By studying an extreme drought that occurred 4,200 years ago in what’s now the island paradise of Mauritius, researchers uncovered a mass mortality event that killed thousands of giant tortoises and dodo birds. They were cramped together in a “deadly cocktail” of their own feces and toxic bacteria, according to findings published in The Holocene earlier this month.
Dodos, who famously went extinct in the 1600s, were only found on that small island east of Madagascar in the Indian Ocean. In the past, a shallow lake on Mauritius called Mare aux Songes was an important source of freshwater that attracted (and concentrated) groups of vertebrates, especially those who don’t migrate. Nowadays, it’s an incredibly fossil-rich area—or lagerstätte, German for “storage space”— that's known as the “dodo swamp”
Many regions around the Indian Ocean depend on monsoon precipitation, but the seasonal rains declined dramatically during a period of climate change between 4,400 and 4,100 years ago. "What I wanted to know was, how did this drought cause this graveyard?” University of Amsterdam’s Erik de Boer says. “How did so many animals die?” So, to reconstruct the ecosystem’s response, de Boer and colleagues combined analyses of geochemistry, pollen, diatoms (a group of algae), and pigments for a multi-proxy approach.
A prolonged megadrought occurred between 4,190 and 4,130 years ago, they found, which led to a mass die-off of island natives in Mare aux Songes—including two species of giant tortoises and dodos.
The abruptly increasing aridity triggered regional fires and salinization, while lowering water levels and shrinking the water surface—all of which resulted in the further concentration of animals who depended on this coastal site. “Annually, the lake would get some fresh water in, however this drinking water turned foul during the dry season,” de Boer tells Science.
Under these hot, dry, salty, and crowded conditions, animal fecal matter built up and up, creating a suitable environment for toxic cyanobacteria. “The animals lived around the edges, and the excrements probably got mixed up in the wetlands," de Boer adds. "It’s like a big toilet.”
In the end, hundreds of thousands of vertebrates died from intoxication, dehydration, and trampling, or they were otherwise mired in the muck that resulted. The giant tortoises and dodos eventually bounced back with the returning rains, Science reports, and they survived for another 3,800 years... until the Dutch arrived.
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