Editing Reindeer (section)

== Physical characteristics ==
Naming in this and following sections follows the taxonomy in the authoritative 2011 reference work ''[[Handbook of the Mammals of the World]] Vol. 2: Hoofed Mammals''.<ref name="HBW" />

=== Antlers ===
[[File:ReindeerLoosingVelvet.jpg|thumb|upright|Losing the velvet layer under which a new antler is growing, an annual process]]
In most [[Deer|cervid]] species, only males grow [[antler]]s; the reindeer is the only cervid species in which females also grow them normally.<ref name=antlers>{{cite web |title=Fun Facts about Reindeer and Caribou |website=U.S. Food and Drug Administration |date=2021-05-21 |url=https://www.fda.gov/animal-veterinary/animal-health-literacy/fun-facts-about-reindeer-and-caribou |access-date=2022-07-21}}</ref> [[Androgen]]s play an essential role in the antler formation of cervids. The antlerogenic genes in reindeer have more sensitivity to androgens in comparison with other cervids.<ref>{{cite journal |last1=Lin |first1=Zeshan |title=Biological adaptations in the Arctic cervid, the reindeer (''Rangifer tarandus'') |journal=Science |date=2019 |volume=364 |issue=6446 |pages=eaav6312 |doi=10.1126/science.aav6312 |pmid=31221829 |bibcode=2019Sci...364.6312L |title-link=doi |doi-access=free}}</ref><ref>{{cite journal |last1=Nasoori |first1=Alireza |date=2020 |title=Formation, structure, and function of extra-skeletal bones in mammals |url=https://archive.org/details/formation-structure-and-function-of-extra-skeletal-bones-in-mammals |journal=Biological Reviews |volume=95 |issue=4 |pages=986–1019 |doi=10.1111/brv.12597 |pmid=32338826 |s2cid=216556342}}</ref>

There is considerable variation among species and subspecies in the size of the antlers (e.g., they are rather small and spindly in the northernmost species and subspecies),<ref name=NorthAmerica/> but on average the bull's antlers are the second largest of any extant deer, after those of the male [[moose]]. In the largest subspecies, the antlers of large bulls can range up to {{cvt|100|cm}} in width and {{cvt|135|cm}} in beam length. They have the largest antlers relative to body size among living deer species.<ref name=antlers/> Antler size measured in number of points reflects the nutritional status of the reindeer and climate variation of its environment.<ref name="Smith1998">{{cite journal |last=Smith |first=B.E. |year=1998 |title=Antler size and winter mortality of elk: effects of environment, birth year, and parasites |journal=[[Journal of Mammalogy]] |volume=79 |issue=3 |pages=1038–1044 |jstor=1383113 |doi=10.2307/1383113 |doi-access=free}}</ref><ref name="Mahony2011">{{cite journal |journal=[[Rangifer (journal)|Rangifer]] |volume=31 |pages=21–34 |number=1 |year=2011 |title=Morphological change in Newfoundland caribou: Effects of abundance and climate |first1=Shane P. |last1=Mahoney |first2=Jackie N. |last2=Weir |first3=J. Glenn |last3=Luther |first4=James A. |last4=Schaefer |first5=Shawn F. |last5=Morrison |url=https://septentrio.uit.no/index.php/rangifer/article/view/1917/1783 |archive-url=https://web.archive.org/web/20141103235739/http://septentrio.uit.no/index.php/rangifer/article/viewFile/1917/1783 |archive-date=3 November 2014 |doi=10.7557/2.31.1.1917 |doi-access=free |url-status=live}}</ref> The number of points on male reindeer increases from birth to 5 years of age and remains relatively constant from then on.<ref name="Mahony2011"/>{{rp|24}} "In male caribou, antler mass (but not the number of tines) varies in concert with body mass."<ref name="MarkussonFolstad1997">{{cite journal|last1=Markusson|first1=Eystein|last2=Folstad|first2=Ivar|title=Reindeer antlers: visual indicators of individual quality?|journal=Oecologia|date=1 May 1997|volume=110|issue=4|pages=501–507 |doi=10.1007/s004420050186|pmid=28307241|issn=0029-8549 |bibcode=1997Oecol.110..501M|s2cid=40646035}}</ref><ref name="ThomasBarry2005">{{cite journal|last1=Thomas|first1=Don|last2=Barry |first2=Sam |title=Antler Mass of Barren-Ground Caribou Relative to Body Condition and Pregnancy Rate|journal=Arctic |date=2005 |volume=58|issue=3 |pages=241–246|jstor=40512709|citeseerx=10.1.1.541.4295}}</ref> While antlers of male woodland caribou are typically smaller than those of male barren-ground caribou, they can be over {{cvt|1|m}} across. They are flattened in cross-section, compact and relatively dense.<ref name="EC2012" /> Geist describes them as frontally emphasized, flat-beamed antlers.<ref name="Geist" /> Woodland caribou antlers are thicker and broader than those of the barren-ground caribou and their legs and heads are longer.<ref name="EC2012" /> Quebec-Labrador male caribou antlers can be significantly larger and wider than other woodland caribou. Central barren-ground male caribou antlers are perhaps the most diverse in configuration and can grow to be very high and wide. Osborn's caribou antlers are typically the most massive, with the largest circumference measurements.<ref name="Allen-1902">Allen, J.A. (1902) Description of a new caribou from northern British Columbia and remarks on ''Rangifer montanus''. Bulletin of the American Museum of Natural History XVI: 149-158.</ref>

