(With Plates 2 and 3)Samples of kibbled (broken) grain have been collected and examined during experiments on rodent control in the field and on rats and mice in the laboratory. The behaviour of rats and mice when eating whole grains has been observed, both in the laboratory and in colonies in conditions approximating to the wild. Kemper (1940) describes similar observations, though in less detail than is given here.The species studied were Rattus norvegicus Berkenhout, R. rattus Linnaeus and Mus musculus Linnaeus. The grains studied were wheat, barley, oats and maize.
RATSRattus norvegicus and R. rattus do not differ greatly in the way in which they eat cereal grains. Direct observation of hungry rats shows that R. norvegicus more often takes mouthfuls of whole wheat back to the nest from an exposed position than does R. rattus; the grains are then disgorged and eaten one by one (Barnett & Spencer, 1951). Both species pick up one grain at a time, generally with their mouths and, squatting on their hind-quarters, hold the grain in their forepaws while eating it. The grain is held with the long axis towards the mouth, as a man holds a cigar, and is bitten across the long axis (P1. 2, figs. 1 and 4-6; P1. 3, figs. 7, 8). Both species leave similar residues of broken grain.Kibbled grain is found only when well-fed rats are supplied with a surplus, and its occasional absence in the field may be due to the fact that all the grain taken is removed to a nesting site or to a point under cover before it is consumed.Wheat. Wheat residues are shown in P1. 2, figs. 1-4. When wheat is kibbled the proportion of a grain discarded usually varies from a quarter to a half of its length. The discarded part is nearly always the bearded end of the grain. Small pieces of the bran consisting of pericarp and testa are also dropped and these may have pieces of endosperm attached (P1. 2, fig. 1).Soaked wheat residues are shown in P1. 2, fig. 3. These residues are quite different from those of plain wheat. Discarded bearded ends though present are less apparent; the more noticeable feature of the kibbling is the large semi-transparent pieces of bran which have been peeled off. There is also a marked increase in the number of grains which have been held at both ends and nibbled along the cheeks: the discarded pieces usually consist of the crease with small pieces of endosperm attached at each end.Oats. Oats contain a relatively large proportion of husk even after threshing. These husks are always bitten off and discarded, and the grain is then eaten from 3.2