About 20,000 years ago, a family of owls lived in a cave. Sometimes, they would cough up owl pellets containing the bones of their prey, which landed on the cave floor. And, researchers have just discovered, ancient bees would use the bones' empty tooth sockets as nests. A new study published in the journal Royal Society Open Science documents this discovery, which represents the first time bees have ever been known to use bones as places to lay their eggs.
The Caribbean island of Hispaniola, which contains Haiti and the Dominican Republic, is full of limestone caves. "In some areas, you'll find a different sinkhole every 100 meters," says Lazaro Viñola López, a postdoctoral researcher at the Field Museum in Chicago and the paper's lead author.
Juan Almonte Milan, the curator of paleobiology at the Dominican Republic's Museo Nacional de Historia Natural, had identified a cave in the southern Dominican Republic as a deposit of lots of fossils, so Viñola López and several of his colleagues explored the cave looking for specimens to study as part of his PhD program at the University of Florida and the Florida Museum of Natural History.
"The initial descent into the cave isn't too deep— we would tie a rope to the side and then rappel down," says Viñola López. "If you go in at night, you see the eyes of the tarantulas that live inside. But once you walk down a ten-meter-long tunnel underground, you start finding the fossils."
There were layers and layers of fossils, separated by carbonate layers resulting from rainy periods in the distant past. Many of the fossils belonged to rodents, but there were also bones from sloths, birds, and reptiles amounting to more than 50 different species. Taken together, these fossils told a story. "We think that this was a cave where owls lived for many generations, maybe for hundreds or thousands of years," says Viñola López. "The owls would go out and hunt, and then come back to the cave and throw up pellets. We find fossils of the animals that they ate, fossils from the owls themselves, and even some turtles and crocodiles who might have fallen into the cave."
Viñola López, a paleontologist, was primarily interested in the bones from the mammals that the owls ate. He was going through the bags and bags of fossil bones that his team retrieved from the cave and cleaning out the dirt and debris when he noticed something odd.
In the empty tooth sockets of the mammal jaws, Viñola López noticed that the sediment in these cavities didn't look like it had just randomly accrued. "It was a smooth surface, and almost concave. That's not how sediment normally fills in, and I kept seeing it in multiple specimens. I was like, 'Okay, there's something weird here,'" he says. "It reminded me of the wasp nest."
Several years earlier, when Viñola López was an undergraduate student, he went on a fossil dig in Montana. A paleontologist there showed him the ancient remains of wasp cocoons: small, thin chambers of dried mud where wasp larvae would metamorphose into adults. These wasp cocoons looked a lot like smooth dirt lining the tooth sockets from the cave fossils Viñola López had found in the Dominican Republic.
Some of the more well-known nests built by bees and wasps belong to social species that live together and raise their young en masse in large colonies— think of paper wasp nests and the wax honeycombs in a honey bee nest. "But actually, most bees are solitary. They lay their eggs in small cavities, and they leave pollen for the larvae to eat," says Viñola López. "Some bee species burrow holes in wood or in the ground, or use empty structures for nests. Some species in Europe and Africa even build their nests in empty snail shells,"
To better examine the potential insect nests present in the cave fossils, Viñola López and his colleagues CT scanned the bones, essentially X-raying the specimens from enough angles that they could produce 3D pictures of the compacted dirt inside the tooth sockets without destroying the fossils or disturbing the sediment.
The shapes and structures of the sediment looked just like the mud nests created by some bee species today; some of these nests even contained grains of ancient pollen that the bee mothers had sealed in the nests for their babies to eat. The researchers hypothesize that the bees mixed their saliva with dirt to make these little individual nests for their eggs; each nest was smaller than the eraser at the tip of a pencil. Building their nests inside the bones of larger animals may have protected the bees' eggs from hungry predators like wasps.
The nests that the scientists found didn't contain any actual fossilized bees; that doesn't surprise Viñola López, as the hot, muggy conditions in this cave would not have been conducive to preserving small, delicate insect bodies.
Since no bees were preserved, Viñola López and his colleagues were not able to assign a species to the bees that made them. However, the nests themselves were different enough from known bees' nests that the researchers were able to give a taxonomic classification to the fossil nests. They classified the nests as Osnidum almontei after Juan Almonte Milan, the scientist who first discovered the cave. Almonte Milan has worked in the area for decades and is the leading paleontologist on the island.
"Since we didn't find any of the bees' bodies, it's possible that they belonged to a species that's still alive today— there's very little known about the ecology of many of the bees on these islands," says Viñola López. "But we know that a lot of the animals whose bones are preserved in the cave are now extinct, so the bees that created these nests might be from a species that has died out."
This study represents the first known case of bees using the hollows in animal bones to build their nests in. Viñola López suspects that this behavior was the result of several combined circumstances: there isn't much soil covering the limestone ground in this region, so the bees may have turned to caves as a place to nest rather than simply burrowing in the ground like many other species. And since this cave happened to be a multi-generational home for owls who coughed up a lot of owl pellets over the years, the bees took advantage of the bones delivered by the owls.
"This discovery shows how weird bees can be— they can surprise you. But it also shows that when you're looking at fossils, you have to be very careful," says Viñola López. If he hadn't previously seen a fossil wasp nest, he might have just scrubbed away the sediment when he was cleaning the fossil bones for this project. "Even if you're looking primarily for fossils of larger, vertebrate animals, you should keep an eye out for trace fossils that can tell you about invertebrates like insects. Knowing about insects can tell you a lot about a whole ecosystem, so you have to pay attention to that part of the story."