The antlers' main beams begin at the brow "extending posterior over the shoulders and bowing so that the tips point forward. The prominent, palmate brow tines extend forward, over the face."<ref name="Virtual">{{citation |url= http://www.lcvirtualwildlife.ca/index.php/caribou |title= Caribou |work= Virtual Wildlife |location= Lethbridge, Alberta |url-status= live |archive-url= https://web.archive.org/web/20141103055205/http://www.lcvirtualwildlife.ca/index.php/caribou |archive-date= 3 November 2014}}</ref> The antlers typically have two separate groups of points, lower and upper.

Antlers begin to grow on male reindeer in March or April and on female reindeer in May or June. This process is called antlerogenesis. Antlers grow very quickly every year on the bulls. As the antlers grow, they are covered in thick [[Velvet antler|velvet]], filled with blood vessels and spongy in texture. The antler velvet of the [[barren-ground caribou]] and the [[boreal woodland caribou]] is dark chocolate brown.<ref name="NWT2012">{{citation |title=Species at Risk in the Northwest Territories 2012 |url=http://www.enr.gov.nt.ca/sites/default/files/reports/species_at_risk_in_the_nwt_2012_eng.pdf |access-date=31 October 2014 |author=GNWT |date=9 November 2023 |publisher=Government of Northwest Territories, Department of Environment and Natural Resources |isbn=978-0-7708-0196-0 |url-status=dead |archive-url=https://web.archive.org/web/20150924001254/http://www.enr.gov.nt.ca/sites/default/files/reports/species_at_risk_in_the_nwt_2012_eng.pdf |archive-date=24 September 2015}}</ref> The velvet that covers growing antlers is a highly vascularised skin. This velvet is dark brown on woodland or barren-ground caribou and slate-grey on Peary caribou and the [[Dolphin-Union caribou]] herd.<ref name="Virtual" /><ref name="GunnSeal1998">{{citation|last1=Gunn |first1=Anne |last2=Nishi |first2=J. |year=1998 |chapter= Review of information for Dolphin and Union caribou herd |title= Population and Habitat Viability Assessment Workshop for the Peary caribou (Rangifer tarandus pearyi) |series= Briefing book |editor-last1=Gunn |editor-first1=A. |editor-last2=Seal |editor-first2=U.S. |editor-last3=Miller |editor-first3=P.S. |location=Apple Valley, Minnesota |publisher=Conservation Breeding Specialist Group (SSC/UCN) |pages=1–22}}</ref><ref name="CanadaArctic2002">{{citation|year=2002a |url=http://www.arctic.uoguelph.ca/cpl/Traditional/traditional_frame.htm|title=Tuktu — Caribou |location=Guelph, Ontario|work=Canada's Arctic|access-date=17 January 2014|url-status=live|archive-url=https://web.archive.org/web/20141115024305/http://www.arctic.uoguelph.ca/cpl/Traditional/traditional_frame.htm|archive-date=15 November 2014}}</ref> Velvet lumps in March can develop into a rack measuring more than a {{cvt|1|m}} in length by August.<ref>{{cite book |title=State of Canada's forests |chapter=Woodland caribou |date=2000}}</ref>{{rp|88}}

