Viviparity

Viviparity is reproduction in which offspring develop within the maternal body and are delivered alive. It contrasts with oviposition, in which an egg is laid before development is completed. Viviparity describes retention and birth; it does not by itself specify how much nourishment is supplied during development.

Some live-bearing animals rely largely on reserves packaged in the ovum, while others receive substantial additional maternal nutrients. A study of the lizard Mabuya heathi compared egg size, fetal growth and placenta structure and found extensive maternal contribution to the developing offspring.^[ref] The comparison illustrates why live birth and the mode of nourishment are separate biological characters, and why a placenta is not exclusive to mammals.

Placental matrotrophy is nourishment of developing offspring by transfer of maternal material across a placenta. It supplements or replaces reliance on reserves packaged in the ovum, such as yolk. Viviparity specifies retention of developing offspring and live birth; placental matrotrophy additionally specifies a route of nutrient supply.

A striking example comes from the lizard described as Mabuya heathi in a 1984 study. Its very small ovum could not account for the large increase in offspring dry mass; the authors estimated that placental transfer supplied more than 99 percent of the near-term fetus's dry mass. Examination of the placenta linked the growth measurements with an anatomical route of exchange.^[ref]

Dry mass matters because uptake of water alone can greatly increase wet mass. In a synthetic comparison, an initial 1 mg of dry mass and a final 10 mg imply a net addition of 9 mg of dry material. Measuring that increase supports transfer beyond the original reserves; determining the particular nutrients and transport mechanisms requires further measurements. The lizard example also makes placental matrotrophy a comparative character across vertebrates, rather than a synonym for being a mammal.