Bison Romance

During North America’s early spring, a reddish-orange creature emerges.  Within minutes of its appearance, the newly-born bison calf rises and begins expending pent up energy.  Its movements constantly watched over by its mother, the largest animal on the North American continent.  Colloquially known as a “red dog,” this little beastie weighs in from 30 to 70 lbs [1] upon its debut, a far cry from its future weight (approx. 2000 lbs for males and 1000 lbs for females), easily attained within a couple of years.  Some may consider this newly arrived entity as “cute.”  They would be deceived.  For this “cute-ness” scampering around in the spring air will transform into potentially the greatest source of animal-induced injuries to humans in North America.  It will scoff at bears, coyotes and mountain lions.  Oh my!!  Of course, wolves are a different matter.  Hunting in packs, they can take down an adult bison. So what is the origin of this “cute” beastie?

Bison Calf – “The Cute Little Beastie”

                Its genesis begins slightly more than 9-1/2 months [2] prior to its emergence. During the summer months of July or August romance rumbles and shakes the earth.  The females of the species begin to gather with the up-until-now aloof males straggling in from their own groups to join the females.  Before now, the bulls do not communicate much with other bison.  Prior to the breeding season, they are usually off on their own or in small groups, being content to graze.  However, with the onset of the breeding season, the rut, the bulls become more communicative.  One may believe only courtship of the female occurs during the rut.  But the male has a tough job ahead.  He not only has to win the attention of a receptive female, but he also has to manage other bulls.  The bull will seek out a female and stay with her.  This behavior is called tending.  While doing so, he must also ward off other bulls by fighting and threatening.

                Males will vie for the females.  Even though a bull is tending a cow, it is not entirely a foregone conclusion this will be the mating pair.  Other bulls may threaten to replace the tending bull.  The one who dominates will take over the tending, but will then also have to ward off anxious bulls looking to mate. But fighting requires much energy which is also required for breeding.  So, the bull will generally threaten first in order to conserve energy.  Threatening may begin with bellowing, a sound the bull makes, which to the undiscriminating ear, may sound like thunder off in the distance.  The intruder may also bellow in return, and if the bellowing competition escalates, as typical of males, the sounds become louder and the bravado more expressive.  During the bellowing, the bulls may paw the ground or wallow [3] [4]. Bison often will lie down in a barren depression and role back-and-forth, throwing up dust.  Though there are other reasons as well for this behavior, it is often used in the bull’s threatening behavior.

                During challenges, wallowing may also engage a strange phenomenon. But males of other species sometimes exhibit bizarre behavior when attempting to win over the female as well — nothing really new here.  A threatening bull may urinate in a wallow and then roll in the moist ground.  The reason is not fully understood.  Dale Lott, a naturalist, speculates the testosterone level is being signaled to the challenging bull.  As the bull uses up physical resources, muscle begins to metabolize with the metabolites entering the urine, which can be detected by the conditioned nose of the challenger.  The opposing bull will know if fat is still being burned and if muscle is still intact [5] of the threatening bull, allowing the opposition to determine if he has the resources to continue the fight against the threatening bull.

Bison Wallowing

                If bellowing and wallowing do not work, then the males will begin to posture.  There are two types of posture: head-to-head and broadside.  Head-to-head posture precedes a charge in which the two bulls will run straight at each other.  As they approach one another, either one or the other may submit by turning  away or they will bang heads.  If they approach slowly, they will engage in a behavior called “nod-threatening”.  The two bulls will stand close enough to each other to reach the other with their horns by turning their heads aside.

                In the broadside posture the bull will keep himself broadside to his opponent and raise his head a little as well as arch his back and bellow.  The idea is to show he is not to be messed with.  However, this posture usually does not lead to a fight.  The broadside and the nod-threatening postures demonstrate the lengths bulls will go to forewarn each other.  The issue is efficiency.  Perhaps even more than offering protection to the herd, producing calves is the bull’s prime directive.  Breeding requires energy, and the bull, instinctually driven, attempts to mate as much as possible to produce  as many calves as possible.  Fighting seriously reduces the bull’s energy.  Further, fighting and breeding burn up fat and muscle mass, which have to be replaced between the end of the rut, which may last into September, and the onset of winter if the bull is to survive the winter [6].

