|Title:||Competitive and Cooperative Behaviors among Forest Elephants in the Presence of a Limited Resource|
Bioacoustics Research Program
Cornell Lab of Ornithology
159 Sapsucker Woods Rd.
Ithaca, NY 14850
In early 2002, ELP (the Elephant Listening Project) collected data on the behavior of forest elephants in the Central African Republic's Dzanga National Park. The field site is a large mineral-rich clearing in the equatorial rainforest; it is a unique center of forest elephant concentration and also the site of much competition. This fact in combination with team member Andrea Turkalo's ability to recognize roughly 80% of the estimated 3000 elephants individually (the result of her 12-year research project here), made Dzanga an ideal place for a detailed study of dominance and aggression in elephants.
ELP conducted focal surveys to measure the incidence and intensity of aggressive interactions among elephants in a particular ("focal") area of the clearing that contained preferred drinking pits and was thus the site of easily defined competition. In 23 focal surveys, 553 aggressive interactions were documented. The same events were recorded acoustically on seven autonomous recording units surrounding the clearing. These are small, self-contained recorders capable of storing months of audio, and time-synchronized via GPS signals. From this composite recording, elephant calls were later located on a computer-generated map. Simultaneous video recordings enabled us to attribute many located calls to the elephants who made them. This includes infrasonic calls, those calls made below the range of human hearing.
Most of the relationships revealed support the hypothesis that elephants use aggression to establish and/or maintain their place in a dominance hierarchy which can be roughly defined in terms of age/sex classes, as follows: Musth males>Adult males>Subadult males>Adult and subadult females>Juveniles>Infants and Newborn calves. Aggressors tend to target individuals in the same or lower rank than themselves. This finding holds true both in cases of active aggression and in displacements. The latter are situations where there is no contact and no visible threat on the part of the displacer. This strongly suggests that elephants are aware of their relative status in a pre-established hierarchy, and that this awareness persists in the absence of aggression.
The 553 aggressive acts documented in 23 focal surveys varied in intensity and nature. Overall, we found a positive correlation between the frequency of aggressive acts and the numbers of elephants in the clearing. This suggests a relationship between aggression and resource limitation and/or crowding and/or the contagion of social excitement.
The negative correlation between the number of displacements per survey and the ratio of adult and subadult males to other elephants in the clearing suggests that members of these dominant age/sex classes can suppress aggressive interactions by members of other less dominant age/sex classes. The presence of older males seems to provide a stabilizing force.
High-frequency calls (screams and trumpets) were correlated with aggression, all trumpets being made by aggressors and most screams by objects of aggression. Most of the screams were associated with tactile active behaviors (stabs, kicks, jousting, and shoves) and non-tactile active interactions (chasing, trunk-shooing and defense of hole). It seems that high-frequency vocal responses can indicate not only the presence but also the intensity of aggressive acts.
None of the contact interactions resulted in obvious serious injuries. It is clear that dominance is quite different in nature from active aggression and violence, at times serving to abort or mediate potentially injurious interactions. In the above analyses we included a set of mildly aggressive interactions labeled as displacement. Without physical contact, these interactions appeared to reinforce elephantsâ awareness of a dominance hierarchy. We postulate for forest elephants that although the cost of maintaining a rigorous dominance hierarchy includes such mildly aggressive interactions, their presence effectively precludes greater losses to the individual, family, and society.