Saguinus geoffroyi has a altitudinal range of 0 till 900m. Rufous-naped tamarins can swim if forced into water. Large grasshoppers are the most important insect pray, comprising over 70 percent of the diet's animal component. The scarcity of both insects and fruits during the dry season and the latter part of the wet season is reflected in the loss of fat reserves and weight by the tamarins. Striking differences in the local availability of food elicit marked changes in tamarin behavior.
Fruit is the most important plant source for most of the year.
During the driest months of the year, fruit may become scarce, and feeding on such secondary resources as nectar and exudate increase and may even overpass that of fruit. Tamarins shift from a preponderance of plant foods during the dry season, to insect prey during the early wet season. Both fruit and insects are at their yearly peak of abundance at this time of year, and seems to denote a preference for insects when they are easily available. Tamarins feed on fruit mainly at the lower and middle canopy levels of the forest. Most fruits eaten are succulent drupes or berries of rather small size and they are usually cropped directly with the mouth.