[[File:Rangifer tarandus 04 MWNH 148.jpg|thumb|left|A ''R. tarandus'' skull]]
When the antler growth is fully grown and hardened, the velvet is shed or rubbed off. To [[Inuit]], for whom the caribou is a "culturally important [[keystone species]]", the months are named after landmarks in the caribou life cycle. For example, ''amiraijaut'' in the [[Igloolik]] region is "when velvet falls off caribou antlers."<ref name="CPL_2002_Tuktu_Caribou"/>

Male reindeer use their antlers to compete with other males during the mating season. Butler (1986) showed that the social requirements of caribou females during the rut determines the mating strategies of males and, consequently, the form of male antlers.<ref>{{cite thesis |last=Butler |first=H.E. |date=1986 |title=Mating strategies of woodland caribou: ''Rangifer tarandus caribou'' |type=Doctoral thesis |place=Calgary, Alberta |publisher=University of Calgary}}</ref> In describing woodland caribou, which have a harem-defense mating system, SARA wrote, "During the rut, males engage in frequent and furious sparring battles with their antlers. Large males with large antlers do most of the mating."<ref name="SARA2014">{{citation|url=http://www.registrelep-sararegistry.gc.ca/species/speciesDetails_e.cfm?sid=636#biology |title=Woodland caribou boreal population – biology |date=October 2014 |access-date=3 November 2014 |publisher=SARA}}</ref> Reindeer continue to migrate until the bulls have spent their back fat.<ref name="CPL_2002_Tuktu_Caribou"/><ref>{{cite book |title=This Is My Country, What's Yours?: A Literary Atlas of Canada |date=29 May 2007 |page=496 |first=Noah |last=Richler |author-link=Noah Richler|publisher=Random House |isbn=9781551994178}}</ref><ref name="IOHP">{{cite interview|series=Igloolik Oral History Project (IOHP) |title=Interview 065 |date=1991 |location=[[Igloolik]], [[Nunavut]]}}</ref> By contrast, barren-ground caribou males tend individual females and their fights are brief and much less intense; consequently, their antlers are long, and thin, round in cross-section and less branched and are designed more for show (or sexual attraction) than fighting.

In late autumn or early winter after the rut, male reindeer lose their antlers, growing a new pair the next summer with a larger rack than the previous year. Female reindeer keep their antlers until they calve. In the [[Scandinavia]]n and [[Arctic Circle]] populations, old bulls' antlers fall off in late December, young bulls' antlers fall off in the early spring, and cows' antlers fall off in the summer.{{citation needed|date=December 2022}}

