*Three Bornean pitcher plant species, Nepenthes lowii, N. rajah and N. macrophylla, produce modified pitchers that 'capture' tree shrew faeces for nutritional benefit. Tree shrews (Tupaia montana) feed on exudates produced by glands on the inner surfaces of the pitcher lids and defecate into the pitchers. *Here, we tested the hypothesis that pitcher geometry in these species is related to tree shrew body size by comparing the pitcher characteristics with those of five other 'typical' (arthropod-trapping) Nepenthes species. *We found that only pitchers with large orifices and lids that are concave, elongated and oriented approximately at right angles to the orifice capture faeces. The distance from the tree shrews' food source (that is, the lid nectar glands) to the front of the pitcher orifice precisely matches the head plus body length of T. montana in the faeces-trapping species, and is a function of orifice size and the angle of lid reflexion. *Substantial changes to nutrient acquisition strategies in carnivorous plants may occur through simple modifications to trap geometry. This extraordinary plant-animal interaction adds to a growing body of evidence that Nepenthes represents a candidate model for adaptive radiation with regard to nitrogen sequestration strategies.