Milene M. Martins 1
Received 30-VI-1999. Corrected 16-VIII-1999. Accepted 28-VIII-1999.
Key words
Callithrix aurita, foraging behavior, invertebrate availability, seasonality, army ants.
Despite their remarkably efficient use of gum resources, marmosets (genus Callithrix) also consume invertebrates and small vertebrates (Stevenson and Rylands 1988, Rylands and Faria 1993). Considering the spatially unpredictable distribution of small prey and the high energetic cost of foraging, marmosets should greatly benefit from concentrations of edible animals. For example, Rylands et al. (1989) reported groups of C. humeralifer, C. kuhli, C. flaviceps and C. geoffroyi taking small prey flushed by army ants Labidus praedator and Eciton burchelli (Ecitoninae, Formicidae).
A group of four buffy-tufted-ear marmosets (C. aurita) was observed following Labidus sp. ants during a 12-month (October/1994 to September/1995) feeding ecology study in a 17-ha semideciduous forest fragment in southern Minas Gerais, Brazil (21o 23’S, 46o 15’W). Here, I describe the ant-following foraging behavior and relate its seasonal occurence to periods of decreased prey availability.
Two seasonal periods were defined according to the 1994/1995 rainfall regime: wet season (November/94 to April/95) and dry season (October/94 and May to September/95). Daily activities were recorded with scan sampling method at five-minute intervals (Altmann 1974), totalling 165 and 141 observation hr, respectively, in the wet and dry season. Foraging success indexes (number of predation records/number of foraging records x 100), after Ferrari et al. (1996), were calculated for each season. Differences were tested by an ANOVA after transformation of the proportions into archsine of their square root (Fowler and Cohen 1990). Monthly abundance of invertebrate prey was evaluated by netting and pit-fall traps. Five-mm and longer body-sized invertebrates were identified to class or order according to Boror and Delong (1988). Associations between number of captured individuals (total invertebrates and invertebrate prey) and monthly accumulated rainfall were tested by Pearson correlation index. ]]>
Marmosets followed Labidus sp. on eight of 29 foraging swarms, all in June, July or August. Each event lasted from 40 to 65 min and ended up suddenly, without shrinking or disappearance of the swarm. Individuals took up positions about 0.5 - 1.0 m above ground and occasionally descended to catch disturbed arthropods. Spiders and unidentified invertebrates were eaten.Number of invertebrate prey (caterpillars, katydids, homopterans and snails) collected monthly was significantly correlated to rainfall (r = 0.69; p = 0.01) as were total invertebrates (r = 0.63; p = 0.03). Temporal scarcity in prey availability resulted in lower foraging success index of the study group during the dry season (13.5%) relative to the wet season (17.5%). This difference was not significant (F = 0.88; p = 0.36; n = 18). Higher frequency of foraging in bromeliads by a group of C. aurita during the dry season was observed by Ferrari et al. (1996) in a more humid coastal forest. I did not observe marmosets foraging in the quite few existing bromeliads. Foraging over swarms may be an attempt to compensate the reduction in prey availability.
Unlike Eciton burchelli, Labidus praedator does not forage intensively during the dry season (Willis and Oniki 1992). No E. burchelli swarm was observed in the study site and, based on the distribution range of L. praedator (see Gotwald 1995), I assume this as the involved species. The absence of ant-following behavior by marmosets during the rainy season is probably due to higher numbers of preferred prey in foliage during the wet months, which may represent a more attractive resource. Associations between army ants and C. aurita had not been registered up to date. Yet, the temporal trend found here may be site-specific and further field investigations elsewhere are demanding.
Acknowledgements
I am grateful to P. S. Oliveira for the identification of the ants. I also thank J. C. Motta-Júnior, E. Willis and two anonymous reviewers for critical comments. Grants were provided by Fundação MB/University of Campinas and the Brazilian Higher Education Authority (Capes).
References
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1 PG Ciências Biológicas/Zoologia. Instituto de Biociências. Universidade Estadual Paulista - Unesp
C.P. 199 13506-900 Rio Claro, SP, Brazil ]]>