When male reindeer shed their antlers in early to mid-winter, the antlered cows acquire the highest ranks in the feeding hierarchy, gaining access to the best forage areas. These cows are healthier than those without antlers.<ref name="Thing297">{{cite journal |last1=Thing |first1=Henning |last2=Olesen |first2=Carsten Riis |last3=Aastrup |first3=Peter |title=Antler possession by west Greenland female caribou in relation to population characteristics |journal=[[Rangifer (journal)|Rangifer]] |year=1986 |volume=6 |issue=2 |page=297 |doi=10.7557/2.6.2.662 |doi-access=free }}</ref> Calves whose mothers do not have antlers are more prone to disease and have a significantly higher mortality.<ref name="Thing297" /> Cows in good nutritional condition, for example, during a mild winter with good winter range quality, may grow new antlers earlier as antler growth requires high intake.<ref name="Thing297"/>

[[File:Mammals of northern Alaska on the Arctic slope (1956) Rangifer arcticus groenlandicus skull.png|thumb|upright|right|A ''R. t. platyrhynchus'' skull]]
According to a respected [[Igloolik]] elder, Noah Piugaattuk, who was one of the last outpost camp leaders,<ref name="nunatsiaqonline">{{cite news |title=Igloolik elders win northern science award |first=Sean |last=McKibbon |newspaper=Nunatsiaq News |location=Igloolik |date=21 January 2000 |access-date=30 October 2017 |url=http://www.nunatsiaqonline.ca/archives/nunavut000131/nvt20121_10.html |quote=Elders in Igloolik were recognized with a national science award last week for their efforts in preserving traditional Inuit knowledge |archive-url=https://web.archive.org/web/20171107002305/http://www.nunatsiaqonline.ca/archives/nunavut000131/nvt20121_10.html |archive-date=7 November 2017 |url-status=dead}}</ref> caribou (''tuktu'') antlers<ref name="CPL_2002_Tuktu_Caribou">{{citation |editor=Hebert, P.D.N. |editor2=Wearing-Wilde, J. |series=Canada's Polar Life (CPL) |publisher=University of Guelph |year=2002 |title=Tuktu — Caribou |quote="Since 1986, elders in the community have worked...the Igloolik Research Centre...to record their knowledge for posterity on paper and audio tape...Noah Piugaattuk contributed 70 to 80 hours of audio tape." Use of antlers (IOHP 037); |url=http://www.arctic.uoguelph.ca/cpl/Traditional/traditional/animals/caribou.htm|access-date=30 October 2017|archive-url=https://web.archive.org/web/20171020081500/http://www.arctic.uoguelph.ca/cpl/Traditional/traditional/animals/caribou.htm |archive-date=20 October 2017 |url-status=live}}</ref>

{{blockquote|...get detached every year...Young males lose the velvet from the antlers much more quickly than female caribou even though they are not fully mature. They start to work with their antlers just as soon as the velvet starts to fall off. The young males engage in fights with their antlers towards autumn...soon after the velvet had fallen off they will be red, as they start to get bleached their colour changes...When the velvet starts to fall off the antler is red because the antler is made from blood. The antler is the blood that has hardened; in fact, the core of the antler is still bloody when the velvet starts to fall off, at least close to the base.|Elder Noah Piugaattuk of Igloolik cited in "Tuktu — Caribou" (2002) "Canada's Polar Life" }}

According to the Igloolik Oral History Project (IOHP), "Caribou antlers provided the Inuit with a myriad of implements, from [[Snow knife|snow knives]] and shovels to drying racks and seal-hunting tools. A complex set of terms describes each part of the antler and relates it to its various uses".<ref name="CPL_2002_Tuktu_Caribou"/> Currently, the larger racks of antlers are used by Inuit in [[Inuit art]] as materials for carving. [[Iqaluit]]-based Jackoposie Oopakak's 1989 carving, entitled ''Nunali'', which means "place where people live", and which is part of the permanent collection of the [[National Gallery of Canada]], includes a massive set of caribou antlers on which he has intricately carved the miniaturized world of the Inuit where "Arctic birds, caribou, polar bears, seals, and whales are interspersed with human activities of fishing, hunting, cleaning skins, stretching boots, and travelling by dog sled and kayak...from the base of the antlers to the tip of each branch".<ref name="NGC_2017_Oopakak">{{citation |url=http://cybermuse.gallery.ca/cybermuse/enthusiast/acquisitions/2003-2004/Oopakak_text_e.jsp |title=Oopakak |date=n.d.|access-date=31 October 2017 |publisher=National Gallery of Canada |url-status=live |archive-url=https://web.archive.org/web/20151012160006/http://cybermuse.gallery.ca/cybermuse/enthusiast/acquisitions/2003-2004/Oopakak_text_e.jsp |archive-date=12 October 2015 }}</ref>

