The Estrous Cycle, Reproduction, Growth, and Development of Temperature Regulation in the Pygmy Mouse, Baiomys taylori

The estrous cycle of Baiomys taylori averages 4.9 days. The gesta-tion period is estimated to be 22 to 23 days. A postpartum estrus is frequent. Puberty occurs as early as 28 days of age. Seven of 10 females between 25 and 29 days of age conceived within a week after pairing. The average weight of 47 individuals at birth was 1.20 ± 0.08 (SD) grams. The adult level of body temperature could be maintained at ages between 10 and 15 or 15 and 20 days for neonates exposed to 30 and 28°C, respectively, for a period of 2 hours. Growth during the first 20 to 35 days occurred in two phases with the first phase exhibiting an instantaneous growth rate of 7.50 ± 0.32 and 7.29 ± 0.42 per cent of body weight per day for experimentals (those young used for metabolic as well as body temperature and body weight measurements) and controls (those young used only for body weight measurements), respectively. The second growth phase commenced at 11.3 days for the experimentals and 17.0 days for the controls. The second growth phase averaged only. 4.01 ± 0.26 and 3.30 °: 0.52 per cent of body weight per day for the experimentals and controls, respectively, and coincided with the full development of homeothermy. At a body temperature of 36 to 37°C and a body weight of 2.0 to 2.5 grams—the lower size limit for homeothermy—the oxygen consumption was 3.6 ° 0.40 (2 SE) cc O2/g/hr if the ambient temperature was 30°C. The oxygen consumption for neonates averaging 22.5 ± 0.7 days of age was 4.31 ± 0.16 cc O2/g/hr at an ambient temperature of 30°C. This relatively high level of oxygen consumption indicated that 30°C was below thermal neutrality, the ambient temperatures at which the oxygen consumption of adults is 1.95 ± 0.25 (2 SE).