Discovery of ancient bee nests in fossils points to a never-before-seen behaviour

For the first time, bees were recorded using bones to nest and lay eggs in a Caribbean cave. Courtesy of Lazaro Viñola López via CNN Newsource

Contrary to popular depictions of bee nests as large, intricate hives that hang from trees, the majority of bee species — about 90 per cent — are actually solitary and build their nests in the ground or within rotting logs and the stems of plants.

But researchers recently discovered what is perhaps one of the most peculiar nesting spots: Inside a limestone cave on the Caribbean island of Hispaniola, a hoard of fossilized bones — some from now-extinct animals — was home to tiny ancient bee nests built within the empty tooth sockets. It is the first time bees have been recorded using bones to nest and lay eggs, according to a study published Tuesday in the journal Royal Society Open Science.

“It was very surprising, because you never find invertebrates there — you find snails, but you don’t find insects,” said lead author Lázaro Viñola-López, a postdoctoral researcher at the Field Museum of Natural History in Chicago. “Usually what you find in this cave are rodents, birds, like all that fauna. So, finding evidence of ancient bees in those cave deposits was very exciting. The first time we have that in the Caribbean.”

The findings help to fill a gap in the fossil record since all other bee fossils described in the Caribbean were discovered within amber and are much older, dating back around 20 million years, Viñola-López explained.

The study authors believe that fossils housing the bee nests date back to around 20,000 years ago and could shed light on how bees have adapted to their environment, even in the ancient past.

“There’s very little known about the ecology of many of the bees on these islands,” Viñola-López said. “This shows that the diversity of nesting processes of bees is actually really big and sometimes expands past what we think is normal. … It also tells that we have to, when we prepare specimens, look closer for all the things that can get preserved inside these ones, which can show very strange behaviours of species that we think that we understand relatively well.”

A fossil within a fossil

Viñola-López was exploring the cave with colleagues in the summer of 2022, looking for specimens to study for his doctoral program at the University of Florida and the Florida Museum of Natural History. The cave itself holds a story, as it contains layer upon layer of fossils from more than 50 species, including rodents, birds and reptiles.

The study authors believe a family of barn owls once lived in the cave and coughed up the bones of their prey, which eventually fossilized. The owls might have remained in the cave for generations, resulting in the accumulation of thousands of fossils. Other species, such as tortoises and crocodiles, could have fallen into the cave — which has an about 8-meter drop at its entrance — and eventually died when they could not climb back out. The environment, protected from the outside weather, served to preserve the specimens further.

But when Viñola-López took a closer look at the fossils, he noticed something odd — dirt lining the insides of the tooth sockets reminded him of how wasps might build their cocoons.

After performing CT scans, X-ray technology that creates detailed 3D imaging, the authors were able to determine that the small nests were made of mud and belonged to a bee. While wasp nests are made from a mix of saliva and chewed plant fibers, the nests from the cave were smooth on the inside, indicative of a bee’s nest-building process, which uses compacted dirt and a secreted waxy substance that coats the inside of the walls.

“This is a very interesting record because usually for bees, sometimes they like shaded areas, some of them can be nocturnal, but nesting in a cave is a very, very strange behaviour,” Viñola-López said. There is only one other recorded instance of a burrowing bee nesting inside a cave, according to a release from the Florida Museum of Natural History. “We also know that it is not one specimen that did it. Even in a single hole of a mandible, there are up to six generations of bees coming back to that same single hole. … So it seems that it was probably a large communal nesting.”

However, while the muggy cave preserved the fossils, the conditions were not ideal for preserving the bodies of any insects. The authors were therefore unable to determine which species of bee was nesting in the cave. It is also unclear whether the nests were built by an extinct bee species or one that is still around today. For now, their nests are the only named specimen, now known as Osnidum almontei, after Juan Almonte Milan, the scientist who first discovered the cave, according to the study.

“This is a nice piece of work presented by these researchers that adds to the record of ‘hidden biodiversity.’ This means that although no body fossils of bees were recovered from the cave, their trace fossils (aka ichnofossils) are diagnostic of soil bees. These bee cells tell us about the environment and ecosystem in which they lived,” said Stephen Hasiotis, a geology professor at the University of Kansas, Lawrence, whose research focuses on paleontology and ichnology, or the study of trace fossils. Hasiotis was not involved with the new study.

“The bees likely built their nests in the cave soils because the humidity within the cave and soils would have been nearly constant,” he added in an email. “The area was likely protected from heavy rains and floodings, not to mention also being protected from many potential predators and omnivores of various sizes cohabiting the soils in which they burrowed.”

Viñola-López said he hopes to return to the area to do further surveys that may help determine whether the bees are still in the area — and if this behaviour is found in other caves and on other islands.

“This discovery is doubly surprising, because modern bees aren’t known to use bones for their nesting, nor are they known to nest in caves. But these fossil bees brooding chambers in sediment-filled parts of bones tell us that their makers could do both, which is exciting to know,” said Anthony Martin, a professor of practice in the department of environmental sciences at Emory University in Atlanta. Martin is the author of the 2023 book, “Life Sculpted: Tales of the Animals, Plants, and Fungi that Drill, Break, and Scrape to Shape Earth.”

“Insects have been adapting to changes in their environments for almost 400 million years, and ground-nesting bees have been around for about the last 100 million years of that time,” said Martin, who was not involved with the new study. “So this exciting fossil discovery of bees nesting in bone cavities, and in caves, serves as a good reminder that when it comes to evolution, bees will keep on being the bees they need to be.”