A Preliminary Analysis of Garbage as Food for the Norway Rat

Martin W. Schein Division of Vertebrate Ecology of the Johns Hopkins School of Hygiene and Public Health

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Holmes Orgain Division of Vertebrate Ecology of the Johns Hopkins School of Hygiene and Public Health

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The total quantity of domestic refuse from a typical residential city block was measured for a period of 18 weeks during the summer of 1950. Samples of the refuse were selected at random for analysis and it was found that approximately 30 per cent of the refuse (by weight) was edible. Assuming that about ⅓ of this ration is always available to the rats, it is estimated that each rat was supplied with about 150 gm. of edible material per day.

For a more detailed analysis of the particular food items, 50 foods were selected on the basis of their ready availability to the rats in domestic garbage and were fed to groups of caged rats. The caloric content per 100 gm. of test food (cal) was obtained from the literature. In addition, three indices were calculated for each food from the results of 3-day feeding trials. The w index represented the percentage, at the end of a 3-day diet, of the rat's initial body weight. The c index represented the average daily consumption, per 100 gm. rat body weight, of a particular food, when no other food was available for a 3-day period. The p index represented the number of grams of test food eaten per hundred gm. of a standard food, both being offered simultaneously and to excess for 3 days. The standard food used was “Fox Checkers,” a complete-diet laboratory food manufactured by the Ralston Purina Company.

The problem of replication of measurements could not be dealt with as fully as might be desired. However, in order to get some indication of the magnitude of variation inherent in the indices, the standard diet was fed to 3 groups simultaneously. The results showed a considerable amount of variation in w and very little in c. Some of the variation in w is tentatively explained by group behavior phenomena.

From the relationship between w and cal, it is determined that the minimum energy requirement of rat food is about 137 calories per 100 gm. food.

The relationship between cal and c is somewhat suggestive of a hyperbolic function. If this hypothesis is valid, then it follows that a rat would strive for a definite amount of food value per day, rather than food volume.

Combining the two relationships described above, it is determined that rats require about 18.9 grams of moderately high caloric food per 100 gm. body weight per day. Therefore, the average (250 gm.) rat would need about 47 grams of food daily, and this figure agrees reasonably well with other estimates of rat intake. Despite the fact that about half (author's estimate) of the 150 gm. daily ration falls below the minimum energy requirement of 137 calories per 100 gm. of food, the quantity of food available was clearly in excess of rat needs. Therefore, it is suggested that food distribution, rather than quantity, may have governed the population of the block under study.

A close relationship was obtained between w and p. This is interpreted as meaning that rats prefer foods on which they will gain weight. It is suggested that some of the scatter in the points is due to avitaminosis, aversion to highly spiced foods, preference for sweetened foods, or a combination of these factors.

Author Notes

Currently with the U. S. Department of Agriculture Livestook Experiment Station, Jeanerette, Louisiana.

Currently with the Maryland State Department of Health, Baltimore, Maryland.