Fetal and Neonatal Growth Patterns and Maternal Reproductive Effort in Ungulates and Subungulates

Abstract

Fetal growth rates, relative proportions of the gravid uterus, average maternal and neonatal weights, neonatal growth rates, and estimates of maternal milk production at the peak of lactation were summarized for species occupying many different environments throughout the world. Interspecific comparisons suggest that very basic controlling mechanisms produce similarities in fetal and neonatal growth of ungulates and subungulates. Fetal growth is a power function characteristic of the general mammalian pattern. Absolute fetal weight at birth increases curvilinearly, and relative fetal weight decreases as average, adult maternal weight increases. Interspecific variations in relative birth weight becomes much greater in smaller ungulates as the selective pressures to modify reproductive effort increase. The thermal environment may not be paramount importance in determining relative birth size of temperate and arctic dwelling species. Maximum growth rates of maternally nursed neonates of different species increase in a very predictable curvilinear manner relative to maternal weight. Apparently the selective pressures of predation, hypothermia, and seasonal food restrictions have produced a relatively high uniform growth rate in the species examined. Peak milk production estimates reflect the general growth pattern relationships. Physiological limits to neonatal growth apparently limit neonatal milk consumption and maternal production when a single neonate is nursed by a lactating female having an ad lib access to high quality feed. The implications of the findings concerning growth rates relevant to livestock selection by the animal scientist were discussed.