Bulls Going Head-to-Head

                Before the tending of the female and the subsequent efforts to win his prize, the bull first selects a female.  Cows come into heat sometime during the rut, and will only be in that state for about two days.  Cows only release one egg at a time.  Achieving fertilization requires attracting a suitable mate and be willing to mate, which means changes for the cow.  When she comes into heat, the vulva becomes swollen and oozes mucus and lymph, signaling to the bull she is ready to mate.  The bull will wander through the herd inspecting each cow to determine which ones are ready to mate.  After inspecting, the bull may curl his lip.  Lip curling is a complex expression.  The bull stretches out his neck, holds his head level, distends his nostrils and curls back his lips.  This occurs after examining the cow’s vulva or sniffing her urine [7].  The cow’s urine will indicate how close she is to ovulation.  The bull detects this through the vomernasal organ, which has an opening in the roof of the bull’s mouth.  This organ seems specialized for analyzing the female’s urine [8].

Bison Lip Curling

                Often the cow rebuffs the bull by the swing of the head, a thrust of horns or sometimes just a good swift kick. After selecting a cow, and if the bull has not been dissuaded, the courting begins.  During courting, the bull stands parallel to her side, warning off intruders which may even include the cow’s previous calf or yearling.  The pairing may last only a few minutes if a more dominant bull intrudes and displaces the tending bull.  Or the pairing could last one or two days.  In either case, the tending bull will move on to another cow, being driven by the bull’s prime directive.

Even though the bull may seem dominant, but the cow rules the courtship. Typically the bull follows behind her.  If he tries to direct her in a particular direction, she can easily dodge him and wander off on her own.  He is resigned to follow her.  Eventually, the herd leaves the pairs behind.  The cow may try to rejoin the herd, but the bull will move to block her, which she can easily evade. Or she may also go for a higher ranking male.  She, too, has a prime directive to breed the best calf.  After all, she only has one shot each season.  So she must choose wisely, and is more receptive to older bulls who have survived winters, predators, disease and battles.  Older bulls have been tested and have been found worthy.  If, however, she stays, she has chosen him.

The bull utters brief panting sounds as he approaches the cow, warning her.  First advances are typically repelled but the bull will be insistent.  His intentions are made known by standing and swinging his head.  Eventually he will attempt to put his chin on her rump, which, in response, she will evade.  He may try to climb upon her, only to have her slip out from under him.  But once receptive, the cow will allow the bull to mount.  He uses his chin as leverage to lift himself onto her rump, leaning his head against her side and pressing his forelegs around her.  After insertion, the thrusts last only 4 to 10 seconds.  Then he either drops off or releases his forelegs so the cow can walk out from underneath him.  The copulation may harm the cow.  Her flanks may have bloody wounds from the bull’s front hoofs striking and rubbing her. The moment of copulation usually attracts the attention of younger males who will follow the couple afterwards.  But the tending bull will stay with the cow, warding off other suiters to prevent them from also mating with her.  This ensures the tending bull’s seed is successful fertilizing the egg and not replaced by the seed of another.  Eventually though, the tending bull will move on to attempt to mate with other cows [9][10]. 

Bison Mating

Nine and-a-half months later, the fruition of the bison romance occurs, as a reddish-orange little beastie emerges.  Upon birth the cow licks, pulls away and even eats the membranes entangling the calf.  After disposing of these membranes, she licks the slimy wet coat of the newborn.  Meanwhile oxytocin, a pituitary gland hormone, has flooded her brain stimulating her endearment of this new creature, which she will nurture and protect [11]. If the cow is only three years old, then this is most likely her first calf.  Females breed when 2 years old and have their first calf at 3 years old.  Cows can live 20-25 years, having a calf each year [12]. 

The cycle completed, the newborn “red-dog”, if male, will repeat the fine tradition of seeking and tending a female, threatening and fighting challenging bulls, and mating with as many females as possible.  If the little beastie is female, she will enter into the honored role of controlling the mating process and producing off-spring, and nurturing the next generation.  And so the cycle continues, sustaining the herd and ensuring the survival of the species.

End Notes:

[1] Retrieved from

[2] The average gestation period is 9-1/2 months.  Retrieved from 5-Jul-2019.

[3] McHugh, Tom. 1972. The Time of the Buffalo. University of Nebraska Press. 191-195.

[4] Lott, Dale.  2003. American Bison: A Natural History. University of California Press. Berkeley. 14-18.

[5] Lott. 9.

[6] McHugh. 192-193.

[7] Lott. 15.

[8] Lott. 20-21.

[9] McHugh. 192-195.

[10] Lott. 15-17.

[11] Lott. 29.


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