<P> The mule deer is the larger of the two Odocoileus species on average, with a height of 80--106 cm (31--42 in) at the shoulders and a nose - to - tail length ranging from 1.2 to 2.1 m (3.9 to 6.9 ft). Of this, the tail may comprise 11.6 to 23 cm (4.6 to 9.1 in). Adult bucks normally weigh 55--150 kg (121--331 lb), averaging around 92 kg (203 lb), although trophy specimens may weigh up to 210 kg (460 lb). Does (female deer) are rather smaller and typically weigh from 43 to 90 kg (95 to 198 lb), with an average of around 68 kg (150 lb). </P> <P> Unlike the whitetail, the mule deer does not generally show marked size variation across its range, although environmental conditions can cause considerable weight fluctuations in any given population . An exception to this is the subspecies, the Sitka black - tailed deer (O. h . sitkensis). This race is markedly smaller than other mule deer, with an average weight of 54.5 kg (120 lb) and 36 kg (79 lb) in males and females, respectively . </P> <P> In addition to movements related to available shelter and food, the breeding cycle is important in understanding deer behavior . The "rut" or mating season usually begins in the fall as does go into estrus for a period of a few days and males become more aggressive, competing for mates . Does may mate with more than one buck and go back into estrus within a month if they did not become pregnant . The gestation period is about 190--200 days, with fawns born in the spring, staying with their mothers during the summer and being weaned in the fall after about 60--75 days . Mule deer females usually give birth to two fawns, although if it is their first time having a fawn, they often have just one . </P> <P> A buck's antlers fall off during the winter, to grow again in preparation for the next season's rut . The annual cycle of antler growth is regulated by changes in the length of the day . For a guide to identify the sex and age class of Rocky Mountain mule deer at various seasons see S1 File . For more information see the main article on deer . </P>

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