In 2019, researchers exploring the southwest coast of Portugal made an unexpected discovery: Hundreds of well-preserved bees had been mummified inside their cocoons for the last 2,975 years. They described their rare find in a new study published in July in the journal Papers in Palaeontology.
Though scientists have discovered other fossilized bee nests, this is the first time they’ve found one with ancient insects still inside, reports Live Science’s Ethan Freedman. Usually, insects decompose quickly after they die, but in this case, the conditions aligned perfectly to preserve the bees’ bodies.
“With a fossil record of 100 million years of nests and hives attributed to the bee family, the truth is that the fossilization of its user is practically non-existent,” says study co-author Andrea Baucon, a paleontologist at the University of Siena in Italy, in a statement.
The team stumbled upon the ancient nest near the present-day town of Odemira while studying how Portugal’s Atlantic coastal ecosystem had evolved over time. As they scoured the coastline, they came across bulb-shaped, fossilized cocoons inside some rocks.
Initially, the scientists couldn’t tell what insect had made the cocoons. But when they took the fossils back to the lab and imaged them using a special kind of CT scan called X-ray microcomputed tomography, they were able to peer inside without damaging the structures. The highly detailed, three-dimensional images revealed the intact remains of young adult bees of the genus Eucera, which spend most of their lives underground.
“What makes this study so cool is that you do have the bee in there and you can see that it’s in the tribe Eucerini, which are the long-horned bees,” says Bryan Danforth, an entomologist at Cornell University who was not involved in the new research, to the New York Times’ Sarah Derouin. “If you look at the CT image, you can see the long antennae, so you know it’s a male.”
Female Eucera bees build subterranean nests and lay their eggs inside. They also leave behind a cache of pollen for their offspring to eat as they mature into adults. The young bees develop inside protective cocoons—which their mother creates by producing a waterproof, silk-like thread—before emerging aboveground and starting the life cycle over again.
In this case, the bees were swaddled safely inside their cocoons when something “catastrophic” happened, as study co-author Carlos Neto de Carvalho, a paleontologist at Portugal’s Naturtejo UNESCO Global Geopark, tells the New York Times. It’s not entirely clear what happened to the insects, but the researchers suspect an abrupt change in weather, such as an unexpected freeze or sudden flood, killed them simultaneously.
The cocoons, which were made of organic polymer, sealed the delicate bee bodies inside and kept oxygen and bacteria out. This allowed the creatures to remain intact for the last three millennia.
From the fossils, the researchers also gleaned that the bees had been eating pollen from a flowering plant in the Brassicaceae family—which includes cabbage and mustard—just before their demise. This further bolsters the theory that a weather event triggered their die-off, rather than mass starvation brought on by a prolonged drought, per the New York Times.
Since more than 25 species of Eucera bees and their relatives still inhabit Portugal, the researchers hope the fossilized nest will offer additional insights into how the insects evolved over time, because they can get “face to face with the users of the nests,” as Neto de Carvalho tells Live Science.