Spatial relationships and matrilineal kinship in African savanna elephant (Loxodonta africana) clans

Abstract

African savanna elephants, Loxodonta africana, live in stable family groups consisting of adult females and their dependent offspring. During the dry season, “clans” consisting of several family groups typically share a common home range. We compared spatial relationships and mitochondrial DNA (mtDNA) haplotypes among 14 adult female elephants within 3 clans during the dry season in northern Zimbabwe. Spatial relationships were studied by radio-tracking. Home-range similarity was quantified by correlating the estimated utilization distributions of all pairs of elephants. Clans were identified by cluster analysis of the home-range similarity values. All three clans contained at least two of the five mtDNA haplotypes that were found, indicating that clan members are not necessarily matrilineally related. Within clans, home ranges of elephants with the same haplotype were not significantly more similar to each other than those of elephants with different haplotypes. Most elephants within each clan used their shared home ranges independently of each other: the distribution of distances between their positions at any given time did not differ from the distribution expected by chance. However, 8 out of the 26 within-clan pairs exhibited long-term coordination of space use by remaining within known hearing distance of each other’s low-frequency calls significantly more often than expected by chance. At least four of these coordinated pairs consisted of animals in different family groups. Elephants in three of the four different-family pairs whose movements were coordinated had different haplotypes. Further research is needed to determine the relationship between these coordinated movements and conventionally defined bond-group behavior.

Appendix

Estimation of mean location error

We estimated the mean location error for each tracking location by the following procedure. For each location, we estimated the probability that the elephant’s true position was at the nominal position where the bearing lines from the two tracking stations crossed. We also estimated the probability that the true position was at each point on a 54×46 km grid of points spaced at 250-m intervals (corresponding to the study area shown in Fig. 1). These probabilities were estimated based on the error function for bearings obtained from a single tracking station. The error function is approximated by a normal distribution of angular tracking errors with mean of 0° and standard deviation of 1.6°, based on earlier tests of the accuracy of the tracking system (R.B. Martin, unpublished data). We then identified the area needed to encompass 95% of the total cumulative probability around each estimated location, which we call the area of uncertainty (AOU). The most likely location for the elephant is where the bearing lines cross, with the probability falling off with increasing distance from there toward the edges of the AOU. The elephant’s true position is somewhere within the AOU in approximately 95% of all cases. In general, the AOU is small and radially symmetrical where the two bearing lines are perpendicular to each other. As the difference between the two bearings diminishes, the AOU becomes more of an elongated ellipse.

For each location, we estimated the average distance between the elephant’s estimated or nominal position and its true position. This distance is called the mean location error (MLE). The MLE is a weighted average of the distances of all gridpoints within the AOU from the nominal location. The weighting value for each point is the ratio of its probability of being the true location to the probability that the nominal location is correct.