Ancient bear penises offer the first hard evidence of how an extinct species of bear lived and mated, a new study says.
Indarctos arctoides, which was about the size of today’s brown bear, roamed what is now France, Spain, and Turkey about 10 to 4.5 million years ago.
Like most bears and many mammals, the ancient bear had a baculum—the scientific term for a penis bone. Bacula are rare in the fossil record, both because they’re found only in males and because their length makes them easily broken and mistaken for rib bones. (See “Biggest Bear Ever Found—’It Blew My Mind,’ Expert Says.”)
But study leader Juan Abella got lucky: He was able to study five bacula at a fossil-rich site that contains remains of many ancient mammal species.
When he compared 3-D scans of the fossil penises with bacula of all eight living bear species, he was surprised to find I. arctoides has the longest known bear baculum. The animal had an average penis-bone length of 9.2 inches (23.3 centimeters), compared with a polar bear‘s average baculum length of 6.6 inches (16.8 centimeters).
“We did not expect these fossil bacula to be so huge—it is longer than those of the biggest [surviving] bears, including Kodiak bears and polar bears,” Abella, a paleobiologist at the Museo Nacional de Ciencias Naturales in Madrid, Spain, said by email.
What’s more, Abella suspects the superlong baculum meant that the bears had longer and fewer sexual encounters than other mammals. That’s because the length helped the bear’s penis to be stiffer and keep the female’s uterus open during mating, he said.
Longer bacula seem to boost fertility, since lengthy penises can better position sperm in the female’s reproductive tract, according to the study, published September 18 in the journal PLoS ONE. That also means that long-baculum genes are passed on to more offspring, Abella added. (See “Study Tracks Science of Penis Preference.”)
Fleshing Out an Ancient Bear
Blaine Schubert, a vertebrate paleontologist at East Tennessee State University who has studied prehistoric bears, noted by email that scientists rarely study bacula. (See “Ancient Bear DNA Mapped—A First for Extinct Species.”)
In fact, this study is the first time that bacula have been used to discern ancient behavior in any fossil species, Abella said.
Most bacula research is limited to reports that make general anatomical comparisons to living species, said Schubert, who was not involved in the study.
But “this paper takes another step,” he said. Not only does it provide detailed descriptions and comparisons of the ancient bacula with those of modern bears, but it also uses the fossils to figure out how the ancient bears behaved.
“The authors do an excellent job here at separating rigid fact from conjecture, [as well as] nicely fleshing out mating behavior possibilities of a long-extinct bear.”