Sekulic, Ranka (1981). The significance of howling in the red howler monkey Alouatta Seniculus.
Ranka Sekulic, Doctor of Philosophy, 1981
Dissertation directed by: Marjorie L. Reaka / Associate Professor / Department of Zoology
John F. Eisenberg / Adjunct Professor / Department of Zoology
The South American howler monkeys (Alouatta spp.) employ an enlarged hyoid bone for the production of loud howls or roars. The roars of free-ranging red howlers (A. seniculus) were studied during a period of one year at Hato Masaguaral, Guarico State, Venezuela. The study subjects were members of four troops with contiguous, overlapping home ranges.
Although the howlers roared in a variety of contexts, most of the daytime roars were directed at visible monkeys, in particular at solitary individuals, and at neighboring troops. These observations indicated that, unlike the loud call of the most forest primates, roars of A. seniculus are use in assessment of their opponents, as an alternative to energetically expensive chases and fights.
It is shown that multimale red howler troops result from the inability of a single male to exclude other competing males from the troop, who not only attempt to replace the dominant male, but also kill unweaned infants. When troops are larger, males have their main competitors for food and mates within the troop; in contrast, when troops are smaller, the single male is under strong selective pressure to produce more, loud, low-pitched aggressive roars in order to repel outside male from entering the troop.
The volume of the hyoid should be reliable predictor of pitch in genus Alouatta. The significant difference in size of male hyoids among five species of howler monkeys was, as predicted, inversely related to the mean number of adult males in the troop. The males responded significantly more to playbacks of female roars that to mixed male/female roars, suggesting that the calls of females act as a strong stimulus for eliciting the call of the male. Females in single male troops should produce loud, low- pitched roars in support of their males against intruding males who can kill their unweaned infants. Female hyoids were significantly smaller than those of males, but were inversely related to the mean number of adult males in the troop.