Ancient Bees Turned a Gruesome Bone Graveyard into a Cozy Home

Solitary bees repurposed the skulls of extinct mammals as multi-generational nurseries.

The floor of the Cueva de Mono, a cave in the Dominican Republic, is a gruesome graveyard. For thousands of years, it served as the dining room for a massive family of now-extinct owls called Tyto ostologa. The owls would swoop down upon the tropical landscape, snatch up hutias (guinea-pig-like rodents) or sloths, and return to the dark to feed. They’d regurgitate the indigestible bits like bones and teeth, creating a grim carpet of skeletal remains.

But this story isn’t about the owls. It’s about bees.

According to fascinating new research published in Royal Society Open Science, these piles of bones became the site of a bustling, vibrant bee community. A team of paleontologists found that solitary bees moved into this ossuary, transforming the bones and skulls of mammals into nurseries for their young.

A Gruesome Cave

The Caribbean island of Hispaniola, which contains Haiti and the Dominican Republic, is full of limestone caves. “In some areas, you’ll find a different sinkhole every 100 meters,” says Lazaro Viñola López, a postdoctoral researcher at the Field Museum in Chicago and the paper’s lead author. But while there’s a lot of limestone rock, there is very little soft, deep soil suitable for burrowing.

For many bees, this is a problem.

Around 90% of bee species are solitary burrowers, not hive-dwellers. They usually dig tunnels in the ground to lay their eggs. But in a landscape of rock, every bit of real estate is important. This is where Cueva de Mono comes in.

Juan Almonte Milan, the curator of paleobiology at the Dominican Republic’s Museo Nacional de Historia Natural, discovered the cave, which was riddled with fossils.

“The initial descent into the cave isn’t too deep — we would tie a rope to the side and then rappel down,” says Viñola López. “If you go in at night, you see the eyes of the tarantulas that live inside. But once you walk down a ten-meter-long tunnel underground, you start finding the fossils.”

There were layers and layers of fossils, stacked one on top of each other, separated by carbonate layers. Many of them belonged to rodents, while sloths, birds, and reptiles were also present. Overall, more than 50 species were represented. Unfortunately for them, these animals didn’t set up shop in the cave; they were brought by owls.

“We think that this was a cave where owls lived for many generations, maybe for hundreds or thousands of years,” says Viñola López. “The owls would go out and hunt, and then come back to the cave and throw up pellets. We find fossils of the animals that they ate, fossils from the owls themselves, and even some turtles and crocodiles who might have fallen into the cave.”

Among this trove of fossils, however, scientists also found something very unusual. Many of them had sediment-filled cavities that didn’t look random.“It was a smooth surface, and almost concave. That’s not how sediment normally fills in, and I kept seeing it in multiple specimens. I was like, ‘Okay, there’s something weird here,’” he says. “It reminded me of the wasp nest.”

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A Waspy World

Years ago, when Viñola López was an undergraduate student, he came across ancient remains of wasp cocoons. They were small, thin chambers of dried mud where wasp larvae would metamorphose into adults. Without this bit of information, they may never have figured it out.

But a hunch is one thing and proving it is another. Identifying the culprit in a cold case that is thousands of years old requires forensic precision. The bees themselves are long gone. Soft-bodied insects rarely fossilize well, especially in these conditions. We don’t have the body; we only have the architecture.

Upon close analysis, the holes appeared to be clearly artificial, made by an insect. The next challenge was figuring out which one.

Researchers used a process of elimination. The cells are ellipsoidal, about 6 millimeters long, with smooth inner walls. This rules out wasps, which generally don’t line their subterranean cells so neatly. It also rules out leafcutter bees (Megachilidae), which typically use foreign materials like leaves or petals to build their nests.

The evidence points strongly to the family Halictidae, commonly known as sweat bees. These bees are known to be burrowers and often line their cells with a wax-like substance that would create the smooth finish seen in the fossils. While they are “solitary,” they are often gregarious, meaning they like to live alone, together, creating aggregations of individual nests in a prime location. The density of the nests in the Cueva de Mono samples supports this. The bees packed as many cells as possible into the available bone “plots”.

“Since we didn’t find any of the bees’ bodies, it’s possible that they belonged to a species that’s still alive today — there’s very little known about the ecology of many of the bees on these islands,” says Viñola López. “But we know that a lot of the animals whose bones are preserved in the cave are now extinct, so the bees that created these nests might be from a species that has died out.”

But the team dug even deeper.

Zoom In

The team used Scanning Electron Microscopy (SEM) to analyze the chemical composition of the cell walls. They found traces of organic compounds and a complex microscopic ecosystem.

The walls were teeming with fossilized microbes. The researchers identified structures resembling cyanobacteria and fungi. Specifically, they found spores related to Canalisporium and Acrodictys, fungi that thrive on decaying wood and organic matter. This suggests a humid environment, rich in organic decay from the owl pellets.

Interestingly, the pollen analysis came up mostly empty. Less than 1% of the material found was pollen. At first glance, this seems odd for a bee nest. Bees store pollen as food for their larvae. But the absence of pollen is actually a clue. It suggests the larvae successfully hatched and ate their packed lunches before leaving. The nursery did its job.

This suggests that the bees had an amazing ability to improvise and adapt to a new environment. Life, as they say, finds a way.

“This discovery shows how weird bees can be — they can surprise you. But it also shows that when you’re looking at fossils, you have to be very careful,” says Viñola López. “Even if you’re looking primarily for fossils of larger, vertebrate animals, you should keep an eye out for trace fossils that can tell you about invertebrates like insects. Knowing about insects can tell you a lot about a whole ecosystem, so you have to pay attention to that part of the story.”

Journal Reference: Trace fossils within mammal remains reveal novel bee nesting behaviour, Royal Society Open Science (2025). DOI: 10.1098/rsos.251748