Predation on Swollen-Thorn Acacia Ants by White-faced Monkeys, Cebus capucinus

Abstract

White-faced monkeys, Cebus capucinus, intensively preyed on the ant Pseudomyrrmex belti, that obligatorily occupies and protects swollen-thorn acacias, primarily Acacia collinsii, at the study site. The Cebus obtained ants and probably ant larvae from acacias by ripping off branches and opening thorns with their teeth. This action often resulted in extensive physical destruction of the acacia, loss of the ant colony, and subsequent death of the plant. Implications of this predation and other interactions between monkeys and myrmecophytes are discussed. IN THE MUTUALISTIC ASSOCIATION between obligate acacia ants (several species of the genus Pseudomyrmex) and swollen-thorn acacias (several species of the genus Acacia) of Central America, the plant provides the ants with food and domicile, while the ant protects the plant from herbivores (primarily insects) by stinging and from invading vegetation by biting it off (Janzen 1966, 1967, 1974). Although some arthropods, lizards, birds, and mammals are known to eat swollen-thorn acacia plant parts, extensive damage of ant-occupied swollen-thorn acacias from these animals is very infrequent. Evidence strongly suggests that large browsing mammals avoid occupied acacias because of the aggressive ants. Similarly, acacia ants and their larvae are seldom eaten from the swollen-thorn acacia by birds and arthropods. No mammal has been reported to intentionally eat acacia ants (Janzen 1969b). During a study of the ecology and behavior of the white-faced monkey, Cebus capucinus, in the tropical dry forest of northwest Costa Rica, Cebus interactions with ant-occupied swollen-thorn acacias provided an apparent exception to the effectiveness of this mutualistic association. These interactions and other observations of interactions between monkeys (Cebus and Ate/es geoffroyi) and myrmecophytes during the study are described in this paper. METHODS AND STUDY AREA From October 1971 to August 1973 observations were made intermittently on the three monkey species, Cebus capucinus, Ateles geoffroyi, and Alouatta palliata, of Santa Rosa National Park in Guanacaste Province, northwest Costa Rica. A more intensive study of Cebus ecology, using the focal animal technique (Altmann 1974), was conducted one week each month from January to August 1973 on a troop of 18 to 21 animals. Focal animals were under observation for about 25-35 hours each study week (except for only 13 hours in the study week in June) with observations generally spanning all hours of the day for a given week. Santa Rosa National Park is located approximately 50 km north-northwest off Liberia, Guanacaste Province. Most of the park is on a plateau of about 280 m, where a mosaic of forests and man-created savannahs occurs. The Cebus study area was located in deciduous, secondary forest averaging 10-15 m tall, the predominant forest type in the plateau area. Annual rainfall averages approximately 1750 mm, and almost all of it falls between mid-May and midDecember. Deciduous trees lose their leaves during the severe dry season. Swollen-thorn acacias (primarily Acacia collinsii) are common in these deciduous