1 Meeting the insect eaters
As you work through this course you will come across boxes, like this one, which give you advice about the study skills that you will be developing as you progress through the course. To avoid breaking up the flow of the text, they will usually appear at the start or end of the sections.
As well as the course text, you will be using The Life of Mammals book (LoM) and related The Life of Mammals DVDs, as described in the introduction to this course. Before you go any further, watch 'The Insect Hunters' on the DVD and read LoM Chapter 2. Unless stated otherwise, all the page references you encounter in this course will be to LoM. When you have watched the TV programme and read LoM Chapter 2, you should have a good 'feel' for the diversity of insect eaters and the range of niches they occupy.
Chapter 2 of LoM begins by describing conditions within a super-continent in the Northern Hemisphere 65 million years ago, during the period of the geological past called the late Cretaceous. As author David Attenborough (DA) points out, both animal and plant life in the late Cretaceous were very different from nowadays. The dense forests were rich in ancient types of coniferous tree, and luxuriant tree ferns and cycads - the latter looking a bit like giant pineapples, with crowns of extensive fronds. Dinosaurs, including large predators, were diverse and abundant; nevertheless, rat-sized placental mammals were living successfully on the ground. They were insect eaters, i.e. insectivores, feeding on the vast numbers of insects and other invertebrates living in soil, leaf litter and low-lying vegetation. Available fossil evidence suggests that placental mammals had diverged from marsupials more than 90 million years ago, during the middle of the Cretaceous. You can think of this divergence as an early branch point in an evolutionary tree, where the two branches (placental and marsupial) sprang from a common ancestral type, each branch further subdividing over geological time into the wonderful array of types described in LoM.
In everyday language, insectivore means 'insect eater', but in LoM and in this course its meaning is much more precise. Insectivores are insect-eating mammals, classified together on the basis of a reasonably close evolutionary relationship. So, for our purposes, an insectivore is a member of the order Insectivora. But as DA points out (and the photograph on p. 39 very convincingly demonstrates), not all insectivores restrict themselves to a diet of insects; it is more accurate to think of insects as a significant proportion of their diet. Insectivores are defined on anatomical grounds, including their small size, their long and mobile snouts and their plantigrade habit, i.e. they walk on the soles of their feet with the heels of their hindlegs on the ground.
Insectivores are often said to be the most primitive of placental mammals, though you may know from S182_1 Studying mammals: a winning design (the first course in OpenLearn's Studying mammals series) that the whole notion of primitiveness is contentious. It is generally agreed that modern-day insectivores most closely resemble, in both their structure and their way of life, those ancestral mammals from which all modern mammals are thought to have evolved. But not all modern-day insectivores are primitive. As you'll see in this course, many insectivores have evolved specialised structures and behaviours that to different degrees disguise an underlying primitiveness.
What group of living mammals does DA suggest resembles the earliest insectivorous mammals?
The earliest mammalian insectivores are described as small and shrew-like [p. 38].
Shrews occupy the same niche as the earliest insectivorous mammals - small and ground-dwelling, feeding on insects in leaf litter and undergrowth. The term 'niche' is used to describe an animal's role within its particular environment. It reflects various characteristics, such as habitat range, how the animal feeds, how it captures its food, its environmental requirements, the food it eats and what are its predators. A niche is, therefore, a description of lifestyle within a particular ecosystem - a term that means a community of living organisms, including different species of plants, animals and fungi, which interact with one another, and with their physical environment. You'll learn more about ecosystems if you work through the remaining units in this series.
A variety of orders of placental mammals exist. Taxonomists argue a great deal about the evolutionary origins of these different groups and how the taxonomic boundaries ought to be drawn, so the exact number of mammalian orders varies from 19 to 27, depending on whose scheme you follow. But LoM focuses largely on members of just four orders of largely insect-eating mammals: Insectivora, Chiroptera, Pholidota and Xenartha. It isn't appropriate here to describe the defining features of each of these four orders in detail, but we can gain an impression of the diversity of insect-eating mammals from a brief exploration of various niches that they occupy, building on the coverage in LoM.
