The ABC Islands bear or Sitka brown bear (''Ursus arctos sitkensis'') is a subspecies or
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
of
brown bear
The brown bear (''Ursus arctos'') is a large bear species found across Eurasia and North America. In North America, the populations of brown bears are called grizzly bears, while the subspecies that inhabits the Kodiak Islands of Alaska is kno ...
that resides in
Southeast Alaska
Southeast Alaska, colloquially referred to as the Alaska(n) Panhandle, is the southeastern portion of the U.S. state of Alaska, bordered to the east and north by the northern half of the Canadian province of British Columbia (and a small part ...
and is found on
Admiralty Island
Admiralty Island is an island in the Alexander Archipelago in Southeast Alaska, at . It is long and wide with an area of , making it the seventh-largest island in the United States and the 132nd largest island in the world. It is one of the ...
,
Baranof Island
Baranof Island is an island in the northern Alexander Archipelago in the Alaska Panhandle, in Alaska. The name Baranof was given in 1805 by Imperial Russian Navy captain Yuri Lisyansky, U. F. Lisianski to honor Alexander Andreyevich Baranov. It ...
, and
Chichagof Island
Chichagof Island (russian: Остров Чичагова), or Shee Kaax, is an island in the Alexander Archipelago of the Alaska Panhandle. At long and wide, it has a land area of , making it the fifth largest island in the United States and ...
in
Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...
(colloquially known as the
ABC Islands), and a part of the
Alexander Archipelago
The Alexander Archipelago (russian: Архипелаг Александра) is a long archipelago (group of islands) in North America lying off the southeastern coast of Alaska. It contains about 1,100 islands, the tops of submerged coastal m ...
. It has a unique genetic structure that relates them not only to brown bears, but also to
polar bears
The polar bear (''Ursus maritimus'') is a hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the largest extant bear species ...
.
Its habitat exists within the
Tongass National Forest
The Tongass National Forest () in Southeast Alaska is the largest U.S. National Forest at . Most of its area is temperate rain forest and is remote enough to be home to many species of endangered and rare flora and fauna. The Tongass, which i ...
, which is part of the perhumid rainforest zone.
Morphology
The ABC Islands bears have the appearance of the typical
Alaska Peninsula brown bear
The Alaska Peninsula brown bear or "peninsular grizzly" is a colloquial nomenclature for a brown bear that lives in the coastal regions of southern Alaska. It is a population of the mainland grizzly bear subspecies (''Ursus arctos horribilis''). ...
, which includes milk chocolate-colored fur and a humped back, with a large size and reputation to match. Although the bear is commonly a milk-chocolate color, it can range from blonde to black. Along with the humped back, the bears also have a slightly dished profile to the face. The bears have long front claws. Adult males on average will weigh , with adult females weighing on average . Adult height will range on average from at the shoulder.
Habitat
As these bears reside within the
Tongass National Forest
The Tongass National Forest () in Southeast Alaska is the largest U.S. National Forest at . Most of its area is temperate rain forest and is remote enough to be home to many species of endangered and rare flora and fauna. The Tongass, which i ...
, the bears depend on species that are found within the ecosystem of the Tongass. Two of the common tree species in this ecosystem include the
Sitka spruce
''Picea sitchensis'', the Sitka spruce, is a large, coniferous, evergreen tree growing to almost tall, with a trunk diameter at breast height that can exceed 5 m (16 ft). It is by far the largest species of spruce and the fifth-larg ...
and the
western hemlock
''Tsuga heterophylla'', the western hemlock or western hemlock-spruce, is a species of hemlock native to the west coast of North America, with its northwestern limit on the Kenai Peninsula, Alaska, and its southeastern limit in northern Sonoma ...
.
Diet
The ABC Islands bears depend on a diet of various flora and fauna that is endemic to the region. For flora this includes grasses; other plants like bulbs; berries like salmonberries and blueberries; and mushrooms. For fauna this includes animals like voles, mice, squirrels, and salmon.
Behavior
The ABC Island bears are usually solitary creatures, except in areas with plentiful food sources. Areas with abundant dietary staples may attract multiple bears. The ABC Islands bears will reach sexual maturity on average at the age of four to seven years, with mating occurring between May and July. Winter hibernation for these bears occurs between the months of October until April, with June at the latest. Pregnant females will give birth to cubs in the den in January or February, with an average litter size of two to three cubs. The cubs remain with the mother for up to three years, during which time she will not get pregnant.
Relationship to polar bears
This brown bear retains all of the physical attributes and behavior associated with the brown bear, however, they do carry mitochondrial DNA that shows a match closer to polar bears than brown bears.
Studies
1996
A study was conducted on the phylogeography of the brown bears in Alaska, specifically the subspecies that exist within Alaska. The study showed that there was no recent flow of genes from Admiralty to any other islands or mainland. The research for the study began in 1982, when bears on Admiralty were equipped with collars and tracked. These bears did not leave the island during the time. It was believed to be related to strong currents existing in the waters around the island which prevented the bears from swimming to nearby land masses. The study was also looking at the possibility of
refugium during the Last Glacial Maximum, in which the
Cordilleran Ice Sheet covered much of the Pacific coast. It is believed that certain populations existed only on the ABC Islands.