=== Pelt ===
The color of the [[fur]] varies considerably, both between individuals and depending on season and species. Northern populations, which usually are relatively small, are whiter, while southern populations, which typically are relatively large, are darker. This can be seen well in North America, where the northernmost subspecies, the [[Peary caribou]], is the whitest and smallest subspecies of the continent, while the Selkirk Mountains caribou (Southern Mountain population DU9)<ref name="Allen-1902" /> is the darkest and nearly the largest,<ref name=NorthAmerica>Reid, F. (2006). ''Mammals of North America''. Peterson Field Guides. {{ISBN|978-0-395-93596-5}}</ref> only exceeded in size by Osborn's caribou (Northern Mountain population DU7).<ref name="Allen-1902" />

The [[Animal coat|coat]] has two layers of fur: a dense [[wool]]ly undercoat and a longer-haired overcoat consisting of hollow, air-filled hairs.<ref name="Bennett116">{{citation|last=Bennett |first=John|publisher=McGill-Queen's University Press|series=McGill-Queen's Native and Northern Series |date=1 June 2008|title=Uqalurait: An Oral History of Nunavut|page=116}}</ref>{{efn|According to Inuit elder Marie Kilunik of the [[Aivilingmiut]], Canadian Inuit preferred the caribou skins from caribou taken in the late summer or autumn, when their coats had thickened. They used it for winter clothing "because each hair is hollow and fills with air trapping heat."<ref>Marie Kilunik, Aivilingmiut, & Crnkovich, 1990, p. 116{{full citation needed|date=November 2022}}</ref> Fur is the primary insulation factor that allows reindeer to regulate their [[Human body temperature#Core temperature|core body temperature]] in relation to their environment, the [[Temperature gradient|thermogradient]], even if the temperature rises to {{cvt|100|F|order=flip}}.<ref name="Moote1955">{{cite journal |last=Moote |first=I. |year=1955 |title=The thermal insulation of caribou pelts |journal=Textile Research Journal |volume=25 |number =10 |pages=832–837 |doi=10.1177/004051755502501002 |s2cid=138926309}}</ref> In 1913, Dugmore noted how the woodland caribou swim so high out of the water, unlike any other mammal, because their hollow, "air-filled, quill-like hair" acts as a supporting "life jacket".<ref name="Dugmore1913">{{citation |first=Arthur Radclyffe |last=Dugmore |title=The romance of the Newfoundland caribou|page=191 |year=1913 |access-date=2 November 2014 |location=Philadelphia |publisher=Lippincott |url=https://archive.org/stream/romanceofnewfoun00dugm/romanceofnewfoun00dugm_djvu.txt}}</ref>

A darker belly color may be caused by two mutations of [[Melanocortin 1 receptor|MC1R]]. They appear to be more common in domestic reindeer herds.<ref name=Vage2014>{{cite journal |title=Two Missense Mutations in Melanocortin 1 Receptor (MC1R) Are Strongly Associated With Dark Ventral Coat Color in Reindeer (''Rangifer Tarandus'') |year=2014 |doi=10.1111/age.12187 |hdl=2164/4960| hdl-access=free| last1=Våge |first1=D. I. |last2=Nieminen |first2=M. |last3=Anderson |first3=D. G. |last4=Røed |first4=K. H. |journal=Animal Genetics |volume=45 |issue=5 |pages=750–753 |pmid=25039753}}</ref>