The order Insectivora comprises a very diverse range of mammals, including shrews, desmans, moles, tenrecs and hedgehogs - which LoM describes in that sequence. (You have the option of rereading pp. 38-44 and pp. 51-54 to refresh your memory.) Other insectivores are the golden moles, which I'll mention later; these animals are wonderfully filmed in the TV programme as 'they swim through sand'. Another group of insect eaters are the elephant shrews. You might recall these animals from the TV programme looking like large versions of true shrews, with their 'path-tidying habit' captured on surveillance camera. Many taxonomists now argue that elephant shrews are sufficiently different from insectivores (and closer to rabbits and hares) to warrant being assigned a separate order, all to themselves.
The long-tailed shrew tenrec [p. 52] also resembles a large shrew, and occupies the same niche, foraging in undergrowth and leaf litter for insects and other invertebrates. As a group, tenrecs (and their close relatives the otter shrews) are very diverse in appearance - a good many are aquatic. The family as a whole displays features that, to some degree, resemble those of ancestral mammals. For example, their body temperature is low and varies throughout a 24-hour period. There is also a cloaca - the common opening for the digestive and urinogenital systems - that is also present in non-mammals such as lizards.
Hedgehogs eat earthworms, slugs, eggs and frogs, in addition to insects. They, of course, are considerably larger than shrews and cannot always hide in undergrowth when foraging, so their niche is not quite the same as that of shrews. DA refers to hedgehogs 'fossicking' around the garden [p. 53] - a delightful term but one with no precise biological meaning, other than the type of 'rummaging around' feeding behaviour evident under DA's gaze in the TV programme.
Other insectivores occupy niches characterised by preying on insects that live in water and soil.
From LoM and the associated TV programme list insectivores that forage for insects (a) underwater, and (b) underground.
(a) Desmans, star-nosed moles and water shrews dive underwater to forage for insects. (b) Moles and golden moles occupy burrowing niches, hunting insects and other invertebrates, such as worms, below the ground.
Bats, grouped in the order Chiroptera, have highly distinctive faces, sharp teeth and furry bodies; their defining feature is possession of wings. In the TV programme DA illustrates the huge range of bat diversity, showing that most bats feed on insects; a few feed on fish; some tropical species feed on fruit, and a few feed on nectar, frogs or mammalian blood. Most insect-eating species are nocturnal, preying on night-flying insects - a very different niche from that occupied by shrews. LoM suggests that bats evolved from a shrew-like ancestor early in the evolutionary history of the Insectivora [p. 44], and highlights the oldest known bat fossil, dated at 50 million years old. You'll probably recall the spectacular film sequences of the New Zealand short-tailed bat in the TV programme (at 44.16), which demonstrate its surprising agility on the ground, helped by a unique ability to tuck its wings tightly against its body. These bats catch prey in the air (using echolocation as described on p. 46), but rely upon hearing and sensitivity to smell when hunting in the leaf litter, which they do in a remarkably 'shrew-like' way.
Armadillos and anteaters, grouped in the order Xenartha, and pangolins, order Pholidota, occupy the 'ant-eating' niche, in which animals break into nests of colonial insect species, and eat large numbers during one hunting foray. In the TV programme, DA demonstrates evidence that shows that pangolins and anteaters have persisted unchanged for many millions of years. Fossil anteaters, tamanduas and pangolins, dated at 50 million years old, were found in the Messel Shales in Germany, and resemble species living today in South America and Africa. The TV sequence of a fossil pangolin 'coming to life' on the museum bench as an African pangolin is memorable (see the programme at 27.22), with just a glimpse of the unique 'shuffling gait' you saw in living specimens.
You have just met a lot of detailed information about the classification of insect eaters. You might find it useful to summarise this material for yourself in a notebook, so that you can refer to it as you progress through this course. You could copy down the four orders and list the names of the species mentioned in each of them. You can add extra species to your lists as you meet them. Similarly, as you read on through Section 2, you might like to make another list of the different sorts of teeth that are found in mammals, with their distinctive features. Teeth will be mentioned in several of the later units so, again, this list may be useful for reference. Whenever you study a section, consider whether it might be useful to summarise any of the information in a list of this type.