With jack frost nipping at your nose and holiday spiced drinks at your local coffee shop, ‘tis the season to be on the lookout for reindeer. Even if you don’t leave carrots by the chimney for Rudolf on Christmas Eve, reindeer are handsome, hardy animals that inspire thoughts of sparkling white snowscapes, the north woods, and the magic of winter.
Wild populations of Rangifer tarandus in North America are known as caribou, while Eurasian populations and domesticated animals of the same species are called reindeer. They belong to the family Cervidae, which also includes deer, moose, and elk. With thick fur coats, sturdy frames, and impressive antlers they use as shovels to uncover food buried in snow, these animals are built for winter.
Reindeer travel in large herds of up to several hundred and graze on mosses, herbs, grass, and even lichens . They have a double-layered coat with an outer layer of hollow hairs that provide insulation and buoyancy while swimming. Both male and female reindeer have antlers, which they regrow each year. Males shed their antlers at the beginning of winter, while pregnant females remain bedecked through winter until their calves are born the following spring. Antlers are bulky, weighing up to 15 kg or 33 pounds , and require a lot of energy to grow and carry around, but they serve as useful tools and valuable weapons in social contests. In addition to their function in male-male competition for mates, antlers are used by females to compete for and defend scarce patches of food over winter . So while we typically think of males as the fighting sex, female reindeer go head-to-head too!
Many animals exhibit differences in behavior throughout the year, particularly between breeding and non-breeding seasons. Reindeer have a polygynous mating system, where a single male can mate with multiple females. During the breeding season, called rut, males become highly territorial and aggressive toward other males; the most dominant individuals defend large harems of up to 15 females. Males devote so much effort to protecting their harems that they lose weight during rutting season: a study in Finland found that male reindeer body mass declined 7% on average (maximum of 27%) during rut . The same study found that body mass loss, activity level, and agonistic behavior (such as chasing and sparring with other males) of dominant male reindeer increased as the number of females in the mating group increased. These findings could be partly explained by the “audience effect”, where animals change their behavior based on their social environment . In other words, male reindeer are show-offs, picking more fights when more females are around to watch.
What about the ladies’ perspective? Female reindeer may evaluate males and choose their mate based on factors such as body size, antler size, vocalizations, or social rank . However, male reindeer can influence female movement by actively herding females in their group, contributing to group stability . Another research study in Norway found that female reindeer tended to associate with close kin during rut, i.e., mother-daughter pairs and sisters stuck together more than unrelated females during the breeding season . One explanation for this pattern could be that older, more dominant females protect their daughters from being harassed by males. Female reindeer also become better mothers as they age . This means that females increase their lifetime fitness not only by living longer to produce more offspring, but also because they gain experience each year: yearly reproductive success, as measured by the number of calves produced that survive to weaning, increased linearly with age .
Next time you hear the jingle of sleigh bells, consider what amazing mothers Prancer and Dancer must be after all these years! And when you see Sven in the movie Frozen, think about the function of those antlers and the poor guy wearing himself thin during the ordeal of rutting season. Keep your eyes peeled on Christmas Eve and you may spot these majestic animals streaking across the starry sky . . . or, perhaps it would be just as magical to see a herd of wild reindeer roaming the frozen tundra on a twinkling winter night.
Jessica Schaefer is a PhD student in the Animal Behavior Graduate Group. She studies the behavioral ecology and mating systems of species from limpets to lizards to birds, and she would love to see a herd of wild reindeer.
Main featured image by Natalia Kollegova [Source]
- San Diego Zoo (2019). Reindeer. San Diego Zoo Animals & Plants, https://animals.sandiegozoo.org/animals/reindeer.
- Schaefer, J.A. & Mahoney, S.P. (2001). Antlers on female caribou: biogeography of the bones of contention. Ecology, 82, 3556–3560.
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