A Graveyard and a Nursery
In a limestone cave in the Dominican Republic’s arid southwest, a intersection of life and death is preserved. The site, Cueva de Mono, contains the first and only known instance of bees building nests inside the bones of other animals. The cave floor consists of red clay packed with the fossilized remains of extinct sloths, birds, reptiles, and rodents. Many of these bones were carried into the cave over thousands of years by a giant barn owl that used the cavern as a roost. The owl would regurgitate pellets of indigestible bones, creating a dense layer of skeletal remains on the cave floor.
Sometime around 20,000 years ago, solitary burrowing bees entered the cave and found this bone-littered floor. In an area where deep soil is scarce, the hollows within these discarded skeletons provided an ideal shelter. Researchers using micro-CT scans revealed that bees built their brood cells inside these natural cavities. They found nests located in the empty tooth sockets of extinct rodents, the vertebral canal of a hutia (a large Caribbean rodent), and even the pulp cavity of an extinct ground sloth's molar. Each tiny nest, smaller than a pencil eraser, was constructed from a mixture of dirt and saliva. Inside these hardened capsules, the mother bee would place a supply of pollen and nectar for her larva to eat after hatching.
Ichnofossils in Body Fossils
The bee nests of Cueva de Mono are a rare type of fossil known as a trace fossil, or ichnofossil. Unlike body fossils (such as bones or shells), trace fossils preserve evidence of an organism's behavior. Footprints, burrows, and nests fall into this category. The Cueva de Mono find is unique is that it is a trace fossil located inside a body fossil, showing a direct interaction between two long-extinct species.
The ground sloths that once roamed Hispaniola, such as Parocnus serus and Acratocnus ye, vanished around 4,500 to 5,000 years ago, shortly after the arrival of humans on the island. While the bee nests are much older, the bones they occupied belonged to these now-extinct megafauna. The specific bee species that created the nests is unknown, as no insect bodies were preserved in the cave's warm, humid environment. It may have been a species that is now extinct or one that still lives on the island today. The discovery shows a survival strategy, where ancient bees adapted to a soil-poor environment by repurposing the remains of the dead as a protected nursery for their young.