=== Heat exchange ===
Blood moving into the legs is cooled by blood returning to the body in a [[Countercurrent exchange|countercurrent heat exchange]] (CCHE), a highly efficient means of minimizing heat loss through the skin's surface. In the CCHE mechanism, in cold weather, blood vessels are closely knotted and intertwined with arteries to the skin and appendages that carry warm blood with veins returning to the body that carry cold blood causing the warm arterial blood to exchange heat with the cold venous blood. In this way, their legs for example are kept cool, maintaining the core body temperature nearly {{cvt|30|C-change}} higher with less heat lost to the environment. Heat is thus recycled instead of being dissipated. The "heart does not have to pump blood as rapidly in order to maintain a constant body core temperature and thus, metabolic rate." CCHE is present in animals like reindeer, fox and moose living in extreme conditions of cold or hot weather as a mechanism for retaining the heat in (or out of) the body. These are countercurrent exchange systems with the same fluid, usually blood, in a circuit, used for both directions of flow.<ref name="Rahiman2009">{{cite web |first=Mohd Hezri Fazalul |last=Rahiman |url=http://hezrif.uitm.edu.my/ese652/51%20Heat%20Exchanger%20090907d.pdf |title=Heat exchanger |year=2009 |access-date=3 November 2014 |location=Malaysia |archive-url=https://web.archive.org/web/20131205011425/http://hezrif.uitm.edu.my/ese652/51%20Heat%20Exchanger%20090907d.pdf |archive-date=5 December 2013 }}</ref>

Reindeer have specialized counter-current vascular heat exchange in their [[Nasal cavity|nasal passage]]s. Temperature gradient along the nasal [[Mucous membrane|mucosa]] is under physiological control. Incoming cold air is warmed by [[Thermoregulation|body heat]] before entering the lungs and water is condensed from the expired air and captured before the reindeer's breath is exhaled, then used to moisten dry incoming air and possibly be absorbed into the blood through the [[mucous membrane]]s.<ref name="BlixJohnsen1983">{{cite journal |last1=Blix |first1=A.S. |last2=Johnsen |first2=Helge Kreiitzer|pmid=6887057|pmc=1199219 |title=Aspects of nasal heat exchange in resting reindeer |journal=Journal of Physiology |volume=340 |year=1983 |pages=445–454 |doi=10.1113/jphysiol.1983.sp014772 }}</ref> Like [[moose]], caribou have specialized noses featuring nasal [[Nasal concha|turbinate bone]]s that dramatically increase the surface area within the [[nostril]]s.

=== Hooves ===
The reindeer has large feet with crescent-shaped cloven hooves for walking in snow or swamps. According to the Species at Risk Public Registry ([[Species at Risk Act|SARA]]), woodland<ref name="SARA2014" />
{{blockquote|"Caribou have large feet with four toes. In addition to two small ones, called "dew claws," they have two large, crescent-shaped toes that support most of their weight and serve as shovels when digging for food under snow. These large concave hooves offer stable support on wet, soggy ground and on crusty snow. The pads of the hoof change from a thick, fleshy shape in the summer to become hard and thin in the winter months, reducing the animal's exposure to the cold ground. Additional winter protection comes from the long hair between the "toes"; it covers the pads so the caribou walks only on the horny rim of the hooves."|source=SARA 2014}}

