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Social Structure
 Permission Pending |
Males In Rocky Mountain bighorn rams, there is no dominance hierarchy, but males do fight to establish dominance during the rut. The literature disagrees as to the duration of dominance relationships. Some argue that dominance is established on a case-by-case basis as males fight for access to specific females (Dewey and Ballenger 1999) Others suggest that dominance is establshed and reinforced in a pre-rut period. (Ballenger et. al. 2007) This could be a result of studying different populations. Perhaps the best known images of bighorn sheep come from male battles. Rams smash into each other at speeds of up to twenty miles per hour with an average of five collisions per hour. Fights have been observed to last up to 25.5 hours. (Dewey and Ballenger 1999) Contrary to fist intuition, injury rarely results from these battles. Male bighorn sheep have double layered skulls reinforced with bone struts and a large tendon connecting the skull to the spine to assist in recoil recovery. (ZooMontana 2007) Older males tend to be the most dominant as a result of their greater body mass and horn size/mass (horns grow for a lifetime and larger horns mean greater head butting ability). (Dewey and Ballenger 1999) Some homosexual activity is observed in rams in which the dominant male mounts the submissive male and |
Females
Hass (1991) found a stable, age related dominance hierarchy in female bighorn sheep. Moreover, many “triangles” are found and it is suggested that dominance doesn’t exist at a “higher level than individual dyads.” In addition females reach behavioral maturity later (four to five years old) than sexual maturity. Behaviorally mature females use a proportionally greater number of threats than contact patterns than do younger individuals. Females do, however, frequently fight to gain higher social status. However, there is no correlation between offspring survival and female rank, although this may be due to the fact that this herd lived in an area with and excess of food resources. In a habitat with limited resources it is likely that higher rank would guarantee greater access to resources and improve survival of offspring. However, dominant individuals do nurse their lambs more frequently and occasionally became helpers to relatives in the herd. Such helping behavior is the only display of kin-helping or kin altruism found in bighorn sheep, and it could be argued that this is not truly altruism since the helper experiences no detriment as a result of the helping behavior. (Hass 1991)
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