When paleontologists examined fossils from a cave in Hispaniola, they discovered more than the remains of extinct animals. The bones also contain evidence of unusual behavior – insects nesting inside the fossils.
This behavior left behind the first known evidence of bees nesting inside pre-existing fossil cavities. Proceedings of the Royal Society B reports. The discovery shows how environmental pressures influenced the insects' behavior and how these adaptations were preserved along with the fossils themselves.
Giant barn owls and the creation of a fossil archive
fossils originate from a limestone cave in the southern Dominican Republic known as Cueva de Mono. Evidence from the site suggests that for many generations it served as a nesting and feeding site for giant barn owls, who brought prey into the cave to feed their chicks.
An illustration of ancient bees found in a cave on the Caribbean island of Hispaniola, nesting inside pre-existing fossil cavities.
(Image credit: Jorge Machuchi/CC BY)
The remains include thousands of bones of hutia, a large Caribbean rodent that is scarce in the island's fossil record. Outside the cave, hutia fossils appear only occasionally, more often in the form of individual teeth or fragments of jaws. There are many of them inside the cave.
By constantly hunting in the same places and returning to the same roost, the owls gradually concentrated the remains of their prey inside the cave. Some animals were probably brought in whole, while others were eaten elsewhere and then regurgitated as compact pellets.
Later, as bees tunneled through shallow, clay-rich sediments, some of them stumbled upon fossilized remains. Instead of abandoning these sites, the insects appear to have used the hollow spaces inside the bones, including empty tooth sockets, as ready-made nesting chambers.
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How burrowing bees turned fossils into nests
Burrowing bees typically dig narrow tunnels in exposed soil leading to small underground chambers where eggs develop. Nesting inside caves is rare, and the use of pre-existing fossil cavities as nesting sites has never been recorded.
Researchers found cavities with smooth lines inside the jaws and vertebrae of a hutia recovered from the cave, as well as inside the pulp cavity of a sloth tooth. The interior lacked the rough texture of bone, instead showing signs of intentional lining.
Many burrowing bees coat the inside of their nests with a waxy secretion produced by special glands. The coating waterproofs the chamber and leaves behind a smooth interior surface, which distinguishes bee nests from nests made by wasps or other insects.
CT scans showed that some cavities were reused several times. In one Hutia jaw, a single tooth socket contained six stacked nesting chambers, each located inside the previous one. This pattern suggests that the bees were returning to existing cavities rather than digging new tunnels after previous occupants emerged.
There is no evidence that insects drilled into the fossils or altered their shape. Instead, they used pre-existing hollow spaces that exactly matched their nests in size and geometry. What remains is not the bees themselves, but physical traces of their behavior – ichnofossils preserved in the remains of much larger animals.
Why environmental pressure changed bees' nesting behavior
Environmental constraints likely influenced the bees' unusual nesting behavior. Much of Hispaniola is dominated by sharp limestone karst, which has little of the fine, stable soil that bees typically need to build nests.
The caves offered a rare alternative. The sediments accumulated in protected pockets, and the fossilized bones and teeth formed protected cavities that were exactly the size and shape of the bees' nesting chambers.
Evidence points to consistency driven by availability rather than preference. Predators concentrated animal remains in the cave, allowing sediment to accumulate around them, and much later, insects began to exploit both the soil and the hollow spaces preserved within the fossils.
Read more: Leeches don't always suck blood: ancient fossils show they swallowed prey whole
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