____________Mating
Photo permission pending, Gary Ferrence, www.pennsylvaniaelkherd.com
A sex ratio that heavily favors females leads elk to be polygamous. Elk practice female defense polygamy. Bull elk compete for females by controlling harems, which are clusters of females controlled by a dominant male specifically for the purpose of mating. Such a mating system does not necessarily increase the fitness of females. However it does allow her to mate with a dominant male, assuring that her offspring will receive good genes. When in a Harem, females are also able to use the male for protection.
The Harem
Harem sizes are dependent upon the bull: cow ratio. It is to each bull’s reproductive advantage to have as many females as possible in his harem. However it is energetically demanding to hold large harems. During rut, agonistic behavior between males increases greatly. Younger bulls and immature bulls will compete for existing harems. Older established bulls will spend much energy competing with these intruders to maintain their harem. These males do not defend a territory, but the cows within it. Bulls generally settle for smaller harems that are less costly to defend (Bender, 2002). The reproductive advantages of defending smaller harems for several years outweigh the advantages of defending a large harem for one year. A smaller harem allows the bull to avoid the excessive energy costs of defense that may leave him unprepared and unfit for winter (2002). Bender documented the relationship between harem size and bull: cow ratio. Harems with a 62:100 ratio generally had between 5 and 6 females, and harems with a 20:100 ratio have between 16 and 20 females (2002). When mature bulls are present in a herd during rut, they usually establish and maintain control of harems. In instances where the bull: cow ratio exceeded 21: 100, mature bulls tended the harems 84-100% of the time (2002).
The Rut
Cow elk begin to breed during the third rutting season after birth, at an average age of 2 years 4 months (Murie, 1951). Bull elk of 2-3 years have reached sexual maturity but have not fully developed. These males have neither the physical maturity nor the dominance to form a harem of their own, and it is rare for them to successfully take the harem of another. These males will attempt to sneak copulations with harem females and occasionally encounter the opportunity. As the young bull becomes older, his ability to maintain a harem and his opportunities to mate increase. The rut begins when most bulls have lost the velvet from their antlers, usually between the August 1 and September 1. This breeding season is most active during September and October (1951). Bull elk eat well for several months preceding rut so that they are well conditioned by the fall. Cow elk continue to eat through the rutting season, but bulls eat little (1951). The majority of the male’s energy during rut is spent in female defense. This behavior causes him to lose weight as his stores of surplus fat are burned.
The rut is initiated by increased testosterone production in males and by an increased production of estrogen in females. During this time the male is capable of producing sperm and the female begins estrus. Estrus lasts only about 12 hours at a time, and can occur several times within a two-month period until fertilization.
Aside from an increase in agonistic interactions, the rutting season is accompanied by flamboyant, energetically costly behavior from the males. Males may be observed in mock battles with trees and bushes and may stomp the ground violently. They spend time wallowing in the mud and will urinate and defecate onto the ground before rolling in it. Males in rut also take time to urinate directly onto their undersides and mane. These males may be observed to have this mud and excrement caked onto their bodies throughout the breeding season.
Males spend much of their energy proving their dominance to other males. These efforts serve as intimidation and as a preventative measure to help them avoid dangerous conflict. Bulls advertise to other males that they are strong, and that a fight would end up in their favor. A bull may communicate with others using visual markers such as broken brush, chemical markers from urine and preorbital gland secretions, and auditory signals created by mock attacks on trees. During the rutting season, bull elk make a bugling sound that starts low and rises to a high pitched whine. This sound serves as a warning to other males, and as an advertisement to nearby females.
Photo permission pending, Gary Ferrence, www.pennsylvaniaelkherd.com |
Agonistic Behavior
The resident bull maintains constant, sometimes aggressive control over his cows. Because estrus lasts for only about 12 hours, the bull must continue to check each female to see if she is ready to mate. At the same time the bull must defend against others that lurk around the perimeter and look to mate with his cows.
Elk use sparring as a last resort method of harem defense or takeover. The fight requires more energy of a bull than other methods of confrontation, and provides too much potential for injury or fatal wounds. In defense of a harem, a dominant bull engages instead in ritual fighting. He first bugles and swells to make himself look bigger, then chases the intruder a short distance. The chase may occur a number of times before an intruder becomes intimidated and gives up. This bluffing rarely gives rise to an actual fight. On the occasion that it does, bulls lock antlers and begin a pushing match that may result in one bull fleeing or one or both bulls becoming seriously injured.