Reindeer [[Hoof|hooves]] adapt to the season: in the summer, when the tundra is soft and wet, the footpads become sponge-like and provide extra traction. In the winter, the pads shrink and tighten, exposing the rim of the hoof, which cuts into the ice and crusted snow to keep it from slipping. This also enables them to dig down (an activity known as "cratering") through the snow to their favourite food, a [[lichen]] known as reindeer lichen (''[[Cladonia rangiferina]]'').<ref>{{cite web |quote=In the winter, the fleshy pads on these toes grow longer and form a tough, hornlike rim. Caribou use these large, sharp-edged hooves to dig through the snow and uncover the lichens that sustain them in winter months. Biologists call this activity "cratering" because of the crater-like cavity the caribou's hooves leave in the snow. |page=5 |url=http://www.taiga.net/projectcaribou/pdf/allaboutcaribou.PDF |title=All About Caribou |archive-url=https://web.archive.org/web/20130706171742/http://taiga.net/projectcaribou/pdf/allaboutcaribou.PDF |archive-date=6 July 2013 |url-status=usurped |work=Project Caribou}}</ref><ref>{{cite web |title=RY.0246-32: A reindeer uses its hooves to clear away snow to reach lichen at its winter pastures. Yamal, Siberia |url=http://www.arcticphoto.co.uk/stories/avkas/ry0246-32.htm |url-status=dead |archive-url=https://web.archive.org/web/20120205212234/http://www.arcticphoto.co.uk/stories/avkas/ry0246-32.htm |archive-date=5 February 2012 |website=Arcticphoto.co.uk |access-date=16 September 2011}}</ref>

=== Size ===
The females (or "cows" as they are often called) usually measure {{cvt|162|-|205|cm}} in length and weigh {{cvt|80|-|120|kg}}.<ref name=Alaska>[http://www.adfg.alaska.gov/index.cfm?adfg=caribou.main Caribou at the Alaska Department of Fish & Game] {{webarchive|url=https://web.archive.org/web/20131230035700/http://www.adfg.alaska.gov/index.cfm?adfg=caribou.main |date=30 December 2013 }}. Adfg.state.ak.us. Retrieved on 16 September 2011.</ref> The males (or "bulls" as they are often called) are typically larger (to an extent which varies between the different species and subspecies), measuring {{cvt|180|-|214|cm}} in length and usually weighing {{cvt|159|-|182|kg}}.<ref name=Alaska/> Exceptionally large bulls have weighed as much as {{cvt|318|kg}}.<ref name=Alaska/> Weight varies drastically between the seasons, with bulls losing as much as 40% of their pre-rut weight.<ref name="Naughton2011">{{citation |title=The Natural History of Canadian Mammals |last=Naughton |first=Donna |pages=543, 562, 567 |publisher=Canadian Museum of Nature and University of Toronto Press |isbn=978-1-4426-4483-0 |year=2011 }}</ref>

The shoulder height is usually {{cvt|85|to|150|cm}}, and the tail is {{cvt|14|to|20|cm}} long.

The [[Svalbard reindeer|reindeer from Svalbard]] are the smallest of all. They are also relatively short-legged and may have a shoulder height of as little as {{cvt|80|cm}},<ref name="Svalbard">{{cite web |last1=Aanes |first1=Ronny |date=2007 |title=Svalbard reindeer |url=http://npweb.npolar.no/english/arter/svalbardrein |website=Norwegian Polar Institute |archive-url=https://web.archive.org/web/20101222190226/http://npweb.npolar.no/english/arter/svalbardrein |archive-date=22 December 2010}}</ref> thereby following [[Allen's rule]].

