Bees are well known for their species and amazing behavioral diversity, from solitary species that nest in burrows to social species that build highly compartmentalized nests. This nesting pattern is partially documented in the fossil record through trace fossils dating from the Cretaceous to the Holocene. In a new paper, Field Museum paleontologist Lazaro Viñola Lopez and colleagues describe a new nesting behavior based on trace fossils found in Late Quaternary cave deposits on the Caribbean island of Hispaniola: isolated brood cells called Osnidum almonteywere found in the cavities of vertebrate remains.
Reconstruction of the life of a trail-building bee nesting inside a cave and using bone cavities as chambers for some of its brooding cells. Image credit: Jorge Mario Macho.
“The initial descent into the cave is not too deep – we tied a rope to the side and then lowered ourselves down,” said Dr. Viñola Lopez.
“If you go there at night, you'll see the eyes of the tarantulas that live inside. But once you go down the ten-metre-long underground tunnel, you start finding fossils.”
There were layers and layers of fossils separated by carbonate layers formed by rainy periods in the distant past.
Many of the fossils were from rodents, but there were also bones from sloths, birds and reptiles, numbering more than 50 different species. Taken together, these fossils told a story.
“We think this was a cave where owls lived for many generations, maybe hundreds or thousands of years,” said Dr. Vignola Lopez.
“The owls would go out to hunt and then return to the cave and drop pellets.”
“We're finding fossils of the animals they ate, fossils of the owls themselves, and even some turtles and crocodiles that may have fallen into the cave.”
In the empty tooth sockets of mammalian jaws, Dr. Vignola Lopez and co-authors observed that the sediment in these cavities did not appear as if it had formed by chance.
“It was a smooth and almost concave surface. This is not how sediment usually fills, and I continued to see this in many samples. I thought, “Okay, there's something weird here.” It reminded me of a hornet’s nest,” said Dr. Vignola Lopez.
Some of the most famous nests built by bees and wasps belong to social species that live together and rear their young en masse in large colonies—think paper wasp nests and the wax honeycomb in a honey bee nest.
“But in reality, most bees are solitary. They lay eggs in small cavities and leave pollen for the larvae to feed on,” said Dr. Vignola Lopez.
“Some species of bees dig holes in wood or soil or use empty structures for nests. Some species in Europe and Africa even build their nests in empty snail shells.”
To better study the potential insect nests present in the cave fossils, the authors scanned the bones using CT scans, essentially shining X-rays through the samples at enough angles that they could create three-dimensional images of the compacted dirt inside the tooth sockets without destroying the fossils or damaging the sediment.
The shapes and structure of the deposits looked exactly like the clay nests created today by some species of bees; Some of these nests even contained grains of ancient pollen that mother bees sealed in the nests for their babies to eat.
They speculate that the bees mixed their saliva with the soil to build small, separate nests for their eggs; each socket was smaller than the eraser on the tip of a pencil.
Building nests inside the bones of large animals could protect bee eggs from hungry predators such as wasps.
Because there were no surviving bees, researchers were unable to identify the species of bee that created them.
However, the nests themselves were so different from the nests of known bees that they were able to give a taxonomic classification to the fossil nests.
They classified the nests as Osnidum almontey in honor of Juan Almonte Milan, the scientist who first discovered the cave.
“Since we didn't find any bee bodies, they may have belonged to a species that is still alive today – very little is known about the ecology of many bees on these islands,” said Dr Vignola Lopez.
Scientists suspect that this behavior was the result of several combined circumstances: There is not much limestone soil in this region, so the bees may have chosen caves as nesting sites rather than simply burrowing into the ground like many other species.
And since this cave was home to several generations of owls who had coughed up a lot of owl droppings over the years, the bees took advantage of the bones provided 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,” Dr Vignola Lopez said.
paper was published today in the magazine Proceedings of the Royal Society of Biological Sciences.
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Lazaro and Vignola-Lopez etc.. 2025. Trace fossils in mammal remains reveal new nesting behavior of bees. R Soc Open Science 12 (12): 251748; two: 10.1098/rsos.251748
