A Precis on Energetics of the Old‐Field Mouse

Old—field mice (Peromyscus polionotus) are as much as 42% less active on clear moonlight nights according to live and snap—trap data. Since food is not always cached in the nest, old—field mice may at times rely upon forms of stored metabolic energy when foraging is not possible. We analyzed 254 mice for metabolizable body fat and 32 for caloric value of stomach contents. The 5 Kcal of metabolizable body fat and 2.8 Kcal of metabolizable stomach contents would last an inactive mouse for at least 2.8 days at basal levels if foraging were to be stopped entirely. The breeding season is evidently a period of increased metabolism for both sexes since body fat reserves are lowest at this time. The availability of arthropod and especially seed foods was seemingly an important factor in counteracting the metabolic drain of breeding as reflected by low body fat reserves. During the breeding season fat reserves of both sexes declined to half of the nonbreeding level. By contrast, reproductively active individuals which were held captive for 3 weeks and supplied with ad libitum food increased mean body fat from 2—4 fold above the levels found in natural populations. Postpartum females seem to have the least amount of body fat because of the high energy demands of lactation. Lactating females, in contrast to other mice, compensate for the necessity of higher energy storage by feeding more intensively (a two fold increase in stomach capacity).