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Mating System
Mating
Every year from late October to early January bachelor and maternal Rocky Mountain bighorn sheep herds come together for the annual rut, or mating season. Most females breed for the first time at thirty months of age, but in highly successful herds ewes have been recorded to give birth as young as one year old. (Beecham et. al. 2007) Males often do not mate until seven years of age because of the dominance level necessary to secure copulations. Most classify the mating system of Rocky Mountain bighorn sheep as polygyny, but as we will see, an argument can be made for polygyandry in some cases. It is thought that the presence of mature males in the maternal group stimulates estrous and may play a role in synchronization of estrous cycles. Females are monestrous, but may copulate many times and with multiple males. (Beecham et. al. 2007)
As described in Social Organization, males compete for access to bighorn ewes. Once dominance has been established, males pursue one of four strategies. These are not evolutionary stable strategies. Instead, males may choose their tactic based on condition at hand (such as dominance status and male:female ratio) and may frequently change tactics during a single mating season. The primary male strategy is tending in which a dominant male defends and copulates with a single estrous ewes (very similar to mate guarding, but only temporary). The ram prevents other males from nearing the ewes using any means necessary, including attack. Females appear to be more receptive to tending males (the more dominant males). Multiple copulations are the norm and retaliatory copulations are common. Tending is the most successful strategy, but only rams in the top third of the dominance hierarchy use tending. One alternative strategy is coursing. In coursing, one or more males fight for temporary access to a defended ewe. Coursing is rarely successful as the ewe attempts to evade the coursing ram. Moreover, injuries from chasing may result and compromise that ram’s success for the remainder of the season. (Hogg and Forbes 1997) Coursing is observed most frequently when the male:female ratio is high, and is probably a last resort to secure copulations. (Beecham et. al. 2007) A third strategy is blocking. A ram will attempt to prevent an ewe from accessing traditional tending areas, often times prior to when the ewe is in estrus. (Hogg and Forbes 1997) Finally, when male:female ratios are low, the males attempt to herd females prior to the start of their estrous period. Multiple males are present and it is not necessarily the dominant male that does the herding. Once the ewes enter estrous, the dominant male will commence tending behavior. (Beecham et. al. 2007)
While females do not actively select rams, they are most receptive to tending rams, as this is an indication of dominance (and hence fitness). (Hogg and Forbes 1997) In essence, the females are assuming that if a male is strong enough to achieve the dominance necessary to engage in tending behavior, then he will father successful offspring.
Prior to an ewe accepting copulation, the ram must perform several courtship behaviors. He must nose her rump and flanks while flicking his tongue, then kick the female with his foreleg followed by resting his chin on rump prior to pushing his chest against her rump and finally rearing to a pre-mount position. (Beecham et. al. 2007)
Rearing of Young
After the completion of the rut, Rocky Mountain bighorn sheep once again split into maternal and bachelor groups. Pregnant ewes moved to the alpine areas earlier in the spring, probably for predator protection during birth. (Ruckstuhl and Festa-Bianchet 1998)Gestation is approximately 175 days after which time ewes leave the main herd and seek isolated, extremely rugged territory (predator protection) to give birth. Labor is short (~20 minutes) and usually one lamb is born but twins do occur. Single births are made up for by pregnancy rates in excess of 90%. (Beecham et. al. 2007) It is probable that females give birth to only one lamb because it enables them to provide greater care. Moreover, single births may be one reason why kin relationships are not important in bighorn sheep. Newborns are precocial, and can walk with their mothers after three hours at which time the mother rejoins the herd.
Ewes provide equal care to both male and female offspring. In addition Blanchard and colleagues (2005) applied the Trivers and Willard model (TWM) predicting that in polygynous mammals mothers that can provide greater care should bias the sex ratio towards sons. However, this was not found to be the case in bighorn sheep. There was a non-significant trend towards fewer males at higher population densities, but no relationship was found between body mass or hierarchical position and sex ration of offspring. They suggest that such is the case because social rank in bighorn ewes does not affect ability to provide care for offspring. (Blanchard et. al. 2005)
Lactating ewes forage more and have a higher bit rate than their non-lactating counterparts, but no difference is found between ewes nursing males and those nursing females. (Ruckstuhl and Festa-Bianchet 1998) This supports the equal care hypothesis and suggests little sexual dimorphism at a young age. In addition, there is no difference between lactating ewes and non-lactating ewes in vigilance behavior, probably resulting from reliance on the herd. (Ruckstuhl and Festa-Bianchet 1998) Finally, pregnant ewes moved to the alpine areas earlier in the spring, probably for predator protection during birth. (Ruckstuhl and Festa-Bianchet 1998)
Finally, all healthy lambs engage in play behavior that serves to develop physical skills and strength. Play resembles patterns used by adults in courtship and intrasexual conflict. Interestingly, males play more than females and use a wider variety of play tactics. Not surprisingly, partners of similar age that provide the most rigorous play are preferred. (Hass and Jenni 1993)
Mating System |
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