ABC island bears were discovered 0.5 million years ago.
2011
A study on matrilineal polar bear hybridization in relationship to Irish lineage was conducted.
The study used the ABC Islands population to support their hypothesis. Comparisons have shown that the ABC Islands had polar bears present during the Last Glacial Maximum, but Ireland most likely did not have proper habitat.
Bone isotopic data shows that Irish bears had a similar terrestrial diet to late Pleistocene brown bears from Alaska, and not similar to the marine diet of polar bears. Thus it is shown that the Irish bears have common ancestry with polar bears and the ABC Islands bears.
2012
An international study led by Penn State University and the University of Buffalo has estimated that polar bears split from brown bears between 4 million and 5 million years ago. This would have been due to climate changes, which included ice ages. Hybrids have also been documented within the Northern
Beaufort Sea
The Beaufort Sea (; french: Mer de Beaufort, Iñupiaq: ''Taġiuq'') is a marginal sea of the Arctic Ocean, located north of the Northwest Territories, the Yukon, and Alaska, and west of Canada's Arctic islands. The sea is named after Sir Fr ...
of
Arctic Canada
Northern Canada, colloquially the North or the Territories, is the vast northernmost region of Canada variously defined by geography and politics. Politically, the term refers to the three territories of Canada: Yukon, Northwest Territories and ...
. This is an area where the range of polar bears overlaps with the range of brown bears. Data was collected using the entire genome of three brown bears and one black bear in comparison to one modern polar bear and one polar bear remains estimated at 120,000 years old.
Data shows that the brown bears and polar bears were isolated at the same time in which black bears became their own species. Brown bears and polar bears continued to remain isolated for a long period of time, theorized due to the presence of ice, before recent changes allowed polar bears and brown bears to interbreed again.
It is estimated that polar bears and brown bears began to interbreed again about 160,000 years ago. The study shows that previous estimates of polar bears only existing 600,000 years ago was wrong, and that they have in fact been around much longer – four to five million years ago. This research analysis in particular discovered more genetic similarities between the polar bears and ABC Islands brown bears.
This indicates that the study of the ABC Islands bears is also important to understanding the evolution of the polar bear.
2013
Research under
Beth Shapiro
Beth Alison Shapiro (born 1976) is an American evolutionary molecular biologist. She is a professor in the Department of Ecology and Evolutionary Biology at the University of California, Santa Cruz. Shapiro's work has centered on the analysis of ...
(Associate Professor) at
University of California, Santa Cruz
The University of California, Santa Cruz (UC Santa Cruz or UCSC) is a public university, public Land-grant university, land-grant research university in Santa Cruz, California. It is one of the ten campuses in the University of California syste ...
shows that around 6.5% of the X chromosomes from the ABC Islands bears have recently come from polar bears, in contrast to 1% of the ABC Islands bears' genomes containing polar bear DNA. The study was conducted using seven polar bears, two brown bears, and one black bear to gather ancestral alleles. Based on research using various simulation scenarios, the team concluded that the ABC Islands bears descended from polar bears that gradually converted into brown bears through hybridization of male brown bears dispersing from the Alaskan mainland.
It is believed that the present polar bear DNA stems from a group of polar bears that were stranded in Southeast Alaska at the end of the last ice age, and the bears were stranded due to receding ice. This would happen with male brown bears swimming over to the islands and mating with the female polar bears.
The study from University of California, Santa Cruz shows that episodes of gene flow between polar bears and brown bears occurred only in isolated populations, without affecting the larger polar bear population, compared to prior hypothesis of past hybridization affecting all polar bear genes to contain brown bear genes as well.
Prior studies have shown that the ABC Islands brown bears are the descendants of polar bears from male-dominated brown bears. The researchers of this study present a model that derives them from a population of polar bears that were stranded after the last glacial period, where brown bears migrated to the island and interbred, leaving the phenotype and genotype of these bears to be primarily brown bear. The research team hypothesizes that this may be a common outcome due to climate change or when an isolated species becomes overrun with an outside species that is able to form hybrids. Final results show that polar bears have no evidence of brown bear ancestry in their genes and ABC Islands bears show clear ancestry of polar bear genes.
Fossil remains from the
Last Glacial Maximum
The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent.
Ice sheets covered much of Northern North America, Northern Eur ...
(LGM) show that there was no habitat suitable for brown bears to occupy during this time, this includes the area with which they need to inhabit.
However, fossils in this area did show a concentration of marine mammals, especially ringed seals, which are a common staple to polar bears diet.
Threats
Conflict with humans is a major staple of disturbance to population numbers for the bears. Conflict can include bears entering city populations or threatening hunters on the islands. Other possible disturbances include climate change. As this subspecies of brown bear proves, climate change can have an effect on the species overall.
References
{{Taxonbar, from=Q11704940
Grizzly bears
Mammals of the Arctic
Arctic land animals
Alexander Archipelago
Mammals of the United States
Endemic fauna of Alaska
Fauna of Alaska
Ursid hybrids