Paleontologists studying fossil jaws in a Caribbean cave found ‘weird’-looking concave cavities; turns out ancient bees laid eggs inside them 20,000 years ago

Amongst the world’s 20,000 species of bees, around 90% of them are solitary, meaning the “hardworking single moms” build their own nests in the ground, instead of having a large hive or colony to defend, as per the World Wildlife Fund.

But researchers in a 2015 study discovered a far more unusual nesting spot: the empty tooth sockets of fossilized animal jaws inside a limestone cave on the Caribbean island of Hispaniola, where tiny bees built nests and laid eggs around 20,000 years ago.

As per the study, which was published in the journal Royal Society Open Science, this is the first time bees have been recorded using bones to nest and lay eggs. Researchers exploring the cave on Hispaniola, a Caribbean island shared by Haiti and the Dominican Republic, discovered thousands of bones, including remains of rodents, sloths, birds, and reptiles.


The cave had likely been home to owls for generations, probably 20,000 years ago, with their regurgitated pellets with bones accumulating on the floor over thousands of years.

Lázaro Viñola-López, a postdoctoral researcher at the Field Museum in Chicago and the study's lead author, was examining mammal bones when he noticed smooth, concave deposits inside several empty tooth sockets.

“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 said in a statement published in Field Museum. “It reminded me of the wasp nest.”

It was a flashback for the researcher; when he visited Montana, where a paleontologist showed him the ancient remains of wasp cocoons that were small chambers of hardened mud where wasp larvae would metamorphose into adults, just like the Caribbean fossils.

How did bees turn fossil jaws into nurseries?

We generally see bees living in large hives, but most bee species are solitary and build individual nests in small cavities, leaving food for their developing offspring. “But actually, most bees are solitary. They lay their eggs in small cavities, and they leave pollen for the larvae to eat,” Viñola-López said. “Some bee species burrow holes in wood or in the ground, or use empty structures for nests. Some species in Europe and Africa even build their nests in empty snail shells.”

Researchers to investigate further CT scanned the fossil bones; it showed detailed 3D images of the compacted sediment inside the tooth sockets

They found that the structures resembled mud nests built by some solitary bees today. Some even contained grains of ancient pollen, which researchers believe mother bees had stored as food for their offspring.

Researchers hypothesize that the bees mixed saliva with dirt to construct the tiny nests, each smaller than the eraser at the tip of a pencil. So, this buildup of nests inside another larger animal may have protected them from predators like wasps.

What happened to the ancient bees?

Researchers, however, did not discover any fossilized bees inside them. The warm, humid conditions inside the cave were not suitable for preserving the delicate bodies of small insects, they said. The researchers named the fossil nests Osnidum almontei, after Juan Almonte Milan, the scientist who first identified the cave.

“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,” Viñola-López said. “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.”

First known evidence of this kind of beehive

As per the team, this discovery is the first known evidence of bees using cavities in animal bones to build nests and lay eggs. López suspected that this behavior was the result of mixed circumstances: one is that there isn’t enough soil covering the limestone ground, so the bees have turned to caves as a place to nest.

Another is that, since this cave was home to owls that coughed up many owl pellets over the years, the bees just took advantage of the bones the owls left.

“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,” Viñola-López said.

“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.”