Fig. 1

a Bornean tarsier (Tarsius bancanus) under nocturnal conditions; note the extreme dilation of the pupil (photograph by David Haring, reproduced with permission). b Anatomical preparation of the eye and brain of T. bancanus (modified from Sprankel 1965), illustrating the comparable volume of the two structures (Castenholz 1984). The eyes of T. bancanus are therefore enormous, both in absolute size and in proportion to the size of the animal. c Geometry of the tarsier eye (modified from Castenholz 1984) illustrating our calculation of the visual angle