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3 Reproduction in marsupials

The study of mammals requires you to deal with measurements, which we call numerical 'data', and you will get practice with compiling and analysing data if you work through all the units in this series. We assume only that you can add, subtract, multiply and divide. In this section, we ask you to use units - grams and kilograms, abbreviated to g and kg, respectively - and to calculate a percentage, for which you will probably need to use a calculator.

In contrast to monotremes, no marsupial lays a shelled egg. You'll know from LoM and the DVD that the embryo develops for a short period inside the uterus (or womb) before transferring to (in most species) a pouch; hence marsupials are sometimes termed 'pouched mammals'. The newborn are tiny and very unlike the adult - so much so, that the description of 'little worms' for the numbat [p. 21] is understandable, though some newborns are considerably more developed, as evident from p. 28. An adult female koala might tip the scales at about 8 kilograms (kg), but the newborn koala weighs just about half a gram, i.e. 0.5 g. Just how tiny this newborn is in relation to the mother becomes clearer if we work out what percentage its weight is of the mother's weight. But to compare 'like with like' we first need to express each measurement in the same units, in this case grams (g). 8.0 kg is equivalent to (8.0 × 1000) grams = 8000 g, so 0.5 g as a percentage of 8000 g is (0.5/8000) × 100%. This calculation comes to a little over 0.006% - compared to the mother, the newborn koala is very small indeed!

Question 2

Suppose a human baby weighs 3.4 kg at birth. If the mother weighs 70 kg, express the newborn's weight in relative terms, i.e. as a percentage of the mother's weight, to the nearest whole number.

Here the units are identical, so you just need to divide the baby's weight by that of the mother, 3.4/70, and multiply by 100%, which gives a value close to 5%. This is a great deal higher than the value for the koala, reflecting the greater relative maturity of the human newborn.

Activity 2

Watch 'A Winning Design' on the DVD, from 20.53-24.04, which focuses on birth in grey kangaroos. Jot down in your notebook the most striking points about the birth.

I was struck by the 'incredible journey' from birth pore to pouch and by the youngster's ability to orientate itself and to move with the aid of well-developed forelimbs. (Though perilous, the journey is relatively brisk - two minutes in all.) I was reminded of the importance of milk to the newborn and noted the fact that the chemical composition of the milk changes during the newborn's development.

After reading LoM, you'll be alert to the danger of thinking of the marsupial method of reproduction as 'primitive' or inferior to that of placental mammals. What we see is a successful reproductive strategy (or rather a range of strategies, because the details vary between different marsupial species) that is very different from our own. What is biologically so interesting is that such a large fraction of the early development of the young occurs after birth, in the pouch.

Question 3

LoM pp. 29-31 suggests that kangaroos 'have brought the marsupial method of reproduction to its most efficient level'. Can you recall what remarkable events offer evidence for such a claim?