In a Caribbean cave where owls once spat out the bones of their prey, scientists have uncovered a nesting strategy that rewrites what we know about bees. Instead of burrowing into soil or wood, some ancient bees appear to have raised their young inside the teeth and spines of dead rodents and other small mammals, turning discarded skeletons into fortified nurseries.
The discovery, preserved in regurgitated pellets and fossilized bone, reveals a behavior that has never been documented before in living or extinct bees. It also opens a rare window into how insects, predators and climate intertwined in a single cave ecosystem thousands of years ago, when the island now known as Hispaniola looked very different from today.
From owl pellets to hidden bee nurseries
The story begins with predators, not pollinators. About 20,000 years ago, a family of owls hunted small mammals around a limestone cavern and then returned to the darkness to cough up compact pellets of fur and bone. Over time, those pellets broke apart and the bones accumulated on the cave floor, building a dense layer of skeletal debris that effectively archived the local food web. The owls were unintentionally curating a collection of tiny skulls, vertebrae and teeth that would later become prime real estate for another species entirely.
When paleontologists finally sifted through that bone bed, they were not just cataloging extinct rodents and sloths. They began to notice Weird pockets of sediment tucked inside the hollow spaces of the skeletons, little plugs and cylinders that did not match the surrounding cave silt. Those odd inclusions, described as EASTER EGGS of ancient behavior, turned out to be the remains of bee brood cells, carefully sealed chambers where larvae once developed in safety, protected from predators like wasps by the armor of bone.
A Caribbean cave and an extinct rodent host
The cave at the center of this discovery, known as Cueva de Mono, sits in the south of what is now the Dominican Rep. Inside, researchers found that the bones were not just scattered randomly but had been reworked and recycled by multiple species over thousands of years. Among the most striking finds were skulls of an extinct rodent whose eye sockets and cranial cavities had been repurposed as nesting chambers, with sediment-filled tubes and partitions marking where bees once laid eggs and stored food for their young. The rodents themselves are long gone, but their skulls became the scaffolding for a second life as insect nurseries.
These cleverly recycled remains were part of a broader pattern across the cave, where bees appear to have targeted the most sheltered and structurally sound cavities. The skulls of the extinct rodent were joined by other bones that showed similar modifications, suggesting that the insects were not opportunistically using a single carcass but repeatedly seeking out the same kinds of hollow spaces. In that sense, the cave functioned as a shared habitat for owls, mammals and bees, with each group exploiting the others’ leftovers in a tightly linked chain of survival.
First fossil bee nests inside bones
What makes the Cueva de Mono material so striking is not just the setting but the behavior it captures. According to the research team, these are the first known Paleontologists to document bee nests built entirely inside fossilized bones, rather than in soil, wood or plant stems. Working on the Caribbean island of Hispaniola, they identified an abundance of accumulated silt within the bones that did not match simple cave infill, but instead showed the layered, compartmentalized structure typical of bee brood cells. That pattern, repeated across multiple specimens, points to a deliberate nesting strategy rather than a geological accident.
The nests themselves preserve a level of detail that is rare in the insect fossil record. In some bones, the researchers could distinguish individual cells stacked end to end, each once containing a single egg and a supply of pollen and nectar. The fact that these cells were carved into the interior of ribs, vertebrae and skulls indicates that the bees were capable of boring into relatively hard material, then sealing their offspring behind sediment plugs. For scientists who usually have to infer ancient insect behavior from scattered impressions in rock, finding intact nest architecture inside bone is like stumbling onto a time capsule of reproductive strategy.
Teeth, spines and a sloth’s pulp cavity
Among the most evocative examples is a nest found inside the pulp cavity of a sloth tooth, a space that would once have been filled with soft tissue and blood vessels. After the animal died and its tooth hollowed out, a bee apparently tunneled in and constructed a series of brood cells, stacking them like Russian dolls inside the narrow chamber. The report notes that Tree sloths were once common in parts of the island, so their teeth would have been a recurring feature in the cave’s bone deposits, offering repeated opportunities for bees to exploit this unusual nesting niche.
Other bones show similar ingenuity. Vertebrae with hollow neural canals, long bones with marrow cavities and even delicate cranial bones became host to sediment-filled tubes that match the dimensions and organization of bee nests. In each case, the insects appear to have selected spaces that were just wide enough to accommodate a line of brood cells, then sealed the openings with carefully packed silt. The result is a cross section of ancient life in which teeth and spines, once part of living mammals, were repurposed as armored cradles for the next generation of bees.
How the nests reveal bee behavior
To interpret these structures, researchers leaned on what is known about modern burrowing bees, which often dig into soil banks or hollow plant stems to create linear series of brood cells. The fossil nests inside the bones show the same basic architecture, with repeated chambers separated by thin partitions and capped by a final plug. In one analysis, scientists described how the bees turned the bones into ready-made burrows, carving only minimal additional space and then filling it with sediment to form the cells, a pattern that matches the behavior of many solitary bees today. The How Burrowing Bees Turned Fossils analysis emphasizes that the bees likely reused the same bones multiple times, returning after earlier occupants had emerged to lay new eggs in the vacated cells.
The nests also hint at the bees’ life cycle and seasonal rhythms. Because each cell would have housed a single larva, the number and arrangement of cells within a bone can suggest how many offspring a female produced in a given nesting attempt. The presence of multiple generations of infill within the same bone implies that the insects treated these skeletal cavities as stable, long term resources, revisiting them year after year. That behavior would make sense in a cave environment where bones accumulated steadily and remained protected from surface erosion, offering a durable alternative to more ephemeral soil burrows.
