Social Spacing |
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Social Spacing
Rocky Mountain bighorn sheep maternal herd |
Herd Formation Rocky Mountain bighorn sheep are gregarious mammals that live in herds most commonly numbering eight to ten individuals, but may have well over 100 members. Herds are fluid and often change membership throughout the year. Bighorn sheep generally form two kinds of herds: bachelor herds and maternal herds. Bachelor herds are comprised of adult rams and sub-adult rams, while maternal herds consist of the ewes, lambs, yearlings, and subadult rams (the dual membership of subadult rams will be discussed shortly). Some mixed groups are found, but they are unstable and most common in the pre-rut period and appear to be the result of males testing female reproductive status, and as such are probably a facet of male reproductive strategy, not predator protection. (Ruckstuhl and Festa-Bianchet 1997) Both male and maternal herds have seasonal home ranges (winter and summer), which only overlap spatially and temporally during the breeding season. (Beecham et. al. 2007) Segregation is driven by different optimal foraging patterns for rams and ewes, which is explained by the “body-size-predation hypothesis” proposed by Ruckstuhl (1998). (see Habitat and Diet) The impetus for forming herds follows the selfish-herd hypothesis and is thought to be based on predator avoidance and greater foraging efficiency. (Beecham et. al. 2007) As population size increases, the size of maternal herds increase, but the size of bachelor herds does not. (Ruckstuhl and Festa-Bianchet 1997) This is most likely because increased herd size provides greater protection for ewes and their young, whereas larger bachelor groups would only mean increased intra-specific competition for food.
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Image coutesy of Montan Fish, Wildlife, and Parks |
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Rocky Mountain bighorn sheep bachelor herd |
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Image Courtesy of Turkey Trot Rare Breeds |
Subadult Male Strategy
Subadult males spend time with both bachelor and maternal herds and occasionally form exclusively subadult male herds. In a sixteen year study Ruckstuhl and Festa-Bianchet (1997) found that the frequency of subadult male herds varies with the number of subadult males in the population (more groups when there are more subadult males). However, much more frequent than subadult male groups is the switching of subadults from the maternal herds to bachelor herds as autumn approaches. Three year old males, who become larger than females as the year progresses, show greater tendency to switch to bachelor groups than do two year old males. Sub-adult males who moved between the maternal and bachelor herds adopted the strategy of the group they were with. (Ruckstuhl 1999) It is unlikely that leaving the maternal group is driven by reproductive strategy (why leave the females?), but instead is explained by Ruckstuhl’s “body size predation hypothesis” and occurs as the optimal foraging strategy and predator protection needs of the growing males transitions to that of adult rams.
Why a home range?
Rocky Mountain bighorn sheep use an undefended home range for three main reasons. First, potential habitats are fragmented. (Beecham et. al. 2007) This means that contact with other herds is rare. Second, they are large mammals that require enough resources so as to make resources indefensible. Moreover, these resources are abundant and do not greatly differ in quality, so that even if contact between herds occurs, it would be deleterious to all to compete for a territory. Finally, bighorn sheep have separate winter and summer ranges and at the times of transition foraging is of utmost importance and energy may be low, so competition would be disadvantageous.
Social Spacing |
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