecology. The things to look at in each squiggle are overall amount of variation across time, and shifts that correspond to the major life history stages of the animal, which are indicated in background color of the graph.

Shifts in Nitrogen values can indicated starvation of lack of food (higher values). A decrease on Carbon value can indicate the same thing. Oxygen can indicated climate change, but the life of one mammoth is too short to spot that signal. It can indicate geographical variation. Strontium represents specific habitats and/or geological location, so as those values change, we assume movement across the landscape to different areas.

From the paper:

Data from the first ~10 cm from the tusk tip showed minimal Sr/ 86Sr variation, suggesting that the young mammoth mostly occupied a range in the lower Yukon River basin in interior Alaska. As a juvenile … the mammoth used a larger range spanning some of the lowlands of interior Alaska between the Alaska and Brooks ranges … The mammoth undertook regular north-south movements within this large core area as well as several long-distance movements, sometimes reaching the eastern end of the Brooks Range and the northern Seward Peninsula in the west … These juvenile-age movements probably represent the movements of a herd (16–18).

With increasing maturity, our study mammoth broadened his range. After ~16 years, a distinctive transition occurred involving higher variance in 87Sr/86Sr along with other isotopic changes. This implied change in the animal’s range probably reflects a transition to reproductive maturity accompanied by long-distance travel between interior Alaska and the North Slope of the Brooks Range. These movements were probably in response
to seasonal changes in resource availability.

One result of this paper is to confirm and underscore the importance of long distance travel in a mammoth’s life. This supports the idea that mammoth confined to small areas (including islands) are selected to become smaller over time (the island effect), which scales the animals to their environment. Isolation to smaller areas of suitable habitat may be linked to the eventual extinction of mammoths.


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