Over years to come, Santa Claus might be well-advised to get more reindeer to pull his sleigh.

A study has shown that reindeer are getting smaller and thus weaker, and that climate change may be the cause.

Ecologist Steve Albon of Scotland's James Hutton Institute, along with his team, have found that adult reindeer living on the Arctic island of Svalbard in Norway born in 2010 weigh on average 12 percent less than those born in 1994.

While the weight loss might not harm the animals itself, it might lead to lowered reproduction, particularly to mothers losing their babies when pregnant.

"If you are small and therefore lose weight over winter, you are more likely to fall below a threshold at which you terminate a reproduction event to save your own life," Steve Albon told DW.

Even if the fetus doesn't die, the baby reindeer that is born will have a hard time surviving.

"It will be smaller at birth, because the mother was in such poor condition during that winter."

Albon presented his results on Monday (12.12.2016) at a meeting of the British Ecological Society.

Food becoming scarcer in winter

One might think that a warmer climate would bring more food for reindeers instead of less - and from spring to autumn, that is indeed the case. In winter, though, climate change takes a toll on the animals.

Warmer temperatures bring more rain instead of snow. While snow in cold winter is powdery and sugar-like and can be shoveled away easily, rain freezes on the ground, creating an ice sheet up to 5 centimeters thick, Albon says.

"The reindeer can't get through that - even we as humans couldn't get through it, we would need an axe."

That's why reindeer can't reach their food which - on Svalbard - is mostly grasses and herbs. So the animals starve.

Albon says this is happening ever more frequently in all Arctic ocean locations - not only on Svalbard, but also on Canadian Arctic islands, for example.

A downward trend

Even if reindeer on Svalbard are getting smaller, Albon is "not worried" at the moment, he says.

That's because the reindeer population on the archipelago is currently increasing, due to warmer summers and more available food.

With other reindeer populations around the world, however, a lack of food in winter could now be having a dramatic impact: Many populations are already decreasing.

Last year, on Christmas Eve - of all days - the International Union for Conservation of Nature and Natural Resources (IUCN) changed the species' conservation status from "least concern" to "vulnerable" - two steps more endangered at once, as the species skipped over the status "near threatened."

"Reindeer population worldwide has plummeted by 40 percent over about 25 years," says Adolf Köhncke, conservation expert at World Wide Fund for Nature in Germany.

According to the IUCN, over that time period numbers have dropped from 4.8 million to 2.9 million individuals, including not only reindeer in Europe and Asia, but also caribou in North America. Both belong to the same species, despite their different names.

"Some caribou herds in Canada have even shrunk by 98 percent," Köhncke told DW.

Threatened by global change

At the semi-annual Committee on the Status of Endangered Wildlife in Canada earlier this December, experts said caribou are "in trouble."

"Many of the great northern caribou herds have now fallen to all-time lows - and there is cause for concern that they will not rebound in the same way they have before," said Justina Ray, co-chair of the terrestrial mammals subcommittee.

A lack of food in winter is not even the biggest problem for these charismatic animals, Köhncke says. It is a mixture of threats.

"As a migratory species, reindeer are particularly sensitive to human disturbances," he says.

Roads may cut through their migratory routes, newly established industry may prevent prospective mothers from giving birth to their babies.

Alessia Uboni, now at the Swedish University of Agricultural Sciences in Uppsala, found earlier this year that while some reindeer populations worldwide are increasing, the species is very sensitive to changes in the environment, including local weather and habitat loss.

They are not only threatened by climate change, Uboni told DW, but also "by global change."

"It is hard to tell which population might suffer the most - but all of them are in danger."

Far north in the Arctic, many reindeer herds cross sea ice. As sea ice is disappearing due to climate change, so are their migratory routes. This November, average Arctic sea ice extent set a record low.

Climate change might also bring unexpected events - like the outbreak of anthrax in Siberia in August, Uboni says. A heat wave had thawed a long-frozen reindeer carcass trapped in permafrost, releasing the deadly bacteria.

The disease killed 2,300 reindeer and one 12-year old boy, and sickened many others.

Polar bears also in danger

Reindeers are not the only species suffering from a changed Arctic climate.

Scientists warn that rising temperatures in the Arctic will probably reduce the polar bear population by a third over the next few decades.

The study was led by the United States Fish and Wildlife Service, and presented during a meeting of the American Geophysical Union in San Francisco, also on Monday (12.12.2016).

Researchers assessed the loss of Arctic sea ice from 1979 to 2015, projecting further declines for both ice and bears over the coming decades.

Polar bears currently number about 26,000 - but their population is expected to diminish by some 8,600 animals over the next 35 to 40 years, the scientists said.

So not only will Santa Claus have to look harder for sledge-pulling reindeer in the future - iconic polar bears might disappear, too.

In some regions of the world, a white Christmas already has.