Midewin National Tallgrass Prairie
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  • 12 Thunder On The Prairie

    Credit Tom Coyne at Midewin

    Envision the sound of the wind upon the prairie. For the prairie wind is a constant, sometimes a gentle whisper, other times roaring, but always moving the sea of grass. Now imagine you hear the rumble of thunder in the distance, except when you look up the sky is blue, the sun is out, and no clouds appear on the horizon. The rumbling noise increases in volume as the ground begins to tremble.  Dust fills the air. You realize the ground shaking thunder and clouds of dust are from no heavenly storm. This approaching melee is caused by powerful hooves of a line of dark, frantic, creatures headed your way.  As people traversed the prairie in the 1800’s, some experienced the awe of these thundering bison hooves. While bison spend most of the time leisurely grazing, a herd can be spooked to stampede. It could be a distant flash of lightening, a roll of thunder, maybe the howl of a coyote, even the scratch of a dry leaf scraping along the ground, basically anything.

    When you see bison grazing you might assume they are huge plodding animals. Your false assumption of these massive mammals could put you in a dangerous situation. Bison can turn on a dime. They can pivot on either front or back legs. When they run, it is at speeds of 35-40 mph. They can sustain this speed easily for 5 miles. A race horse can run at 40 mph for one mile therefore a bison can easily outdistance a horse and rider. Bison can swim great distances across water either as individuals or in a herd. They can jump from a standstill up and over a 6 foot height. Bison are athletes in the bovine world. 

    As athletic as bison can be, most of their life is spent eating. They eat and walk, then eat and walk more. If given the space, a migrating herd of bison can travel ten to fifteen miles in a day, moving at a speed of 5-6 mph. This mobile style of foraging typically occurs for 9-11 hours a day. While bison are always chewing, they do have food preferences. Their grazing choices are chiefly grasses; 93% of their diet is grass, 5% flowering plants, and 2% browse, the tips of woody plants. Bison wrap their tongue around a clump of grass, bite down with their bottom teeth and upper gums causing the grass to tear. They are similar to deer in the absence of incisors, the front teeth in the top front of jaw. 

    Bison are ruminants. This means they have a four chambered stomach and have a “cud chewing” behavior. After a quick crunch with the molars, the grass is swallowed, sending it to the compartment of the stomach called the rumen. The rumen is a large vat for fermentation. It is filled with billions of microorganisms, including bacteria and protozoa. Here fermentation of the plant material occurs to helps break down tough cellular walls of grasses and other plant materials. After fermenting in the rumen for a time, the bison

    regurgitate a cud, or bolus of partially chewed grass. 

    If you see a bison relaxed and reclined on the ground, they are usually busy chewing. As they rest, bison chew this cud to further breakdown the plant material. The twice masticated food is then re-swallowed and sent to chambers 3 and 4 of the stomach. A great deal of a bison’s time is spent chewing. Bison have adapted to eat and survive on grasses less nutritious than other bovines require. These less nutritious grasses require 80 hours to pass through the digestive track of a bison. Their bodies have adapted to squeeze out all the nutrition possible from difficult to digest prairie grasses that comprise the majority of a bison’s diet.

    Bison are described in terms similar to those used for cattle. A yearling is a male or female calf that has reached its first birthday and is in its second year. A heifer is a female that has not conceived her first calf. A cow is a full grown female that has borne a calf. A full grown male is a bull.  

    In bison social structure, the herding behaviors of these categories of bison change with the season. Bulls tend to form small groups of up to five or brave a solitary existence for most of the year. The winter winds find cow or matriarchal bands forming. These cow-calf matriarchal herds average 10 to 20 bison but can reach numbers of 50 to 70 animals. Winter is a time of poor quality and reduced forage. Food needed for survival is the primary reason to scatter and decrease herd size in winter. 

    Spring breezes alter herd composition again. Green grass heralds the arrival of new calves. Weaned yearlings form temporary bands, while expectant mothers form nursery groups as they await the arrival of babes. This in turn relegates barren females and older cows into spinster bands.

    The heat of summer and urgency to procreate modifies the herd configuration, yet again. All bands of female and male bison congregate during the breeding season to form one large herd. The breeding congregation splits up as the heat of summer wains. The herd division of fall & winter seasons return sending off pregnant cows to forage enough grasses to survive the harsh conditions to come. Then to repeat the cycle of new life again in spring.

    Bison are a long lived animal. The average lifespan of bison range from 15-20 years.  For bulls, prime breeding age is 6-10 years. Female bison reach sexual maturity by two years of age. Cows have one calf every one to three years until the age of 16. The gestation period for a cow lasts 9.5 months, about 285 days. Single births are the norm, twins being a rarer occurrence. Bison in the wild have evolved to be survivors.  Cows usually have only enough milk for one calf. If this is the case and twins are born, it is likely that one twin will be dominant and survive or both twins will be compromised. Having only one calf in the wild means survival is more likely.   

    Calving season runs from March to the end of May. The average bison baby weighs between 33-66 lbs. These orange-red balls of fur come into life with their eyes open. Within 30 minutes of birth, they are standing and feeding from mom. Amazingly, within a couple of hours of birth, they are traveling with the herd. The ability of calves to stand quickly and keep up with the herd so rapidly is a survival strategy that keeps predators at bay. The calves sport a cinnamon colored coat and have been nicknamed “red dogs”. Within four months’ time, the calves’ coats darken to the brown of adults. The weaning of calves is completed between 7 to 12 months of age. At this age, calves weigh between 350 to 425 lbs., equivalent to the weight of a full grown lion.    

    In spring, these bouncing bundles of energy are all cuteness on the surface. Within these adorable bovine calves exists the potential to shake the earth. Calves grow quickly into the mighty beings that once shook the prairie. The existence of bison is more complex than just hooves thundering across the land. They spend the majority of time foraging for enough food year round. This instinct to survive is needed to create another generation. While we may never again see the massive thundering herds crossing an endless prairie, Midewin’s bison still inspire awe in all who view them. Power still resides in the muscle and sinew of their being. This potential is waiting to again thunder across the land.   

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