About 20,000 years ago, the cave was home to generations of owls, regularly spitting out pellets containing the bones of their prey. Those discarded bones later became an unexpected resource for another group of animals. According to a new study published in Royal Society Open ScienceAncient bees used the empty tooth sockets of their fossilized jaws as small nests for their offspring.
The discovery provides the first known evidence that honey bees use animal bones as a place to lay eggs, revealing an unusual nesting strategy never before documented.
A cave rich in fossils that preserves an ancient ecosystem
The Caribbean island of Hispaniola, shared by Haiti and the Dominican Republic, is home to thousands of limestone caves.
“In some areas, you can find different sinkholes every 100 meters,” says Lázaro Vignola López, a postdoctoral fellow at the Field Museum in Chicago and lead author of the study.
The cave investigated in this study had previously been identified by Juan Almonte Milan, curator of paleontology at the National Museum of Natural History of the Dominican Republic, as having an extremely rich deposit of fossils. Vignola López and colleagues explored this site while he completed his doctoral research at the University of Florida and the Florida Museum of Natural History.
“The first descent into the cave is not very deep. You tie a rope to the side and then rappel down,” says Vignola López. “When you go inside at night, you can see the eyes of the tarantulas that live inside. But when you walk through the 10-metre-long tunnel underground, you start to find fossils.”
The cave preserved multiple fossil layers separated by carbonate deposits formed during an ancient rainy season. Most of the remains were of rodents, but researchers also found fossils of sloths, birds, reptiles, and many other animals from more than 50 species.
These fossils revealed how the cave was used over a long period of time.
“We believe this was a cave where owls lived for generations, perhaps hundreds or even thousands of years,” Vignola López said. “Owls would go out and hunt, then come back into the cave and spit out pellets. We’ve found fossils of animals they ate, fossils of the owls themselves, and even turtles and crocodiles that may have fallen into the cave.”
A rare discovery inside the tooth socket
Vignola López primarily studies mammal bones left behind by owls, but as she was cleaning up the fossils, she noticed something unusual.
Some jawbones contained smooth deposits within the empty tooth sockets, which looked different from naturally accumulated deposits.
“It was a smooth surface, almost concave, and that’s not how deposits usually fill in. I kept looking at it in multiple specimens and thought, ‘Okay, there’s something weird here,'” he says. “It reminded me of a hornet’s nest.”
This observation immediately reminded me of an earlier experience during an undergraduate fossil dig in Montana. There, another paleontologist showed me a fossilized hornet cocoon. A cocoon is a small mud chamber in which the developing larva grows into an adult. The structure was very similar to what he had seen inside fossil jaws.
Ancient beehive hidden in bones
Although honey bees and paper wasps are well known for building large communal nests, most bees actually live alone.
“But in reality, most bees are solitary. They lay eggs in small cavities and leave pollen for the larvae to eat,” says Vignola López. “Some bee species dig holes in trees or the ground, or use empty structures for their nests. Some European and African species even nest in empty snail shells.”
To investigate further, the research team performed CT scans of the fossil bones. The scan produced a detailed three-dimensional image of the compressed material inside the tooth socket without damaging the fossil or sediment.
Scans showed that the structure was consistent with mud nests built by modern solitary bees. Some hives also contained ancient grains of pollen that mother bees stored as food for their developing offspring.
Researchers believe the bees created each tiny nest, smaller than a pencil eraser, by mixing soil and saliva. Nesting in the hollow bones of large animals may also help protect eggs from natural predators such as wasps.
A new type of fossil nest
The nest contained no fossilized bees, which is not surprising, the researchers said, since the warm, humid environment of caves is not ideal for preserving delicate insect bodies.
Without the preserved bees, scientists could not pinpoint exactly which species built the nest. However, the nest structure itself was distinct enough to receive its own taxonomic classification.
The fossil nest was given a name Osnidam Almontei In honor of Juan Almonte Milan, who first identified this cave and who, as one of Hispaniola’s leading paleontologists, has spent decades studying this area.
“We did not find any dead bees, so they may have belonged to a species that is still alive today. Little is known about the ecology of many bees on these islands,” says Vignola López. “However, we know that many of the animals whose bones are preserved in this cave are now extinct, so the bees that built these nests may be from extinct species.”
First known example of a bee nesting inside a bone
Researchers say this is the first recorded case of honeybees using animal bones as nesting sites.
Vignola López believes that several environmental factors may have enabled this behavior. The area’s limestone landscape has little soil and traditional underground nesting sites are few and far between. At the same time, generations of owls continued to deposit bones throughout the cave, providing countless hollow tooth sockets that solitary bees could use.
“This discovery shows how strange bees are. They can surprise you, but it also shows that you need to be very careful when looking at fossils,” says Vignola López.
He points out that without previous experience in recognizing fossilized wasp nests, he could have simply removed the unusual deposits during fossil preparation.
“Even if you’re primarily looking for fossils of large vertebrates, you still need to pay attention to trace fossils that can tell you about invertebrates such as insects. Knowing about insects can tell you a lot about the ecosystem as a whole, so you need to pay attention to that part.”
