In mammals, the degree to which the maternal environment shapes developmental trajectories of fetuses is largely unknown. This study examined such characteristics in utero in the nine-banded armadillo (Dasypus novemcinctus). In addition, little is known about the energetics of reproduction in D. novemcinctus. This study is the first to examine the impacts of gestation and lactation on the energy budget of armadillos.
Armadillos have a unique reproductive physiology in that they are the only known mammals to exhibit obligate polyembryony, with each pregnancy producing a set of genetically identical quadruplets. Because genetic variation is negligible, the maternal environment produces the majority of variation measured during development. It has been established that physiological variation exists within and between litters of D. novemcinctus as early as day four post-parturition. The first aim of this study was to determine if and when developmental differences arise in utero in the armadillo. Skull size and heart rate were not significantly different between fetuses of the same litter. There were, however, significant differences in some skull measurements between whole litters. There was also a significant negative correlation between maternal mass and fetal skull size. Maternal and fetal heart rates were significantly correlated.
In addition to a unique reproductive physiology, D. novemcinctus has an unusually low body temperature (~34.5°C) and basal metabolic rate (BMR) (~50% lower per g) when compared to other mammals. There is a trend in other mammals for the non-mass-specific BMR to increase significantly during pregnancy and lactation. The
second aim of this study was to investigate the metabolic costs of pregnancy and lactation in armadillos by calculating BMR in pregnant, lactating, and non-pregnant individuals using flow-through respirometry. Comparison of BMR within pregnant and post-pregnant animals indicated that reproduction resulted in a 39.7% increase in mass-specific O2 consumption and a 17.2% increase in mass-specific CO2 excretion. Not surprisingly, pregnant armadillos were an average of 23% heavier than non-pregnant armadillos. Pregnant armadillos had lower mass-specific BMRs before and up to parturition, possibly indicating that weight gained during pregnancy had comparatively low metabolic activity. Other mammals commonly gain adipose tissue prior to pregnancy to buffer the daily costs of fetal growth, so it is likely a similar mechanism is used by D. novemcinctus. These data along with additional energy budget data further elucidate the effects of energetically costly conditions on the ecology and evolution of this species.