A Post-Glacial matriarch
The Shiretoko Peninsula, a UNESCO World Natural Heritage site on the northeastern edge of Hokkaido, is a rugged wilderness with one of the highest densities of brown bears in the world. An estimated 500 Ussuri brown bears (Ursus arctos lasiotus) roam this terrain of volcanic peaks and dense forests. Genetic analysis of this thriving population has revealed a remarkable story stretching back to the end of the last Ice Age.
Studies of the bears' mitochondrial DNA (mtDNA) show an exceptionally low level of genetic diversity. Unlike nuclear DNA, which is inherited from both parents, mtDNA is passed down exclusively from mother to offspring. This maternal inheritance makes it a powerful tool for tracing ancestry. Across the peninsula, the bears share a strikingly similar mtDNA haplotype, showing a single common maternal ancestor for the entire population. This genetic uniformity is a "founder effect"—a loss of genetic variation that occurs when a new population is established by a very small number of individuals.
After the glaciers retreated thousands of years ago, the Shiretoko Peninsula became a newly habitable frontier. Scientists theorize that a single pregnant female, or a very small group of related females, colonized the isolated peninsula. Every one of the hundreds of bears on the peninsula today carries the mitochondrial signature of that original founding matriarch.
A tale of two genomes
The story of the Shiretoko bears' genetics becomes more complex when looking at their nuclear DNA. While their maternal lineage is uniform, analysis of nuclear DNA, which comes from both parents, shows greater diversity. This indicates that while females tend to stay within their home territories—a behavior known as female philopatry—males have been moving onto the peninsula from other parts of Hokkaido.
This male-biased dispersal helps the population's health. The influx of new genes from outside males prevents the widespread inbreeding that could otherwise result from such a narrow maternal lineage. It creates a genetic scenario: a population with a single maternal origin that is simultaneously maintained by genetic input from males from separate lineages. Studies have identified at least seven mtDNA haplotypes on the peninsula, but the distribution is highly structured, with females rarely moving between regions, while males distribute their genes far and wide.
The peninsula's ecosystem, with abundant salmon runs and diverse vegetation, supports the high density of bears and allows them to have some of the smallest home ranges in the world. An adult female's home range averages a mere 15 square kilometers. This unique environment, combined with its dramatic genetic history, makes the Shiretoko bear population a globally significant natural laboratory for studying population dynamics and evolution.