Nature & Environment

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Introducing mammals

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# 4 Reproduction in marsupials

The study of mammals requires you to deal with measurements. This course assumes only that you can add, subtract, multiply and divide. This section uses units – grams and kilograms, abbreviated to g and kg, respectively – and asks you to calculate a percentage, for which you will probably need to use a calculator. If you wish to develop these kinds of skills further, you may like to study our badged course, Mathematics for science and technology [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .

In contrast to monotremes, no marsupial lays a shelled egg. 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. The numbat is a marsupial that feeds on insects. The young are born live, but at a very early stage in their development and are tiny. They have no food reserves at all and must rapidly locate one of their mother’s teats hidden in her fur. Some do not succeed, and interestingly, the number of young exceeds the number of teats. The term ‘survival of the fittest’ seems apt here.

Figure 16 A young numbat, Myrmecobius fasciatus, showing its long, flexible tongue

Many newborn marsupials are very small at birth. 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 becomes clearer when you work out its weight as a percentage of the mother’s weight. But to compare ‘like with like’ you first need to express each measurement in the same units, in this case grams (g). 8 kg is equivalent to (8 × 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. Note your answer below.

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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 this video clip about the birth of a baby kangaroo, described by Sir David Attenborough.

http://www.bbc.co.uk/ earth/ story/ 20141001-newborn-baby-kangaroo

Jot down some notes on the most striking points about the birth.

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