11. Humans and mammals: the past and the future

From the very start of the spread of humans, the world’s mammals have been irreversibly, and mostly detrimentally, affected through direct exploitation for food and for skins to make clothes and shelter. Our domestication of certain mammal species has also had a huge impact on the rest of the world’s mammalian fauna, and indeed on its whole biota. ‘Humans and mammals: the past and the future’ considers how human activity has caused the latest megafaunal extinction and looks at the future crisis facing many mammalian species. A quarter of mammalian species are, today, faced with severe population decreases that may result in extinction. What can be done to conserve them?

9. Flying mammals

Being able to fly opens up a whole new range of habits and lifestyles. A flyer can seek food from the tops of trees and airborne insects. It can avoid ground-based predators and rear its young safely out of reach. But flying is the most demanding form of locomotion, requiring a lot of power. ‘Flying mammals’ describes the single mammalian Order that has evolved flight—Chiroptera—with almost 1,200 species of bat worldwide, often living in colossal colonies. It also describes other airborne mammals that glide passively (using a membrane stretched between the limbs) rather than by actively flapping wings (e.g. colugos of Southeast Asia, Australian opossums, and flying squirrels).

6. Herbivorous mammals

Compared to a predator’s diet, plant food has two great advantages: it is abundant and it does not run away. ‘Herbivorous mammals’ explains how these advantages are matched by difficulties: plants are generally of low nutritional value and must be eaten in large amounts; leaves with protective abrasive particles can quickly wear down herbivores’ chewing teeth; and mammals cannot make their own cellulase enzymes for breaking down cellulose to sugars. The eating habits and the challenges of small herbivores (e.g. rodents, rabbits, and hyraxes) are considered, as well as those of large ungulates and elephants; marsupial herbivores (e.g. kangaroos, wombats, and koalas); and specialist herbivores (pandas, dugongs, and manatees).

4. The radiation of mammals

For two-thirds of their entire history, right through the 135 million years left of the Mesozoic Era, mammals were all small, secretive animals feeding at night on insects, worms, seeds, and plant tubers. Mammals were restricted in size due to their competition with the dinosaurs. But mammalian evolution continued and early examples of placentals and marsupials have been dated to this period. ‘The radiation of mammals’ outlines the end-Cretaceous mass extinction period 65 mya when over 60 per cent of all living species disappeared. It also describes the Tertiary radiation of mammals through changing climates, and the mammalian evolution on the island continents of Australia and South America.

1. What is a mammal?

‘What is a mammal?’ considers the diverse range of animals of varying forms, lifestyles, and habitats that make up the Class Mammalia. Despite the diversity, these animals share key characteristics: they have a single bone in the lower jaw; three little, sound-conducting ear bones; a very large forebrain; a permanently warm body and a high energy-expenditure level; and females have mammary glands for feeding milk to their young. The c.5,500 species of mammals alive today fall into three very unequal groups: the monotremes found in Australasia; the marsupials found in Australasia and South America; and the placentals, of which there are 5,000 species classified into twenty Orders.

10. Primates

‘Primates’ considers how a group of small, rather insignificant, tree-dwelling mammals living 60 mya eventually evolved the highest level of expression of the mammalian characteristic of adaptable behaviour by means of a large brain. It first discusses lemurs, lorises, bush babies, and tarsiers. It then describes the differences between New World and Old World monkeys, part of the Anthropoidea, which started their separate evolutionary journeys around 30 mya. Finally, it considers the rest of the Anthropoidea—the lesser apes (gibbons) and the great apes (orang-utans, chimpanzees, gorillas, and humans). The two most important new adaptations to evolve in humans are bipedalism and a huge brain.

8. Aquatic mammals

Virtually all mammals can swim at least sufficiently well to cross bodies of water, but the extent of specific adaptation for aquatic life varies amongst different mammals from no more than modified behaviour like the Japanese macaque monkeys spending their days sitting in hot springs, through different degrees of amphibiousness to the permanently marine whales and dolphins, dugongs and manatees that cannot come out onto land at all. ‘Aquatic mammals’ first discusses the mammals living in freshwater that have evolved various degrees of aquatic adaptation before considering the marine mammals such as pinnipeds (sea lions, seals, and walruses), sirenians (dugongs and manatees), and cetaceans (whales, dolphins, and porpoises).

5. Carnivorous mammals

‘Carnivorous mammals’ considers the various carnivorous mammals that have evolved from the common ancestor of all modern mammals—the small, nocturnal, insectivorous mammal. Several mammals still follow this mode of life today: the placental shrews, moles, and hedgehogs; the tenrecs of Madagascar; and many of the opossums of South America and Australia. These have sharp, pointed incisors and canines for capturing prey, followed by sharp-crested premolar and molar teeth. The evolution of a small, insectivorous ancestor into larger bodied, predaceous mammals required relatively few anatomical changes, the main adaptations being for capture of its prey—by stealth hunting by solitary animals or pack hunting by organized social groups.

2. The biology of mammals

In order to understand mammals and their amazing variety, we must first learn about their shared biology. ‘The biology of mammals’ first considers how and why mammals stay warm and active. Mammals are endotherms, meaning they have a high metabolic rate, high and constant body temperature, and high maximum activity level. A high metabolic rate requires a lot of food and oxygen, but the benefits are that mammals can remain active for longer and they are more complex internally. Their ability to save water, methods of obtaining food, modes of locomotion, sense organs, and methods of reproduction are also considered alongside their brain and behaviour.

7. Diggers and burrowers

‘Diggers and burrowers’ considers mammals at the opposite extreme to the high-speed running ungulate mammals. It describes mammals with short, powerfully built legs that bear large feet equipped with strong, blunt claws. When the limb muscles contract, they move the limb relatively slowly but powerfully, and so a large force is applied to the ground. Most of these animals are adapted for digging burrows to live in and find underground food such as beetle larvae or plant rhizomes. Examples are moles and mole-rats. With aardvarks, pangolins, and anteaters, the limbs are adapted for digging into extremely hard ant and termite nests, when particularly powerful muscles and claws are needed.