The American mastodon roamed North America until its extinction around 10,000 to 12,000 years ago. Reconstructing the life of these extinct animals is challenging because their behavior cannot be directly observed. Scientists analyze chemical signatures locked within their fossilized remains to peer into the past.
Mastodons possessed massive tusks and teeth that grew continuously throughout their lives. These mineralized tissues serve as a biological archive, recording the animal’s lifetime history. By sampling these tissues sequentially, researchers access a chronological record of the mastodon’s diet, environment, and movement using oxygen and strontium isotopes.
The Basic Science of Oxygen and Strontium Isotopes
Isotopes are atoms of the same element with slight variations in atomic mass due to differing numbers of neutrons. The ratio of heavy to light isotopes in an animal’s body reflects the ratio present in the water and food it consumes. This ratio is permanently locked into biomineralized tissues like tusks and teeth as they form.
Mastodon tusks are continuously growing incisor teeth, forming in concentric, cone-shaped layers throughout the animal’s life. Each layer represents a specific time period, much like the rings of a tree. As the mastodon consumes local water and plants, oxygen and strontium from these sources are incorporated into the newly formed tusk tissue.
Because the tusk grows sequentially, samples taken from the tip represent the animal’s earlier life, while samples from the base reflect its final years. Analyzing a series of samples along the tusk’s growth axis provides a high-resolution, chronological record. This allows scientists to track changes in the animal’s environment and behavior over its lifespan.
Tracking Diet and Seasonal Changes with Oxygen Isotopes
Oxygen isotopes (\(\delta^{18}O\)) are primarily influenced by the oxygen in the mastodon’s body water. The isotopic composition of this body water is directly linked to the water the animal drinks and the moisture content of the food it eats. Consequently, the \(\delta^{18}O\) values recorded in the tusk fluctuate with environmental changes.
In continental regions, the \(\delta^{18}O\) value of precipitation and surface water exhibits a predictable seasonal cycle. Warmer seasons have higher values, and colder seasons have lower values. These seasonal variations are captured as distinct peaks and valleys in the \(\delta^{18}O\) record along the tusk. By measuring these cyclical changes, scientists can determine the season in which a specific layer of the tusk was grown.
The oxygen isotope signature also provides insights into the mastodon’s diet and water consumption patterns. A sudden, sustained shift in \(\delta^{18}O\) can indicate a change in the animal’s primary water source or a major shift in its feeding habits. These data help reconstruct the local environment, such as the severity of cold periods or the availability of water, and reveal seasonal dietary adjustments.
Mapping Geographical Movement with Strontium Isotopes
Strontium isotopes (\(^{87}Sr/^{86}Sr\) ratio) serve as a geographical marker. This ratio varies systematically based on the age and elemental composition of the underlying geological bedrock. Older rocks, which contain more radiogenic elements, tend to have higher \(^{87}Sr/^{86}Sr\) ratios than younger rocks.
When a mastodon feeds on plants and drinks water, it absorbs the local strontium signature, which is incorporated into the mineral structure of its tusks. Since the strontium ratio is tied to the local geology, the isotopic value in the tusk acts like a geological “zip code” for the feeding location. Scientists create geographical maps, known as isoscapes, that plot the expected strontium ratios across a landscape.
By comparing the \(^{87}Sr/^{86}Sr\) ratios measured in sequential tusk samples to these isoscapes, researchers reconstruct the mastodon’s physical movement over its lifetime. A significant shift in the strontium ratio indicates that the animal moved from one distinct geological region to another. This technique enables the mapping of migration routes, home ranges, and the extent of individual travel.
Synthesizing Isotopic Records for Behavioral Insights
The isotopic analysis is most powerful when combining oxygen and strontium records from the same tusk layer. The \(\delta^{18}O\) data provide the timing of the growth layer (e.g., summer or winter), while the \(^{87}Sr/^{86}Sr\) data provide the location where that layer was formed. Integrating these two records allows for the reconstruction of complex, annual behavioral patterns.
A sudden drop in \(\delta^{18}O\) (indicating winter) that consistently coincides with a specific \(^{87}Sr/^{86}Sr\) value suggests a predictable, seasonal migration pattern. This approach documented the annual migration of individual mastodons, such as the Buesching mastodon. This mastodon repeatedly traveled to a northern region during the warm season, with researchers identifying its home range in central Indiana and a summer range likely used for mating in northeastern Indiana.
Analyzing abrupt shifts in both isotope systems in juvenile tusks can reveal significant life history events. A sudden, simultaneous change in both signatures often marks the time of weaning, as the calf shifts from a milk-based diet to consuming local forage and water. Changes in movement patterns, such as an adolescent male leaving the matriarchal herd, are recorded as an increase in the variability of the strontium signature. This combined evidence provides a detailed autobiography of an extinct animal, revealing its life history and response to its ancient environment.