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!
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.
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.
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?
LoM describes a very productive and seemingly appropriate method of producing young. Recall what was said on pp. 31-32 about the three young of the kangaroo, each at very different stages of development. In addition to the dormant or developing embryo in the uterus, the mother can support a youngster suckling in the pouch, plus yet another offspring that is largely independent but, as LoM points out [p. 32], returns to feed from time to time, for milk of a particular composition. (The timings of reproduction described at this point in LoM refer to the continuous breeding of the red kangaroo; the details of reproduction vary considerably amongst the 14 species within the subfamily that comprise the kangaroos, wallaroos and wallabies.)
To describe fully these complex events requires some new vocabulary. In many kangaroos, females mate very soon after giving birth. In the event of conception, the tiny ball of dividing cells, called a blastocyst, stops developing after a few days and the process of attachment to the inner lining of the uterus is prevented. In most forms of mammalian reproduction, a blastocyst would undergo such implantation without significant delay - indeed, in humans it's seen as marking the beginnings of true pregnancy. But in kangaroos the blastocyst remains 'frozen in time' in what is technically termed embryonic diapause. Some time just before the youngster in the pouch is ready to leave, the blastocyst implants and development proceeds to the point of birth. At about that point, the mother actively encourages the older offspring to spend less time in the pouch and prepares the pouch for the new arrival. Soon after the birth, mating is likely to lead to a further conception, and so on.
It's impossible to know for sure why embryonic diapause evolved, but the link with drought mentioned by DA [p. 32] is plausible. (If you work through the units in this series, you'll encounter many speculative explanations of unconventional mammalian strategies, trying to explain what selective advantage they might confer; if this mode of thinking is unfamiliar to you, then don't worry - it will be covered later.) During a severe drought, a suckling young in the pouch may be expelled and the heavy energetic demands of producing milk temporarily suspended. As DA points out [p. 32], this reduced energy expenditure will 'make the minimum demand on the meagre pasture around her and she has a fertile dormant egg within her womb ready to start its development just as soon as conditions improve'.