The Ontogeny of Homeothermy in Neonatal Peromyscus leucopus

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Abstract
Oxygen consumption ( ) and body temperature ( ) were examined in 2-20-day-old Peromyscus leucopus exposed as individuals to ambient temperatures ( ) of 0-35 C for 2.5-3.0 h. The same measures were taken on 4-14-day-old litters of four young enclosed in a cotton nest during exposure for 2.5-3.0 h to (measured outside the nest) of 0-30 C. Among 2- and 4-day-olds tested as individuals (without a nest) some animals at 30 C maintained 3.7-4.7 C above , and their , was 2.9-3.7 times higher than the , of animals at 35 C . In 2- and 4-day-olds tested individually fell with as was lowered from 30 to 0 C anal closely approximated in tests at 0-15 C; , fell with below 25 C . Individual 18- and 20-day-olds, in contrast to 2- and 4-day-olds, usually maintained above 36 C when tested at 0-35 C , and their , increased steadily as To was lowered toward 0 C. The homeothermic capabilities of 18- and 20-day-olds developed gradually over the postnatal period, the rate of development being particularly rapid at 10-14 days of age. The developmental advance in thermoregulatory ability of individuals between 2 and 20 days of age can be attributed to a 2.8-fold increase in overall weight-specific insulation and a doubling ot weight-specific metabolic capacity. Although 4-10-day-olds exhibited meager abilities to thermoregulate at low , when tested as individuals, litters of these ages tested in cotton nests usually maintained above 34 C over the entire range of from 0 to 30 C. Litters, even those as young as 4 days, also generally exhibited a graded increase in as was lowered from 30 to 0 C. It is concluded that early-age young in their natural environment, living together as a litter in a nest, can remain homeothermic at low for at least 2.5-3.0 h despite their inadequacies when tested as individuals and that the ability for individuals to remain homeothermic at low Ta develops rapidly at about the same age (14 days) as eye-opening permits young to wander safely from the nest.