Why Did Santa Chose Female Reindeer to Help Him Deliver Presents?

In the cold Scandinavian territories, in icy Finland, , like every year during this time, Santa Claus is getting ready to go worldwide from house to house to deliver presents to all.

In order to carry out such an important task, he had to carefully select which animal would help to travel across the world the night of Christmas Eve. After giving it a lot of thought, he chose reindeer. What is special about reindeer? And, why did Santa choose them instead of horses, dogs or oxen which can also resist cold temperatures and pull the snow sleigh?

Santa’s reindeer are most often depicted with long, complete antlers

Reindeer are perfectly adapted to survive in a hostile environment –that is with small amounts of food and in very cold temperatures- and in order to do that they have developed some unique physical characteristics:

First of all, their fur is very dense, they have over 13,000 hairs per in2 (people have 1,600/in2). Furthermore, these hairs are hollow. Their interior is filled with air, a characteristic that allows them to perfectly isolate the body from the icy temperatures which can reach -60ºF.

Reindeer hair under a microscope. See hollow, bubble-like internal structure. Longitudinal and transversal cuts. Source: image on the left FBI files, image on the right thefeatherbender.com

On the other hand, the reindeer legs have a very smart vascularization system: when temperatures get really cold, the amount of blood that gets to the limbs is reduced to the minimum necessary so that the loss of heat is severely reduced.

Besides, the arteries (blood vessels that go from the heart to the extremities) and the veins (blood vessels that go from the extremities to the heart) of the legs are very close together, and because of that, the arterial blood, which is at the animal’s body temperature, warms up the blood in the veins and goes up to the heart already warmed up although coming from a very cold leg.


Blood circulation in reindeer leg. Adapted from Olsen doctoral thesis

One more particularity of reindeer is that –contrary to cows- they can exhale cold air, they are able to keep most of the heat of the air that goes out of the lungs thanks to a recovery system that they have in their nasal system. Air gets in very cold, it is warmed up in the turbinate system: a collection of narrow, convoluted passages of nasal mucose that air has to go through before reaching the windpipe. As air gets warmer, the nasal mucose becomes cooler and drier.

When the animal breathes out,  the warm air coming from the lungs is full of moisture, then it makes contact with the cooler nasal mucose. This makes moisture to condense making the mucosa warmer. This moisture is absorbed. So the reindeer doesn’t lose so much water or so much heat as it would without this mechanism. All in all, 75% of the heat and 80% of water are recovered thanks to this peculiar physiological feature. When ambient temperature is less than 50ºF, the exhaled air is 70ºF lower than body temperature.

Respiratory mechanism that enables heat recovery in cold ambient in reindeer.

It is no surprise, then, that reindeer -or caribou which is a word also used to name them- can handle the cold with stoicism and that they can travel incredibly long distances to find the best pasture in the Siberian tundra -they can go across over 80 miles per day and can travel up to 3,000 miles in a year during their different migrations.

However, how do we know that the reindeer that Santa Claus chose are female? The reason lies in that adult males lose their antlers every year towards the beginning of December -to have them grown again a few months later.

Conversely, females keep them until the spring to protect their young ones from predators. Therefore, when it comes to distributing gifts, towards the end of December, only females still have their horns, and since the reindeer that accompany Santa Claus are usually depicted with beautiful antlers, there is no doubt that they must be female.

Santa’s trip, nevertheless, presents some risks: we all know that Santa Claus’ snow sleigh is magical and flies through the air, but this makes high-tension power lines very dangerous because the sleigh could hit them and electrocute Santa, the deer and all the presents. The Christmas spirit would be lost. How could mankind survive such a disaster?

And here we find one more reason that makes Santa Claus so fond of reindeer: amongst mammals they have the uncommon ability to see ultraviolet light (UV). The high-power lines ionize the air around them causing an emission of UV rays which humans cannot detect, but reindeer can. Furthermore, caribous can visualize the length of the waves hundreds of meters away. This rare capability explains why the reindeer that help Santa never crash against electrical wiring.

So, without a doubt, this unique vision was another reason why Santa Claus decided to use these animals in his long travels every Christmas Day. Conversely, horse’s eye lenses remove most of the UV radiation, so they cannot see UV light which explains why Santa didn’t pick them.

But why do reindeer need this skill in their day to day lives -since out of the three million reindeer in the world, only a few were selected to work for Santa? It is necessary to understand that UV vision gives them significant advantages in the Nordic forests: snow reflects all UV light while lichen -the main source of food for reindeer during the cold winter- absorb this light, creating a chromatic contrast that makes them easier to find. The polar winter is characterized by having many months with very little light, having UV vision, therefore, means an important competitive advantage. Moreover, wolves also absorb UV light, so, just like lichen, they are visible in an environment covered by snow, and at the same time easier to avoid.

To all who have taken the time to read these lines: Merry Christmas and Happy 2018!!!



  1. Sami and Fins. Finland. Reindeer Herding.


2. Olson, E.; Peripheral Heterothermia in Reindeer. Doctoral Thesis. 2011

https://munin.uit.no/handle/10037/3718FBI Archives

  1. FBI archives


4. Cressey, D. Why reindeer steer clear of power lines. Nature News. March 2014


5. Hodge, A.M. A skill better than Rudolph’s. Scientific American. August 2011


6. Tyler N. J. C.; Cryptic impact: Visual detection of corona light and avoidance of power lines by reindeer. Wildlife Society Bulletin. Vol 40, Issue 1, March 2016, Pages 50–58


7. Douglas R. H. et al, The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals. Proceedings of the Royal Society, 281: 20132995

8. Langman, V.A, Nasal heat exchange in a Northern ungulate, the reindeer (Rangifer Tarandus). Respiratory Physiology, (1985) 59, 279-287

9. Hogg, C. et al. Arctic reindeer extend their visual range into the ultraviolet. The journal of experimental biology, 214, 2014-2019. 2011





4 thoughts on “Why Did Santa Chose Female Reindeer to Help Him Deliver Presents?”

  1. Thank you so much. I linked to you from scienceblog.com
    Sent it around to my regulars as a Who Knew story for the holidays
    I wonder how these amazing abilities of reindeer will be affected by the warming of earth
    Definitely wish to receive further posts from you
    Best wishes


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