ABSTRACT
The role of genetics on neonatal physiological variability was examined in the nine-banded armadillo (Dasypus novemcinctus). Since armadillos give birth to only monozygous quadruplets, the genetic variation within litters is essentially zero. Quadruplets born in captivity were isolated a.nd weighed within 8 h of birth. Oxygen consumption 
was measured in resting neonates by flow-through respirometry, heart rate obtained from an electrocardiogram and ventilation was measured by impedance techniques. Following the measurements, neonates were returned to the mother. Measurements were repeated at 4 and 8 days after birth. Mean heart rate significantly increased from 132 beats min−1 on the day of birth to 169 beats min−1 on day 8. Mean ventilation rate significantly decreased from 81 breaths min−1 on the day of birth to 54 breaths min−1 on day 8. During this same developmental period, mean mass significantly increased from 100 g to 129 g, and mean mass-specific oxygen consumption significantly decreased from ml O2 kg−1 min−1 to 28.6 ml O2 kg−1 min−1. For all variables measured, within-litter variability was always significantly less than between-litter variability, confirming a ‘sibling effect’ that we attribute to the genetic components determining physiological characters.
