Often portrayed as pulling Santa's sleigh, reindeer are a Christmas staple. Now, ecologists have found that reindeer are shrinking due to the impact of climate change on their food supplies.

Speaking at the British Ecological Society annual meeting in Liverpool this week, researchers will reveal that over the past 20 years -- during a period of noticeable summer and winter warming in the Arctic -- reindeer on Svalbard have got smaller and lighter.

Ecologists from the James Hutton Institute, Norwegian Institute for Nature Research and Norwegian University of Life Sciences have worked in the high Arctic since 1994, measuring and weighing the reindeer. Each winter they catch, mark and measure 10-month-old calves, returning each year to recapture them and track their size and weight as adults.

The survey shows that over 16 years, the adult reindeers' weight declined by 12% -- from 55kg for those born in 1994 to just over 48kg for those born in 2010.

Professor Steve Albon who led the study believes that three factors -- all influenced by climate change -- are responsible for the shrinking reindeer.

"In Svalbard, snow covers the ground for eight months of the year, and low temperatures typically limit grass growth to June and July. But as summer temperatures have increased by 1.5°C, pastures have become more productive, allowing female reindeer to gain more weight by the autumn and therefore conceive more calves," he explains.

Warmer winters, however, mean more rain. The rain falls on snow, where it freezes, thus locking-out the reindeer from the food beneath the snow. As a result, the reindeer starve, aborting their calves or giving birth to much lighter young.

The third factor at play is that over the past 20 years, reindeer numbers have doubled, so greater competition for food in winter could also help explain the shrinking reindeer.

All of which could spell disaster for this iconic Christmas species, Albon warns: "The implications are that there may well be more smaller reindeer in the Arctic in the coming decades but possibly at risk of catastrophic die-offs because of increased ice on the ground."

Professor Albon will discuss the study at the British Ecological Society annual meeting in Liverpool on Monday 12 December 2016.