Effects of reproductive and social context on vocal communication in captive female African elephants (Loxodonta africana)

Investigating the impact of rank and ovarian activity on the social behavior of captive female African elephants

Over a third of captive female African elephants in North America fail to exhibit normal estrous cycles based on long-term serum progestagen analyses. Why acyclicity occurs is unknown; however, the majority of noncycling females are ranked by keepers as the dominant individual within the group. To investigate the relationship between ovarian cyclicity status and keeper-determined social rank, observations were conducted on 33 female African elephants (18 cycling, 15 noncycling). Based on keeper evaluations, five cycling elephants were ranked dominant, seven in the middle and six as subordinate. In contrast, 10 noncycling elephants were ranked as dominate and five as subordinate with none ranked as middle. When comparing the behavior of the elephants by their keeper-determined rank, the dominant females dominant were significantly more likely to approach, displace and push. Similarly, keeper-determined subordinate females more frequently presented their hind end and held their ears erect. Behaviors initiated by one elephant toward another did not vary between cycling and noncycling females, except when the interaction with social rank was tested. Dominant, noncycling females initiated a higher percentage of "approach" and "displace" behaviors than both cycling and noncycling, subordinate elephants. Subordinate, noncycling elephants displayed the highest percentage of "ears erect." Social rank drives the interactions of ex situ female African elephants more than ovarian cyclicity status. Thus, behavioral interactions cannot be used to predict which cycling elephants are most likely to become acyclic.

Vocal communication in African elephants (Loxodonta africana)

Research on vocal communication in African elephants has increased in recent years, both in the wild and in captivity, providing an opportunity to present a comprehensive review of research related to their vocal behavior. Current data indicate that the vocal repertoire consists of perhaps nine acoustically distinct call types, "rumbles" being the most common and acoustically variable. Large vocal production anatomy is responsible for the low-frequency nature of rumbles, with fundamental frequencies in the infrasonic range. Additionally, resonant frequencies of rumbles implicate the trunk in addition to the oral cavity in shaping the acoustic structure of rumbles. Long-distance communication is thought possible because low-frequency sounds propagate more faithfully than high-frequency sounds, and elephants respond to rumbles at distances of up to 2.5 km. Elephant ear anatomy appears designed for detecting low frequencies, and experiments demonstrate that elephants can detect infrasonic tones and discriminate small frequency differences. Two vocal communication functions in the African elephant now have reasonable empirical support. First, closely bonded but spatially separated females engage in rumble exchanges, or "contact calls," that function to coordinate movement or reunite animals. Second, both males and females produce "mate attraction" rumbles that may advertise reproductive states to the opposite sex. Additionally, there is evidence that the structural variation in rumbles reflects the individual identity, reproductive state, and emotional state of callers. Growth in knowledge about the communication system of the African elephant has occurred from a rich combination of research on wild elephants in national parks and captive elephants in zoological parks.