Why bones made the perfect bee real estate
From the bees’ perspective, bones offered several advantages over more typical nesting sites. The hard outer layer of a skull or vertebra provided physical protection against predators and parasites, while the interior cavities were already hollow or partially hollow, reducing the amount of excavation required. In the cave environment, these bones were also buffered from temperature swings and direct rainfall, creating a relatively stable microclimate for developing larvae. One study described how Bones of now extinct species became a haven for bee babies, with the insects exploiting the abundance of skeletal material they found in excess on the cave floor.
There is also a strategic element to nesting inside bone. Many modern bees face intense competition for nesting sites, and in a cave crowded with other insects and scavengers, any structure that offered a secure, enclosed space would have been valuable. By choosing the interior of teeth and spines, the bees effectively hid their offspring in plain sight, in places that predators and kleptoparasitic insects might overlook. The sediment plugs at the entrances of the nests further camouflaged the brood cells, making them look like ordinary bits of cave silt rather than a cache of developing larvae and stored food.
Piecing together a 20,000-year-old ecosystem
The nests do more than document an odd behavior; they help reconstruct an entire ecosystem at a moment in deep time. The presence of owl pellets, small mammal bones, sloth teeth and bee nests in the same stratigraphic layers shows how predators, prey and pollinators interacted in and around the cave. Earlier reporting notes that Sometimes the owls would cough up pellets containing the bones of rodents and other animals, which then broke apart and accumulated on the cave floor, creating a rich substrate for bees searching for places to lay their eggs. That chain of events links aerial predators, terrestrial mammals and ground nesting insects in a single, tightly coupled system.
The age of the deposits, identified as 20,000-year-old fossils, places this ecosystem near the end of the last ice age, when global climates and sea levels were shifting. On the island, cooler and perhaps drier conditions may have influenced vegetation patterns, which in turn would have shaped the availability of pollen and nesting sites for bees. By examining which mammals were present in the owl pellets and which bones were reused by bees, scientists can infer how biodiversity and species interactions changed as the climate warmed and some lineages, including the extinct rodent and local sloths, disappeared from the landscape.
What this means for bee evolution and diversity
For bee evolution, the discovery expands the known repertoire of nesting strategies and underscores how flexible these insects can be in exploiting new materials. Modern bees already show remarkable diversity in where they raise their young, from underground burrows to hollow twigs and even snail shells. The fossil nests inside bones suggest that at least some ancient lineages were willing to push that flexibility further, turning vertebrate remains into a novel resource. In interviews about the work, one researcher noted that the behavior documented in the fossils is unlike anything we have seen before in living bees, a point echoed in coverage of Fossilized Bee Nests Inside Skeletons Are Unlike Anything We have Seen Before.
That novelty matters because it hints at ecological experiments that may have flourished and then vanished without leaving living descendants. If bone nesting was once more widespread, it could have provided a competitive edge in environments where traditional nesting substrates were scarce. Alternatively, it might have been a highly specialized adaptation tied to the unique conditions of caves like Cueva de Mono, disappearing when those conditions changed. Either way, the behavior captured in the fossils broadens the baseline for what scientists consider possible in bee biology, reminding us that the evolutionary playbook is often more inventive than modern ecosystems alone would suggest.
How scientists cracked the case
Unraveling this story required a mix of fieldwork, microscopy and comparative anatomy. Researchers first had to recognize that the sediment inside the bones was not just random infill but something patterned and biological. They compared the internal structures to known bee nests, looking for telltale features such as repeated cell size, thin partitions and terminal plugs. In one account, a scientist explained that the team initially thought they were dealing with simple silt deposits until closer inspection revealed the organized architecture of brood cells, a realization that shifted the interpretation from geology to behavior. Reporting on how Dec fieldwork unfolded highlights how careful observation turned a pile of bones into evidence of insect engineering.
Once the team identified the nests, they used imaging techniques to map the internal cavities without destroying the fossils, then correlated those images with external bone features to understand how the bees accessed and modified the spaces. The work culminated in a peer reviewed study in the journal Royal Society Open Science, which framed the find as a Discovery of ancient bee nests in fossils that points to a never before seen behavior. By Taylor Nicioli reported that the study’s authors see the nests as a rare convergence of taphonomy, ecology and insect life history, preserved only because the cave environment protected both the bones and the delicate sediment structures inside them. Updated Dec coverage underscores how unusual it is to catch insect behavior in the act at this timescale.
A new lens on how fragile behaviors vanish
For me, the most striking aspect of the Cueva de Mono nests is how they capture a behavior that might otherwise have vanished without a trace. Insects leave behind far fewer fossils than vertebrates, and behaviors like nesting are even harder to preserve. Here, a chain of coincidences had to line up: owls had to hunt and regurgitate their prey in the same cave where bees were active; bones had to accumulate rather than wash away; bees had to choose those bones as nesting sites; and the entire assemblage had to be buried and protected long enough to fossilize. The fact that all of those steps occurred in sequence is what allows us to see how ancient bees once lived inside the teeth and spines of dead animals.
The find also serves as a reminder that many ecological interactions are inherently fragile. If the owls had nested elsewhere, or if the cave had flooded more frequently, the bees might still have used bones as nests, but we would never know. By documenting this one case in detail, the researchers give us a template for what to look for in other fossil sites: subtle patterns of sediment inside bones, repeated cell like structures, and associations between predator accumulations and insect traces. Future work may reveal that bone nesting was not a one off quirk but part of a broader, now mostly invisible repertoire of insect strategies that helped species survive in challenging environments.
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