Brother Bear: Ursidae Family

Giant panda cub Yun Zi

The variety of behavioral and physiological traits among the eight living bear species is amazing. On one extreme we have the giant panda, a relatively docile bear whose dependence on plant material has benefited from unique physiological features like a gripping pseudothumb and an enlarged sagittal crest on the skull that anchors the powerful jaw muscles needed to chew the bamboo. On the other extreme, we have the entirely carnivorous polar bear, with adaptations specialized for its frozen habitat: large size, big paws with gripping surfaces, and insulating fur that keeps the bear warm even when swimming in frigid water. In between these two species we have a range of bears with unique characteristics.

Polar bear Kalluk

To envision the bigger picture of bear evolution, it is helpful to use a “tree of life” analogy. Imagine you are a little bug sitting on the far tip of a branch of a big tree. Every gardener knows that the tip of the branch is where you find the new buds, the most recent growth on the tree. On the branch that is the bear family Ursidae, the newest buds represent the living species of bears. Let’s start at the polar bear bud, since polar bears are thought to be the most recently diverged species. Crawl back toward the trunk of the tree of life, and the first branching you encounter would be that of the polar and brown bear lineages. These two species are the most closely related of the living bears, and their lines diverged from each other less than one million years ago. In evolutionary terms, that’s not a long time! No wonder they share so many similar features: large size, a diet typically heavy in meat (for the browns, at least part of the year), and in some areas, overlapping habitat.

Sun bear youngster Pagi

Now, little bug, crawl back toward the trunk of the tree and the next branching you encounter will be the point that separates the brown/polar group from the sun, sloth, and black bears, both American and Asiatic. This branching occurred between five and eight million years ago, and the timeline for divergence of each species is debated. One well-regarded theory holds that sun bears split from sloth bears first, and then the black bear lineages diverged. Other studies reveal little difference between lineages. These bears have a range of feeding strategies, from the highly insectivorous sloth bear to the fruit- and insect-eating sun bear and American black bear, which eat plants, bugs, roots, and, occasionally, meat.

Andean bear Tommy

If the little bug continues to crawl back toward the trunk, the next major branching that occurs is the divergence of the bears above (the subfamily Ursinae) from the subfamily Tremarctinae, the family of the Andean (or spectacled) bear. This divergence occurred about 15 million years ago. The Andean bear is the only species with 52 chromosomes in its genome, while the Ursinae all have 74 chromosomes. Andean bears are highly herbivorous, feeding on bromeliads, sapote, and even corn in planted fields in their native areas. There is much yet to be learned about this species.

The final split on the family tree of modern bears occurs closest to the trunk: the divergence of the subfamily Ailuripodidae from other bears, which occurred about 20 million years ago. The modern giant panda has 42 chromosomes and is the most herbivorous bear, subsisting almost exclusively on bamboo. The whole of this bear family tree diverged from its common ancestor with the Procyonids (the raccoon family) more than 30 million years ago.

So there you have it: a map of the bear family tree. It’s important to note that while we have eight living species of bear, there are many bear species that have not survived until modern time. The giant short-faced bear is perhaps the most well known, and it inhabited North America a mere 15,000 years ago! It was highly carnivorous. The cave bears of Europe, also quite large, lived at about the same time as the giant short-faced bear but were thought to be more herbivorous.

Most of our modern bears are conservation dependent: only the American black bear and brown bear are considered at little risk of extinction. It is our hope to conserve the rare bears of today so that they may continue to be with us tomorrow.

Suzanne Hall is a senior research technician for the San Diego Zoo’s Institute for Conservation Research.

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