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                <article style="max-width: 500px; margin: 0 auto">
  <h1>The Polar Bear</h1>
    The polar bear (Ursus maritimus) is a hypercarnivorous species of bear. Its native range lies largely within
    the Arctic Circle, encompassing the Arctic Ocean and its surrounding seas and landmasses, which includes the
    northernmost regions of North America and Eurasia. It is the largest extant bear species, as well as the
    largest extant land carnivore. A boar (adult male) weighs around 350–700 kg (770–1,540 lb), while a sow
    (adult female) is about half that size. Although it is the sister species of the brown bear, it has evolved
    to occupy a narrower ecological niche, with many body characteristics adapted for cold temperatures, for
    moving across snow, ice and open water, and for hunting seals, which make up most of its diet. Although most
    polar bears are born on land, they spend most of their time on the sea ice. Their scientific name means
    "maritime bear" and derives from this fact. Polar bears hunt their preferred food of seals from the edge of
    sea ice, often living off fat reserves when no sea ice is present. Because of their dependence on the sea
    ice, polar bears are classified as marine mammals. Because of expected habitat loss caused by climate
    change, the polar bear is classified as a vulnerable species. For decades, large-scale hunting raised
    international concern for the future of the species, but populations rebounded after controls and quotas
    began to take effect. For thousands of years, the polar bear has been a key figure in the material,
    spiritual, and cultural life of circumpolar peoples, and polar bears remain important in their cultures.
    Historically, the polar bear has also been known as the "white bear". It is sometimes referred to as the
    "nanook", based on the Inuit term nanuq.
  <div class="revealing-image">
    <img style="max-width: 100%" src="" />
  <h2>Naming and etymology</h2>
    Constantine John Phipps was the first to describe the polar bear as a distinct species in 1774 in his report
    about his 1773 expedition towards the North Pole. He chose the scientific name Ursus maritimus, the Latin
    for "maritime bear", due to the animal's native habitat. The Inuit refer to the animal as nanook
    (transliterated as nanuq in the Inupiat language). The Yupik also refer to the bear as nanuuk in Siberian
    Yupik. In the Chukotko-Kamchatkan languages of Alyutor and Koryak, the name of the polar bear is umqa, while
    in the related Chukchi, it is umqə. In Russian, it is usually called бе́лый медве́дь (bélyj medvédj, 'white
    bear'), though an older word still in use is ошку́й (Oshkúj, which comes from the Komi oski, "bear"). In
    Quebec, the polar bear is referred to by the french terms ours blanc ('white bear') or ours polaire ('polar
    bear'). In Norwegian, one of the primary languages of the Svalbard archipelago, the polar bear is referred
    to as isbjørn ('ice bear') or kvitbjørn ('white bear'). The polar bear was previously considered to be in
    its own genus, Thalarctos. However, evidence of hybrids between polar bears and brown bears, and of the
    recent evolutionary divergence of the two species, does not support the establishment of this separate
    genus, and the accepted scientific name is now therefore Ursus maritimus, as Phipps originally proposed.
  <div class="revealing-image">
    <img style="max-width: 100%" src="" />
  <h2>Taxonomy and evolution</h2>
    The bear family, Ursidae, is thought to have split from other carnivorans about 38 million years ago. The
    subfamily Ursinae originated approximately 4.2 million years ago. The oldest known polar bear fossil is a
    130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004. Fossils show that between
    10,000 and 20,000 years ago, the polar bear's molar teeth changed significantly from those of the brown
    bear. Polar bears are thought to have diverged from a population of brown bears that became isolated during
    a period of glaciation in the Pleistocenefrom the eastern part of Siberia (from Kamchatka and the Kolym
    Peninsula). The evidence from DNA analysis is more complex. The mitochondrial DNA (mtDNA) of the polar bear
    diverged from the brown bear, Ursus arctos, roughly 150,000 years ago. Further, some clades of brown bear,
    as assessed by their mtDNA, were thought to be more closely related to polar bears than to other brown
    bears, meaning that the brown bear might not be considered a species under some species concepts, but
    paraphyletic. The mtDNA of extinct Irish brown bears is particularly close to polar bears. A comparison of
    the nuclear genome of polar bears with that of brown bears revealed a different pattern, the two forming
    genetically distinct clades that diverged approximately 603,000 years ago, although the latest research is
    based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of
    polar and brown bears, and establishes the divergence of polar and brown bears at 400,000 years ago.
    However, the two species have mated intermittently for all that time, most likely coming into contact with
    each other during warming periods, when polar bears were driven onto land and brown bears migrated
    northward. Most brown bears have about 2 percent genetic material from polar bears, but one population, the
    ABC Islands bears, has between 5 percent and 10 percent polar bear genes, indicating more frequent and
    recent mating. Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids; rather
    than indicating that they have only recently diverged, the new evidence suggests more frequent mating has
    continued over a longer period of time, and thus the two bears remain genetically similar. However, because
    neither species can survive long in the other's ecological niche, and because they have different
    morphology, metabolism, social and feeding behaviours, and other phenotypic characteristics, the two bears
    are generally classified as separate species. When the polar bear was originally documented, two subspecies
    were identified: the American polar bear (Ursus maritimus maritimus) by Constantine J. Phipps in 1774, and
    the Siberian polar bear (Ursus maritimus marinus) by Peter Simon Pallas in 1776. This distinction has since
    been invalidated. One alleged fossil subspecies has been identified: Ursus maritimus tyrannus, which became
    extinct during the Pleistocene. U.m. tyrannus was significantly larger than the living subspecies. However,
    recent reanalysis of the fossil suggests that it was actually a brown bear.


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  to {
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.revealing-image {
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  view-timeline-axis: block;

  animation: auto linear reveal both;
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  animation-range: entry 25% cover 50%;

/* 效果相同 */
/* .revealing-image {
    animation: auto linear reveal both;
    animation-timeline: view();
    animation-range: entry 25% cover 50%;
} */