Caribou Migration

Off the Top of Your Head...
Why Do You Suppose Caribou Have Antlers?

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Imagine balancing heavy bones on your head and carrying them everywhere you go! Any idea why caribou have such amazing antlers?
Antler development is a unique phenomenon of nature. In some species such as moose and deer only males grow antlers whereas in caribou both males and females grow antlers each year. In all cases it is an annual process that separates antlered animals from horned animals that usually grow a set of horns over a lifetime. The bulls can develop spectacular antlers at times exceeding 125 cm in length, 112 cm across and as much as 15 to 20 pounds. While all mature caribou bulls develop antlers, fewer than 5% of females fail to develop antlers.

Photo: Gov't. of the Northwest Territories, Division of Natural Resources.

Year after year, the antlers are grown and shed, then re-grown and shed again. The entire process of antler development begins each spring and starts from two permanent stumps of bone called pedicles on the head of each caribou. As the antlers grow they are covered in a hairy skin called velvet. Beneath this outer layer of furry skin are thousand of blood vessels and nerves that carry calcium and other minerals to the developing, soft cartilage-like tissue. During this phase they remain soft and fragile but as a measure of compensation they also grow at an astounding rate, as much as 2.5 cm in a day. This makes them the fastest growing tissue in the animal world. This growth uses hard-earned energy--one can't help but wonder why.

Enormous Energy
This growth process occurs throughout the entire summer months. At times the growth is so rapid that the dietary intake of food cannot supply enough minerals and the bull’s body actually uses calcium from its entire skeletal system. The bull’s body then replaces the lost calcium when antlers growth ceases and while prime forage is still abundant.

For everything in nature there is a season--and there's also a reason! Here caribou biologist Doug Urquart describes the seasonal differences between the antlers of males and females:

"Antler development is 3-6 months out of phase between the sexes. For example, the male's antlers begin developing in March, grow rapidly from May to July, and are completely hardend and out of velvet by mid-September. Following the rut, antlers are shed in early November by older males, but may be kept until April by some of the younger ones. Female antlers develop from June to September and are out of velvet by late September. The females' antlers are retained throughout the winter. Pregnant females drop their antlers within days of calving. Barren cows shed their antlers before the spring." (Provided courtesy of the Government of the Northwest Territories, Division of Natural Resources.)

Did You Know?
The developing antlers also offer a unique and built in cooling system. As the warm blood rushes to the antlers ensuring rapid growth it is quickly cooled by the outside air, which then allows the bull’s entire body to cool. Mother Nature’s built in air conditioner.

Journaling Questions

"For what reasons might female caribou keep their antlers during the winter, while males drop theirs in the fall?"
"Why are the male caribou antlers generally much larger than the female's?"
"Why might the females antlers begin to develop after calving?"
"What some reasons why both male and female caribou have antlers, but in other kinds of deer only the male animals have antlers?"

Remember, WHEN something happens often helps explain WHY it happens, so try to think of what kinds of energy that goes into growing antlers and the different ways antlers can be used. Then guess the reasons for the the differences Urquart describes.

Try This!

Make a life-sized pair of caribou antlers and put them on display. Find something that weighs the same as caribou antlers do, and try to balance them on your head.
Proportionate to your own weight, calculate how heavy your antlers would be. (Average body weights for Porcupine caribou are: male 130 kg/285 lbs, and female 90 kg/198 lbs.)
Create a chart showing when caribou have their antlers. Use headings like developing/velvet, hardened/out of velvet, no antlers. Compare male and female, with extra headings for pregnant and barren females and young animals. Use the chart to create a list of generalizations about caribou and their antlers.