Environmental stresses could be causing physical changes in the bears
Polar bears are one of the most polluted mammals on the globe
The team used skulls from the Zoological Museum of Copenhagen
The maximum sea ice extent is declining by about 2.7% per decade
2009-08-27 中國時報 【潘勛／綜合廿五日外電報導】
丹麥科學家比對百餘年來的北極熊顱骨變化後推斷，因為環境惡化，現存北極熊的軀體相較於一百多年前的熊祖先，已經縮小。
研究團隊比對「哥本哈根動物博物館」收藏的近三百具北極熊顱骨，發現過去一百廿年間，顱骨尺寸縮減的幅度在二％至九％之間。
研究報告主撰人「奧胡斯大學」生物學教授裴托迪表示，丹麥團隊研究那批顱骨、顱骨形狀及尺寸變化，可以判斷北極熊軀體大小改變。研究成果發表於《動物學期刊》（Jounal of Zoology）。
裴托迪表示，北極熊軀體縮小，按推斷原因在環境惡化，造成「生理壓力」。首先，全球暖化，北極海冰層因消融而縮小，北極熊耗費在覓食的力氣增加，因此沒有額外精力讓身軀長大。
再來，人類製造的化學汙染物存留在北極熊體內，也造成北極熊生育率減少。由於生育率降低，導致交配對象減少，北極熊近親繁殖現象增加，連帶改變了北極熊顱骨的形狀。
裴托迪表示，北極熊受化學汙染影響特別重，原因是牠們位居食物鏈頂端；食物鏈較低層生物，如魚及海豹等凡沾染汙染物者，都匯聚到北極熊脂肪內。
'Stress' is shrinking polar bears
By Victoria Gill
Science reporter, BBC News
Polar bears have shrunk over the last century, according to research.
Scientists compared bear skulls from the early 20th Century with those from the latter half of the century.
Their study, in the Journal of Zoology, describes changes in size and shape that could be linked an increase in pollution and the reduction in sea ice.
Physical "stress" caused by pollutants in the bears' bodies, and the increased effort needed to find food, could limit the animals' growth, the team said.
The researchers used the skulls as indicators of body size. The skulls from the later period were between two and 9% smaller.
"Because the ice is melting, the bears have to use much more energy to hunt their prey," explained Cino Pertoldi, professor of biology from Aarhus University and the Polish Academy of Science, and lead scientist in this study.
"Imagine you have two twins - one is well fed during its growth and one is starving. (The starving) one will be much smaller, because it will not have enough energy to allocate to growth."
The team, which included colleagues from Aarhus University's Department of Arctic Environment, also found shape differences between the skulls from the different periods.
This development was slightly more mysterious, said Dr Pertoldi.
He explained that it was not possible to determine the cause, but that the changes could be linked to the environment - more specifically to pollutants that have built up in the Arctic, and in the polar bears' bodies.
The aim of the study was to compare two groups of animals that lived during periods when sea ice extent and pollution levels were very different.
The pollutants that the scientists focused on were compounds containing carbon and halogens - fluorine, chlorine, bromine or iodine.
Some of these compounds have already been phased out, but many still have important uses in industry. These include solvents, pesticides, refrigerants, adhesives and coatings.
Genetic brink
The changes, the team says, could also be related to a reduction in the genetic diversity of the species.
Hunting over the last century, said Dr Pertoldi, could have depleted the gene pool, leaving polar bears to suffer the effects of inbreeding.
"We also know from previous studies that some chlorinated chemical pollutants have affected the fertility of the females," he continued.
Rune Dietz from Aarhus University was another member of the research team.
He explained that he and his colleagues had already determined a link between man-made "persistent organic pollutants" and reduced bone mineral density in polar bears - which could leave the animals vulnerable to injury and to the bone disease osteoporosis.
Skull collection
The collection of almost 300 polar bear skulls was provided by the Zoological Museum of Copenhagen in Denmark.
Christian Sonne, a veterinary scientist from Aarhus University who worked with the team, said that this provided a unique and "fantastic sample", providing a window into the bears' development over a whole century.
During that time, he said, concentrations of many man-made pollutants in the Arctic have significantly increased.
He said: "Polar bears are one of the most polluted mammals on the globe."
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