Social Organization and Habitat Selection in Bison

Social Organization in North American Bison

The level of social organization in bison is widely debated. Some believe bison live in extended families, while others believe social interactions among bison occur randomly. Ryan Shaw examined social organization in bison. His objective was to determine if mother-daughter relationships are allowed to develop, do bison form long-term associations with related individuals? The study was conducted from 2007-2009 on the 350,000 acre Armendaris Ranch during spring, summer, and fall of each year. Twenty-five treatment mothers were selected from the free-ranging herd. Their calves were not weaned and allowed to stay with their mothers. Calves from 25 control females were weaned and removed from the herd, a typical practice for this and most other bison operations.

The strongest social associations were among treatment mothers and their offspring. Further analysis revealed treatment mothers and their offspring had close associations over multiple generations. There were more mother-female than mother-male offspring relationships. Of the 27 prominent kin relationships observed, only five were with males.

 
When and Where Should We Go?
 

Ryan also examined initiation of movement and direction by bison after they rest periods. He found that select older females consistently made decisions for the group and initiated the direction of movement. In 93% of the 31 recorded choices, group directions were within 95% confidence intervals generated by these older females. Furthermore, these directions were different from directions chosen by other group members in 52% of cases. An entire bison group would not move after rest periods until an average of 47% of adult cows departed the group and waited for the rest of the group to catch-up. Interestingly, the oldest females initiated movement after rest in 81% of cases, verifying their importance in the decision-making process.

 
Management Practices Increase Stress in Young Male Bison

In his last study, Ryan examined stress levels of bison in three different management scenarios. Male bison are typically weaned and placed in feedlots at a time when they would ordinarily be leaving their mothers and joining small bachelor groups. Ryan determined levels of stress for males under tight vs. loose confinement in feedlots and under free-ranging conditions. Yearling bison males were weaned and placed in either tight (TC) or loose (LC) confinement. The free-ranging group (FR) was weaned from their mothers and then returned to the herd. Fecal cortisol metabolites (FCM) were used as an index of stress. Pre-trial FCM were determined from samples taken prior to weaning. Fecal samples were collected every 2 weeks from mid-January to the end of April 2009.

Fecal cortisol levels were lowest for FR (23 ng/g DM), intermediate for LC (39 ng/g DM), and highest for TC (63 ng/g DM). Pre-trial FCM levels increased dramatically from Period 1 to Periods 2-8 for TC and LC, but not for FR.

These results support the idea that bison live in extended families of mothers and their daughters and young males. Older females strongly influenced the initiation and direction of group movements across landscapes. Disruption of social dynamics greatly increases stress in young male bison.