=== Clicking sound ===
The knees of many species and subspecies of reindeer are adapted to produce a clicking sound as they walk.<ref name=clicking>Banfield, Alexander William Francis (1966) "The caribou", pp. 25–28 in ''The Unbelievable Land''. Smith, I.N. (ed.) Ottawa: Queen's Press, cited in {{Cite journal
| pmid = 18986518
| year = 2008
| last1 = Bro-Jørgensen
| first1 = J.
| title = Knee-clicks and visual traits indicate fighting ability in eland antelopes: Multiple messages and back-up signals
| journal = BMC Biology
| volume = 6
| page = 47
| last2 = Dabelsteen
| first2 = T.
| doi = 10.1186/1741-7007-6-47
| pmc = 2596769
| doi-access = free
}}</ref> The sounds originate in the tendons of the knees and may be audible from several hundred meters away. The frequency of the knee-clicks is one of a range of signals that establish relative positions on a dominance scale among reindeer. "Specifically, loud knee-clicking is discovered to be an honest signal of body size, providing an exceptional example of the potential for non-vocal acoustic communication in mammals."<ref name=clicking/> The clicking sound made by reindeer as they walk is caused by small tendons slipping over bone protuberances (sesamoid bones) in their feet.<ref name="Shackleton2013">{{citation|title=Hoofed Mammals of British Columbia |first=David |last=Shackleton |date= May 2013 |publisher=Royal BC Museum |isbn=978-0-7726-6638-3 |orig-year=1999}}</ref><ref name="Banfield1966">{{citation|last=Banfield |first=Alexander William Francis|chapter=The caribou|title=The Unbelievable Land|editor-last=Smith|editor-first=I.N.|location=Ottawa|publisher=Queen's Press|year=1966|pages=25–28}}</ref> The sound is made when a reindeer is walking or running, occurring when the full weight of the foot is on the ground or just after it is relieved of the weight.<ref name="Dugmore1913" />

=== Eyes ===
A study by researchers from [[University College London]] in 2011 revealed that reindeer can see light with wavelengths as short as 320&nbsp;nm (i.e. in the ultraviolet range), considerably below the human threshold of 400&nbsp;nm. It is thought that this ability helps them to survive in the Arctic, because many objects that blend into the landscape in light visible to humans, such as urine and fur, produce sharp contrasts in ultraviolet.<ref name=ultraviolet>[http://www.ucl.ac.uk/news/news-articles/1105/11052502 Reindeer use UV light to survive in the wild] {{webarchive |url=https://web.archive.org/web/20111129042727/http://www.ucl.ac.uk/news/news-articles/1105/11052502 |date=29 November 2011 }}. Ucl.ac.uk (26 May 2011). Retrieved on 16 September 2011.</ref> It has been proposed that [[Corona discharge#Problems|UV flashes on power lines]] are responsible for reindeer avoiding power lines because "...in darkness these animals see power lines not as dim, passive structures but, rather, as lines of flickering light stretching across the terrain."<ref name="Tyler2014">{{cite journal |author1=Tyler, N. |author2=Stokkan, K.A. |author3=Hogg, C. |author4=Nellemann, C. |author5=Vistnes, A.I. |author6=Jeffery, G. |year=2014 |title=Ultraviolet vision and avoidance of power lines in birds and mammals |journal=Conservation Biology |volume=28 |issue=3 |pages=630–631 |doi=10.1111/cobi.12262|pmid=24621320 |pmc=4232876 |bibcode=2014ConBi..28..630T }}</ref>

In 2023, researchers studying reindeer living in [[Cairngorms National Park]], Scotland, suggested that UV visual sensitivity in reindeer helps them detect UV-absorbing lichens against a background of UV-reflecting snows.<ref>{{cite journal |last1=Dominy |first1=Nathaniel J. |last2=Hobaiter |first2=Catherine  |last3=Harris |first3=Julie M. |date=2023 |title=Reindeer and the quest for Scottish enlichenment |journal=i-Perception |volume=14 |issue=6 |pages=1–6 |doi=10.1177/20416695231218520 |pmid=38107029 |pmc=10725117 }} (note pun with [[Scottish Enlightenment]]).</ref>

The [[tapetum lucidum]] of [[Arctic reindeer]] eyes changes in color from gold in summer to blue in winter to improve their vision during times of continuous darkness, and perhaps enable them to better spot predators.<ref>{{cite journal |last1= Stokken|first1= Karl-Arne|last2= Folkow|first2= Lars|date= December 2013|title= Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer|journal= Proceedings of the Royal Society B|volume= 280|issue= 1773|page= 20132451|doi= 10.1098/rspb.2013.2451|pmid= 24174115|pmc= 3826237}}</ref>