Caribou are large-bodied deer, with adult bulls in the biggest subspecies weighing upward of 250 kg (roughly 550 pounds) and standing about 1.2 meters (four feet) at the shoulder. Cows are considerably smaller, and body size varies dramatically across the dozen-plus subspecies and ecotypes spread from Alaska to Scandinavia to the high Arctic islands. What makes caribou especially interesting is that their weight is a moving target, swinging by a third or more across a single year, and that both sexes grow antlers, a trait unique among the world’s deer.
A Range of Sizes Across Subspecies
Caribou and reindeer are the same species, Rangifer tarandus, divided into multiple subspecies that differ strikingly in body size. The largest-bodied caribou are the woodland and mountain ecotypes found in boreal Canada and parts of Alaska, where mature bulls commonly reach 180 to 250 kg and occasionally exceed 270 kg. Barren-ground caribou, the migratory herds of the open tundra, are somewhat lighter, with bulls in the range of 130 to 200 kg. At the other end of the spectrum, Svalbard reindeer, the small, stocky subspecies found on the Arctic archipelago north of Norway, are dramatically smaller. Adult females in that population weigh roughly 46 to 58 kg, which is about the size of a large dog.1Functional Ecology. Fat storage influences fasting endurance more than body size in an ungulate That four-to-one weight difference between the biggest bull woodland caribou and a Svalbard female illustrates just how much body size varies within a single species.
Shoulder height follows a similar pattern. Large-bodied males stand 1.1 to 1.3 meters at the shoulder, while females of the same subspecies are typically 0.9 to 1.1 meters. The compact Svalbard reindeer stand noticeably lower, with proportionally shorter legs, an adaptation that conserves heat and lowers their center of gravity in wind-blasted Arctic terrain. Semi-domesticated Scandinavian reindeer fall somewhere in between, their size partly shaped by centuries of selective herding.
Males Versus Females
Sexual dimorphism in caribou is pronounced. Bulls outweigh cows by roughly 40 to 80 percent, depending on the subspecies and the time of year. The difference begins early. At birth, male and female calves weigh about the same, but by autumn of their first year, male calves are already about 9 kg heavier than their female counterparts.2Canadian Journal of Zoology. Body composition and nutrient reserves of arctic caribou That gap only widens with age.
The size difference is driven partly by the mating system. Caribou are polygynous, meaning dominant bulls compete for access to multiple females during the autumn rut. Larger, heavier males have a competitive advantage in sparring and display, so natural selection has pushed male body size upward over evolutionary time. Females, by contrast, face different pressures: they need to be large enough to carry a calf through a harsh winter pregnancy and fuel early lactation, but beyond a certain body mass, the cost of hauling extra weight across long migrations outweighs the benefit.
Weight That Changes with the Seasons
One of the most remarkable things about caribou physiology is how dramatically their weight fluctuates through the year. These are not animals that hold a steady mass. A bull caribou in peak autumn condition, fat-laden and muscular ahead of the rut, can be a very different animal from the same bull in late winter.
During the rut, bulls essentially stop eating. They spend weeks sparring, chasing, and guarding females, burning through stored energy at a staggering rate. Research on semi-domesticated reindeer bulls showed that males lost about 34 percent of their ingesta-free body mass over a 77-day rutting period. That loss included roughly 78 percent of their body fat and 23 percent of their body protein.3PubMed. Polygynous mating impairs body condition and homeostasis in male reindeer (Rangifer tarandus tarandus) Even when food was available after the rut ended, body mass did not recover through the rest of the winter. The rut essentially leaves bulls in a depleted state for months.
Females experience their own version of seasonal weight change, driven by pregnancy and lactation rather than mating competition. Pregnant females heading into winter are significantly heavier and carry more fat than non-pregnant females.4Journal of Zoology. Pregnancy of adult caribou (Rangifer tarandus): evidence for lactational infertility That extra condition matters because caribou calve in spring, often before the landscape has greened up enough to supply high-quality forage. A cow that enters winter too lean may not have the reserves to support both her own survival and a growing fetus.
The Energetic Demands of Being a Caribou Cow
Lactation is the single most energy-intensive phase of a female caribou’s year, even more demanding than late pregnancy. Peak food intake roughly doubles from winter pregnancy through summer lactation, and the digestible energy a cow needs to maintain her body rises by about 85 percent during that transition.5PubMed. Robust Responses of Female Caribou to Changes in Food Supply Nitrogen demands, which largely reflect the need to produce protein-rich milk, spike even more sharply.
To meet those costs, caribou use a strategy that mixes stored body reserves with incoming food. During winter, when forage quality is poor, both reindeer and caribou conserve body protein by minimizing how much amino acid nitrogen they burn for energy. Maternal protein stored from early winter supports the vast majority of fetal growth. Once the calf is born and nursing begins, lactating females continue to lose body protein even as non-reproductive females are already regaining it.6PubMed. Allocating protein to reproduction in arctic reindeer and caribou The protein cost of making milk is so high that it exceeds the energy cost in relative terms. In other words, running out of protein is the more immediate threat for a nursing cow.
