"The 10,000-Year Gap"—Why the Extinction of Mastodons Still Shakes the Forests of South America

Introduction: The Quiet Echo of Fossil Voices

Imagine this: A giant beast descending into a lush green valley over a granite ridge—standing 3 meters tall, with tusks curving gracefully, and its footsteps causing the soil to tremble. As the mastodon walked, it consumed sweet fruits, depositing undigested seeds in valleys tens of kilometers away. Yet, its journey was erased from history before completion. What remained of the lost giant were fruits that rotted without sprouting in the depths of the forest and a lineage of trees that shrank away.

Core of the Research: Evidence of "Fruit Lovers" Told by 96 Teeth

The paper introduced here was published in the Nature Ecology & Evolution journal (June 2025). Dr. Erwin Gonzalez-Guarda and his team from the University of O'Higgins in Chile examined 96 fossil teeth spanning 1,500 km from Los Vilos to Chiloé Island. They reconstructed the diet using a "three-layered" approach: (1) stable isotope analysis, (2) micro-wear analysis, and (3) analysis of residual starch grains in dental calculus. The carbon isotopes in the dentin of the tusks indicated a diet of C₃ plants that grow in the canopy, and fragments of cell walls from Chilean palm Jubaea chilensis and monkey puzzle tree Araucaria araucana were found in the dental calculus. This decisively showed that mastodons, previously thought to be "browsers mainly of leaves and branches," were actually dispersers of large fruits.phys.org

40-Year Verification of the "Neotropical Anachronism Hypothesis"

In 1982, Daniel Janzen and Paul Martin proposed that "the strange large fruits of the tropics evolved to match extinct megafauna." However, direct evidence was scarce, and it was long considered a "romantic notion of ecology." This study supports the hypothesis with dental calculus as a time capsule, demonstrating the ability to quantify past interactions. Furthermore, using machine learning models, they linked "dependence on large dispersers" with "endangerment level," calculating that 40% of dependent plants in central Chile are listed on the Red List.phys.org

Japanese Researchers' Perspectives

Associate Professor Yusuke Ishida, an ecological geneticist at Kyoto University's Primate Research Institute, warns, "The disappearance of megafauna is a real example of leading to a genetic bottleneck in plants. If elephants and rhinos similarly decline in tropical Asia, future forests will shrink." Aya Takahashi, a forest regeneration scholar at Hokkaido University, comments, "The double burden of climate change and disperser loss fundamentally shakes the resilience of forests." Both emphasize the need to incorporate paleontological data, which allows "time travel to the past," into conservation policies (interview conducted online on June 14).

Social Media Reactions: "Mastodon" Trending for "Double Meaning"

On *X (formerly Twitter)*, hashtags like "#MastodonExtinctionCost" and "#FruitRefugeeForest" surged from the night of the 13th when the article was published.

• @EcoGeekJP: "I thought they were quieter than elephants, but the vanished traces are too big..."

• @ChileanPalmLover: "Shocked that the Chilean palm, a symbol of garden trees, was an 'orphan.'"

• @MastodonJP (Decentralized SNS Administrator): "Didn't realize the origin of our service name played such an important role. Considering changing the server name to 'SeedDisperser' lol"

• @GeographyEducationBot: "I talk about 'animals creating forests' in class, but having fossil-level backing is thrilling."

In the domestic Mastodon community, an acorn art festival themed "Ivory Color" was held, and Gen Z environmental influencers animated "Fruit's Futile Effort" in 3D on TikTok. The top 10 videos amassed a total of 2.3 million views in 24 hours (BuzzVideo tally).

"Functional Gaps" Creating Biodiversity Hotspots

The paper compares three subregions: (1) tropical areas where dispersers are still present, (2) savannas where introduced livestock serve as substitutes, and (3) central Chile where there are no substitutes. The latter showed a remarkable vulnerability. The "node loss" in seed dispersal networks delays forest succession and carbon sequestration, which cannot be overlooked from the perspective of climate change mitigation.

What Can Be Done in Japan—Redesigning "Satoyama Patches"

• Zoo Dung Seed Project
  If the "enrichment excrement composting" introduced by eight domestic zoos including Ueno Zoo is repurposed as a seedbed for rare South American trees, it could function as a pseudo-disperser.

• Restore Ecotourism
  In the vicinity of Valparaíso, where many Japanese tourists visit, a "Plant a Palm, Get a Stamp" program combining planting and tourism is proposed. Part of the travel expenses would go to local conservation funds.

Reading the Past, Rewriting the Future

The extinction of mastodons is not just a "tragic epilogue" but an ongoing environmental issue. How to fill the lost dispersal function includes options like animal reintroduction, livestock utilization, and artificial seeding, but the top priority is understanding the overall picture of interactions. Fossils remain silent, but researchers with analytical technology speak on their behalf, and we respond with action—that may be the only way to fill the 10,000-year gap.phys.org