Paleontologists discover first known example of ancient bees nesting inside vertebrate fossils

Bees are well known for their species and remarkable behavioral diversity, ranging from solitary species that nest in burrows to social species that build highly compartmentalized nests. This nesting variation is partially documented in the fossil record by trace fossils dating from the Cretaceous to the Holocene. In a new paper, Field Museum paleontologist Lazaro Viñola López and colleagues described a new nesting behavior based on trace fossils recovered from a Late Quaternary cave deposit on the Caribbean island of Hispaniola: isolated brood cells, named Osnidum almonteihave been found inside cavities of vertebrate remains.

“The initial descent into the cave is not too deep: we attach a rope to the side and then rappel down,” explained Dr. Viñola López.

“If you go in at night, you see the eyes of the tarantulas that live inside. But once you go through a ten-meter-long underground tunnel, you start to find the fossils.”

There were layers and layers of fossils, separated by carbonate layers resulting from rainy periods in the distant past.

Most of the fossils belonged to rodents, but there were also bones of sloths, birds and reptiles representing more than 50 different species. Together, these fossils told a story.

“We think this was a cave where owls lived for many generations, perhaps for hundreds or thousands of years,” said Dr. Viñola López.

“The owls would go out to hunt, then come back to the cave and vomit pellets.”

“We find fossils of the animals they ate, fossils of the owls themselves and even turtles and crocodiles that may have fallen into the cave.”

In the empty dental sockets of mammals’ jaws, Dr. Viñola López and her co-authors noticed that the sediment in these cavities did not appear to have accumulated randomly.

“It was a smooth, almost concave surface. That’s not how sediment normally fills, and I kept seeing it on several specimens. I was like, ‘Okay, there’s something weird here.’ It reminded me of the wasps’ nest,” said Dr. Viñola López.

Some of the most well-known nests built by bees and wasps belong to social species that live together and raise their young en masse in large colonies – think paper wasp nests and wax bee nests in a honey bee nest.

“But in reality, most bees are solitary. They lay their eggs in small cavities and leave pollen for the larvae,” said Dr. Viñola López.

“Some species of bees dig holes in wood or ground, or use empty structures for their nests. Some species from Europe and Africa even build their nests in empty snail shells.”

To better examine potential insect nests present in the cave fossils, the authors scanned the bones, essentially X-raying the specimens from enough angles that they could produce 3D images of the compacted dirt inside the tooth sockets without destroying the fossils or disturbing the sediment.

The shapes and structures of the sediments resembled the mud nests created today by certain species of bees; some of these nests even contained grains of ancient pollen that mother bees had stuffed into the nests so their babies could eat them.

They hypothesize that the bees mixed their saliva with soil to make these small individual nests for their eggs; each nest was smaller than the eraser on the end of a pencil.

Building their nests inside the bones of larger animals could have protected the bees’ eggs from hungry predators like wasps.

Since no bees were preserved, researchers were unable to assign a species to the bees that made them.

However, the nests themselves were different enough from known bee nests that a taxonomic classification could be given to the fossil nests.

They classified the nests as Osnidum almontei named after Juan Almonte Milan, the scientist who first discovered the cave.

“As we did not find any bee bodies, it is possible that they belong to a species still alive today. Very little is known about the ecology of most of the bees on these islands,” said Dr Viñola López.

Scientists suspect that this behavior is the result of several circumstances combined: There isn’t much soil covering the limestone soil in this region, so the bees may have turned to caves as a nesting location rather than simply burrowing into the ground like many other species.

And since this cave was a multi-generational home to owls that spit out lots of owl pellets over the years, the bees took advantage of the bones delivered by the owls.

“This discovery shows how strange bees can be: they can surprise you. But it also shows that when you look at fossils you have to be very careful,” said Dr Viñola López.

The article was published today in the Proceedings of the Royal Society B Biological Sciences.

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Lázaro and Viñola-Lopez and others. 2025. Trace fossils in mammal remains reveal new bee nesting behavior. R Soc Open Sci 12 (12): 251748; two: 10.1098/rsos.251748