Many primates sustain moderate drops in food availability by increasing day range (Marsh 1981; Robinson 1986; Boinski 1987), increasing foraging time, (Terborgh 1983; Overdorff 1993; Doran 1997), or increasing home-range size (Clutton-Brock 1977). As preferred resources become increasingly scarce, most primates are forced to alter their diet by including greater amounts of lower-quality items, often labeled fallback foods (van Schaik and Brockman 2005; Hanya et al. 2006). For frugivores this might entail feeding more heavily on nonreproductive plant parts, such as leaves, bark, pith, or sap. Alternatively some species are able to maintain high rates of fruit in the diet by making increased use of keystone foods (Gilbert 1980). Keystone plant resources represent a special class of fallback food that Terborgh (1986) defines as any reliable food item that plays a prominent role “in sustaining frugivores through periods of general food scarcity” (p. 339). Among primates palm nuts and figs are the foods most often identified as keystone resources (Terborgh 1983; Hemingway and Bynum 2005). For example Terborgh (1986) found that two frugivorous primates at Cocha Cashu, Peru, coped with seasonal food shortages differently. Whereas capuchins exploited palm nuts during lean periods, squirrel monkeys, which are too small to break into the hard outer husk of palm nuts, fed heavily from large fig trees.