This explains why body condition entering winter is such a strong predictor of reproductive success. Females that are too lean going into the cold months are less likely to conceive or carry a pregnancy to term, and those that lactated the previous summer are leaner than females that skipped reproduction.4Journal of Zoology. Pregnancy of adult caribou (Rangifer tarandus): evidence for lactational infertility The result is that many caribou cows do not breed in consecutive years, especially in herds where summer forage is limited. Size and fatness are, in a very real sense, currencies that determine whether a female gets to reproduce.
Antlers on Both Sexes
Caribou are the only living deer species in which females routinely grow antlers.7Bulletin of the Florida Museum of Natural History. Shed Female Antlers as a Mineral Resource for Caribou on Spring Calving Grounds In every other deer species, antlers are an exclusively male feature. This quirk has drawn researchers’ attention for decades, partly because it raises an obvious question: why would a female invest in growing a structure that costs significant energy and minerals if she is not using it to compete with other females for mates?
The timing of antler growth and shedding differs between the sexes in ways that hint at the answer. Males grow their antlers over summer and carry them through the autumn rut, when they use them for sparring. Bulls typically shed their antlers in late autumn or early winter, after the breeding season ends. Females, on the other hand, grow their antlers in summer and retain them through the entire winter, shedding them around the time they give birth in spring.8Ecosphere. Addled by antlers: Synchronous disruption to female caribou antler phenology The leading hypothesis is that antlers help pregnant females compete for access to limited feeding craters in the snow. A cow with antlers can defend a dug-out foraging hole against antlerless bulls and younger animals. Since she is pregnant and needs every calorie she can get, that competitive advantage in winter feeding may be significant enough to justify the cost of antler production.
Female antler shedding is roughly synchronized with calving and tends to happen at the same time and place each year within a population.7Bulletin of the Florida Museum of Natural History. Shed Female Antlers as a Mineral Resource for Caribou on Spring Calving Grounds The shed antlers pile up on traditional calving grounds and serve as a mineral resource: other caribou and calves chew on them to recover calcium and phosphorus, both of which are scarce in the tundra diet. So even discarded antlers have an ecological role.
How Big Antlers Get and What They Cost
Male caribou antlers are the most dramatic feature of the species. A large bull can carry a rack spanning over a meter wide, with long, sweeping beams and distinctive flattened “shovels” near the brow. Total antler mass in big males can exceed 10 kg, and the largest racks on record approach 15 kg. Female antlers are much more modest, typically thin, short, and irregular compared to the sweeping architecture of a mature bull’s set.
Growing antlers of that size is metabolically expensive. Simulations of antler growth in caribou found that antler production raised energy requirements by 8 to 16 percent in males during the growth period, depending on how efficiently the animal converts energy into antler tissue.9Rangifer. Simulating antler growth and energy, nitrogen, calcium and phosphorus metabolism in caribou The mineral demands are even more striking. At peak growth rates, males deposit more than 25 grams of calcium and more than 12 grams of phosphorus per day into their developing antlers. Dietary intake alone cannot supply that much mineral, so males actually resorb calcium and phosphorus from their own skeleton to keep up with antler growth. They are, in effect, temporarily weakening their bones to build their headgear.
Females face a milder version of the same mineral crunch. They can meet their calcium needs for antler growth without borrowing from bone, but phosphorus still gets resorbed from the skeleton during the final stages of antler hardening.9Rangifer. Simulating antler growth and energy, nitrogen, calcium and phosphorus metabolism in caribou Across all the nutrients modeled, phosphorus emerged as the mineral most likely to limit antler size in both sexes, after energy. This is consistent with the fact that caribou in areas with richer mineral soils tend to grow larger antlers than those in mineral-poor habitats.
Why Some Caribou Are Shrinking
Caribou size is not fixed within a population. It responds to both population density and climate, and in some herds, the trend has been downward. A long-term study of Newfoundland caribou tracked body and antler measurements across about 40 years and a 20-fold swing in population size. As the population grew, adults of both sexes showed reduced stature: jawbone length declined, and the number of antler points at harvest dropped.10Rangifer. Morphological change in Newfoundland caribou: Effects of abundance and climate This pattern is consistent with density-dependent competition, where more caribou sharing the same finite habitat means less food per individual, leading to slower growth and smaller adult body size.
But density alone did not explain the full pattern. Large-scale winter climate patterns, specifically the North Atlantic Oscillation, were also negatively associated with jawbone size in that population.10Rangifer. Morphological change in Newfoundland caribou: Effects of abundance and climate Warmer or more variable winters can reduce the availability of lichens (a key winter food), increase the frequency of icing events that lock forage under a hard crust, and disrupt the seasonal timing of plant growth. Calves born after a harsh or mistimed winter tend to be smaller, and since early-life growth strongly influences adult body size, the effects can persist throughout the animal’s life.
Similar patterns of body-size reduction have been documented in Svalbard reindeer and Norwegian populations. The trend is worrying because smaller females carry less fat into winter, which, as discussed earlier, reduces their odds of successfully reproducing. A population of shrinking caribou is not just cosmetically different; it may be less productive and less resilient to further environmental stress.
