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Climate change offsets evolution
to shrink the wild sheep of St. Kilda
More lambs survive milder winters, upping food competition
By Susan Milius
Climate change now hits home for tongue
twister fans. Shorter, sweeter winters
shrink sheep, scientists say (slowly).
Female wild Soay sheep on the remote
North Atlantic island of Hirta in the
St. Kilda archipelago have shrunk by
about 5 percent during the past two
decades, says Tim Coulson of Imperial
College London’s campus in Berkshire.
To see what’s driving that change — a
weight loss averaging 81 grams per
year—Coulson and his colleagues
applied a new analytical approach to a
mountain of data. It turns out that evolutionary forces favor the opposite trend,
toward bigger sheep. But environmental changes have softened winters, overwhelming those evolutionary effects, the
team reports online July 2 in Science.
For climate change, “the effects people tend to focus on are the ecological
ones,” Coulson says. Studies have documented creatures shifting their ranges
or changing the timing of migrations
or blooming. “We’re showing that the
effects extend beyond the ecology, down
to individual attributes,” Coulson says.
The results show that influences on
size are complex, says Kaustuv Roy of
the University of California, San Diego.
“ We urgently need more case studies like
this to really make sense of how populations and species will respond to ongoing
warming,” he says.
Thanks to detailed monitoring that
began on Hirta in 1986, the team could figure out why females in this population are
shrinking. Soay sheep, with brown coats
and curling rams’ horns, resemble early
forms of domesticated sheep and have
roamed for several thousand years on
the archipelago, “a group of godforsaken
rocks halfway to Iceland,” Coulson says.
To parse out what governs body size
in such a harsh climate, Coulson and his
colleagues started with basic equations
that population biologists use to describe
how traits change over time. The team
combined and refined these equations to
create a type of bookkeeper’s ledger that
describes all the factors that in theory
could cause a trait to vary. Plugging in
12 | SCIENCE NEWS | August 1, 2009
Soay sheep have lived as isolated popu-
lations on the islands of St. Kilda archi-
pelago for thousands of years.
female sheep data, the researchers found
conflicting forces at work.
The evolutionary force of natural
selection favored bigger body sizes, the
researchers concluded. Size is partly
inherited, and larger youngsters survived better than smaller, weaker ones
because sheep need to draw on fat
reserves during the winters.
A quirky effect of milder winters, however, drove body size toward the diminutive. Over the past 25 years, spring has
shifted two to three weeks earlier in
Northern Europe. In years with shorter
winters, more of the small, weak lambs
survived. With more sheep competing
for food in spring, growth rates slowed
among the surviving youngsters. Environmental factors linked to less-violent
winters were the most important determinant of body size and overcame the
evolutionary effect, the researchers say.
Biologists have been trying to distinguish genetic from environmental effects
for decades, often by comparing the specific forms, or phenotypes, of individuals
of the same species at different altitudes,
says evolutionary geneticist Mark
Rausher of Duke University in Durham,
N. C. “What’s nice about this new study is
that it may be the first that convincingly
shows that a change in phenotype over
time is driven by the direct action of the
environment on phenotype,” he says.
Data reveal that environmental factors
are to blame for the shrinking of female
Soay sheep over the past two decades.
BOTH: ARPAT OZGUL