1Department of Biology, Regis University, 2Department of Anthropology, University of Waterloo
April 18, 2020 , Platinum Ballroom
Howler monkey long calls have been explained by several non-mutually exclusive hypotheses including ecological resource defense (howls are used to defend food resources) and mate defense (howls are used to defend females from extra-group males). We investigated the relationship between howling and resource defense in mantled howler monkeys in two Costa Rican rainforests, La Selva and La Suerte. Only ~25km apart and sharing similar wildlife, La Selva is much larger and has bigger, more abundant fruiting trees than La Suerte. Due to greater food resource abundance at La Selva, we predicted that group size would be larger and monkeys would howl at higher rates, for longer durations, with more howls per bout than at La Suerte. We conducted all-occurrences sampling of howling behavior at La Suerte (June-August 2017/2018, January 2019) and La Selva (November 2018-February 2019). Group size was significantly larger at La Selva (12.7 monkeys) than La Suerte (9.1) although there was no difference in number of males/group. Mean howl length and number of howls/bout were both significantly higher at La Selva (156.6sec; 9.2howls/bout) than La Suerte (121.4; 5.2), but howling rate was significantly higher at La Suerte (1.1howl/hour) than La Selva (0.6). Rate was positively correlated with group size at both sites but number of males/group did not vary, suggesting that howling rate relates to mate defense with differences in operational sex ratio driving how often males howl. In contrast, howl length and number/bout are associated with ecological resource defense, with longer howling bouts occurring in richer habitats.