SOCIAL-FACTORS INFLUENCE THE ACOUSTIC VARIABILITY IN THE LONG-DISTANCE CALLS OF MALE CHIMPANZEES

Acoustic variability in the species-typical long-distance calls of mal e chimpanzees was investigated. Analysis of 13 acoustic features revea led that calls varied more within than between individuals. Although o nly a limited amount of acoustic variability was attributable to diffe rent populations, discriminant-function analysis confirmed that males from two populations produce acoustically distinguishable calls. Addit ional investigation suggested that social factors may contribute to th e patterns of acoustic variation both between and within individuals. Matrix-permutation tests revealed a positive association between the a mount of time males spent together and a measure of call similarity. V ocal similarities between individuals appeared to result from a dynami c process involving chorusing behaviour: males matched the acoustic ch aracteristics of each other's vocalizations when calling together. Cho rusing may also affect the degree of within-individual acoustic variat ion in calls. Males who chorused often with others produced more varia ble calls than individuals who chorused less often or called alone.