The natural diet of moustached tamarins consists of fruits, arthropods, exudates, nectar, and small vertebrates (GARBER, 1988; HEYMANN, in prep.; RAMIREZ, 1989). Feeding on leaves or predation of seeds has not been observed so far (Seeds of many fruits are ingested by moustached tamarins but are eliminated unharmed with the feces and thus are dispersed (GARBER, 1986, pers. obs.)).
Therefore, moustached tamarins are not confronted with high levels of secondary plant compounds and the problems of leaf digestion. Thus, functions (1) (absorption of plant toxins; see Introduction) and (2) (adjustment of pH in the stomach) do not seem to be important for geophagy in moustached tamarins. No data are available on the mineral content of the diet of S. mystax. Therefore, function (3) (mineral supplementation) is difficult to evaluate. Mineral (especially Ca) supplementation has been proposed as the function of exudate feeding in Panamanian tamarins (S. geoffroyi) (GARBER, 1984). Moustached tamarins consistently feed on exudates, but its proportion in the overall diet is very small (GARBER, 1988; HEYMANN, in prep.; RAMIREZ, 1989). The months July to September receive on average less rain than the other months and also represent a period of relative fruit scarcity (RAMIREZ, 1989). Perhaps overall mineral uptake of the moustached tamarins is reduced during this period.
The physical appearance of the three soil samples is different (sample A is very fine-grained, samples B and C are sandy), so it seems unlikely that soil is consumed to obtain a tactile stimulus (function 4). It is also very improbable that soil is fed for gustatory stimuli (salt). The three soil samples had no special taste, and the threshold for the taste quality "salty" is even lower in callitrichids than in man (GLASER, 1978).
The geochemical analyses of the soil samples revealed that the concentration of many elements was much higher in the sample from the broken mound of leaf-cutting ants. There are two reasons for this increase. First, the soil material used in construction by the ants may stem from deeper soil layers (VERHAAGH, pers. comm.) which are less leached than the surface soil. Second, the decomposition of organic matter in the underground nests of leaf-cutting ants may lead to increased levels of potassium compounds and exchangeable cations in ant mounds (PETAL, 1978). It has been reported for other primates that they feed on soils from termitaria, which are similarly modified in their geochemical composition (DAVIES & BAILLIE, 1988; HIRABUKI & IZAWA, 1990; HLADIK & HLADIK, 1972; UEHARA, 1982).
In summary, the most likely functional explanation for geophagy in S. mystax is mineral supplementation. Whether a general need for mineral supplementation or a special need for one or several trace elements is responsible for geophagy remains unanswered. The observations reported here on geophagy in moustached tamarins are too limited to draw any further conclusion on the function of geophagy in this species, but demonstrate that this alimentary habit is not restricted to primates that include foliage in their diet.
Acknowledgements. The field study by EWH was conducted under authorization No. 001-90-GRASRAPE-DRRRNN of the Dirección Regional de Recursos Naturales, Regional Government of Amazonia. EWH likes to thank the Proyecto Peruano de Primatologia, especially Drs. JAIME MORO S., FILOMENO ENCARNACION C., and LUIS MOYA I., for their help and support, and EMERITA TIRADO H., JUAN HUANAQUIRI H., and WILLIAM ASIPALI M. for skilled field assistance. We are thankful to Dr. MANFRED VERHAAGH for information and literature on ants, and to Prof. Dr. HANS-JÜRG KUHN and Dr. URSULA BARTECKI for commenting on the manuscript. Meteorological data were provided by the Servicio Nacional de Meteorologia e Hidrologia (SENAMHI) in Iquitos. This study was made possible through an award by the Förderkreis des Deutschen Primatenzentrums e.V. and was financed by the Deutsches Primatenzentrum GmbH.
