COMMENT
the presence of humans, from the hybridiza-
tion of two other species. But hybridization
driven by human activities tends to occur
quickly and to reduce genomic and species
diversity. When mallard ducks were intro-
duced to New Zealand in the 1860s, they
began mating with native grey ducks. Now
few, if any, pure native populations remain.
Diversity loss may be minor if, say, North
Pacific and North Atlantic minke whale
subspecies interbreed in an Arctic with
diminished ice. Other crosses will be more
problematic. Interbreeding between the
North Pacific right whale, of which there
are probably fewer than 200, and the more
numerous bowhead whale could quickly
push the former to extinction. If polar bears
survive climate change in secluded refuges
 which is far from certain  interbreeding
The polar bear (possible hybrid pictured) is one of several species vulnerable to hybridization. could be the final straw.
Cross-breeding might affect social
and ecological interactions. The appar-
ent narwhal beluga hybrid discovered in
The Arctic melting pot
Greenland had teeth combining qualities of
each species, but lacked the narwhal s tusk 
Hybridization in polar species could hit biodiversity hard,
an important determinant of narwhal breed-
ing success. Polar grizzly hybrid bears in a
say Brendan Kelly, Andrew Whiteley and David Tallmon.
German zoo exhibited behaviour associated
with seal hunting, but not the strong swim-
n 2006, a white bear with patches of developed immediately to monitor the genet- ming abilities of polar bears. First-generation
brown fur was shot by hunters in the ics of Arctic animals and to deal with hybrids crosses can have  hybrid vigour , but later
IArctic. DNA tests confirmed what many before currently discrete populations merge generations are likely to be less fit than their
suspected  it was a hybrid of a polar bear and at-risk species are bred out of existence. ancestors ( outbreeding depression ).
and a grizzly. A media frenzy quoted biolo- We have counted at least 34 possible The International Union for the Con-
gists as saying that although they knew in hybridizations between discrete populations, servation of Nature should develop a com-
theory such cross-breeding could happen, species and genera of Arctic and near-Arctic prehensive policy for managing hybrids,
they didn t expect to see it in the wild. In marine mammals (see Supplementary Infor- including determining when it is practical
2010, another hybrid was killed by a hunter in mation). Of the 22 species involved, 14 are to prevent or limit hybridizations. Red wolf
the western Canadian Arctic. This time, the listed  or are candidates for listing  as and coyote hybrids, for example, have been
animal was a second-generation cross  its endangered, threatened or of special con- culled in the United States in the past decade
mother was a hybrid and its father a grizzly. cern by one or more nations. Twelve cases to help preserve distinct species.
More cases are probably out there. are of hybridization between different spe- Researchers should combine models of
Biologists should not be surprised. There cies  half involving crosses between what sea-ice loss, oceanography and landscape
have been hints of Arctic hybrids before. In the are normally classified as distinct genera. genomics to predict when and where hybrid-
late 1980s, a whale thought to be a narwhal Twenty-two cases involve isolated popula- ization is most likely, and to monitor the
beluga mix was found in west Greenland. tions at risk of intra-species mixing, nine of genetics of at-risk populations. National and
In 2009, an apparent bowhead right-whale which are classified as distinct subspecies. tribal governments should work together:
hybrid was photographed in the Bering Sea, The Arctic Ocean is predicted to be some indigenous groups actively monitor
between Alaska and Russia. Dall s porpoises ice-free in summer before the end of the the harvests of Arctic marine mammals,
are known to be mating with harbour por- century, removing a continent-sized barrier and they could collect genetic samples in
poises off the coast of British Columbia, and to interbreeding. Polar bears are spending remote areas. The rapid disappearance of
seal hybrids have been identified in museum more time in the same areas as grizzlies; seals sea ice leaves little time to lose. %
specimens and in the wild. and whales currently isolated by sea ice will
These are just the first of many hybridiza- soon be likely to share the same waters. Brendan P. Kelly is at the National Marine
tions that will threaten polar biodiversity. Not all cross-species matings will produce Mammal Laboratory National Oceanic
Rapidly melting Arctic sea ice imperils viable  or indeed any  offspring. The and Atmospheric Administration, Juneau,
species through interbreeding as well as chance is enhanced in Arctic marine mammals, Alaska 99801, USA. Andrew Whiteley
through habitat loss. As more isolated popu- because their number of chromosomes has is in the Department of Environmental
lations and species come into contact, they changed little over time. There is evidence of Conservation, University of Massachusetts,
will mate, hybrids will form and rare species hybridization across species (such as between Amherst, Massachusetts 01003, USA.
are likely to go extinct. As the genomes of spotted and harbour seals) as well as across David Tallmon is in the Faculty of Biology
species become mixed, adaptive gene com- genera (such as harp and hooded seals). and Marine Biology, University of Alaska
binations will be lost. Hybridization is not necessarily a bad thing. Southeast, Juneau, Alaska 99801, USA.
Researchers have little idea how much It has been an important source of evolution- e-mail: [e-mail]
hybridization is occurring, let alone how ary novelty. For example, a new species of
Further reading and Supplementary Information
it will affect populations. Plans must be chub originated in the Colorado River before accompanies this article at go.nature.com/h4bksj
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