About 20,000 years ago, a cave was home to generations of owls that regularly coughed up 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 in fossilized jaws as little nests for their offspring.
The discovery marks the first known evidence that bees use animal bones to lay their eggs, revealing an unusual nesting strategy that has never been documented before.
Fossil rich cave preserves an ancient ecosystem
Hispaniola, the Caribbean island shared by Haiti and the Dominican Republic, contains thousands of limestone caves.
“In some areas, you’ll find a different sinkhole every 100 meters,” says Lazaro Vinola Lopez, a postdoctoral researcher at the Field Museum in Chicago and lead author of the study.
The cave investigated in this study was first identified as containing exceptionally rich fossil deposits by Juan Almonte Milan, curator of paleobiology at the Museo Nacional de Historia Natural in the Dominican Republic. Vinola Lopez and colleagues discovered the site while he was completing his PhD research at the University of Florida and the Florida Museum of Natural History.
“The initial descent into the cave is not very deep – we have to tie a rope to the edge and then descend,” says Vinola Lopez. “If you go inside at night, you see the eyes of the tarantulas that live inside. But once you walk into the ten-meter-long tunnel underground, you start to find fossils.”
The cave preserves several fossil layers separated by carbonate deposits formed during ancient rainy periods. Most of the remains were of rodents, but researchers also recovered fossils of sloths, birds, reptiles, and many other animals, representing more than 50 species.
Together, the fossils revealed how the cave was used over a long period of time.
Vinola Lopez says, “We think it was a cave where owls lived for many generations, maybe hundreds or thousands of years.” “The owls would go out and hunt, and then come back into the cave and shed pellets. We found fossils of the animals they ate, fossils of owls, and even some turtles and crocodiles that may have fallen into the cave.”
An unusual finding inside the tooth socket
Vinola Lopez was studying mammal bones left primarily by owls when she noticed something unusual while cleaning the fossils.
Many of the jaw bones contained smooth deposits inside their empty tooth sockets that looked different from naturally deposited sediments.
“It had a smooth surface, and was almost concave. Usually sediments don’t fill out like this, and I kept seeing it in several samples. I thought, ‘Okay, there’s something strange here,'” he says. “It reminded me of a wasp’s nest.”
The observation immediately reminded him of an earlier experience during an undergraduate fossil excavation in Montana. There, another paleontologist had shown him fossilized wasp cocoons, which are tiny mud chambers where larvae develop and mature into adults. The structures were very similar to what he was seeing inside the fossil jaw.
Ancient bee nests hidden in bones
Although honey bees and paper wasps are known for building large communal nests, most bee species actually live alone.
“But in reality, most bees live alone. They lay their eggs in small cavities, and they leave pollen for the larvae to eat,” says Vinola Lopez. “Some species of bees burrow into wood or the ground, or use empty structures for nesting. Some species in Europe and Africa also build their nests in empty snail shells,”
To investigate further, the research team CT scanned the fossilized bones. The scans produced detailed three-dimensional images of the compressed material inside the tooth socket without damaging the fossil or sediment.
Scans revealed that the structures matched mud nests made by some modern solitary bees. Some nests also preserved grains of ancient pollen that mother bees had stored as food for their developing offspring.
Researchers believe the bees mixed dirt with saliva to build each tiny nest, the size of which was smaller than a pencil eraser. Building nests inside the hollow bones of larger animals may also have helped protect their eggs from predators such as wasps.
A new type of fossil nest
There were no fossils of bees in the nests, which the researchers say is not surprising since the cave’s hot, humid conditions are poor for preserving the delicate insect bodies.
Without preserved bees, scientists could not determine which species built the nests. However, nest structures varied sufficiently to receive their own taxonomic classification.
Fossil nests were named Osnidum almontei In honor of Juan Almonte Milans, who first identified the cave and spent decades studying the area as one of Hispaniola’s leading paleontologists.
Vinola López says, “Since we did not find any bodies of bees, it is possible that they belong to a species that still survives today – little is known about the ecology of many bees on these islands.” “But we know that many of the animals whose bones are preserved in the cave are now extinct, so these nesting bees may have belonged to a species that is now extinct.”
First known example of bees nesting in bones
According to the researchers, this is the first documented case of bees using animal bones as nesting sites.
Vinola Lopez believes that several environmental factors likely made this behavior possible. The limestone landscape in this area has very little soil, making traditional underground nesting sites rare. At the same time, generations of owls continuously deposited bones throughout the cave, providing countless hollow tooth sockets that solitary bees could use.
Vinola Lopez says, “This discovery shows how strange bees can be – they can surprise you. But it also shows that when you are looking at fossils, you have to be very careful.”
He noted that without his previous experience identifying fossil wasp nests, he would have simply cleaned up the unusual sediment during fossil preparation.
“Even if you’re primarily looking for fossils of large, vertebrate animals, you should also keep an eye out for fossils that can tell you about invertebrates like insects. Learning about insects can tell you a lot about the entire ecosystem, so you have to pay attention to that part of the story.”