Studying mammals: A winning design
Studying mammals: A winning design

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Studying mammals: A winning design

2 How should we think of monotremes?

This section contains the first of the activities, Activity 1. If possible, you should do each activity as you come to it; the text that follows it assumes you have done so. However, if this is not possible, then try to do the activity at the first opportunity, rather than leaving all the activities to do together at the end of the course. The activities usually need you to write something down and you will probably find it best to use a (hard-backed) notebook for your notes to keep them safe and together. Activity 1 asks you to watch a sequence on the DVD and make some notes from it on a particular subject. Pause the DVD if you need to, while you write things down. Don't spend too much time on this and don't worry if you find it difficult - you will get a lot more practice later on if you work through all the units in the series. Again, keep the answer concealed while you are doing the activity.

Recall the 'stars' of the early part of LoM and the TV programme - the echidna and the duck-billed platypus. These modern mammals are so distinct that they are assigned to a discrete subclass, the Prototheria, which includes the order Monotremata, separate from the more familiar and well-studied mammals that occupy most of our attention in this series of units. In LoM [p. 20] these two animals are termed 'part-reptile, part-mammal', a phrase that we'll look at more critically in a moment.

Activity 1

Watch 'A Winning Design' on the DVD from 06.32-17.40 and briefly write down the mammalian features evident in these monotremes. Is their egg-laying habit similar in all ways to that displayed by reptiles, such as snakes?

Answer

  • Both echidnas and the platypus have fur - though in the former, some of the hairs are thickened in the form of spines.

  • Both animals produce milk - they have mammary glands, though well defined nipples are not evident.

  • Both are 'warm-blooded' - but you'll see in Section 5 why we should be wary of using this everyday term too freely.

  • The egg-tooth in the hatchling [p. 17] is reminiscent of reptiles, but immature young hatch from their soft-shelled eggs after about 10 days; snakes typically hatch in a much more mature state. Thereafter, a young monotreme, though no longer carried, remains dependent on the mother's milk - for more than six months in the case of echidnas. You'll have watched that wonderful sequence of DA peering electronically into a platypus' breeding burrow, with an immature newborn obviously dependent upon prolonged maternal support.

Years ago, biologists often thought of the term 'egg-laying mammal' as synonymous with 'reptile-like mammal' or 'primitive mammal'. Now, with our greater understanding of monotreme biology, these emotive terms are disapproved of, since these animals have so many authentic mammalian features. For example, if echidnas didn't lay eggs, you might be forgiven for thinking of this animal as (in DA's description [p. 16]) 'little more remarkable than a rather large and slightly chilly hedgehog'; the hedgehog is a 'true' (or placental) mammal, as you'll see if you study the next course. I've mentioned that the period of development within the egg is relatively brief, but many aspects of reproduction and maternal care in the monotremes are distinctly mammalian.

Neither monotreme species is genuinely primitive or unsuccessful. They are specialist feeders; the platypus feeding on invertebrates (e.g. 'freshwater shrimp, an insect larva or a small mollusc' [p. 18]) living in the bottom of streams, while echidnas are terrestrial carnivores. The word 'primitive' implies a similarity with ancestral types, but in terms of lifestyle and anatomy, there's not a lot to link monotremes with the ancestral mammals typified by Megazostrodon.

There are further reasons not to think of monotremes as ineffective species that haven't quite 'made it' in an evolutionary sense. As part of their specialist forms of feeding, monotremes have highly developed sense organs - remember the 'remote-sensing' device of the platypus [p. 18]. If we think of numbers as a measure of an animal's success, monotremes are certainly successful. Echidnas - at least the short-nosed species - are described as 'quite common across their geographical range'. The platypus inhabits a particular type of environment (i.e. it occupies an environmental niche) that is threatened by human habitation, so numbers fell soon after the beginnings of European settlement in Australia in the 18th century. Recent conservation measures have meant that the species is no longer under severe threat.

And how long have monotremes existed? The platypus fossil that DA shows us in the TV programme (see 18.51), and mentions in LoM [p. 20], is about 25 million years old, but the oldest monotreme fossil (a jaw bone) is over 100 million years old. Their evolution could therefore be described as conservative, with little evidence of major changes over time. However, biologists remain ignorant about what the ancestors of monotremes looked like and what historic relationship they had with the evolutionary lines that gave rise to marsupial and placental mammals.

In the section you have just read, there were a number of references to pages in LoM given in square brackets [ ]. Indeed, unless stated otherwise, all the page references you encounter will be to LoM. Sometimes you may feel that you remember enough and do not need to go back and look again. If a particular species is mentioned, you may want to find more information about it. Use the index of mammals in LoM [pp. 316-320] to look it up. For example, echidnas were mentioned earlier in this section. Page 317 of the index gives you its scientific name, Tachyglossus aculeatus, together with references to text on p. 14 and illustrations (page numbers given in bold type) on pp. 15 and 17.

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