Highly dynamic fission-fusion species can exhibit leadership when traveling

This post was written by QBIC sophomore, Mariluz Soula.

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Lewis JS, Wartzok D, Heithaus MR. (2011) Highly dynamic fission-fusion species can exhibit leadership when traveling. Behavior Ecology Sociobiology 65: 1061-1069

Leadership in tight-knit social groups is known to enhance the fitness of the entire group. Leadership by specific individuals usually occurs in species groups that are stable and related, and that inhabit complex habitats (Payne, 2003). However, leadership has not been studied in fission-fusion groups, or social groups that change in size and composition as time passes. In a species exhibiting fission-fusion dynamics, the group might disperse to forage during the day (fission), then reunite to sleep in the same place (fusion). Although these groups tend to be unstable, complex habitats may allow leadership to develop regardless. Leaders may have variable effects: they may lead the group to areas rich in resources but may also lead the group away from forage sites (King et al., 2009). Leaders of fission-fusion groups may also influence group size and may actually select individuals from the group that will maximize the leader’s fitness since these types of species frequently change in composition and size (Conradt & Roper, 2005).

Scientists from Florida International University sought to elucidate whether a group of bottlenose dolphins off the Florida coast exhibited consistent leadership. This group was ideal for the study because dolphin pods are known to exhibit fission-fusion behavior yet the complex habitat of the Lower Florida Keys (LFK) may prompt these groups to establish leadership roles for the enhanced fitness of the group. The area is mostly shallow and divided up by small islands and most foraging grounds are accessible by channels traversable only during high tide. Thus, more experienced individuals may prevent straying and enhance foraging opportunities. Lewis et al. (2011) studied pod dynamics to see whether individuals led by position, if leaders controlled direction change more often, and what factors contributed to number of leading individuals.

Data collection was accomplished by locating dolphins during surveys conducted in the LFK over six years. Individual dolphin groups were defined as those with dolphins of different ages within 100 m of each other; a total of 171 different group samples were analyzed. Individuals more than 100 meters apart or physically separated by geography were defined as exhibiting fission. Individual dolphins were identified by unique patterns on the dorsal fin. The scientists filmed and recorded relative positions of individuals when the dolphins surfaced as well as direction change attempts and success of each individual. When fission occurred, the observers followed the group that exhibited fission earliest as a result of a leading dolphin’s directional change.

Photo courtesy of the Heithaus Lab

For data analysis purposes, a threshold was established to identify leading individuals. This threshold was influenced by two factors: the proportion an individual spent in a leading position versus the time all others spent in leading positions during the first half-hour of each group sample and comparison of the average leadership values for each individual sampled at least five times. The threshold value was calculated to be 20% of the group sample spent in lead position. This value was then used to analyze if the same individuals consistently led during more group samples than expected by chance. To examine whether positional leaders controlled group movement, success rates of directional changes were quantified using a logistic regression. The numbers of leaders per group sample were compared to group sample duration in order to see if sample length affected the number of leaders.

Most individuals spent little time as the leader but a small number of specific individuals consistently spent more time leading and these dolphins were more successful at affecting group sample directional change. The physiological state of individuals did not seem to motivate leadership. Hunger or higher energy demand of larger or lactating individuals was not observed to be a motivating force since particular individuals were consistent leaders and there were no obvious differences in size. Two leaders were lactating mothers with calves yet other females also had calves yet did not lead. Also, the number of leaders never exceeded two individuals, perhaps for efficient travel.

          In the LFK population, leadership is crucial to survival since the pod must travel for prolonged periods of time to find prey. Time and energy efficiency, and high chances of food location is very important to these populations. The high interaction rates of dolphins in pods allow specific individuals to be recognized and thus more experienced and efficient leaders can be identified and followed. It is important to realize that the many pressures on bottlenose dolphins such as habitat alteration, contamination and vessel traffic may affect populations more severely than previously assumed if leaders are lost.