In a cave on the Caribbean island of Hispaniola, ancient bees commandeered the jawbones and other skeletal remains of now-extinct rodents and sloths as ready-made chambers for their nests. These fossilized brood cells, dating to the late Quaternary period, reveal a rare opportunistic nesting strategy in solitary bees, including repeated use of the same bone cavities across generations and large-scale aggregations in a soil-scarce landscape, researchers report December 17 in Royal Society Open Science.
Bees are celebrated for their diverse lifestyles, from the complex hives of honeybees to the simple burrows of solitary species that make up about 90 percent of the group’s roughly 20,000 species. While most ground-nesting bees dig into soil, some exploit preexisting holes in wood, rocks, or even snail shells. Fossil evidence of bee nests, preserved as trace fossils or ichnofossils, offers glimpses into ancient behaviors, but examples from caves or inside bones are virtually unknown.
The new finds come from Cueva de Mono, a dry cave in the southwestern Dominican Republic, where red clay sediments hold a trove of vertebrate fossils, likely accumulated from owl pellets. Amid the bones of extinct rodents like Plagiodontia araeum and sloths like Acratocnus ye, researchers spotted oval-shaped structures filling tooth sockets, skull cavities, and vertebral canals.
Named Osnidum almontei, a new ichnogenus and species meaning bone nest, these isolated cells measure about 6 millimeters long and 4 millimeters wide, with smooth, multilayered inner walls that suggest a waterproof lining applied by the bee. Micro-CT scans revealed up to six stacked generations in a single socket, with only the curved bases of older cells surviving as the bees reused the space. Scanning electron microscopy showed fine clay particles packed densely inside, along with microbial and fungal traces that may have aided preservation or colonized later.
No tunnels connected to the cells were found, but their abundance, restricted to the upper three sediment layers, separated by carbonate bands from wetter periods, points to long-term use of the cave as a nesting hub. “The presence of these nesting cells within multiple layers of the deposits, along with the presence of multiple generations of cells in the same bony cavity, suggests a high level of nesting site fidelity likely spanning numerous generations,” the researchers write.
Palynomorph analysis uncovered rare pollen grains but abundant fungal spores and microbial structures, including cyanobacteria-like tubes and filaments, hinting at a phosphate-rich environment from decaying animal matter that supported mineralization.
This behavior stands out in a karst region of exposed limestone with thin soils, where cave deposits offered the best substrate. “The use of vertebrate remains in cave soils represents a poorly documented behaviour in modern bees and may reflect the scarcity of suitable soil substrates in a karstic landscape dominated by denuded limestone,” the team notes.
Citations
L. W. Viñola-López et al. Trace fossils within mammal remains reveal novel bee nesting behaviour. Royal Society Open Science. Published online December 17, 2025. DOI: 10.1098/rsos.251748
