Off
the Top of Your Head...
Why Do You Suppose Caribou Have Antlers?
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See: Reading
and Writing Connections >>
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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.
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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
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Bull
caribou in velvet
Credit Amy Gulick
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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.
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.
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