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

Most bees, about 90% of all species, live solitary lives, creating nests in the ground, decaying wood, or plant stems, unlike the popular image of large, communal hives hanging from trees. Recently, researchers uncovered a unique nesting location: fossilized bones inside a limestone cave on the island of Hispaniola.

The ancient bee nests were found within empty tooth sockets of the fossils, some of which belonged to extinct animals. Published in the journal *Royal Society Open Science*, the study marks the first known instance of bees using bones for nesting and egg-laying.

Lázaro Viñola-López, a postdoctoral researcher at the Field Museum of Natural History in Chicago and lead author of the study, expressed surprise at the find. He noted that while snails are sometimes found in the cave, insects are rare. The cave typically contains fossils of rodents and birds, making the discovery of ancient bee evidence particularly exciting, and a first for the Caribbean.

The discovery helps expand the fossil record, as previous bee fossils found in the Caribbean were encased in amber and date back approximately 20 million years, according to Viñola-López.

The researchers estimate that the fossils containing the bee nests are around 20,000 years old. These nests could provide insights into how bees adapted to their environment in the distant past.

Viñola-López emphasized the limited knowledge about the ecology of many bee species on these islands. This discovery illustrates the wide diversity of bee nesting behaviors, which sometimes goes beyond what is considered normal. He also noted the importance of careful specimen examination to uncover unusual behaviors of seemingly well-understood species.

**A Fossil Within a Fossil**

Viñola-López and his colleagues explored the cave in the summer of 2022 while searching for specimens for his doctoral program at the University of Florida and the Florida Museum of Natural History. The cave itself is a treasure trove of fossils, containing remains from over 50 species, including rodents, birds, and reptiles.

The researchers propose that barn owls once inhabited the cave, regurgitating the bones of their prey, which subsequently fossilized. Over generations, this process led to the accumulation of thousands of fossils. Other animals, like tortoises and crocodiles, may have fallen into the cave—which has an approximately 8-meter drop at its entrance—and died after being unable to escape. The cave's protected environment further aided in the preservation of the specimens.

While examining the fossils, Viñola-López noticed dirt inside the tooth sockets, reminiscent of wasp cocoons. CT scans, which use X-ray technology to create detailed 3D images, confirmed that the small nests were constructed from mud and belonged to bees. Unlike wasp nests, which are made of saliva and chewed plant fibers, the cave nests had smooth interiors, characteristic of bee nests built with compacted dirt and a waxy secretion that coats the inner walls.

Viñola-López noted that while some bees prefer shaded areas and some are nocturnal, nesting inside a cave is highly unusual. There is only one other documented case of a burrowing bee nesting in a cave, according to the Florida Museum of Natural History. He also pointed out that multiple generations of bees used the same holes, suggesting a large communal nesting site.

That said, the reality is a bit more complicated. the cave's humid conditions, while preserving the fossils, were not conducive to preserving insect bodies. Consequently, the researchers were unable to identify the specific bee species responsible for the nests. It remains unknown whether the nests were built by an extinct species or one that still exists today. For now, the nests are classified as *Osnidum almontei*, named after Juan Almonte Milan, who initially discovered the cave, according to the study.

Stephen Hasiotis, a geology professor at the University of Kansas, Lawrence, who specializes in paleontology and ichnology (the study of trace fossils) but was not involved in the study, called it a valuable contribution to the record of "hidden biodiversity." He explained that these bee cells, or trace fossils, provide information about the environment and ecosystem in which the bees lived.

Hasiotis added that the consistent humidity and protection from rain, floods, predators, and omnivores likely made the cave soils an ideal nesting location for the bees.

Viñola-López hopes to conduct further surveys in the area to determine if the bees still inhabit the region and whether this behavior occurs in other caves and on other islands.

Anthony Martin, a professor of practice in the department of environmental sciences at Emory University, who was also not involved in the study, stated that the discovery is surprising because modern bees are not known to use bones for nesting or to nest in caves. The fossil bee brooding chambers indicate that these bees were capable of both, which Martin finds exciting.

Martin, author of “Life Sculpted: Tales of the Animals, Plants, and Fungi that Drill, Break, and Scrape to Shape Earth,” noted that insects have been adapting to environmental changes for nearly 400 million years, and ground-nesting bees have existed for about 100 million years. This fossil discovery serves as a reminder that bees will continue to adapt and evolve as needed.