Built for Snow and Cold
Beyond raw mass and height, caribou have body proportions shaped by life in extreme northern environments. Their legs are relatively long for their body mass compared to most temperate deer, and their feet are oversized. Each hoof is broad and concave, functioning almost like a snowshoe. The dew claws (small secondary toes higher up the foot) splay out when the foot hits the ground, increasing the total area of contact with snow or soft substrate.
Measurements of semi-domesticated reindeer showed strong correlations between body weight and hoof area, which makes intuitive sense: heavier animals need bigger platforms to avoid sinking. Wild forest reindeer had significantly larger hooves and foot areas than semi-domesticated reindeer of similar age and sex classes.11Rangifer. Hoof and foot loads for reindeer (Rangifer tarandus) Wild forest reindeer stag foot areas averaged about 259 square centimeters, compared to roughly 214 square centimeters for semi-domesticated hinds. The difference probably reflects the fact that wild forest reindeer move through deeper snow in more rugged terrain, where foot loading matters more. In semi-domesticated herds, where animals are partly managed and may be supplementally fed, the selection pressure for enormous feet is somewhat relaxed.
Front hooves are consistently larger than hind hooves in all caribou, and males have larger hooves than females from calf age onward.11Rangifer. Hoof and foot loads for reindeer (Rangifer tarandus) The front-heavy distribution makes sense given that caribou carry most of their weight, including a heavy head and antler rack in bulls, over their front quarters. Those broad, splayed hooves also double as digging tools. In winter, caribou paw through snow to reach the lichens and ground vegetation buried beneath, and the wide, hard-edged hoof is well-suited for scraping through crusted snow surfaces.
How Wild Caribou Compare to Domesticated Reindeer
People sometimes treat “caribou” and “reindeer” as completely different animals. They are not. Both are Rangifer tarandus, and the main line between them is cultural rather than biological: “reindeer” typically refers to the semi-domesticated or Eurasian populations, while “caribou” is used for wild North American animals. That said, domestication has nudged reindeer body shape and size in subtle ways over several thousand years of herding in Scandinavia and Russia.
Semi-domesticated reindeer tend to be somewhat stockier and shorter-legged than their wild counterparts. They have smaller hooves, as noted in the foot-load research above, and often carry more subcutaneous fat heading into winter because herders may provide supplemental feeding during harsh periods. Wild caribou, particularly the long-distance migratory ecotypes like barren-ground caribou and Alaskan Grant’s caribou, are leaner and leggier, built for covering hundreds of kilometers between seasonal ranges. A barren-ground caribou might migrate over a thousand kilometers in a year, which selects for a body plan that balances endurance with the ability to store enough fat and protein to survive a long winter.
The protein economy of the two forms also differs in interesting ways. Caribou that calve later in spring, after some green-up has begun, rely somewhat less on stored maternal protein for fetal growth compared to reindeer that calve earlier. In reindeer, about 96 percent of the protein needed for the fetus comes from the mother’s own body stores, while in caribou the figure is closer to 84 percent.6PubMed. Allocating protein to reproduction in arctic reindeer and caribou Both figures are remarkably high, but the gap reflects a real difference in calving strategy. Reindeer calve earlier, before spring forage is available, and depend almost entirely on body reserves to build the calf. Caribou time their births a few weeks later, allowing them to supplement stored protein with incoming food. Either way, body size at the start of winter is a critical determinant of whether a female has enough reserves to bring a healthy calf to term.
Svalbard Reindeer and the Limits of Miniaturization
The Svalbard reindeer deserves its own mention because it represents one extreme of the caribou size spectrum. Isolated on an Arctic archipelago roughly 800 kilometers north of the Norwegian mainland, this subspecies has evolved in a habitat with no large predators and no possibility of long-distance migration. The result is an animal that looks almost comically compact: short-legged, barrel-bodied, and much lighter than any mainland caribou.
Adult females on Svalbard weigh in the range of 46 to 58 kg, and males are only modestly larger.1Functional Ecology. Fat storage influences fasting endurance more than body size in an ungulate Their survival strategy depends heavily on fat storage. Research on fasting endurance in Svalbard reindeer found that initial fat mass, not body size, was the dominant factor determining how long an animal could survive without food. Across a sample of 140 individuals, the estimated fasting endurance averaged about 85 days, with a range from 48 to 137 days. Fatter animals lasted longer regardless of their overall body mass, which runs counter to the common ecological expectation that larger animals should tolerate fasting better simply because they have more total reserves relative to their metabolic rate.
The Svalbard example is a reminder that “bigger is better” is an oversimplification when it comes to caribou size. In a predator-free, island environment where food availability swings violently between summer abundance and winter scarcity, being small and very fat can be a more effective survival strategy than being large and moderately lean. The compact body shape also reduces heat loss, and the short legs reduce the energetic cost of moving through deep snow. Svalbard reindeer are, in many ways, the most extreme Arctic adaptation within Rangifer.