The per capita behavioral responses of predators to variation in prey species abundance is characterized as the functional response (FR). Most vertebrates exhibit either Type II (consumption increases to a plateau with increasing prey abundance) or Type III (sigmoidal, reflecting prey switching as prey increases from low to modest abundance) FR. A Type IV FR is similar to a Type II or Type III except that per capita consumption declines at very high prey abundance. Type IV FR are best known among invertebrates but were originally postulated for vertebrates foraging on group-living prey such as schooling fish. Reflecting this, many prey species capitalize on group living or group foraging to increase proportional vigilance. The degu (Octodon degus) is a social hystricomorph rodent that has been subject to extensive behavioral research. Degus foraging in large groups detect predators more rapidly than those in smaller groups, suggesting that predation rates should decline at high degu densities. A key predator of degus is the culpeo fox (Lycalopex culpaeus), which is characterised as a generalist rodent predator that exhibits numerical but no FR. We monitored degu and culpeo numbers over two decades, and we characterized culpeo diet with traditional scat analysis. Using 23 years of observations, our data show a clear Type II FR for rodents in general, but for the highly social degu the per-capita consumption clearly declines at the highest degu density. To our knowledge this is the first demonstration of a Type IV FR for a mammalian predator.